The ocean is critical to climate change prediction and mitigation, but major knowledge gaps remain. The IOC-R Report identifies clear global priorities for integrated ocean carbon research, observation, and cooperation to close these knowledge gaps. Supported by interdisciplinary research, understanding of the ocean carbon cycle has advanced significantly since the last release of a report from the IOC-R community (IOC of UNESCO, 2021; Sabine et al., 2024). However, fundamental knowledge gaps remain leading to a discrepancy of about 25% between model and observation based estimates of the magnitude of the ocean inorganic carbon sink. Future projections of the change in ocean uptake, however, are highly uncertain. Likewise, there is a ~ 40% difference in data driven model predictions of the size and even the direction of the organic carbon sink mediated by the marine foodweb. These uncertainties severely hamper the development of climate change adaptation and mitigation strategies, including those involving ocean based solutions. This report has brought together the foremost international scientists in ocean carbon research to provide a consensus on the priority research needs required to close these fundamental knowledge gaps. These include operationalising ocean carbon observations, undertaking multiple driver manipulation and adaptation studies on marine plankton, creating a land-ocean observational programme which incorporates local and indigenous knowledge, and evaluating the impact of future ocean based industrial processes including fisheries. Improved quantification of the ocean carbon sink can then be used to assess whether ocean based climate interventions are feasible, safe and effective. By setting the priority needs for integrated ocean carbon research, observation, and cooperation, the report allows the international community to identify those essential actions for moving ocean carbon research forward to which they can contribute. The report invites the global community of countries, policymakers, funding bodies, local communities, and researchers, to partake and address the interdisciplinary challenges in understanding, predicting and mitigating inevitable changes of the global carbon cycle.

Abstract The Arctic region is experiencing rapid warming and an increase in the frequency and intensity of marine heatwaves (MHWs), posing an increasing risk to coastal ecosystems. Kelp communities in the Arctic are vital habitats that support biodiversity and resilience but are particularly vulnerable to increasing temperature anomalies. In this study, we conducted a mesocosm experiment over 23 days in summer to assess the effects of warming and MHWs on three kelp species ( Saccharina latissima , Laminaria digitata and Alaria esculenta ) and associated community members: coralline algae, snails, sea urchins and mussels. Three warming scenarios were tested, all of which were offset from a naturally variable control. The four conditions were a control (i.e. ambient seawater) which ranged from ~7.5 to 10°C (naturally variable), a constant high‐temperature treatment (+1.8°C from the control), a treatment simulating a single 13‐day heatwave with a maximum offset temperature of +2.8°C from the control, and a treatment simulating two consecutive 5‐day MHWs with a maximum offset of +3.9°C. For the three kelp species, net photosynthetic rate, maximum quantum yield, chlorophyll a content, carbon to nitrogen ratio and growth rate were investigated. For the associated species, only growth rates were evaluated. Laminaria digitata showed no significant changes in any of the measured parameters, suggesting a robust tolerance to heat stress. Conversely, the maximum quantum yield for S. latissima decreased in the two heatwaves treatment. In addition, A. esculenta displayed a decrease in chlorophyll a content and net photosynthesis over time in the two heatwaves treatment. The growth rates of coralline algae, mussels, snails and sea urchins were not affected by the treatments. Synthesis : These results suggest that Arctic kelp communities are fairly tolerant to warming and short‐term MHWs, despite the few minor effects on photosynthesis by some of the kelp species. We note that the future response of kelp communities must account for the potential indirect effects of climate change, such as predation and competition, along with any consequential effects of warming anomalies, as those presented here.

Rapid, deep and sustained reductions in carbon dioxide (CO₂) emissions are essential to achieve the goals of the Paris Climate Agreement of keeping the long-term global average surface temperature increase well below 2°C above pre-industrial levels and pursue efforts to limit it to 1.5°C1. In addition, the 2021 IPCC Report explains that carbon dioxide removal (CDR) will be needed to offset residual CO₂ emissions from activities and sectors that are difficult to decarbonize by 2050 (Arias et al., 2021). The objective of CDR is removal of atmospheric CO2 from residual emissions and its durable storage in reservoirs, which is an additional critical element towards achieving carbon neutrality by 2050 and thereby ensure less than 2°C global warming. The annual estimates of CDR required in 2030 and by 2050 are 3.6 Gt and 9.4 Gt, respectively (Lamb et al., 2024), leaving a CDR gap of 1 Gt by 2030 and 6.8 Gt by 2050. How much of this gap can be filled sustainably by land-based CDR is unknown. Novel CDR methods include direct air carbon capture and storage (DACCS), biochar, and various marine approaches. Although these novel methods currently account for <0.1% of CDR worldwide, many are being tested through model simulations and small-scale pilot projects. Despite the ocean’s critical role in regulating Earth’s climate, mCDR offers substantial untapped opportunities that have so far been overlooked. Modeling indicates that several mCDR methods could scale to a billion tonnes annually, but their potential ecological side-effects are poorly known. Exploration of the potential of safe, durable and verifiable mCDR and its scalability within sustainability limits is urgently required, even though the process of testing, refining, verifying, and scaling mCDR will take at least a decade. (Boyd et al., 2023a). Time is short, and policymakers must therefore prioritize an ambitious timeline to deliver safe, sustainable, durable, and verifiable mCDR solutions that can potentially scale in parallel with land-based efforts, together with a regulatory framework for deployment.

Mediterranean open marine and coastal ecosystems face multiple risks that impact their unique biodiversity, with climate change representing a major ongoing threat. While these ecosystems are also under pressure from non-climatic anthropogenic drivers (e.g., overfishing, pollution), this study primarily focuses on risks related to climate change. To assess these risks and evaluate their confidence levels, we adopt the scenario-based approach of the Intergovernmental Panel on Climate Change (IPCC), relying on a review of literature projecting changes in Mediterranean Sea ecosystems. The main drivers of environmental change are sea level rise, ocean warming and acidification. Similar to global conditions, all Mediterranean ecosystems face high risks under all climate scenarios, with coastal ecosystems being more strongly impacted than open marine ecosystems. For these coastal ecosystems, risk levels are expected to become very high already once global warming exceeds 0.8 °C with respect to the 1976–2005 period. A few Mediterranean ecosystems (e.g., coralligenous and rocky coasts) are relatively more resilient compared to others, probably because of their long evolutionary history and the presence of a variety of climatic and hydrological conditions. However, high-emission scenarios in specific sub-basins, in addition to acidification impacts, could reduce this resilience, decreasing both habitat extent and ecosystem function dramatically. Overall, due to the higher observed and projected rates of climate change in the Mediterranean, compared to global trends, for variables such as seawater temperature and pH, marine ecosystems (particularly coastal) are projected to be under higher risks compared to the global ocean.

Abstract Coral ecosystems support a diverse array of marine life and healthy ecological functioning, yet they are vulnerable to decreases in ocean pH caused by anthropogenic carbon dioxide emissions. In temperate rocky reefs of the Mediterranean, the corals Cladocora caespitosa and Astroides calycularis live at sites with ambient seawater pH and at adjacent submarine volcanic CO 2 vent sites with low seawater pH where it is more energetically demanding to grow. We collected corals from distinct ambient pH (average pH T 8.05) and lower pH CO 2 vent sites (average pH T 7.74–7.90) and quantified their physiological health and heterotrophic capacity (i.e., feeding capacity). Both species at CO 2 vent sites had higher heterotrophic capacity than their ambient site counterparts, enabling them to maintain energy reserves. Our results indicate that high heterotrophic capacity underlies the success of these two temperate corals at CO 2 vent sites. Therefore, conservation of CO 2 vent coral could be strategically important to maintaining rocky reef ecosystem function and ecological resilience in the Mediterranean.

Recognizing the unprecedented severity of mass mortality of corals in 2023 and 2024, where global ocean temperature exceeded previous records by a large margin, and the severe risk of catastrophic loss of coral reefs, the International Scientific Committee of the One Ocean Science Congress calls on the nations gathering at Third United Nations Ocean Conference to take urgent actions.

Unraveling the functional future of marine ecosystems amid global change poses a pressing challenge. This is particularly critical in the Mediterranean Sea, which is highly impacted by global and local drivers. Utilizing extensive mass mortality events (MMEs) datasets spanning from 1986 to 2020 across the Mediterranean Sea, we investigated the trait vulnerability of benthic species that suffered from MMEs induced by nine distinct mortality drivers. By analyzing changes in ten ecological traits across 389 benthic species—constituting an extensive compendium of Mediterranean ecological traits to date—we identified 228 functional entities (FEs), defined as groups of species sharing the same trait values. Our findings indicate that of these 55 FEs were impacted by MMEs, accentuating a heightened vulnerability within specific trait categories. Notably, more than half of the mortality records showed severe impacts on calcifying and larger species with slower growth which mostly account for tree-like and massive forms. Altogether, we highlight that 29 FEs suffered extreme mortality, leading to a maximum increase of 19.1% of the global trait volume vulnerability over 35 years. We also reveal that 10.8% of the trait volume may have been temporarily lost over the last five years, emphasizing the risk of a rapid ecological transformation in the Mediterranean Sea.

The Paris Agreement to limit global warming to well below 2°C requires drastic reductions in greenhouse gas emissions and the balancing of any remaining emissions by carbon dioxide removal (CDR). Due to uncertainties about the potential and durability of many land-based approaches to deliver sufficient CDR, marine CDR options are receiving more and more interest. We present the current state of knowledge regarding the potentials, risks, side effects as well as challenges associated with technical feasibility, governance, monitoring, reporting and accounting of marine CDR, covering a range of biotic and geochemical approaches. We specifically discuss to what extent a comparison with direct injection of CO 2 into seawater, which had been proposed decades ago and is now prohibited by international agreements, may provide guidance for evaluating some of the biotic marine CDR approaches.

We outline the critical importance of the ocean for planetary health, economic prosperity, and human well-being, while highlighting the urgent threats it faces from climate change, pollution, overfishing, and poor governance. It presents the One Ocean Science Congress (OOSC) and the 3rd United Nations Ocean Conference (UNOC3), scheduled for June 2025 in Nice, France, as pivotal events aimed at promoting ocean sustainability through science-based policy, innovation, and global cooperation. The OOSC will deliver scientific recommendations to Heads of States and Govern-ments, while additional high-level events—the Ocean Rise and Coastal Resilience Summit and the Blue Economy and Finance Forum—will address climate adaptation and sustainable investment. The outcome of these efforts will be the Nice Ocean Action Plan, comprising a political declaration, voluntary commitments, and strategic priorities focused on multilateral processes, financial mobilization, and enhanced scientific knowledge to advance the Sustainable Develop-ment Goal 14 and ensure a resilient, thriving ocean for future generations.

Abstract Kelps have an extensive distribution in Arctic coastal waters. However, quantifying their role in the Arctic food web and carbon cycle is challenged by the scarcity of documented geographical distribution, standing stocks and production. Here we present a framework based on an empirical function to predict the potential kelp distribution and their summer biomass as a function of seafloor irradiance and bathymetry. Predictions of biomass were limited to the lower‐depth, light‐limited range of the kelp distribution, where the fit of the empirical function was significant (from the depth of maximum biomass to the deepest kelp extent). The model was developed and tested in Kongsfjorden, Svalbard, and applied in six additional fjords in the Arctic. The predicted potential kelp biomass in the fjords ranged from 0.6 to 4.7 kg WW m −2 and was in good agreement with published values. The resulting kelp standing stock ranged from 0.4 to 300 Gg DW, corresponding to 0.2–109 Gg C. These potential estimates account for light limitation, but do not consider substrata or other factors limiting the kelp distribution area. We identified fjord‐specific dependencies between predicted standing stocks and seafloor irradiance and between seafloor irradiance and its drivers (surface irradiance and water column light attenuation) but found no significant change between 2004 and 2022. Our framework provides a baseline for estimating potential kelp biomass from seafloor irradiance, which is expected to change with increasing sediment runoff causing coastal darkening.

Established in 2018, the High Level Panel for a Sustainable Ocean Economy (Ocean Panel) is a unique initiative made up of serving world leaders who are building momentum for a sustainable ocean economy in which effective protection, sustainable production and equitable prosperity go hand in hand. By working collaboratively with a wide array of stakeholders, the Ocean Panel aims to identify bold solutions that bridge ocean health, wealth and equity and accelerate and scale responsive action worldwide. This Blue Paper was prepared in support of the work of the Ocean Panel to provide a robust science and knowledge base and practical opportunities for action across issues central to the attainment of a sustainable ocean economy. The Blue Paper was developed by consensus of the authors who have balanced their individual academic and other perspectives. The arguments, findings and opportunities outlined in this Blue Paper represent the views of the authors alone. Ocean Panel members have not been asked to formally endorse the Blue Paper and should not be taken as having done so.

This paper reviews the outcomes of the One Ocean Science Congress (OOSC) and its strategic alignment with the Third United Nations Ocean Conference (UNOC3) as a case study in ocean science diplomacy. Convened in June 2025 in Nice, France, as a UN Special Event immediately preceding UNOC3, the OOSC introduced an innovative model for multilateral conferences by providing a direct channel for transmitting science-based recommendations to policymakers. This sequencing sought to ensure that political deliberations were anchored in the most current knowledge. The extent to which OOSC outputs influenced UNOC3 discussions and outcomes is analyzed. While causality cannot be firmly established, evidence shows that 44 of the 60 OOSC recommendations are reflected in UNOC3 documents, indicating substantial uptake. Notable alignments include recognition of the importance of international cooperation in ocean research and the inclusion of indigenous knowledge systems in governance frameworks. Voluntary commitments, such as a precautionary pause on deep-sea mining, further mirrored OOSC's core messages. The paper concludes that the OOSC-UNOC3 sequence demonstrates the potential of strategically timed, policy-ready scientific inputs to enhance the science-policy interface. Interest from future UNOC hosts in replicating this approach underscores its promise as a model for embedding science into high-level ocean governance.

Abstract. Total alkalinity (AT) and dissolved inorganic carbon (CT) in the oceans are important properties to understand the ocean carbon cycle and its link with global change (ocean carbon sinks and sources, ocean acidification) and ultimately to find carbon-based solutions or mitigation procedures (marine carbon removal). We present an extended database (SNAPO-CO2; Metzl et al., 2024c) with 24 700 new additional data for the period 2002 to 2023. The full database now includes more than 67 000 AT and CT observations along with basic ancillary data (time and space location, depth, temperature, and salinity) in various oceanic regions obtained since 1993 mainly in the framework of French research projects. This includes both surface and water columns data acquired in open oceans, coastal zones, rivers, the Mediterranean Sea, and either from time series stations or punctual cruises. Most AT and CT data in this synthesis were measured from discrete samples using the same closed-cell potentiometric titration calibrated with certified reference material, with an overall accuracy of ±4 µmol kg−1 for both AT and CT. The same technique was used on board for underway measurements during cruises conducted in the southern Indian and Southern oceans. The AT and CT data from these cruises are also added to this synthesis. The data are provided in one dataset for the global ocean (https://doi.org/10.17882/102337, Metzl et al., 2024c) that offers a direct use for regional or global purposes, e.g., AT–salinity relationships, long-term CT estimates, constraint and validation of diagnostics CT and AT reconstructed fields, ocean carbon and coupled climate–carbon models simulations, and data derived from Biogeochemical Argo (BGC-Argo) floats. These data can also be used to calculate pH, fugacity of CO2 (fCO2), and other carbon system properties to derive ocean acidification rates or air–sea CO2 fluxes.

Arctic fjords are considered to be one of the ecosystems changing most rapidly in response to climate change. In the Svalbard archipelago, fjords are experiencing a shift in environmental conditions due to the Atlantification of Arctic waters and the retreat of sea-terminating glaciers. These environmental changes are predicted to facilitate expansion of large, brown macroalgae, into new ice-free regions. The potential resilience of macroalgal benthic communities in these fjord systems will depend on their response to combined pressures from freshening due to glacial melt, exposure to warmer waters, and increased turbidity from meltwater runoff which reduces light penetration. Current predictions, however, have a limited ability to elucidate the future impacts of multiple-drivers on macroalgal communities with respect to ecosystem function and biogeochemical cycling in Arctic fjords. To assess the impact of these combined future environmental changes on benthic productivity and resilience, we conducted a two-month mesocosm experiment exposing mixed kelp communities to three future conditions comprising increased temperature (+ 3.3 and + 5.3°C), seawater freshening by ∼ 3.0 and ∼ 5.0 units (i.e., salinity of 30 and 28, respectively), and decreased photosynthetically active radiation (PAR, - 25 and - 40 %). Exposure to these combined treatments resulted in non-significant differences in short-term productivity, and a tolerance of the photosynthetic capacity across the treatment conditions. We present the first robust estimates of mixed kelp community production in Kongsfjorden and place a median compensation irradiance of ∼12.5 mmol photons m$^−$$^2$ h$^−$$^1$ as the threshold for positive net community productivity. These results are discussed in the context of ecosystem productivity and biological tolerance of kelp communities in future Arctic fjord systems.

Global environmental change drives diversity loss and shifts in community structure. A key challenge is to better understand the impacts on ecosystem function and to connect species and trait diversity of assemblages with ecosystem properties that are in turn linked to ecosystem functioning. Here we quantify shifts in species composition and trait diversity associated with ocean acidification (OA) by using field measurements at marine CO2 vent systems spanning four reef habitats across different depths in a temperate coastal ecosystem. We find that both species and trait diversity decreased, and that ecosystem properties (understood as the interplay between species, traits, and ecosystem function) shifted with acidification. Furthermore, shifts in trait categories such as autotrophs, filter feeders, herbivores, and habitat-forming species were habitat-specific, indicating that OA may produce divergent responses across habitats and depths. Combined, these findings reveal the importance of connecting species and trait diversity of marine benthic habitats with key ecosystem properties to anticipate the impacts of global environmental change. Our results also generate new insights on the predicted general and habitat-specific ecological consequences of OA.

Since 2004, the Service facility SNAPO-CO2 (Service National d’Analyse des Paramètres Océaniques du CO2) housed by the LOCEAN laboratory (Paris, France) has been in charge for the analysis of Total Alkalinity (AT) and Total dissolved inorganic carbon (CT) of seawater samples on a series of cruises or ships of opportunity conducted in different regions in the frame of French projects. More than 44000 observations are synthetized in this work. Sampling was performed either from CTD-Rosette casts (Niskin bottles) or collected from the ship’s seawater supply (intake at about 5m depth). After completion of each cruise, discrete samples were returned back at LOCEAN laboratory and stored in a dark room at 4 °C before analysis generally within 2-3 months after sampling (sometimes within a week). AT and CT were analyzed simultaneously by potentiometric titration using a closed cell (Edmond, 1970). Certified Reference Materials (CRMs) provided by Pr. A. Dickson (Scripps Institution of Oceanography, San Diego, USA) were used to calibrate the measurements. The same instrumentation was used for underway measurements during OISO cruises (https://doi.org/10.18142/228) and OISO AT-CT data for 1998-2018 in the South Indian Ocean added in this synthesis. The synthesis is organized in two files (one for Global ocean and the Coastal Zones, one for the Mediterranean Sea) with the same format: Cruise name, Ship name, day, month, year, hour, minute, second, latitude, longitude, depth, AT (µmol/kg), Flag-AT, CT (µmol/kg), Flag-CT, Temperature (°C), Flag-Temp, Salinity (PSU), Flag-Salinity, nsample/cruise, nsample on file, sampling method.

Abstract Fjord systems in the Norwegian Arctic are experiencing an increasing frequency and magnitude of marine heatwaves. These episodic heat stress events can have varying degrees of acute impacts on primary production and nutrient uptake of mixed kelp communities, as well as modifying the biogeochemical cycling in nearshore systems where vast areas of kelp create structural habitat. To assess the impact of future marine heatwaves on kelp communities, we conducted a 23 day mesocosm experiment exposing mixed kelp communities to warming and heatwave scenarios projected for the year 2100. Three treatments were considered: a constant warming (+1.8°C from the control), a medium magnitude and long duration heatwave event (+2.8°C from the control for 13 days), and two short‐term, more intense, heatwaves(5 day long scenarios with temperature peaks at +3.9°C from the control). The results show that both marine heatwave treatments reduced net community production, whereas the constant warm temperature treatment displayed no difference from the control. The long marine heatwave scenario resulted in reduced accumulated net community production, indicating that prolonged exposure had a greater severity than two high magnitude, short‐term heatwave events. We estimated an 11°C temperature threshold at which negative effects to primary production appeared present. We highlight that marine heatwaves can induce sublethal effects on kelp communities by depressing net community production. These results are placed in the context of potential physiological resilience of kelp communities and implications of reduced net community production to future Arctic fjord environmental conditions.

Abstract. Total alkalinity (AT) and dissolved inorganic carbon (CT) in the oceans are important properties with respect to understanding the ocean carbon cycle and its link to global change (ocean carbon sinks and sources, ocean acidification) and ultimately finding carbon-based solutions or mitigation procedures (marine carbon removal). We present a database of more than 44 400 AT and CT observations along with basic ancillary data (spatiotemporal location, depth, temperature and salinity) from various ocean regions obtained, mainly in the framework of French projects, since 1993. This includes both surface and water column data acquired in the open ocean, coastal zones and in the Mediterranean Sea and either from time series or dedicated one-off cruises. Most AT and CT data in this synthesis were measured from discrete samples using the same closed-cell potentiometric titration calibrated with Certified Reference Material, with an overall accuracy of ±4 µmol kg−1 for both AT and CT. The data are provided in two separate datasets – for the Global Ocean and the Mediterranean Sea (https://doi.org/10.17882/95414, Metzl et al., 2023), respectively – that offer a direct use for regional or global purposes, e.g., AT–salinity relationships, long-term CT estimates, and constraint and validation of diagnostic CT and AT reconstructed fields or ocean carbon and coupled climate–carbon models simulations as well as data derived from Biogeochemical-Argo (BGC-Argo) floats. When associated with other properties, these data can also be used to calculate pH, the fugacity of CO2 (fCO2) and other carbon system properties to derive ocean acidification rates or air–sea CO2 fluxes.

Since the beginning of the industrial revolution, atmospheric carbon dioxide (CO2) concentrations have risen steadily and have induced a decrease of the averaged surface ocean pH by 0.1 units, corresponding to an increase in ocean acidity of about 30 %. In addition to ocean warming, ocean acidification poses a tremendous challenge to some marine organisms, especially calcifiers. The need for long-term oceanic observations of pH and temperature is a key element to assess the vulnerability of marine communities and ecosystems to these pressures. Nearshore productive environments, where a large majority of shellfish farming activities are conducted, are known to present pH levels as well as amplitudes of daily and seasonal variations that are much larger than those observed in the open ocean. Yet, to date, there are very few coastal observation sites where these parameters are measured simultaneously and at high frequency. To bridge this gap, an observation network was initiated in 2021 in the framework of the CocoriCO2 project. Six sites were selected along the French Atlantic and Mediterranean coastlines based on their importance in terms of shellfish production and the presence of high- and low-frequency monitoring activities. At each site, autonomous pH sensors were deployed, both inside and outside shellfish production areas, next to high-frequency CTD (conductivity–temperature–depth) probes operated through two operating monitoring networks. pH sensors were set to an acquisition rate of 15 min, and discrete seawater samples were collected biweekly in order to control the quality of pH data (laboratory spectrophotometric measurements) as well as to measure total alkalinity and dissolved inorganic carbon concentrations for full characterization of the carbonate system. While this network has been up and running for more than 2 years, the acquired dataset has already revealed important differences in terms of pH variations between monitored sites related to the influence of diverse processes (freshwater inputs, tides, temperature, biological processes). Data are available at https://doi.org/10.17882/96982 (Petton et al., 2023a).

Abstract. The rapid environmental changes in aquatic systems as a result of anthropogenic forcings are creating a multitude of challenging conditions for organisms and communities. The need to better understand the interaction of environmental stressors now, and in the future, is fundamental to determining the response of ecosystems to these perturbations. This work describes an automated ex situ mesocosm perturbation system that can manipulate several variables of aquatic media in a controlled setting. This perturbation system was deployed in Kongsfjorden (Svalbard); within this system, ambient water from the fjord was heated and mixed with freshwater in a multifactorial design to investigate the response of mixed-kelp communities in mesocosms to projected future Arctic conditions. The system employed an automated dynamic offset scenario in which a nominal temperature increase was programmed as a set value above real-time ambient conditions in order to simulate future warming. A freshening component was applied in a similar manner: a decrease in salinity was coupled to track the temperature offset based on a temperature–salinity relationship in the fjord. The system functioned as an automated mixing manifold that adjusted flow rates of warmed and chilled ambient seawater, with unmanipulated ambient seawater and freshwater delivered as a single source of mixed media to individual mesocosms. These conditions were maintained via continuously measured temperature and salinity in 12 mesocosms (1 control and 3 treatments, all in triplicate) for 54 d. System regulation was robust, as median deviations from nominal conditions were < 0.15 for both temperature (∘C) and salinity across the three replicates per treatment. Regulation further improved during a second deployment that mimicked three marine heat wave scenarios in which a dynamic temperature regulation held median deviations to < 0.036 ∘C from the nominal value for all treatment conditions and replicates. This perturbation system has the potential to be implemented across a wide range of conditions to test single or multi-stressor drivers (e.g., increased temperature, freshening, and high CO2) while maintaining natural variability. The automated and independent control for each experimental unit (if desired) provides a large breadth of versatility with respect to experimental design.

Bivalve farming was usually considered as a CO$_2$ source through respiration and calcification, but recent studies suggest its potential as a CO$_2$ sink, prompting exploration of its inclusion in carbon markets. Here we reviewed the scientific basis behind this idea and found that it is not supported by observational and experimental studies. This idea indeed arises from carbon budget models that are based on theoretical misconceptions regarding seawater carbonate chemistry. The main misunderstanding consists of assuming that the carbon trapped in the shell originates from atmospheric CO$_2$ when it mostly comes from (bi)carbonate ions. While these ions originate from atmospheric CO$_2$ through the erosion of minerals over geological time scales, their incorporation into shells does not prompt short‐term CO$_2$ compensation. The opposite occurs—calcification releases CO$_2$ in seawater and limits or even prevents the uptake of atmospheric CO$_2$ . Some authors suggest that considering the bivalve farm ecosystem could change the perspective on the source/sink issue but there is no evidence for that now. Most ecosystem‐based carbon budget models rely on several unverified assumptions and estimates. Although challenging, field measurements must be conducted for monitoring, reporting, and verifying atmospheric CO$_2$ uptake before qualifying for carbon credits. To achieve scientific consensus, we need reinforcing measurement‐based studies of CO$_2$ fluxes in shellfish ecosystems, integrating carbon balance models with observational and experimental science, and fostering interdisciplinary collaboration. Although bivalve farming provides numerous environmental benefits and is vital for sustainable aquaculture, there is currently no evidence that it contributes to CO$_2$ capture.

Many governments and industries are relying on future large-scale, land-based carbon dioxide (CO2) removal (CDR) to avoid making necessary steep greenhouse gas (GHG) emission cuts today (1, 2). Not only does this risk locking us into a high overshoot above 1.5°C (3), but it will also increase biodiversity loss, imperiling the Kunming-Montreal Global Biodiversity Framework (KMGBF) goals (4). Such CDR deployments also pose major economic, technological, and social feasibility challenges; threaten food security and human rights; and risk overstepping multiple planetary boundaries, with potentially irreversible consequences (1, 5, 6). We propose three ways to build on the Intergovernmental Panel on Climate Change (IPCC) analyses of CDR mitigation potential by assessing sustainability risks associated with land-use change and biodiversity loss: estimate the sustainable CDR budget based on socioecological thresholds; identify viable mitigation pathways that do not overstep these thresholds; and reframe governance around allocating limited CDR supply to the most legitimate uses.

Background Ocean-related options (OROs) to mitigate and adapt to climate change are receiving increasing attention from practitioners, decision-makers, and researchers. In order to guide future ORO development and implementation, a catalogue of scientific evidence addressing outcomes related to different ORO types is critical. However, until now, such a synthesis has been hindered by the large size of the evidence base. Here, we detail a protocol using a machine learning-based approach to systematically map the extent and distribution of academic evidence relevant to the development, implementation, and outcomes of OROs. Method To produce this systematic map, literature searches will be conducted in English across two bibliographic databases using a string of search terms relating to the ocean, climate change, and OROs. A sample of articles from the resulting de-duplicated corpus will be manually screened at the title and abstract level for inclusion or exclusion against a set of predefined eligibility criteria in order to select all relevant literature on marine and coastal socio-ecological systems, the type of ORO and its outcomes. Descriptive metadata on the type and location of intervention, study methodology, and outcomes will be coded from the included articles in the sample. This sample of screening and coding decisions will be used to train a machine learning model that will be used to estimate these labels for all the remaining unseen publications. The results will be reported in a narrative synthesis summarising key trends, knowledge gaps, and knowledge clusters.

Phytoplankton and sea ice algae are traditionally considered to be the main primary producers in the Arctic Ocean. In this Perspective, we explore the importance of benthic primary producers (BPPs) encompassing microalgae, macroalgae, and seagrasses, which represent a poorly quantified source of Arctic marine primary production. Despite scarce observations, models predict that BPPs are widespread, colonizing ~3 million km 2 of the extensive Arctic coastal and shelf seas. Using a synthesis of published data and a novel model, we estimate that BPPs currently contribute ~77 Tg C y −1 of primary production to the Arctic, equivalent to ~20 to 35% of annual phytoplankton production. Macroalgae contribute ~43 Tg C y −1 , seagrasses contribute ~23 Tg C y −1 , and microalgae-dominated shelf habitats contribute ~11 to 16 Tg C y −1 . Since 2003, the Arctic seafloor area exposed to sunlight has increased by ~47,000 km 2 y −1 , expanding the realm of BPPs in a warming Arctic. Increased macrophyte abundance and productivity is expected along Arctic coastlines with continued ocean warming and sea ice loss. However, microalgal benthic primary production has increased in only a few shelf regions despite substantial sea ice loss over the past 20 y, as higher solar irradiance in the ice-free ocean is counterbalanced by reduced water transparency. This suggests complex impacts of climate change on Arctic light availability and marine primary production. Despite significant knowledge gaps on Arctic BPPs, their widespread presence and obvious contribution to coastal and shelf ecosystem production call for further investigation and for their inclusion in Arctic ecosystem models and carbon budgets.

The coastal ocean represents an important global carbon sink and is a focus for interventions to mitigate climate change and meet the Paris Agreement targets while supporting biodiversity and other ecosystem functions. However, the fate of the flux of carbon exported from seaweed forests—the world’s largest coastal vegetated ecosystem—is a key unknown in marine carbon budgets. Here we provide national and global estimates for seaweed-derived particulate carbon export below 200 m depth, which totalled 3–4% of the ocean carbon sink capacity. We characterized export using models of seaweed forest extent, production and decomposition, as well as shelf–open ocean water exchange. On average, 15% of seaweed production is estimated to be exported across the continental shelf, which equates to 56 TgC yr−1 (range: 10–170 TgC yr−1). Using modelled sequestration timescales below 200 m depth, we estimated that each year, 4–44 Tg seaweed-derived carbon could be sequestered for 100 years. Determining the full extent of seaweed carbon sequestration remains challenging, but critical to guide efforts to conserve seaweed forests, which are in decline globally. Our estimate does not include shelf burial and dissolved and refractory carbon pathways; still it highlights a relevant potential contribution of seaweed to natural carbon sinks.

The global increase in anthropogenic CO 2 is leading to ocean warming and acidification, which is threatening corals. In Ischia, Italy, two species of Mediterranean scleractinian corals–the symbiotic Cladocora caespitosa and the asymbiotic Astroides calycularis –were collected from ambient pH sites (average pH T = 8.05) and adjacent CO 2 vent sites (average pH T = 7.8) to evaluate their response to ocean acidification. Coral colonies from both sites were reared in a laboratory setting for six months at present day pH (pH T ~ 8.08) or low pH (pH T ~7.72). Previous work showed that these corals were tolerant of low pH and maintained positive calcification rates throughout the experiment. We hypothesized that these corals cope with low pH by increasing their heterotrophic capacity (i.e., feeding and/or proportion of heterotrophically derived compounds incorporated in their tissues), irrespective of site of origin, which was quantified indirectly by measuring δ 13 C, δ 15 N, and sterols. To further characterize coral health, we quantified energy reserves by measuring biomass, total lipids, and lipid classes. Additional analysis for C . caespitosa included carbohydrates (an energy reserve) and chlorophyll a (an indicator of photosynthetic capacity). Isotopic evidence shows that ambient-sourced Mediterranean corals, of both species, decreased heterotrophy in response to six months of low pH. Despite maintaining energy reserves, lower net photosynthesis ( C . caespitosa) and a trend of declining calcification ( A . calycularis ) suggest a long-term cost to low heterotrophy under ocean acidification conditions. Conversely, vent-sourced corals maintained moderate ( C . caespitosa ) or high ( A . calycularis ) heterotrophic capacity and increased photosynthesis rates ( C . caespitosa ) in response to six months at low pH, allowing them to sustain themselves physiologically. Provided there is sufficient zooplankton and/or organic matter to meet their heterotrophic needs, vent-sourced corals are more likely to persist this century and potentially be a source for new corals in the Mediterranean.

Abstract To limit global warming below 2°C by 2100, we must drastically reduce greenhouse gas emissions and additionally remove ~100–900 Gt CO 2 from the atmosphere (carbon dioxide removal, CDR) to compensate for unavoidable emissions. Seaweeds (marine macroalgae) naturally grow in coastal regions worldwide where they are crucial for primary production and carbon cycling. They are being considered as a biological method for CDR and for use in carbon trading schemes as offsets. To use seaweeds in carbon trading schemes requires verification that seaweed photosynthesis that fixes CO 2 into organic carbon results in CDR, along with the safe and secure storage of the carbon removed from the atmosphere for more than 100 years (sequestration). There is much ongoing research into the magnitude of seaweed carbon storage pools (e.g., as living biomass and as particulate and dissolved organic carbon in sediments and the deep ocean), but these pools do not equate to CDR unless the amount of CO 2 removed from the atmosphere as a result of seaweed primary production can be quantified and verified. The draw‐down of atmospheric CO 2 into seawater is via air‐sea CO 2 equilibrium, which operates on time scales of weeks to years depending upon the ecosystem considered. Here, we explain why quantifying air‐sea CO 2 equilibrium and linking this process to seaweed carbon storage pools is the critical step needed to verify CDR by discrete seaweed beds and nearshore and open ocean aquaculture systems prior to their use in carbon trading.

Abstract. The Arctic is projected to warm by 2 to 5 °C by the end of the century. Warming causes melting of glaciers, shrinking of the areas covered by sea ice, and increased terrestrial runoff from snowfields and permafrost thawing. Warming, decreasing coastal underwater irradiance, and lower salinity are potentially threatening polar marine organisms, including kelps, that are key species of hard-bottom shallow communities. The present study investigates the physiological responses of four kelp species (Alaria esculenta, Laminaria digitata, Saccharina latissima, and Hedophyllum nigripes) to these environmental changes through a perturbation experiment in ex situ mesocosms. Kelps were exposed for 6 weeks to four experimental treatments: an unmanipulated control; a warming condition under the CO2 emission scenario SSP5-8.5; and two multifactorial conditions combining warming, low salinity, and low irradiance reproducing the future coastal Arctic exposed to terrestrial runoff under two CO2 emission scenarios (SSP2-4.5 and SSP5-8.5). The physiological effects on A. esculenta, L. digitata, and S. latissima were investigated, and gene expression patterns of S. latissima and H. nigripes were analyzed. Across all species and experimental treatments, growth rates were similar, underlying the acclimation potential of these species to future Arctic conditions. Specimens of A. esculenta increased their chlorophyll a content when exposed to low irradiance conditions, suggesting that they may be resilient to an increase in glacier and river runoff with the potential to become more dominant at greater depths. S. latissima showed a lower carbon : nitrogen (C : N) ratio under the SSP5-8.5 multifactorial conditions' treatment, suggesting tolerance to coastal erosion and permafrost thawing. In contrast, L. digitata showed no response to the conditions tested on any of the investigated physiological parameters. The down-regulation of genes coding for heat-shock proteins in H. nigripes and S. latissima underscores their ability to acclimate to heat stress, which portrays temperature as a key influencing factor. Based on these results, it is expected that kelp communities will undergo changes in species composition that will vary at local scale as a function of the changes in environmental drivers.

The Mediterranean Sea is a hotspot of global change, particularly exposed to ocean warming and the increasing occurrence of marine heatwaves (MHWs). However, experiments based on long-term temperature data from the field are scarce. Here, we investigate the response of the zooxanthellate coral Cladocora caespitosa and the azooxanthellate coral Astroides calycularis to future warming and MHWs based on 8 years of in situ data. Corals were maintained in the laboratory for five months under four temperature conditions: Warming (3.2°C above the in situ mean from 2012 to 2020), Heatwave (temperatures of 2018 with two heatwaves), Ambient ( in situ mean) and Cool (deeper water temperatures). Under the Warming treatment, some C. caespitosa colonies severely bleached and A. calycularis colonies presented necrosis. Cladocora caespitosa symbiosis was impaired by temperature with a decrease in the density of endosymbiotic algae and an increase in per cent whiteness in all the treatments except for the coolest. Recovery for both species was observed through different mechanisms such as regrowth of polyps of A. calycularis and recovery of pigmentation for C. caespitosa . These results suggest that A. calycularis and C. caespitosa may be resilient to heat stress and can recover from physiological stresses caused by heatwaves in the laboratory.

The Paris Agreement to limit global warming to well below 2 °C requires ambitious emission reduction and the balancing of remaining emissions through carbon sinks, i.e. the deployment of carbon dioxide removal (CDR). While ambitious climate protection scenarios until now consider primarily land-based CDR methods, there is growing concern about their potential to deliver sufficient CDR, and marine CDR options receive more and more interest. Based on idealized theoretical studies, Ocean Alkalinity Enhancement (OAE) appears as a promising marine CDR method. However, the knowledge base is insufficient for a robust assessment of its practical feasibility, of its side effects, social and governance aspects as well as monitoring, reporting and verification issues. A number of research efforts aim to improve this in a timely manner. We provide an overview on the current situation of developing OAE as marine CDR method, and describe the history that has led to the creation of the OAE research Best Practices Guide.

Abstract. The collection of in situ data is generally a costly process, with the Arctic being no exception. Indeed, there has been a perception that the Arctic is lacking in situ sampling; however, after many years of concerted effort and international collaboration, the Arctic is now rather well sampled, with many cruise expeditions every year. For example, the GLODAP (Global Ocean Data Analysis Project) product has a greater density of in situ sampling points within the Arctic than along the Equator. While this is useful for open-ocean processes, the fjords of the Arctic, which serve as crucially important intersections of terrestrial, coastal, and marine processes, are sampled in a much more ad hoc process. This is not to say they are not well sampled but rather that the data are more difficult to source and combine for further analysis. It was therefore noted that the fjords of the Arctic are lacking in FAIR (findable, accessible, interoperable, and reusable) data. To address this issue, a single dataset has been created from publicly available, predominantly in situ data from seven study sites in Svalbard and Greenland. After finding and accessing the data from a number of online platforms, they were amalgamated into a single project-wide standard, ensuring their interoperability. The dataset was then uploaded to PANGAEA so that it can be findable and reusable in the future. The focus of the data collection was driven by the key drivers of change in Arctic fjords identified in a companion review paper. To demonstrate the usability of this dataset, an analysis of the relationship between the different drivers was performed. Via the use of an Arctic biogeochemical model, these relationships were projected forward to 2100 via Representative Carbon Pathways (RCPs) 2.6, 4.5, and 8.5. This dataset is a work in progress, and as new datasets containing the relevant key drivers are released, they will be added to an updated version planned for the middle of 2024. The dataset (Schlegel and Gattuso, 2022) is available on PANGAEA at https://doi.org/10.1594/PANGAEA.953115. A live version is available at the FACE-IT WP1 site and can be accessed by clicking the “Data access” tab: https://face-it-project.github.io/WP1/ (last access: 17 August 2023).

Human activities are causing a sustained increase in the concentration of carbon dioxide (CO2) and other greenhouse gases in the atmosphere. The resulting harmful effects on Earth’s climate require decarbonizing the economy and, given the slow pace and inherent limitations of decarbonization of some industries such as aviation, also the active removal and safe sequestration of CO2 away from the atmosphere (i.e., carbon dioxide removal or CDR; NASEM, 2022). Limiting global warming to 1.5°C—a target that may already have been exceeded—would require CDR on the order of 100–1000 Gt CO2 over the twenty-first century (IPCC, 2018).

<p>Climate change and biodiversity loss are two of humanity’s greatest challenges. Blue carbon, i.e. the carbon captured and stored by marine living organisms and ecosystems, has the potential to help mitigate both challenges, because marine ecosystems that are important for sequestering carbon often also harbour rich biodiversity. Expanding and protecting Blue Carbon ecosystems has therefore been proposed as a Nature-based Solution to complement climate change mitigation efforts on land and to protect and restore marine biodiversity. In addition, securing and rebuilding Blue Carbon ecosystems can stabilise livelihoods, protect coasts, and support other societal needs such as food provision from the Ocean.</p> <p>However, the effectiveness of Blue Carbon ecosystems as a Nature-based Solution depends on the available space and ecosystem productivity, which can be impacted by climate change. Moreover, the overall carbon sequestration potential of Blue Carbon ecosystems is low and their contribution to climate stabilisation will only be significant once greenhouse gas emissions are strongly limited. Therefore, a drastic reduction of greenhouse gas emissions to keep global warming close to 1.5°C above pre-industrial levels is essential to maintain the health and long-term functionality of Blue Carbon ecosystems as a Nature-based Solution.</p> <p>This document describes examples and benefits of Blue Carbon ecosystems, and discusses uncertainties and challenges for the conservation and restoration of Blue Carbon ecosystems as a climate change solution. It also highlights the wider role of the Ocean in mitigating climate change through the carbon cycle, and closes with key research and management recommendations.</p>

Abstract Effective climate policy that addresses carbon dioxide emissions is essential to minimizing and addressing the impacts of ocean acidification (OA). Here we present a framework to assess the readiness of OA policy, using coral reefs as a focal system. Six dimensions encompass comprehensive preparation by ecosystems and societies for the impacts of OA and other anthropogenic hazards: (1) climate protection measures, (2) OA literacy, (3) area-based management, (4) research and development, (5) adaptive capacity of dependent sectors, and (6) policy coherence. We define standardized indicators, identify leading countries, and evaluate the case study of Australia, the country with the largest coral reef system. The framework provides a rubric for a government unit to self- assess strengths and weaknesses in policy preparedness and to prioritize future endeavors.

Abstract. The rapid environmental changes in aquatic systems as a result of anthropogenic forcings are creating a multitude of challenging conditions for organisms and communities. The need to better understand the interaction of environmental stressors now, and in the future, is fundamental to determining the response of ecosystems to these perturbations. This work describes an in situ mesocosm perturbation system that can manipulate aquatic media in a controlled setting on land. The employed system manipulated ambient water from Kongsfjorden, (Svalbard) by increasing temperature and freshening the seawater to investigate the response of mixed kelp communities to projected future Arctic conditions. This system manipulated temperature and salinity in real-time as an offset from incoming ambient seawater to conditions simulating future Arctic fjords. The system adjusted flow rates and mixing regimes of chilled, heated, ambient seawater, and freshwater, based on continuously measured conditions in a total of 12 mesocosms (1 ambient-control and 3 treatments, all in triplicates) for 54 days. System regulation was robust as median deviations from setpoint conditions were < 0.15 for both temperature (°C) and salinity across the 3 replicates per treatment. The implementation of this system has a wide range of versatility and can be deployed in a range of conditions to test single or multi-stressor conditions while maintaining natural variability.

Arctic coastal ecosystems include benthic communities that hold an important role within the marine food chain. Kelps, fucoid species, and coralline algae dominate rocky habitats, offering food and shelter for various species. Kelps and fucoid species also aid in carbon sequestration, sediment stabilization, and erosion mitigation. In summer, the influx of freshwater from glacier and permafrost melt alters coastal waters conditions. The input of turbid freshwater influences underwater light, salinity, and substrate, impacting benthic organism distribution. This study investigates possible link between environmental conditions and benthic diversity through environmental DNA (eDNA). Six sites were monitored along Kongsfjorden (Svalbard,Norway) during the summer of 2021. Contrary to expectations, macroalgal distribution didn’t correlate with light, and suspension feeders showed no clear links with chlorophyll a or nutrient concentrations. Glacial influence may have contributed to higher benthic diversity. Predators’ presence, tied to glacier proximity, possibly explained this trend. However, further studies are needed to validate these observations and assumptions.

Effective management of data is essential for successful ocean alkalinity enhancement (OAE) research, as it guarantees the long-term preservation, interoperability, discoverability, and accessibility of data. OAE research generates various types of data, such as discrete bottle measurements, autonomous measurements from surface underway and uncrewed platforms (e.g., moorings, Saildrones, gliders, Argo floats), physiological response studies (e.g., laboratory, mesocosms, and field experiments, and natural analogues), and model outputs. This chapter covers data and metadata standards for all these types of OAE data. As part of this study, existing data standards have been updated to accommodate OAE research needs, and a completely new physiological response data standard has been introduced. Additionally, an existing ocean acidification metadata template has been upgraded to be applicable to OAE research. This chapter also presents controlled vocabularies for OAE research, including types of studies, alkalinization methods, platforms, and instruments. These guidelines will aid OAE researchers in preparing their metadata and data for submission to permanent archives. Finally, the chapter provides information about available data assembly centers (DACs) that OAE researchers can utilize for their data needs.

Abstract. The Arctic Ocean is subject to high rates of ocean warming and acidification, with critical implications for marine organisms as well as ecosystems and the services they provide. Carbonate system data in the Arctic realm are spotty in space and time, and, until recently, there was no time-series station measuring the carbonate chemistry at high frequency in this region, particularly in coastal waters. We report here on the first high-frequency (1 h), multi-year (5 years) dataset of salinity, temperature, CO2 partial pressure (pCO2) and pH at a coastal site (bottom depth of 12 m) in a high-Arctic fjord (Kongsfjorden, Svalbard). Discrete measurements of dissolved inorganic carbon and total alkalinity were also performed. We show that (1) the choice of formulations for calculating the dissociation constants of the carbonic acid remains unsettled for polar waters, (2) the water column is generally somewhat stratified despite the shallow depth, (3) the saturation state of calcium carbonate is subject to large seasonal changes but never reaches undersaturation (Ωa ranges between 1.4 and 3.0) and (4) pCO2 is lower than atmospheric CO2 at all seasons, making this site a sink for atmospheric CO2 (−9 to −16.8 molCO2m-2yr-1, depending on the parameterisation of the gas transfer velocity). Data are available on PANGAEA: https://doi.org/10.1594/PANGAEA.960131 (Gattuso et al., 2023a).

Seagrasses are important primary producers in oceans worldwide. They live in shallow coastal waters that are experiencing carbon dioxide enrichment and ocean acidification. Posidonia oceanica, an endemic seagrass species that dominates the Mediterranean Sea, achieves high abundances in seawater with relatively low concentrations of dissolved inorganic nitrogen. Here we tested whether microbial metabolisms associated with P. oceanica and surrounding seawater enhance seagrass access to nitrogen. Using stable isotope enrichments of intact seagrass with amino acids, we showed that ammonification by free-living and seagrass-associated microbes produce ammonium that is likely used by seagrass and surrounding particulate organic matter. Metagenomic analysis of the epiphytic biofilm on the blades and rhizomes support the ubiquity of microbial ammonification genes in this system. Further, we leveraged the presence of natural carbon dioxide vents and show that the presence of P. oceanica enhanced the uptake of nitrogen by water column particulate organic matter, increasing carbon fixation by a factor of 8.6-17.4 with the greatest effect at CO 2 vent sites. However, microbial ammonification was reduced at lower pH, suggesting that future ocean climate change will compromise this microbial process. Thus, the seagrass holobiont enhances water column productivity, even in the context of ocean acidification.

There are now serious concerns about the growing risk of shortfalls in open-ocean fish stocks, and the effects on economic imperatives to access the remaining stocks, global-scale inflation and resulting inequalities in access to marine proteins for the most deprived people in society, as well as fractured regional and international relationships beyond the fishery sector itself.

The magnitude and distribution of net primary production (NPP) in the coastal ocean remains poorly constrained, particularly for shallow marine vegetation. Here, using a compilation of in situ annual NPP measurements across >400 sites in 72 geographic ecoregions, we provide global predictions of the productivity of seaweed habitats, which form the largest vegetated coastal biome on the planet. We find that seaweed NPP is strongly coupled to climatic variables, peaks at temperate latitudes, and is dominated by forests of large brown seaweeds. Seaweed forests exhibit exceptionally high per-area production rates (a global average of 656 and 1711 gC m −2 year −1 in the subtidal and intertidal, respectively), being up to 10 times higher than coastal phytoplankton in temperate and polar seas. Our results show that seaweed NPP is a strong driver of production in the coastal ocean and call for its integration in the oceanic carbon cycle, where it has traditionally been overlooked.

Along with global warming and anthropogenic impacts, ocean acidification deoxygenation and sea level rising have resulted in tremendous ecological losses at species, diversity and ecosystem levels, leading to serious consequences. This calls for a dramatic scaling up of efforts towards ambitious mitigation and adaptation. A joint working group from the Chinese Academy of Sciences (CAS) and the European Academy of Sciences (EurASc) on ocean-based measures for climate action is thus formed at the moment.

The study of early life stages such as larval development, settlement, survivorship, and recruit growth iscritical to better understand the resilience and persistence of coral populations. While the deleterious effectsof ocean acidification on calcification and growth on adult corals are well known, the impacts on early lifestages are still poorly documented. Here, we investigate whether past-exposure of parental colonies tonaturally acidified environments at CO2 vents can increase the offspring's tolerance to low pH. Larvae of theMediterranean azooxanthellate coral Astroides calycularis were obtained from parent colonies collected fromlow and ambient pH sites in Ischia, Italy. Larvae were exposed in the laboratory to three pH treatments:ambient (pHT ~8.05), low (pHT ~7.7, pH projected for the end of the century under RCP 8.5) and extreme low(pHT ~7.5, pH as extreme condition, only for the larvae from the CO2 vent site). Several traits of the early lifestages were monitored for 5 months: larval size, settlement success, survival and growth of the recruits. Keydifferences were observed between the site of origin of the larvae. For instance, larvae from the CO2 ventsite were shorter and exhibited a higher rate of mortality than the larvae from the ambient pH site regardlessof the pH treatment they were exposed to. Also, larvae from the CO2 vent site had a lower settlementsuccess at low and extreme low pH. Our results suggest that future acidification conditions will have animpact on survival and settlement of early life stages. These findings provide new insights into coralrecruitment and the ability to respond to present and future ocean acidification conditions

Volcanic CO2 vents are ideal sites to study the long-term effects of ocean acidification on long- lived calcified species, such as corals. At the Island of Ischia (Italy), we investigated, both in situ and ex situ, whether past exposure to low pH confers physiological tolerance to ocean acidification in different life stages and affects sexual reproduction. Adult colonies of Astroides calycularis (azooxanthellate) and Cladocora caespitosa zooxanthellate), and larvae of A. calycularis were sampled at CO2 vent and ambient pH sites. They were then maintained under controlled conditions in the laboratory, during two distinct 6-month experiments, under ambient (pHT 8.0), low (pHT 7.7) and extreme low pH (pHT 7.5, only for larvae from CO2 vents). Adult net calcification and respiration of both species, and net and gross photosynthesis of C. caespitosa were unaffected by the pH treatments regardless of their environmental history. Larvae of A. calycularis from the CO2 vents were smaller and had a lower survivorship and settlement success than larvae from the ambient pH site regardless of experimental pH treatment. Colonies of C. caespitosa were smaller and less abundant near CO2 vents. Histological sections of C. caespitosa from the vents demonstrated an asynchrony of spawning between male and female, which could have major effects on fertilization. Our results suggest that adult colonies of A. calycularis and C. caespitosa exhibit tolerance to acidification. However, long exposition to low pH appears to decrease sexual reproduction and early life stages development, which are both of particular relevance for the persistence of healthy populations.

The Arctic region faces a warming rate that is more than twice the global average. Seaice loss, increase in precipitation and freshwater discharge, changes in underwater light, and amplification of ocean acidification modify benthic habitats and the communities they host. Here we synthesize existing information on the impacts of climate change on the macroalgal communities of Arctic coasts. We review the shortand long-term changes in environmental characteristics of shallow hard-bottomed Arctic coasts, the floristics of Arctic macroalgae (description, distribution, life-cycle, adaptations), the responses of their biological and ecological processes to climate change, the resulting winning and losing species, and the effects on ecosystem functioning. The focus of this review is on fucoid species, kelps, and coralline algae which are key ecosystem engineers in hard-bottom shallow areas of the Arctic, providing food, substrate, shelter, and nursery ground for many species. Changes in seasonality, benthic functional diversity, food-web structure, and carbon cycle are already occurring and are reshaping Arctic benthic ecosystems. Shallow communities are projected to shift from invertebrate-to algal-dominated communities. Fucoid and several kelp species are expected to largely spread and dominate the area with possible extinctions of native species. A considerable amount of functional diversity could be lost impacting the processing of land-derived nutrients and organic matter and significantly altering trophic structure and energy flow up to the apex consumers. However, many factors are not well understood yet, making it difficult to appreciate the current situation and predict the future coastal Arctic ecosystem. Efforts must be made to improve knowledge in key regions with proper seasonal coverage, taking into account interactions between stressors and across species.

Mangrove forests, seagrass meadows and tidal saltmarshes are vegetated coastal ecosystems that accumulate and store large quantities of carbon in their sediments. Many recent studies and reviews have favorably identified the potential for such coastal “blue carbon” ecosystems to provide a natural climate solution in two ways: by conservation, reducing the greenhouse gas emissions arising from the loss and degradation of such habitats, and by restoration, to increase carbon dioxide drawdown and its long-term storage. The focus here is on the latter, assessing the feasibility of achieving quantified and secure carbon removal (negative emissions) through the restoration of coastal vegetation. Seven issues that affect the reliability of carbon accounting for this approach are considered: high variability in carbon burial rates; errors in determining carbon burial rates; lateral carbon transport; fluxes of methane and nitrous oxide; carbonate formation and dissolution; vulnerability to future climate change; and vulnerability to non-climatic factors. Information on restoration costs is also reviewed, with the conclusion that costs are highly uncertain, with lower-range estimates unrealistic for wider application. CO 2 removal using coastal blue carbon restoration therefore has questionable cost-effectiveness when considered only as a climate mitigation action, either for carbon-offsetting or for inclusion in Nationally Determined Contributions. Many important issues relating to the measurement of carbon fluxes and storage have yet to be resolved, affecting certification and resulting in potential over-crediting. The restoration of coastal blue carbon ecosystems is nevertheless highly advantageous for climate adaptation, coastal protection, food provision and biodiversity conservation. Such action can therefore be societally justified in very many circumstances, based on the multiple benefits that such habitats provide at the local scale.

Effective data management plays a key role in oceanographic research as cruise-based data, collected from different laboratories and expeditions, are commonly compiled to investigate regional to global oceanographic processes. Here we describe new and updated best practice data standards for discrete chemical oceanographic observations, specifically those dealing with column header abbreviations, quality control flags, missing value indicators, and standardized calculation of certain properties. These data standards have been developed with the goals of improving the current practices of the scientific community and promoting their international usage. These guidelines are intended to standardize data files for data sharing and submission into permanent archives. They will facilitate future quality control and synthesis efforts and lead to better data interpretation. In turn, this will promote research in ocean biogeochemistry, such as studies of carbon cycling and ocean acidification, on regional to global scales. These best practice standards are not mandatory. Agencies, institutes, universities, or research vessels can continue using different data standards if it is important for them to maintain historical consistency. However, it is hoped that they will be adopted as widely as possible to facilitate consistency and to achieve the goals stated above.

Abstract. The ability of coral populations to recover from disturbance depends on larval dispersion and recruitment. While ocean warming and acidification effects on adult corals are well documented, information on early life stages is comparatively scarce. Here, we investigate whether ocean warming and acidification can affect the larval and recruit development of the Mediterranean azooxanthellate coral Astroides calycularis. Larvae and recruits were raised for 9 months at ambient (23 ∘C) and warm (26 ∘C) temperatures and ambient (8.0) and low pH (7.7, on the total scale). The timing of the larval metamorphosis, growth of the recruit polyp by linear extension and budding, and skeletal characteristics of the 9-month-old polyps were monitored. Settlement and metamorphosis were more successful and hastened under a warm temperature. In contrast, low pH delayed the metamorphosis and affected the growth of the recruits by reducing the calcified area of attachment to the substrate as well as by diminishing the skeleton volume and the number of septa. However, skeleton density was higher under low pH and ambient temperature. The warm temperature and low-pH treatment had a negative impact on the survival, settlement, and growth of recruits. This study provides evidence of the threat represented by ocean warming and acidification for the larval recruitment and the growth of recruits of A. calycularis.

Predicting the potential for species adaption to climate change is challenged by the need to identify the physiological mechanisms that underpin species vulnerability. Here, we investigated the sensitivity to ocean acidification in marine mussels during early development, and specifically the trochophore stage. Using RNA and DNA sequencing and in situ RNA hybridization, we identified developmental processes associated with abnormal development and rapid adaptation to low pH. Trochophores exposed to low pH seawater exhibited 43 differentially expressed genes. Gene annotation and in situ hybridization of differentially expressed genes point to pH sensitivity of (1) shell field development and (2) cellular stress response. Five genes within these two processes exhibited shifts in allele frequencies indicative of a potential for rapid adaptation. This case study contributes direct evidence that protecting species' existing genetic diversity is a critical management action to facilitate species resilience to climate change.

Ocean acidification (OA) is a serious consequence of climate change with complex organism-to-ecosystem effects that have been observed through field observations but are mainly derived from experimental studies. Although OA trends and the resulting biological impacts are likely exacerbated in the semi-enclosed and highly populated Mediterranean Sea, some fundamental knowledge gaps still exist. These gaps are at tributed to both the uneven capacity for OA research that exists between Mediterranean countries, as well as to the subtle and long-term biological, physical and chemical interactions that define OA impacts. In this paper, we systematically analyzed the different aspects of OA research in the Mediterranean region based on two sources: the United Nation’s International Atomic Energy Agency’s (IAEA) Ocean Acidification International Coordination Center (OA-ICC) database, and an extensive survey. Our analysis shows that 1) there is an uneven geographic capacity in OA research, and illustrates that both the Algero-Provencal and Ionian sub-basins are currently the least studied Mediterranean areas, 2) the carbonate system is still poorly quantified in coastal zones, and long-term time-series are still sparse across the Mediterranean Sea, which is a challenge for studying its variability and assessing coastal OA trends, 3) the most studied groups of organisms are autotrophs (algae, phanerogams, phytoplankton), mollusks, and corals, while microbes, small mollusks (mainly pteropods), and sponges are among the least studied, 4) there is an overall paucity in socio-economic, paleontological, and modeling studies in the Mediterranean Sea, and 5) in spite of general resource availability and the agreement for improved and coordinated OA governance, there is a lack of consistent OA policies in the Mediterranean Sea. In addition to highlighting the current status, trends and gaps of OA research, this work also provides recommendations, based on both our literature assessment and a survey that targeted the Mediterranean OA scientific community. In light of the ongoing 2021-2030 United Nations Decade of Ocean Science for Sustainable Development, this work might provide a guideline to close gaps of knowledge in the Mediterranean OA research. Systematic Review Registration https://www.oceandecade.org

Using Argo profiling floats, cruises and mooring data, we reconstructed the dissolved oxygen (O2) dynamics in the Gulf of Lion and the Ligurian Sea, with a focus on the intermediate waters. By applying the CANYON-MED neural network-based method on the large network of O2-equipped Argo floats we derived nutrients and carbonate system variables in the Gulf of Lion and the Ligurian Sea at different depths in the water column and derived trends over the 2012-2020 period. In these waters, the O2 minimum is strongly affected by the intermittent convection process, and the two areas show dissimilar responses to the mixing events. In the absence of deep convection events, the O2-depleted layer tends to spread vertically and intensify even more so in the Ligurian than in the Gulf of Lion. In both areas, over the 2012-2020 period, nutrients increase overall in deep layers, with a concomitant impact on nutrient molar ratios tending toward an increase in P-limitation. Acidification estimates derived in different layers of the water column show an overall increase in dissolved inorganic carbon and a concurrent pH decrease. These trends were strongly affected by convection events slowing down the overall acidification trend.

A Correction to this paper has been published: https://doi.org/10.1038/s41586-021-03271-2.

Can experimental studies on the behavioural impacts of ocean acidification be trusted? That question was raised in early 2020 when a high-profile paper failed to corroborate previously observed responses of coral reef fish to high CO2. New information on the methodologies used in the “replicated” studies now provides a plausible explanation: the experimental conditions were substantially different. High sensitivity to test conditions is characteristic of ocean acidification research; such response variability shows that effects are complex, interacting with many other factors. Open-minded assessment of all research results, both negative and positive, remains the best way to develop process-based understanding. As in other fields, replication studies in ocean acidification are most likely to contribute to scientific advancement when carried out in a spirit of collaboration rather than confrontation.

É rosion de la biodiversité marine : quelles solutions ? Plus de 300 000 espèces de plantes et d'animaux ont été décrits dans les mers et océans. Il est probable que le nombre total d'espèces, y compris celles qui restent à décrire, atteigne 1 à 10 millions, car la biodiversité marine est beaucoup moins bien connue que la biodiversité terrestre. Cela ne représente que 15 % de toutes les espèces décrites, mais la vie a débuté en mer et un grand nombre de groupes sont essentiellement ou exclusivement marins. Par ailleurs, ces estimations ne concernent que les espèces évoluées et pas les unicellulaires, ni les bactéries, archées et virus. De nombreux facteurs érodent cette biodiversité, mais des solutions éprouvées existent pour interrompre ce déclin et restaurer la vie marine.Érosion de la biodiversité marine : quelles solutions ? Plus de 300 000 espèces de plantes et d'animaux ont été décrits dans les mers et océans. Il est probable que le nombre total d'espèces, y compris celles qui restent à décrire, atteigne 1 à 10 millions, car la biodiversité marine est beaucoup moins bien connue que la biodiversité terrestre. Cela ne représente que 15 % de toutes les espèces décrites, mais la vie a débuté en mer et un grand nombre de groupes sont essentiellement ou exclusivement marins. Par ailleurs, ces estimations ne concernent que les espèces évoluées et pas les unicellulaires, ni les bactéries, archées et virus. De nombreux facteurs érodent cette biodiversité, mais des solutions éprouvées existent pour interrompre ce déclin et restaurer la vie marine.

Ocean acidification is perceived to be a major threat for many calcifying organisms, including scleractinian corals. Here we investigate (1) whether past exposure to low pH environments associated with CO2 vents could increase corals tolerance to low pH and (2) whether zooxanthellate corals are more tolerant to low pH than azooxanthellate corals. To test these hypotheses, two Mediterranean colonial corals Cladocora caespitosa (zooxanthellate) and Astroides calycularis (azooxanthellate) were collected from CO2 vents and reference sites and incubated in the laboratory under present-day (pH on the total scale, pHT 8.07) and low pH conditions (pHT 7.70). Rates of net calcification, dark respiration and photosynthesis were monitored during a six-month experiment. Monthly net calcification was assessed every 27 to 35 d using the buoyant weight technique, whereas light and dark net calcification was estimated using the alkalinity anomaly technique during 1 h incubations. Neither species showed any change in net calcification rates, respiration, and photosynthesis regardless of their environmental history, pH treatment and trophic strategy. Our results indicate that C. caespitosa and A. calycularis could tolerate future ocean acidification conditions for at least 6 months. These results will aid in predicting species' future responses to ocean acidification, and thus improve the management and conservation of Mediterranean corals.

Word count: 295For millennia, coastal and marine ecosystems have adapted and flourished in the unique environment of the Red Sea. Surrounded by deserts on all sides, the Red Sea is subjected to high dust inputs receiving very little freshwater input and so harbors a high salinity. Coral reefs, seagrass meadows, and mangroves flourish in this environment and provide socio-economic and environmental benefits to the bordering coastlines and countries of Saudi Arabia, Yemen, Sudan, Egypt, Eritrea, and Djibouti. Interestingly, while coral reef ecosystems are currently experiencing rapid decline in health on a global scale, those in the Red Sea appear relatively in better shape. That said, they are certainly not immune to the stressors which cause degradation. In many regions, ecosystems are already severely deteriorating and are further threatened by increasing population pressure and large coastal development projects. Degradation of these marine habitats will lead to environmental costs, as well as significant economic losses, and will therefore result in a missed opportunity for the bordering countries to develop a sustainable blue economy and integrate innovative nature-based solutions. Recognizing that securing the future of the Red Sea ecosystems must occur in synergy with continued social and economic development, we developed an action plan for the conservation, restoration, and growth of marine environments of the Red Sea. We then investigated the level of resources for financial and economic investment that may incentivize these activities. This study presents a set of commercially viable financial investment strategies, ecological innovations, and opportunities for sustainable development, which can, if implemented strategically, help ensure long-term economic benefits and promote environmental conservation. We make a case for investing in blue natural capital and propose a strategic development model that relies on maintaining the health of natural ecosystems to safeguard the sustainable development of the Red Sea.

Maintaining coral reef ecosystems is a social imperative, because so many people depend on coral reefs for food production, shoreline protection, and livelihoods. The survival of reefs this century, however, is threatened by the mounting effects of climate change. Climate mitigation is the foremost and essential action to prevent coral reef ecosystem collapse. Without it, reefs will become extremely diminished within the next 20-30 years. Even with strong climate mitigation, however, existing conservation measures such as marine protected areas and fisheries management are no longer sufficient to sustain the ecosystem and many additional and innovative actions to increase reef resilience must also be taken. In this paper we assess the suite of protections and actions in terms of their potential be effective according to a set of criteria that include effectiveness, readiness, co-benefits and disbenefits. Even with the best scientific innovation, saving coral reefs will require a well-funded, welldesigned, and rapidly executed strategy with political and social commitments at the level of other grand challenges.

Environmentally-induced changes in fitness are mediated by direct effects on physiology and behaviour, which are tightly linked. We investigated how predicted ocean warming (OW) and acidification (OA) affect key ecological behaviours (locomotion speed and foraging success) and metabolic rate of a keystone marine mollusc, the sea hare Stylocheilus striatus, a specialist grazer of the toxic cyanobacterium Lyngbya majuscula. We acclimated sea hares to OW and/or OA across three developmental stages (metamorphic, juvenile, and adult) or as adults only, and compare these to sea hares maintained under current-day conditions. Generally, locomotion speed and time to locate food were reduced ~1.5-to 2-fold when the stressors (OW or OA) were experienced in isolation, but reduced ~3-fold when combined. Decision-making was also severely altered, with correct foraging choice nearly 40% lower under combined stressors. Metabolic rate appeared to acclimate to the stressors in isolation, but was significantly elevated under combined stressors. Overall, sea hares that developed under OW and/or OA exhibited a less severe impact, indicating beneficial phenotypic plasticity. Reduced foraging success coupled with increased metabolic demands may impact fitness in this species and highlight potentially large ecological consequences under unabated OW and OA, namely in regulating toxic cyanobacteria blooms on coral reefs. Rising atmospheric carbon dioxide (CO 2) levels may lead to continued and accelerated global warming over the coming century 1. The resulting elevated sea surface temperature (SST) (i.e. ocean warming; OW) is accompanied by increased partial pressure of CO 2 (pCO 2) in the ocean, thus rapidly changing the marine environment by increasing acidity (i.e. ocean acidification; OA) at unprecedented rates 2. The Intergovernmental Panel on Climate Change (IPCC) has indicated that, by year 2100, global mean SST will increase by 2-4 °C and seawater pH decrease by 0.14-0.43 units 3 , with concomitant effects on marine ecosystems. Marine invertebrates are critical

Cabled coastal observatories are often seen as future-oriented marine technology that enables science to conduct observational and experimental studies under water year-round, independent of physical accessibility to the target area. Additionally, the availability of (unrestricted) electricity and an Internet connection under water allows the operation of complex experimental setups and sensor systems for longer periods of time, thus creating a kind of laboratory beneath the water. After successful operation for several decades in the terrestrial and atmospheric research field, remote controlled observatory technology finally also enables marine scientists to take advantage of the rapidly developing communication technology. The continuous operation of two cabled observatories in the southern North Sea and off the Svalbard coast since 2012 shows that even highly complex sensor systems, such as stereo-optical cameras, video plankton recorders or systems for measuring the marine carbonate system, can be successfully operated remotely year-round facilitating continuous scientific access to areas that are difficult to reach, such as the polar seas or the North Sea. Experience also shows, however, that the challenges of operating a cabled coastal observatory go far beyond the provision of electricity and network connection under water. In this manuscript, the essential developmental stages of the "COSYNA Shallow Water Underwater Node" system are presented, and the difficulties and solutions that have arisen in the course of operation since 2012 are addressed with regard to technical, organizational and scientific aspects.

A 21 year (1998–2018) continuous monthly data set of the global distribution of light (photosyntheti-cally available radiation, PAR, or irradiance) reaching the seabed is presented. This product uses ocean color andbathymetric data to estimate benthic irradiance, offering critical improvements on a previous data set. The timeseries is 4 times longer (21 versus 5 years), the spatial resolution is better (pixel size of 4.6 versus 9.3 km at theEquator), and the bathymetric resolution is also better (pixel size of 0.46 versus 3.7 km at the Equator). The paperdescribes the theoretical and methodological bases and data processing. This new product is used to estimate thesurface area of the seafloor where (1) light does not limit the distribution of photosynthetic benthic organismsand (2) net community production is positive. The complete data set is provided as 14 netCDF files available onPANGAEA (Gentili and Gattuso, 2020a, https://doi.org/10.1594/PANGAEA.910898). The R packageCoastal-Light, available on GitHub (https://github.com/jpgattuso/CoastalLight.git, last access: 29 July 2020), allows us(1) to download geographical and optical data from PANGAEA and (2) to calculate the surface area that receivesmore than a given threshold of irradiance in three regions (nonpolar, Arctic, and Antarctic). Such surface areascan also be calculated for any subregion after downloading data from a remotely and freely accessible server

The study of the oceanic carbonate system is linked to two important environmental issues: ocean CO$_2$ uptake and ocean acidification and its impact on organisms, ecosystems and ecosystem services. This chapter mainly focuses on the seasonal cycles and long‐term trends of the ocean carbonate system based on a synthesis of data collected in the Ligurian Sea from 1998 to 2016. In addition to the effect of potential T (theta) on CO$_2$ solubility, the distribution of dissolved inorganic carbon CT in the water column is driven by the antagonistic effects of the biological carbon pump that increases the vertical gradient of CT (lowering CT at the surface and increasing it in the ocean interior) and the exchange of CO$_2$ at the air–sea interface. The chapter also presents the seasonal cycle of the carbonate system and ancillary variables in the surface water.

A regional neural network-based method, "CANYON-MED" is developed to estimate nutrients and carbonate system variables specifically in the Mediterranean Sea over the water column from pressure, temperature, salinity, and oxygen together with geolocation and date of sampling. Six neural network ensembles were developed, one for each variable (i.e., three macronutrients: nitrates (NO − 3), phosphates (PO 3− 4) and silicates (SiOH 4), and three carbonate system variables: pH on the total scale (pH T), total alkalinity (A T), and dissolved inorganic carbon or total carbon (C T), trained using a specific quality-controlled dataset of reference "bottle" data in the Mediterranean Sea. This dataset is representative of the peculiar conditions of this semi-enclosed sea, as opposed to the global ocean. For each variable, the neural networks were trained on 80% of the data chosen randomly and validated using the remaining 20%. CANYON-MED retrieved the variables with good accuracies (Root Mean Squared Error): 0.73 µmol.kg −1 for NO − 3 , 0.045 µmol.kg −1 for PO 3− 4 and 0.70 µmol.kg −1 for Si(OH) 4 , 0.016 units for pH T , 11 µmol.kg −1 for A T and 10 µmol.kg −1 for C T. A second validation on the ANTARES independent time series confirmed the method's applicability in the Mediterranean Sea. After comparison to other existing methods to estimate nutrients and carbonate system variables, CANYON-MED stood out as the most robust, using the aforementioned inputs. The application of CANYON-MED on the Mediterranean Sea data from autonomous observing systems (integrated network of Biogeochemical-Argo floats, Eulerian moorings and ocean gliders measuring hydrological properties together with oxygen concentration) could have a wide range of applications. These include data quality control or filling gaps in time series, as well as biogeochemical data assimilation and/or the initialization and validation of regional biogeochemical models still lacking crucial reference data. Matlab and R code are available at https:// github.com/MarineFou/CANYON-MED/.

Coral reefs are constructed by calcifiers that precipitate calcium carbonate to build their shells or skeletons through the process of calcification. Accurately assessing coral calcification rates is crucial to determine the health of these ecosystems and their response to major environmental changes such as ocean warming and acidification. Several approaches have been used to assess rates of coral calcification, but there is a real need to compare these approaches in order to ascertain that high-quality and intercomparable results can be produced. Here, we assessed four methods (total alkalin-ity anomaly, calcium anomaly, 45 Ca incorporation, and 13 C incorporation) to determine coral calcification of the reef-building coral Stylophora pistillata. Given the importance of environmental conditions for this process, the study was performed under two starting pH levels (ambient: 8.05 and low: 7.2) and two light (light and dark) conditions. Under all conditions , calcification rates estimated using the alkalinity and calcium anomaly techniques as well as 45 Ca incorporation were highly correlated. Such a strong correlation between the alkalinity anomaly and 45 Ca incorporation techniques has not been observed in previous studies and most probably results from improvements described in the present paper. The only method which provided calcification rates significantly different from the other three techniques was 13 C incorporation. Calcification rates based on this method were consistently higher than those measured using the other techniques. Although reasons for these discrepancies remain unclear, the use of this technique for assessing calcification rates in corals is not recommended without further investigations.

Can experimental studies on the impacts of ocean acidification be trusted? That question was raised in early 2020 when a high-profile paper failed to corroborate previously-observed impacts of high CO 2 on the behaviour of coral reef fish. New information on the methodologies used in the 'replicated' studies now provides the explanation: the experimental conditions were substantially different. High sensitivity to test conditions is characteristic of ocean acidification research; such response variability shows that effects are complex, interacting with many other factors. Open-minded assessment of all research results, both negative and positive, remains the best way to develop process-based understanding of those responses. Whilst replication studies can provide valuable insights and challenges, they can unfortunately also be counter-productive to scientific advancement if carried out in a spirit of confrontation rather than collaboration.

The carbon fluxes between phytoplankton and heterotrophic bacterioplankton were studied in two coastal oligotrophic sites in the NW Mediterranean. Phytoplankton and bacterial production rates were measured under natural conditions using different methods. In the Bay of Villefranche, the temporal variability revealed net heterotrophy in July-October and net autotrophy in December-March. The spatial variability was studied in the Bay of Palma, showing net autotrophic areas in the west and heterotrophic areas in the east. On average bacterial respiration, represented 62% of the total community respiration. Bacterial growth efficiency (BGE) values were significantly higher in autotrophic conditions than in heterotrophic ones. During autotrophic periods, dissolved primary production (DPP) was enough to sustained bacterial metabolism, although it showed a positive correlation with organic carbon stock (DOC). Under heterotrophic conditions, DPP did not sustain bacterial metabolism but bacterial respiration correlated with DPP and bacterial production with DOC. Temperature affected positively, DOC, BGE, bacterial respiration and production when the trophic status was autotrophic. To summarize, the response of bacterial metabolism to temperature and carbon sources depends on the trophic status within these oligotrophic coastal systems.

Global climate change has intensified the need to assess the capacity for natural populations to adapt to abrupt shifts in the environment. Reductions in seawater pH constitute a conspicuous global change stressor that is affecting marine ecosystems globally. Here, we quantify the phenotypic and genetic modifications associated with rapid adaptation to reduced seawater pH in the Mediterranean mussel, Mytilus galloprovincialis. We reared a genetically diverse larval population in two pH treatments (pH T 8.1 and 7.4) and tracked changes in the shell-size distribution and genetic variation through settlement. Additionally, we identified differences in the signatures of selection on shell growth in each pH environment. Both phenotypic and genetic data show that standing variation can facilitate adaptation to declines in seawater pH. This work provides insight into the processes underpinning rapid evolution, and demonstrates the importance of maintaining variation within natural populations to bolster species' adaptive capacity as global change progresses.

This chapter addresses the question as to how Mediterranean cold-water corals might fare in the future under anthropogenically-induced global climate change. The focus on three most prominent scleractinian cold-water corals species, the two branching and habitat-forming forms Madrepora oculata, Lophelia pertusa and the solitary cup coral Desmophyllum dianthus. We provide an introduction to climate change principals, highlight the current status of the marine environment with regard to global climate change, and describe how climate change impacts such as ocean acidification are predicted to affect key calcifiers such as scleractinian cold-water corals in the Mediterranean region. A synthesis of the experimental cold-water coral studies conducted to date on climate change impacts: The present state of knowledge reviewed in this chapter takes into account the number of experiments that have been carried out in the Mediterranean as well as for comparative purposes in other parts of the world, to examine the effects of climate change on the corals. We assess the statistical robustness of these experiments and what challenges the presented experiments. A comprehensive multi-study comparison is provided in order to inform on the present state of knowledge , and knowledge gaps, in understanding the effects of global climate change on cold-water corals. Finally we describe what the fate could be for the important sclerac-tinian coral group in the Mediterranean region.

Worldwide, coral reef ecosystems are experiencing increasing pressure from a variety of anthropogenic perturbations including ocean warming and acidification, increased sedimentation, eutrophication, and overfishing, which could shift reefs to a condition of net calcium carbonate (CaCO3) dissolution and erosion. Herein, we determine the net calcification potential and the relative balance of net organic carbon metabolism (net community production; NCP) and net inorganic carbon metabolism (net community calcification; NCC) within 23 coral reef locations across the globe. In light of these results, we consider the suitability of using these two metrics developed from total alkalinity (TA) and dissolved inorganic carbon (DIC) measurements collected on different spatiotemporal scales to monitor coral reef biogeochemistry under anthropogenic change. All reefs in this study were net calcifying for the majority of observations as inferred from alkalinity depletion relative to offshore, although occasional observations of net dissolution occurred at most locations. However, reefs with lower net calcification potential (i.e., lower TA depletion) could shift towards net dissolution sooner than reefs with a higher potential. The percent influence of organic carbon fluxes on total changes in dissolved inorganic carbon (DIC) (i.e., NCP compared to the sum of NCP and NCC) ranged from 32% to 88% and reflected inherent biogeochemical differences between reefs. Reefs with the largest relative percentage of NCP experienced the largest variability in seawater pH for a given change in DIC, which is directly related to the reefs ability to elevate or suppress local pH relative to the open ocean. This work highlights the value of measuring coral reef carbonate chemistry when evaluating their susceptibility to ongoing global environmental change and offers a baseline from which to guide future conservation efforts aimed at preserving these valuable ecosystems.

Coastal marine ecosystems experience dynamic fluctuations in seawater carbonate chemistry. The importance of this variation in the context of ocean acidification requires knowing what aspect of variability biological processes respond to. We conducted four experiments (ranging from 3 to 22 days) with different variability regimes (pH T 7.4-8.1) assessing the impact of diel fluctuations in carbonate chemistry on the early development of the mussel Mytilus galloprovincialis. Larval shell growth was consistently correlated to mean exposures, regardless of variability regimes, indicating that calcification responds instantaneously to seawater chemistry. Larval development was impacted by timing of exposure, revealing sensitivity of two developmental processes: development of the shell field, and transition from the first to the second larval shell. Fluorescent staining revealed developmental delay of the shell field at low pH, and abnormal development thereof was correlated with hinge defects in D-veligers. This study shows, for the first time, that ocean acidification affects larval soft-tissue development, independent from calcification. Multiple developmental processes additively underpin the teratogenic effect of ocean acidification on bivalve larvae. These results explain why trochophores are the most sensitive life-history stage in marine bivalves and suggest that short-term variability in carbonate chemistry can impact early larval development.

Recent accelerated climate change has exacerbated existing environmental problems in the Mediterranean Basin that are caused by the combination of changes in land use, increasing pollution and declining biodiversity. For five broad and interconnected impact domains (water, ecosystems, food, health and security), current change and future scenarios consistently point to significant and increasing risks during the coming decades. Policies for the sustainable development of Mediterranean countries need to mitigate these risks and consider adaptation options, but currently lack adequate information — particularly for the most vulnerable southern Mediterranean societies, where fewer systematic observations schemes and impact models are based. A dedicated effort to synthesize existing scientific knowledge across disciplines is underway and aims to provide a better understanding of the combined risks posed.

Pairs of marine carbonate system variables are often used to calculate others, but those results are seldom reported with estimates of uncertainties. Although the procedure to propagate these uncertainties is well known, it has not been offered in public packages that compute marine carbonate chemistry, fundamental tools that are relied on by the community. To remedy this shortcoming, four of these packages were expanded to calculate sensitivities of computed variables with respect to each input variable and to use those sensitivities along with user-specified estimates of input uncertainties (standard uncertainties) to propagate uncertainties of calculated variables (combined standard uncertainties). Sensitivities from these packages agree with one another and with analytical solutions to within 0.01%; similar agreement among packages was found for the combined standard uncertainties. One package was used to quantify how propagated uncertainties vary among computed variables, seawater conditions, and the chosen pair of carbonate system variables that is used as input. The relative contributions to propagated uncertainties from the standard uncertainties of the input pair of measurements and various other input data (equilibrium constants etc) were explored with a new type of diagram. These error-space diagrams illustrate that further improvement beyond today's state-of-the-art measurement uncertainties for the input pair would generally be ineffective at reducing the combined standard uncertainties because the contribution from the constants is larger. Likewise, using much more uncertain measurements of the input pair does not always substantially worsen combined standard uncertainty. The constants that contribute most to combined standard uncertainties are generally K 1 and K 2 , as expected. Yet more of the propagated uncertainty in the computed saturation states of aragonite and calcite comes from their solubility products. Thus percent relative combined standard uncertainties for the saturation states are larger than for the carbonate ion concentration. Routine propagation of these uncertainties should become standard practice.

Marine life is controlled by multiple physical and chemical drivers and by diverse ecological processes. Many of these oceanic properties are being altered by climate change and other anthropogenic pressures. Hence, identifying the influences of multifaceted ocean change, from local to global scales, is a complex task. To guide policy‐making and make projections of the future of the marine biosphere, it is essential to understand biological responses at physiological, evolutionary and ecological levels. Here, we contrast and compare different approaches to multiple driver experiments that aim to elucidate biological responses to a complex matrix of ocean global change. We present the benefits and the challenges of each approach with a focus on marine research, and guidelines to navigate through these different categories to help identify strategies that might best address research questions in fundamental physiology, experimental evolutionary biology and community ecology. Our review reveals that the field of multiple driver research is being pulled in complementary directions: the need for reductionist approaches to obtain process‐oriented, mechanistic understanding and a requirement to quantify responses to projected future scenarios of ocean change. We conclude the review with recommendations on how best to align different experimental approaches to contribute fundamental information needed for science‐based policy formulation.

The Paris Agreement target of limiting global surface warming to 1.5–2∘C compared to pre-industrial levels by 2100 will still heavily impact the ocean. While ambitious mitigation and adaptation are both needed, the ocean provides major opportunities for action to reduce climate change globally and its impacts on vital ecosystems and ecosystem services. A comprehensive and systematic assessment of 13 global- and local-scale, ocean-based measures was performed to help steer the development and implementation of technologies and actions toward a sustainable outcome. We show that (1) all measures have tradeoffs and multiple criteria must be used for a comprehensive assessment of their potential, (2) greatest benefit is derived by combining global and local solutions, some of which could be implemented or scaled-up immediately, (3) some measures are too uncertain to be recommended yet, (4) political consistency must be achieved through effective cross-scale governance mechanisms, (5) scientific effort must focus on effectiveness, co-benefits, disbenefits, and costs of poorly tested as well as new and emerging measures.

which shows a significant, high latitude-intensified increase between +0.1 and +0.4 units per decade. This shows the utility that such transfer functions with realistic uncertainty estimates provide to ocean biogeochemistry and global climate change research. In addition, CANYON-B provides robust and accurate estimates of nitrate, phosphate, and silicate. Matlab and R code are available at https://github.com/HCBScienceProducts/.

Coastal time series of ocean carbonate chemistry are critical for understanding how global anthropogenic change manifests in near-shore ecosystems. Yet, they are few and have low temporal resolution. At the time series station Point B in the northwestern Mediterranean Sea, seawater was sampled weekly from 2007 through 2015, at 1 and 50 m, and analyzed for total dissolved inorganic carbon (C T) and total alkalinity (A T). Parameters of the carbonate system such as pH (pH T , total hydrogen ion scale) were calculated and a deconvolution analysis was performed to identify drivers of change. The rate of surface ocean acidification was −0.0028 ± 0.0003 units pH T yr −1. This rate is larger than previously identified open-ocean trends due to rapid warming that occurred over the study period (0.072 ± 0.022 • C yr −1). The total pH T change over the study period was of similar magnitude as the diel pH T variability at this site. The acidification trend can be attributed to atmospheric carbon dioxide (CO 2) forcing (59 %, 2.08 ± 0.01 ppm CO 2 yr −1) and warming (41 %). Similar trends were observed at 50 m but rates were generally slower. At 1 m depth, the increase in atmospheric CO 2 accounted for approximately 40 % of the observed increase in C T (2.97 ± 0.20 µmol kg −1 yr −1). The remaining increase in C T may have been driven by the same unidentified process that caused an increase in A T (2.08 ± 0.19 µmol kg −1 yr −1). Based on the analysis of monthly trends, synchronous increases in C T and A T were fastest in the spring–summer transition. The driving process of the interannual increase in A T has a seasonal and shallow component, which may indicate riverine or groundwater influence. This study exemplifies the importance of understanding changes in coastal carbonate chemistry through the lens of biogeochemical cycling at the land–sea interface. This is the first coastal acidification time series providing multi-year data at high temporal resolution. The data confirm rapid warming in the Mediterranean Sea and demonstrate coastal acidification with a synchronous increase in total alkalinity.

Planet Earth has entered a new geological era, the Anthropocene, in which geologically significant conditions and processes are profoundly altered by human activities (Waters et al., 2016). Among many impacts, human activities have released excessive amounts of carbon dioxide (CO2) in the atmosphere leading to warming and ocean acidification: a decrease in pH and CO32- concentration and an increase in CO2 and HCO3- concentrations (Gattuso and Hansson, 2011). On average, at the global scale, surface ocean pH has decreased by 0.1 units since the beginning of the industrial era, equivalent to an increased acidity of 26% (Ciais et al., 2013). An additional decrease of pH is expected by 2100, ranging from 0.07 to 0.33, depending on the CO2 emission scenario considered (Gattuso et al., 2015).

Anthropogenic carbon perturbation has caused decreases in seawater pH and increases in global temperatures since the start of the 20 th century. The subsequent lowering of the saturation state of CaCO 3 may make the secretion of skeletons more problematic for marine cal-cifiers. As organisms that precipitate thin aragonite shells, thecosome pteropods have been identified as being particularly vulnerable to climate change effects. Coupled with their global distribution, this makes them ideal for use as sentinel organisms. Recent studies have highlighted shell dissolution as a potential indicator of ocean acidification; however, this metric is not applicable for monitoring pH changes in supersaturated basins. In this study, the novel approach of high resolution computed tomography (CT) scanning was used to produce quantitative 3-dimensional renderings pteropod shells to assess the potential of using this method to monitor small changes in shell biometrics that may be driven by climate change drivers. An ontogenetic analysis of the shells of Cavolinia inflexa and Styliola subula collected from the Mediterranean was used to identify suitable monitoring metrics. Modern samples were then compared to historical samples of the same species, collected during the Mediterranean leg of the Thor (1910) and Dana (1921) cruises to assess whether any empirical differences could be detected. Shell densities were calculated and scanning electron microscopy was used to compare the aragonite crystal morphology. pH for the collection years was hind-cast using temperature and salinity time series with atmospheric CO 2 concentrations from ice core data. Historical samples of S. subula were thicker than S. sub-ula shells of the same size from 2012 and C. inflexa shells collected in 1910 were significantly denser than those from 2012. These results provide a baseline for future work to develop monitoring techniques for climate change in the oceans using the novel approach of high-resolution CT scanning.

In order to fully constrain paleo-carbonate systems, proxies for two out of seven parameters, plus temperature and salinity, are required. The boron isotopic composition (δ11B) of planktonic foraminifera shells is a powerful tool for reconstructing changes in past surface ocean pH. As B(OH)4− is substituted into the biogenic calcite lattice in place of CO3 2−, and both borate and carbonate ions are more abundant at higher pH, it was suggested early on that B ∕ Ca ratios in biogenic calcite may serve as a proxy for [CO3 2−]. Although several recent studies have shown that a direct connection of B ∕ Ca to carbonate system parameters may be masked by other environmental factors in the field, there is ample evidence for a mechanistic relationship between B ∕ Ca and carbonate system parameters. Here, we focus on investigating the primary relationship to develop a mechanistic understanding of boron uptake. Differentiating between the effects of pH and [CO3 2−] is problematic, as they co-vary closely in natural systems, so the major control on boron incorporation remains unclear. To deconvolve the effects of pH and [CO3 2−] and to investigate their impact on the B ∕ Ca ratio and δ11B, we conducted culture experiments with the planktonic foraminifer Orbulina universa in manipulated culture media: constant pH (8.05), but changing [CO3 2−] (238, 286 and 534 µmol kg−1 CO3 2−) and at constant [CO3 2−] (276 ± 19.5 µmol kg−1) and varying pH (7.7, 7.9 and 8.05). Measurements of the isotopic composition of boron and the B ∕ Ca ratio were performed simultaneously using a femtosecond laser ablation system coupled to a MC-ICP-MS (multiple-collector inductively coupled plasma mass spectrometer). Our results show that, as expected, δ11B is controlled by pH but it is also modulated by [CO3 2−]. On the other hand, the B ∕ Ca ratio is driven by [HCO3−], independently of pH. This suggests that B ∕ Ca ratios in foraminiferal calcite can possibly be used as a second, independent, proxy for complete paleo-carbonate system reconstructions. This is discussed in light of recent literature demonstrating that the primary relationship between B ∕ Ca and [HCO3−] can be obscured by other environmental parameters

Alterations to colonization or early post-settlement stages may cause the reorganization of communities under future ocean acidification conditions. Yet, this hypothesis has been little tested by in situ pH manipulation. A Free Ocean Carbon Dioxide Enrichment (FOCE) system was used to lower pH by a ~ 0.3 unit offset within a partially enclosed portion (1.7 m3) of a Posidonia oceanica seagrass meadow (11 m depth) between 21 June and 3 November 2014. Epibiont colonization and early post settlement stages were assessed within the FOCE setup, as part of the larger community-level study, to better understand the outcome for a multispecies assemblage and the ecological processes that result in reported community shifts under altered carbonate chemistry. Two types of artificial collectors (tiles and scourers) were placed within three treatments: a pH-manipulated enclosure, an un-manipulated control enclosure, and an open plot in the ambient meadow. Tiles and scourers were collected after one to four months. Additionally, to see whether the outcome differed for communities in a later successional stage, previously settled scourer-collectors were also placed in the same three treatments. Enclosures acted to reduce settlement and migrant colonization. Scourers deployed for one to four months within the open-plot contained a community assemblage that could be distinguished from the assemblages within the enclosures. However, a comparison of enclosure assemblages on tiles showed evidence of a pH effect. There was lowered coverage of crustose coralline algae and fewer calcareous tube-forming polychaetes (Spirorbis sp. and Spirobranchus sp.) on tiles placed in the pH-manipulated enclosure compared to the un-manipulated enclosure. For assemblages in scourer collectors, shared and common taxa, in all treatments, were invertebrate polychaetes Psamathe fusca, Sphaerosyllis sp., Chrysopetalum sp., arthropods Harpacticoida, and Amphipoda, and the juvenile bivalve Lyonsia sp. Similar organism composition and abundance, as well as taxonomic richness and evenness, were found in scourers from both enclosures. Pre-settled scourers contained greater numbers of individuals and more calcified members, but the assemblage, as well as the growth rate of a juvenile bivalve Lyonsia sp., appeared unaffected by a two-month exposure to lowered pH and calcium carbonate saturation state. Results from this case study support the hypothesis that early stages of specific calcifiers (crustose coralline algae and calcareous tube-forming polychaetes) are sensitive to near future ocean acidification conditions yet suggest that negative effects on sessile micro-invertebrate assemblages will be minimal.

Alterations in seagrass epiphytic communities are expected under future ocean acidification conditions, yet this hypothesis has been little tested in situ. A Free Ocean Carbon Dioxide Enrichment system was used to lower pH by a ~0.3 unit offset within a partially enclosed portion (1.7 m3) of a Posidonia oceanica meadow (11 m depth) between June 21 and November 3, 2014. Leaf epiphytic community composition (% cover) and bulk epiphytic mineralogy were compared every 4 weeks within three treatments, located in the same meadow: a pH-manipulated (experimental enclosure) and a control enclosure, as well as a nearby ambient area. Percent coverage of invertebrate calcifiers and crustose coralline algae (CCA) did not appear to be affected by the lowered pH. Furthermore, fleshy algae did not proliferate at lowered pH. Only Foraminifera, which covered less than 3% of leaf surfaces, declined in manner consistent with ocean acidification predictions. Bulk epiphytic magnesium carbonate composition was similar between treatments and percentage of magnesium appeared to increase from summer to autumn. CCA did not exhibit any visible skeleton dissolution or mineral alteration at lowered pH and carbonate saturation state. Negative impacts from ocean acidification on P. oceanica epiphytic communities were smaller than expected. Epiphytic calcifiers were possibly protected from the pH treatment due to host plant photosynthesis inside the enclosure where water flow is slowed. The more positive outcome than expected suggests that calcareous members of epiphytic communities may find refuge in some conditions and be resilient to environmentally relevant changes in carbonate chemistry.

Research assessing the biological impacts of global ocean change often requires a burdensome characterization of seawater carbonate chemistry. For laboratory-based ocean acidification research, this impedes the scope of experimental design. Honeywell Durafet ® III pH electrodes provide precise and continuous seawater pH measurements. In addition to use in oceanographic sensor packages, Durafets can also be used in the laboratory to track and control seawater treatments via Honeywell Universal Dual Analyzers (UDAs). Here we provide performance data, instructions, and step-by-step recommendations for use of multiple UDA-Durafets. Durafet pH measurements were within ±0.005 units pH T of spectrophotometric measurements and agreement among eight Durafets was better than ±0.005 units pH T. These results indicate equal performance to Durafets in oceanographic sensor packages, but methods for calibration and quality control differ. Use of UDA-Durafets vastly improves time-course documentation of experimental conditions and reduces person-hours dedicated to this activity. Due to the versatility of integrating Durafets in laboratory seawater systems, this technology opens the door to advance the scale of questions that the ocean acidification research community aims to address.

Oligotrophic areas account for about 30% of oceanic primary production and are projected to expand in a warm, high-CO2 world. Changes in primary production in these areas could have important impacts on future global carbon cycling. To assess the response of primary production and respiration of plankton communities to increasing partial pressure of CO2 (pCO2) levels in Low Nutrient Low Chorophyll areas, two mesocosm experiments were conducted in the Bay of Calvi (Corsica, France) and in the Bay of Villefranche (France) in June–July 2012 and February–March 2013 under different trophic state, temperature and irradiance conditions. Nine mesocosms of 50 m3 were deployed for 20 and 12 days, respectively, and were subjected to seven pCO2 levels (3 control and 6 elevated levels). The metabolism of the community was studied using several methods based on in situ incubations (oxygen light–dark, 18O and 14C uptake). Increasing pCO2 had no significant effect on gross primary production, net community production, particulate and dissolved carbon production, as well as on community respiration. These two mesocosm experiments, the first performed under maintained low nutrient and low chlorophyll, suggest that in large areas of the ocean, increasing pCO2 levels may not lead to a significant change in plankton metabolic rates and sea surface biological carbon fixation.

Despite an increasing number of experiments, no consensus has emerged on the effect of ocean acidification on plankton communities and carbon flow. During two experiments, performed in the Bay of Calvi (France, Corsica; summer 2012) and the Bay of Villefranche (France; winter 2013), nine off-shore mesocosms (∼50 m3) were deployed among which three served as controls and six were enriched with CO2 to reach partial pressure of CO2 (pCO2) levels from 450 to 1350 μatm and 350–1250 μatm in the Bay of Calvi and the Bay of Villefranche, respectively. In each mesocosm, inorganic 13C was added in order to follow carbon transfer from inorganic via bulk particulate organic carbon and phytoplankton to bacteria by means of biomarkers as well as to zooplankton and settling particles. Despite very low plankton biomasses, labelled carbon was clearly transferred through plankton communities. Incorporation rates in the various plankton compartments suggested a slow-growing community based on regenerated production in the Bay of Calvi while in the Bay of Villefranche, fast-growing species were clearly dominating community production at the start with a shift toward slow-growing species during the experiment due to nutrient limitation. Both bulk and group-specific productions rates did not respond to increasing pCO2 levels. These experiments were the first conducted in the Mediterranean Sea under low nutrient concentrations and phytoplankton biomasses and suggest that ocean acidification may not significantly impact plankton carbon flows in low nutrient low chlorophyll (LNLC) areas.

There are concerns about the future of coral reefs in the face of ocean acidification and warming, and although studies of these phenomena have advanced quickly, efforts have focused on pieces of the puzzle rather than integrating them to evaluate ecosystem-level effects. The field is now poised to begin this task, but there are information gaps that first must be overcome before progress can be made. Many of these gaps focus on calcification at the levels of cells, organisms, populations, communities, and ecosystem, and their closure will be made difficult by the complexity of the interdependent processes by which coral reefs respond to ocean acidification, with effects scaling from cells to ecosystems and from microns to kilometers. Existing ecological theories provide an important and largely untapped resource for overcoming these difficulties, and they offer great potential for integrating the effects of ocean acidification across scales on coral reefs.

Climate change has significant implications for biodiversity and ecosystems. With slow progress towards reducing greenhouse gas emissions, climate engineering (or ‘geoengineering’) is receiving increasing attention for its potential to limit anthropogenic climate change and its damaging effects. Proposed techniques, such as ocean fertilization for carbon dioxide removal or stratospheric sulfate injections to reduce incoming solar radiation, would significantly alter atmospheric, terrestrial and marine environments, yet potential side-effects of their implementation for ecosystems and biodiversity have received little attention. A literature review was carried out to identify details of the potential ecological effects of climate engineering techniques. A group of biodiversity and environmental change researchers then employed a modified Delphi expert consultation technique to evaluate this evidence and prioritize the effects based on the relative importance of, and scientific understanding about, their biodiversity and ecosystem consequences. The key issues and knowledge gaps are used to shape a discussion of the biodiversity and ecosystem implications of climate engineering, including novel climatic conditions, alterations to marine systems and substantial terrestrial habitat change. This review highlights several current research priorities in which the climate engineering context is crucial to consider, as well as identifying some novel topics for ecological investigation.

Ocean acidification is a major threat to calcifying marine organisms such as deep-sea cold-water corals (CWCs), but related knowledge is scarce. The aragonite saturation threshold (Ω a) for calcification, respiration and organic matter fluxes were investigated experimentally in the Mediterranean Madrepora oculata. Over 10 weeks, colonies were maintained under two feeding regimes (uptake of 36.75 and 7.46 µmol C polyp −1 week −1) and exposed in 2 week intervals to a consecutively changing air–CO 2 mix (pCO 2) of 400, 1600, 800, 2000 and 400 ppm. There was a significant effect of feeding on calcification at initial ambient pCO 2 , while with consecutive pCO 2 treatments, feeding had no effect on calcification. Respiration was not significantly affected by feeding or pCO 2 levels. Coral skeletons started to dissolve at an average Ω a threshold of 0.92, but recovered and started to calcify again at Ω a ≥1. The surplus energy required to counteract dissolution at elevated pCO 2 (≥1600 µatm) was twice that at ambient pCO 2. Yet, feeding had no mitigating effect at increasing pCO 2 levels. This could be due to the fact that the energy required for calcification is a small fraction (1–3%) of the total metabolic energy demand and corals even under low food conditions might therefore still be able to allocate this small portion of energy to calcification. The response and resistance to ocean acidification are consequently not controlled by feeding in this species, but more likely by chemical reactions at the site of calcification and exchange processes between the calicoblastic layer and ambient seawater.

Shelled pteropods play key roles in the global carbon cycle and food webs of various ecosystems. Their thin external shell is sensitive to small changes in pH, and shell dissolution has already been observed in areas where aragonite saturation state is ~1. A decline in pteropod abundance has the potential to disrupt trophic networks and directly impact commercial fisheries. Therefore, it is crucial to understand how pteropods will be affected by global environmental change, particularly ocean acidification. In this study, physiological and molecular approaches were used to investigate the response of the Mediterranean pteropod, Heliconoides inflatus, to pH values projected for 2100 under a moderate emissions trajectory (RCP6.0). Pteropods were subjected to pHT 7.9 for 3 days, and gene expression levels, calcification and respiration rates were measured relative to pHT 8.1 controls. Gross calcification decreased markedly under low pH conditions, while genes potentially involved in calcification were up-regulated, reflecting the inability of pteropods to maintain calcification rates. Gene expression data imply that under low pH conditions, both metabolic processes and protein synthesis may be compromised, while genes involved in acid–base regulation were up-regulated. A large number of genes related to nervous system structure and function were also up-regulated in the low pH treatment, including a GABAA receptor subunit. This observation is particularly interesting because GABAA receptor disturbances, leading to altered behavior, have been documented in several other marine animals after exposure to elevated CO2. The up-regulation of many genes involved in nervous system function suggests that exposure to low pH could have major effects on pteropod behavior. This study illustrates the power of combining physiological and molecular approaches. It also reveals the importance of behavioral analyses in studies aimed at understanding the impacts of low pH on marine animals.

Seagrass is expected to benefit from increased carbon availability under future ocean acidification. This hypothesis has been little tested by in situ manipulation. To test for ocean acidification effects on seagrass meadows under controlled CO2/pH conditions, we used a Free Ocean Carbon Dioxide Enrichment (FOCE) system which allows for the manipulation of pH as continuous offset from ambient. It was deployed in a Posidonia oceanica meadow at 11 m depth in the Northwestern Mediterranean Sea. It consisted of two benthic enclosures, an experimental and a control unit both 1.7 m3, and an additional reference plot in the ambient environment (2 m2) to account for structural artifacts. The meadow was monitored from April to November 2014. The pH of the experimental enclosure was lowered by 0.26 pH units for the second half of the 8-month study. The greatest magnitude of change in P. oceanica leaf biometrics, photosynthesis, and leaf growth accompanied seasonal changes recorded in the environment and values were similar between the two enclosures. Leaf thickness may change in response to lower pH but this requires further testing. Results are congruent with other short-term and natural studies that have investigated the response of P. oceanica over a wide range of pH. They suggest any benefit from ocean acidification, over the next century (at a pH of ∼ 7.7 on the total scale), on Posidonia physiology and growth may be minimal and difficult to detect without increased replication or longer experimental duration. The limited stimulation, which did not surpass any enclosure or seasonal effect, casts doubts on speculations that elevated CO2 would confer resistance to thermal stress and increase the buffering capacity of meadows.

Abstract. The exponential growth of studies on the biological response to ocean acidification over the last few decades has generated a large amount of data. To facilitate data comparison, a data compilation hosted at the data publisher PANGAEA was initiated in 2008 and is updated on a regular basis (doi:10.1594/PANGAEA.149999). By January 2015, a total of 581 data sets (over 4 000 000 data points) from 539 papers had been archived. Here we present the developments of this data compilation 5 years since its first description by Nisumaa et al. (2010). Most of the study sites from which data have been archived are in the Northern Hemisphere and the number of archived data from studies from the Southern Hemisphere and polar oceans is still relatively low. Data from 60 studies that investigated the response of a mix of organisms or natural communities were all added after 2010, indicating a welcome shift from the study of individual organisms to communities and ecosystems. The initial imbalance of considerably more data archived on calcification and primary production than on other processes has improved. There is also a clear tendency towards more data archived from multifactorial studies after 2010. For easier and more effective access to ocean acidification data, the ocean acidification community is strongly encouraged to contribute to the data archiving effort, and help develop standard vocabularies describing the variables and define best practices for archiving ocean acidification data.

Red calcareous coralline algae are thought to be among the organisms most vulnerable to ocean acidifi-cation due to the high solubility of their magnesium cal-cite skeleton. Although skeletal mineralogy is proposed to change as CO 2 and temperature continue to rise, there is currently very little information available on the response of coralline algal carbonate mineralogy to near-future changes in pCO 2 and temperature. Here we present results from a 1-year controlled laboratory experiment to test mineralogi-cal responses to pCO 2 and temperature in the Mediterranean crustose coralline alga (CCA) Lithophyllum cabiochae. Our results show that Mg incorporation is mainly constrained by temperature (+1 mol % MgCO 3 for an increase of 3 • C), and there was no response to pCO 2. This suggests that L. cabiochae thalli have the ability to buffer their calcifying medium against ocean acidification, thereby enabling them to continue to deposit magnesium calcite with a significant mol % MgCO 3 under elevated pCO 2. Analyses of CCA dissolution chips showed a decrease in Mg content after 1 year for all treatments, but this was affected neither by pCO 2 nor by temperature. Our findings suggest that biological processes exert a strong control on calcification on magnesium calcite and that CCA may be more resilient under rising CO 2 than previously thought. However, previously demonstrated increased skeletal dissolution with ocean acidification will still have major consequences for the stability and maintenance of Mediterranean coralligenous habitats.

The ocean moderates anthropogenic climate change at the cost of profound alterations of its physics, chemistry, ecology, and services. Here, we evaluate and compare the risks of impacts on marine and coastal ecosystems—and the goods and services they provide—for growing cumulative carbon emissions under two contrasting emissions scenarios. The current emissions trajectory would rapidly and significantly alter many ecosystems and the associated services on which humans heavily depend. A reduced emissions scenario—consistent with the Copenhagen Accord’s goal of a global temperature increase of less than 2°C—is much more favorable to the ocean but still substantially alters important marine ecosystems and associated goods and services. The management options to address ocean impacts narrow as the ocean warms and acidifies. Consequently, any new climate regime that fails to minimize ocean impacts would be incomplete and inadequate.

An updated synthesis of the observed and projected impacts of climate change on the chemical, physical and biological processes in the oceans.

Agulhas rings provide the principal route for ocean waters to circulate from the Indo-Pacific to the Atlantic basin. Their influence on global ocean circulation is well known, but their role in plankton transport is largely unexplored. We show that, although the coarse taxonomic structure of plankton communities is continuous across the Agulhas choke point, South Atlantic plankton diversity is altered compared with Indian Ocean source populations. Modeling and in situ sampling of a young Agulhas ring indicate that strong vertical mixing drives complex nitrogen cycling, shaping community metabolism and biogeochemical signatures as the ring and associated plankton transit westward. The peculiar local environment inside Agulhas rings may provide a selective mechanism contributing to the limited dispersal of Indian Ocean plankton populations into the Atlantic.

Cet article analyse les impacts observés et projetés du changement climatique sur les systèmes humains et naturels, leur vulnérabilité et les possibilités d'adaptation. Il donne un aperçu des principaux résultats pour l'hydrologie, l'agriculture, les écosystèmes naturels, les infrastructures, le transport, l'énergie, le tourisme, la santé et les aspects sociaux. L'article reprend les principales conclusions du chapitre Europe de la contribution du Groupe II au 5e rapport du Giec paru en 2014. Plusieurs études traitant spécifiquement de la France métropolitaine sont également présentées, mais sans prétendre à l'exhaustivité, le sujet étant particulièrement vaste.

Abstract The effect of ocean warming and acidification was investigated on a natural plankton assemblage from an oligotrophic area, the bay of Villefranche (NW Mediterranean Sea). The assemblage was sampled in March 2012 and exposed to the following four treatments for 12 days: control (∼360 μatm, 14°C), elevated pCO2 (∼610 μatm, 14°C), elevated temperature (∼410 μatm, 17°C), and elevated pCO2 and temperature (∼690 μatm, 17°C). Nutrients were already depleted at the beginning of the experiment and the concentrations of chlorophyll a (chl a), heterotrophic prokaryotes and viruses decreased, under all treatments, throughout the experiment. There were no statistically significant effects of ocean warming and acidification, whether in isolation or combined, on the concentrations of nutrients, particulate organic matter, chl a and most of the photosynthetic pigments. Furthermore, 13C labelling showed that the carbon transfer rates from 13C-sodium bicarbonate into particulate organic carbon were not affected by seawater warming nor acidification. Rates of gross primary production followed the general decreasing trend of chl a concentrations and were significantly higher under elevated temperature, an effect exacerbated when combined to elevated pCO2 level. In contrast to the other algal groups, the picophytoplankton population (cyanobacteria, mostly Synechococcus) increased throughout the experiment and was more abundant in the warmer treatment though to a lesser extent when combined to high pCO2 level. These results suggest that under nutrient-depleted conditions in the Mediterranean Sea, ocean acidification has a very limited impact on the plankton community and that small species will benefit from warming with a potential decrease of the export and energy transfer to higher trophic levels.

Marine scientists often use two measured or modeled carbonate system variables to compute others. These carbonate chemistry calculations, based on well-known thermodynamic equilibria, are now available in a dozen public packages. Ten of those were compared using common input data and the set of equilibrium constants recommended for best practices. Current versions of all 10 packages agree within 0.2 μatm for pCO2, 0.0002 units for pH, and 0.1 μmol kg−1 for CO32− in terms of surface zonal-mean values. That represents more than a 10-fold improvement relative to outdated versions of the same packages. Differences between packages grow with depth for some computed variables but remain small. Discrepancies derive largely from differences in equilibrium constants. Analysis of the sensitivity of each computed variable to changes in each constant reveals the general dominance of K1 and K2 but also the comparable sensitivity to KB for the AT–CT input pair. Best-practice formulations for K1 and K2 are implemented consistently among packages. Yet with more recent formulations designed to cover a wider range of salinity, packages disagree by up to 8 μatm in pCO2, 0.006 units in pH, and 1 μmol kg−1 in CO32− under typical surface conditions. They use different proposed sets of coefficients for these formulations, all of which are inconsistent. Users would do well to use up-to-date versions of packages and the constants recommended for best practices.

The effects of elevated CO2 partial pressure (pCO2) on otolith calcification and on the coupling between the somatic and otolith growth were investigated in juvenile gilthead seabream Sparus aurata. Six-month old individuals were raised during seven weeks under four pCO2 conditions set according to projected future ocean acidification scenarios. Body and otolith biometric parameters were measured throughout the experiment along with the otolith biomineralization monitored using a radiotracer technique based on 45Ca incorporation. Seabream exhibited somatic growth resilience to all treatments. In contrast, increased growth rate and shape complexity of otoliths were observed with a pHT drop from 8.1 to 7.5. Hypercalcification was observed under lowered pH, with a rate of calcium incorporation increasing by up to 18% between pHT 8.1 and pHT 7.7. This work highlighted an uncoupling of otolith and body growth of juvenile seabream within 40 d at pHT 7.9 projected to be reached by the end of the century. As the otolith is an essential tool used in reconstructing fish life history, this work suggests that information resulting from otolith studies should be interpreted with caution with respect to the potential impacts that ocean acidification projected modifications could have on otolith biomineralization.

All living organisms contribute to the biogeochemical cycles, but microorganisms, due to their high abundance, their tremendous metabolic capacities and adaptation potential, play a key role in the functioning and the evolution of biogeochemical cycles. Consequently, they are keyplayers in adaptation, resistance and resilience of ecosystems. The role of microorganisms in the main biogeochemical cycles (carbon, nitrogen, sulfur, phosphorus, silicon, metals), in soils, freshwater and marine ecosystems is presented. Microbial processes involved in the turnover of biogeochemical cycles are discussed from gene to ecosystem (natural and anthropogenic ecosystems), at global, regional and local scales, as well as in targeted microenvironments (such as particles or microniches). The biodiversity of microorganisms is highlighted and their metabolic pathways on which are based exchanges and biotransformations of organic and mineral components within ecosystems are described in details. The impacts of human activities on the microbial actors and processes of biogeochemical cycles, and the cascading ecological effects (greenhouse gas emissions, acid rains, dystrophic crises, etc.), are also discussed.

Ocean acidification (OA) refers to the general decrease in pH of the global ocean as a result of absorbing anthropogenic CO2 emitted in the atmosphere since preindustrial times (Sabine et al., 2004). There is, however, considerable variability in ocean acidification, and many careful measurements need to be made and compared in order to obtain scientifically valid information for the assessment of patterns, trends, and impacts over a range of spatial and temporal scales, and to understand the processes involved. A single country or institution cannot undertake measurements of worldwide coastal and open ocean OA changes; therefore, international cooperation is needed to achieve that goal. The OA data that have been, and are being, collected represent a significant public investment. To this end, it is critically important that researchers (and others) around the world are easily able to find and use reliable OA information that range from observing data (from time-series moorings, process studies, and research cruises), to biological response experiments (e.g., mesocosm), data products, and model output. [...]

Free-ocean CO2 enrichment (FOCE) systems are designed to assess the impact of ocean acidification on biological communities in situ for extended periods of time (weeks to months). They overcome some of the drawbacks of laboratory experiments and field observations by enabling (1) precise control of CO2 enrichment by monitoring pH as an offset of ambient pH, (2) consideration of indirect effects such as those mediated through interspecific relationships and food webs, and (3) relatively long experiments with intact communities. Bringing perturbation experiments from the laboratory to the field is, however, extremely challenging. The main goal of this paper is to provide guidelines on the general design, engineering, and sensor options required to conduct FOCE experiments. Another goal is to introduce xFOCE, a community-led initiative to promote awareness, provide resources for in situ perturbation experiments, and build a user community. Present and existing FOCE systems are briefly described and examples of data collected presented. Future developments are also addressed as it is anticipated that the next generation of FOCE systems will include, in addition to pH, options for oxygen and/or temperature control. FOCE systems should become an important experimental approach for projecting the future response of marine ecosystems to environmental change.

Continuous anthropogenic CO2 emissions to the atmosphere and uptake by the oceans will cause a reduction of seawater pH and saturation state (Omega) of CaCO3 minerals from which marine calcifiers build their shells and skeletons. Sea urchins use the most soluble form of calcium carbonate, high-magnesium calcite, to build their skeleton, spines and grazing apparatus. In order to highlight the effects of increased pCO(2) on the test thickness and carbonate elemental composition of juvenile sea urchins and potential differences in their responses linked to the diet, we performed a laboratory experiment on juvenile Paracentrotus lividus, grazing on calcifying (Corallina elongata) and non-calcifying (Cystoseira amentacea, Dictyota dichotoma) macroalgae, under different pH (corresponding to pCO(2) values of 390, 550, 750 and 1000 mu atm). Results highlighted the importance of the diet in determining sea urchin size irrespectively of the pCO(2) level, and the relevance of macroalgal diet in modulating urchin Mg/Ca ratio. The present study provides relevant clues both in terms of the mechanism of mineral incorporation and in terms of bottom-up processes (algal diet) affecting top-down ones (fish predation) in rocky subtidal communities. (C) 2013 Elsevier Ltd. All rights reserved.

The widespread distribution of pteropods, their role in ocean food webs and their sensitivity to ocean acidification and warming has renewed scientific interest in this group of zooplankton. Unfortunately, their fragile shell, sensitivity to handling, unknowns surrounding buoyancy regulation and poorly described feeding mechanisms make thecosome pteropods notoriously difficult to maintain in the laboratory. The resultant high mortality rates and unnatural behaviours may confound experimental findings. The high mortality rate also discourages the use of periods of acclimation to experimental conditions and precludes vital long-term studies. Here we summarize the current status of culture methodology to provide a comprehensive basis for future experimental work and culture system development.

The present data publication provides permanent links to original and updated versions of validated data files. The data files include properties of seawater, particulate matter and dissolved matter that were measured from discrete water samples collected with Niskin bottles during the 2009-2013 Tara Oceans expedition. Properties include pigment concentrations from HPLC analysis (10 depths per vertical profile, 25 pigments per depth), the carbonate system (Surface and 400m; pH (total scale), CO₂, pCO₂, <em>f</em>CO₂, HCO₃, CO₃, Total alkalinity, Total carbon, OmegaAragonite, OmegaCalcite, and dosage Flags), nutrients (10 depths per vertical profile; NO₂, PO₄, NO₂/NO₃, SI, quality Flags), DOC, CDOM, and dissolved oxygen isotopes. The Service National d'Analyse des Paramètres Océaniques du CO₂, at the Université Pierre et Marie Curie, determined CT and AT potentiometrically (Edmond 1970; DOE 1994) on samples preserved according to Dickson et al. (2007). More than 250 vertical profiles of these properties were made across the world ocean. DOC, CDOM and dissolved oxygen isotopes are available only for the Arctic Ocean and Arctic Seas (2013).

The effect of salinity on the elemental and isotopic skeletal composition of modern zooxanthellate scleractinian corals (Acropora sp., Montipora verrucosa and Stylophora pistillata) was investigated in order to evaluate potential salinity proxies. Corals were cultured in the laboratory at three salinities (36, 38 and 40). The other environmental parameters were kept constant. For all species analyzed, Sr/Ca, Mg/Ca, U/Ca and Li/Ca ratios were not influenced by salinity changes. The Ba/Ca ratio also lacks a systematic relationship with salinity and exhibits high inter-generic variations, up to one order of magnitude. On the contrary, the Cd/Ca ratio decreases as a function of increasing salinity, and delta O-18 and delta C-13 also presented a significant response, but with opposite trends to salinity variations. Since Cd/Ca is usually considered as an upwelling proxy, its salinity dependence could compromise the upwelling signal, unless some corrections can be carried out. Regardless, if the dependence found in the present dataset proved to be widespread and systematic, the Cd/Ca ratio could represent a promising salinometer awaiting further investigation. This study also confirmed the reliability of the well-established temperature proxies Sr/Ca, Mg/Ca and U/Ca, as these ratios were insensitive to salinity variations. Moreover, our results showed that delta O-18 or delta C-13 can be considered as reliable temperature recorders as far as the salinity effect is removed from the parameter reconstructed (e.g., temperature). Investigating the influence of salinity on the skeletal chemistry of scleractinian corals grown under controlled environmental conditions confirmed previous results, validated isotopic corrections, and identified a promising proxy of salinity.

The present study investigates the influence of environmental (temperature, salinity) and biological (growth rate, inter-generic variations) parameters on calcium isotope fractionation (delta Ca-44/40) in scleractinian coral skeleton to better constrain this record. Previous studies focused on the delta Ca-44/40 record in different marine organisms to reconstruct seawater composition or temperature, but only few studies investigated corals. This study presents measurements performed on modern corals from natural environments (from the Maldives for modem and from Tahiti for fossil corals) as well as from laboratory cultures (Centre Scientifique de Monaco). Measurements on Porites sp., Acropora sp., Montipora verrucosa and Stylophora pistillata allow constraining inter-generic variability. Our results show that the fractionation of delta Ca-44/40 ranges from 0.6 to 0.1%., independent of the genus or the environmental conditions. No significant relationship between the rate of calcification and delta Ca-44/40 was found. The weak temperature dependence reported in earlier studies is most probably not the only parameter that is responsible for the fractionation. Indeed, sub-seasonal temperature variations reconstructed by delta O-18 and Sr/Ca ratio using a multi-proxy approach, are not mirrored in the coral's delta Ca-44/40 variations. The intergeneric variability and intrageneric variability among the studied samples are weak except for S. pistillate, which shows calcium isotopic values increasing with salinity. The variability between samples cultured at a salinity of 40 is higher than those cultured at a salinity of 36 for this species. The present study reveals a strong biological control of the skeletal calcium isotope composition by the polyp and a weak influence of environmental factors, specifically temperature and salinity (except for S. pistillate). Vital effects have to be investigated in situ to better constrain their influence on-the calcium isotopic signal. If vital effects could be extracted from the isotopic signal, the calcium isotopic composition of coral skeletons could provide reliable information on the calcium composition and budget in ocean. (c) 2012 Elsevier B.V. All rights reserved.

The anthropogenic increase of carbon dioxide (CO 2) alters the seawater carbonate chemistry, with a decline of pH and an increase in the partial pressure of CO 2 (pCO 2). Although bacteria play a major role in carbon cycling, little is known about the impact of rising pCO 2 on bacterial carbon metabolism, especially for natural bacterial communities. In this study, we investigated the effect of rising pCO 2 on bacterial production (BP), bacterial respiration (BR) and bacterial carbon metabolism during a mesocosm experiment performed in Kongsfjorden (Svalbard) in 2010. Nine meso-cosms with pCO 2 levels ranging from ca. 180 to 1400 µatm were deployed in the fjord and monitored for 30 days. Generally BP gradually decreased in all mesocosms in an initial phase, showed a large (3.6-fold average) but temporary increase on day 10, and increased slightly after inorganic nutrient addition. Over the wide range of pCO 2 investigated, the patterns in BP and growth rate of bulk and free-living communities were generally similar over time. However, BP of the bulk community significantly decreased with increasing pCO 2 after nutrient addition (day 14). In addition, increasing pCO 2 enhanced the leucine to thymidine (Leu : TdR) ratio at the end of experiment, suggesting that pCO 2 may alter the growth balance of bacteria. Stepwise multiple regression analysis suggests that multiple factors, including pCO 2 , explained the changes of BP, growth rate and Leu : TdR ratio at the end of the experiment. In contrast to BP, no clear trend and effect of changes of pCO 2 was observed for BR, bacterial carbon demand and bacterial growth efficiency. Overall, the results suggest that changes in pCO 2 potentially influence bacterial production, growth rate and growth balance rather than the conversion of dissolved organic matter into CO 2 .

Ocean acidification has emerged over the last two decades as one of the largest threats to marine organisms and ecosystems. However, most research efforts on ocean acidification have so far neglected management and related policy issues to focus instead on understanding its ecological and biogeochemical implications. This shortfall is addressed here with a systematic, international and critical review of management and policy options. In particular, we investigate the assumption that fighting acidification is mainly, but not only, about reducing CO2 emissions, and explore the leeway that this emerging problem may open in old environmental issues. We review nine types of management responses, initially grouped under four categories: preventing ocean acidification; strengthening ecosystem resilience; adapting human activities; and repairing damages. Connecting and comparing options leads to classifying them, in a qualitative way, according to their potential and feasibility. While reducing CO2 emissions is confirmed as the key action that must be taken against acidification, some of the other options appear to have the potential to buy time, e.g. by relieving the pressure of other stressors, and help marine life face unavoidable acidification. Although the existing legal basis to take action shows few gaps, policy challenges are significant: tackling them will mean succeeding in various areas of environmental management where we failed to a large extent so far.

Ocean acidification represents a threat to marine species worldwide, and forecasting the ecological impacts of acidification is a high priority for science, management, and policy. As research on the topic expands at an exponential rate, a comprehensive understanding of the variability in organisms' responses and corresponding levels of certainty is necessary to forecast the ecological effects. Here, we perform the most comprehensive meta-analysis to date by synthesizing the results of 228 studies examining biological responses to ocean acidification. The results reveal decreased survival, calcification, growth, development and abundance in response to acidification when the broad range of marine organisms is pooled together. However, the magnitude of these responses varies among taxonomic groups, suggesting there is some predictable trait-based variation in sensitivity, despite the investigation of approximately 100 new species in recent research. The results also reveal an enhanced sensitivity of mollusk larvae, but suggest that an enhanced sensitivity of early life history stages is not universal across all taxonomic groups. In addition, the variability in species' responses is enhanced when they are exposed to acidification in multi-species assemblages, suggesting that it is important to consider indirect effects and exercise caution when forecasting abundance patterns from single-species laboratory experiments. Furthermore, the results suggest that other factors, such as nutritional status or source population, could cause substantial variation in organisms' responses. Last, the results highlight a trend towards enhanced sensitivity to acidification when taxa are concurrently exposed to elevated seawater temperature.

Ocean acidification caused by anthropogenic uptake of CO2 is perceived to be a major threat to calcifying organisms. Cold-water corals were thought to be strongly affected by a decrease in ocean pH due to their abundance in deep and cold waters which, in contrast to tropical coral reef waters, will soon become corrosive to calcium carbonate. Calcification rates of two Mediterranean cold-water coral species, Lophelia pertusa and Madrepora oculata, were measured under variable partial pressure of CO2 (pCO(2)) that ranged between 380 mu atm for present-day conditions and 930 matm for the end of the century. The present study addressed both short- and long-term responses by repeatedly determining calcification rates on the same specimens over a period of 9 months. Besides studying the direct, short-term response to elevated pCO(2) levels, the study aimed to elucidate the potential for acclimation of calcification of cold-water corals to ocean acidification. Net calcification of both species was unaffected by the levels of pCO(2) investigated and revealed no short-term shock and, therefore, no long-term acclimation in calcification to changes in the carbonate chemistry. There was an effect of time during repeated experiments with increasing net calcification rates for both species, however, as this pattern was found in all treatments, there is no indication that acclimation of calcification to ocean acidification occurred. The use of controls (initial and ambient net calcification rates) indicated that this increase was not caused by acclimation in calcification response to higher pCO(2). An extrapolation of these data suggests that calcification of these two cold-water corals will not be affected by the pCO(2) level projected at the end of the century.

The oceans moderate the rate and severity of climate change by absorbing massive amounts of anthropogenic CO2 but this results in large-scale changes in seawater chemistry, which are collectively referred to as anthropogenic ocean acidification. Despite its potentially widespread consequences, the problem of ocean acidification has been largely absent from most policy discussions of CO2 emissions, both because the science is relatively new and because the research community has yet to deliver a clear message to decision makers regarding its impacts. Here we report the results of the first expert survey in the field of ocean acidification. Fifty-three experts, who had previously participated in an IPCC workshop, were asked to assess 22 declarative statements about ocean acidification and its consequences. We find a relatively strong consensus on most issues related to past, present and future chemical aspects of ocean acidification: non-anthropogenic ocean acidification events have occurred in the geological past, anthropogenic CO2 emissions are the main (but not the only) mechanism generating the current ocean acidification event, and anthropogenic ocean acidification that has occurred due to historical fossil fuel emissions will be felt for centuries. Experts generally agreed that there will be impacts on biological and ecological processes and biogeochemical feedbacks but levels of agreement were lower, with more variability across responses. Levels of agreement were higher for statements regarding calcification, primary production and nitrogen fixation than for those about impacts on foodwebs. The levels of agreement for statements pertaining to socio-economic impacts, such as impacts on food security, and to more normative policy issues, were relatively low.

Temperate marine rocky habitats may be alternatively characterized by well vegetated macroalgal assemblages or barren grounds, as a consequence of direct and indirect human impacts (e.g. overfishing) and grazing pressure by herbivorous organisms. In future scenarios of ocean acidification, calcifying organisms are expected to be less competitive: among these two key elements of the rocky subtidal food web, coralline algae and sea urchins. In order to highlight how the effects of increased pCO 2 on individual calcifying species will be exacerbated by interactions with other trophic levels, we performed an experiment simultaneously testing ocean acidification effects on primary producers (calcifying and non-calcifying algae) and their grazers (sea urchins). Artificial communities, composed by juveniles of the sea urchin Paracentrotus lividus and calcifying (Corallina elongata) and non-calcifying (Cystoseira amentacea var stricta, Dictyota dichotoma) macroalgae, were subjected to pCO 2 levels of 390, 550, 750 and 1000 matm in the laboratory. Our study highlighted a direct pCO 2 effect on coralline algae and on sea urchin defense from predation (test robustness). There was no direct effect on the non-calcifying macroalgae. More interestingly, we highlighted diet-mediated effects on test robustness and on the Aristotle's lantern size. In a future scenario of ocean acidification a decrease of sea urchins' density is expected, due to lower defense from predation, as a direct consequence of pH decrease, and to a reduced availability of calcifying macroalgae, important component of urchins' diet. The effects of ocean acidification may therefore be contrasting on well vegetated macroalgal assemblages and barren grounds: in the absence of other human impacts, a decrease of biodiversity can be predicted in vegetated macroalgal assemblages, whereas a lower density of sea urchin could help the recovery of shallow subtidal rocky areas affected by overfishing from barren grounds to assemblages dominated by fleshy macroalgae.

Over the next century, elevated quantities of atmospheric CO2 are expected to penetrate into the oceans, causing a reduction in pH (-0.3/-0.4 pH unit in the surface ocean) and in the concentration of carbonate ions (so-called ocean acidification). Of growing concern are the impacts that this will have on marine and estuarine organisms and ecosystems. Marine shelled molluscs, which colonized a large latitudinal gradient and can be found from intertidal to deep-sea habitats, are economically and ecologically important species providing essential ecosystem services including habitat structure for benthic organisms, water purification and a food source for other organisms. The effects of ocean acidification on the growth and shell production by juvenile and adult shelled molluscs are variable among species and even within the same species, precluding the drawing of a general picture. This is, however, not the case for pteropods, with all species tested so far, being negatively impacted by ocean acidification. The blood of shelled molluscs may exhibit lower pH with consequences for several physiological processes (e.g. respiration, excretion, etc.) and, in some cases, increased mortality in the long term. While fertilization may remain unaffected by elevated pCO(2), embryonic and larval development will be highly sensitive with important reductions in size and decreased survival of larvae, increases in the number of abnormal larvae and an increase in the developmental time. There are big gaps in the current understanding of the biological consequences of an acidifying ocean on shelled molluscs. For instance, the natural variability of pH and the interactions of changes in the carbonate chemistry with changes in other environmental stressors such as increased temperature and changing salinity, the effects of species interactions, as well as the capacity of the organisms to acclimate and/or adapt to changing environmental conditions are poorly described.

The Service d'Observation de la Rade de Villefranche-sur-Mer is designed to study the temporal variability of hydrological conditions as well as the abundance and composition of holo- and meroplankton at a fixed station in this bay of the northwest Mediterranean. The weekly data collected at this site, designated as ``Point B'' since 1957, represent a long-term time series of hydrological conditions in a coastal environment. Since 2007, the historical measurements of hydrological and biological conditions have been complemented by measurements of the CO2-carbonic acid system parameters. In this contribution, CO2-carbonic acid system parameters and ancillary data are presented for the period 2007-2011. The data are evaluated in the context of the physical and biogeochemical processes that contribute to variations in CO2 in the water column and exchange of this gas between the ocean and atmosphere. Seasonal cycles of the partial pressure of CO2 in seawater (pCO(2)) are controlled principally by variations in temperature, showing maxima in the summer and minima during the winter. Normalization of pCO(2) to the mean seawater temperature (18.5 degrees C), however, reveals an apparent reversal of the seasonal cycle with maxima observed in the winter and minima in the summer, consistent with a biogeochemical control of pCO(2) by primary production. Calculations of fluxes of CO2 show this area to be a weak source of CO2 to the atmosphere during the summer and a weak sink during the winter but near neutral overall (range -0.3 to +0.3 mmol CO2 m(-2) h(-1), average 0.02 mmol CO2 m(-2) h(-1)). We also provide an assessment of errors incurred from the estimation of annual fluxes of CO2 as a function of sampling frequency (3-hourly, daily, weekly), using data obtained at the Hawaii Kilo Nalu coastal time-series station, which shows similar behavior to the Point B location despite significant differences in climate and hydrological conditions and the proximity of a coral reef ecosystem.

This study investigated the effects of seawater pH (i.e. 8.10, 7.85 and 7.60) and temperature (16 and 19°C) on (i) the abiotic conditions in the fluid surrounding the embryo (viz. the perivitelline fluid), (ii) growth, development and (iii) cuttlebone calcification of embryonic and juvenile stages of the cephalopod Sepia officinalis. Egg swelling increased in response to acidification or warming, leading to an increase in egg surface while the interactive effects suggested a limited plasticity of the swelling modulation. Embryos experienced elevated pCO2 conditions in the perivitelline fluid (> 3-fold higher pCO2 than that of ambient seawater), rendering the medium under-saturated even under ambient conditions. The growth of both embryos and juveniles was unaffected by pH, whereas 45Ca incorporation in cuttlebone increased significantly with decreasing pH at both temperatures. This phenomenon of hypercalcification is limited to only a number of animals but does not guarantee functional performance and calls for better mechanistic understanding of calcification processes.

The rise of CO 2 has been identified as a major threat to life in the ocean. About one-third of the anthro-pogenic CO 2 produced in the last 200 yr has been taken up by the ocean, leading to ocean acidification. Surface seawater pH is projected to decrease by about 0.4 units between the pre-industrial revolution and 2100. The branching cold-water corals Madrepora oculata and Lophelia pertusa are important, habitat-forming species in the deep Mediter-ranean Sea. Although previous research has investigated the abundance and distribution of these species, little is known regarding their ecophysiology and potential responses to global environmental change. A previous study indicated that the rate of calcification of these two species remained constant up to 1000 µatm CO 2 , a value that is at the upper end of changes projected to occur by 2100. We examined whether the ability to maintain calcification rates in the face of rising pCO 2 affected the energetic requirements of these corals. Over the course of three months, rates of respiration were measured at a pCO 2 ranging between 350 and 1100 µatm to distinguish between short-term response and longer-term acclimation. Respiration rates ranged from 0.074 to 0.266 µmol O 2 (g skeletal dry weight) −1 h −1 and 0.095 to 0.725 µmol O 2 (g skeletal dry weight) −1 h −1 for L. pertusa and M. oculata, respectively, and were independent of pCO 2. Respiration increased with time likely due to regular feeding, which may have provided an increased energy supply to sustain coral metabolism. Future studies are needed to confirm whether the insensitivity of respiration to increasing pCO 2 is a general feature of deep-sea corals in other regions.

The response of respiration, photosynthesis, and calcification to elevated pCO2 and temperature was investigated in isolation and in combination in the Mediterranean crustose coralline alga Lithophyllum cabiochae. Algae were maintained in aquaria during 1year at near-ambient conditions of irradiance, at ambient or elevated temperature (+3 degrees C), and at ambient (ca. 400atm) or elevated pCO2 (ca. 700atm). Respiration, photosynthesis, and net calcification showed a strong seasonal pattern following the seasonal variations of temperature and irradiance, with higher rates in summer than in winter. Respiration was unaffected by pCO2 but showed a general trend of increase at elevated temperature at all seasons, except in summer under elevated pCO2. Conversely, photosynthesis was strongly affected by pCO2 with a decline under elevated pCO2 in summer, autumn, and winter. In particular, photosynthetic efficiency was reduced under elevated pCO2. Net calcification showed different responses depending on the season. In summer, net calcification increased with rising temperature under ambient pCO2 but decreased with rising temperature under elevated pCO2. Surprisingly, the highest rates in summer were found under elevated pCO2 and ambient temperature. In autumn, winter, and spring, net calcification exhibited a positive or no response at elevated temperature but was unaffected by pCO2. The rate of calcification of L.cabiochae was thus maintained or even enhanced under increased pCO2. However, there is likely a trade-off with other physiological processes. For example, photosynthesis declines in response to increased pCO2 under ambient irradiance. The present study reports only on the physiological response of healthy specimens to ocean warming and acidification, however, these environmental changes may affect the vulnerability of coralline algae to other stresses such as pathogens and necroses that can cause major dissolution, which would have critical consequence for the sustainability of coralligenous habitats and the budgets of carbon and calcium carbonate in coastal Mediterranean ecosystems.

Primary production and calcification responses to irradiance were investigated in Lithophyllum cabiochae, a crustose coralline alga from Mediterranean coralligenous communities, collected at c. 25m depth in the Bay of Villefranche. Algae were maintained in aquaria at temperature and irradiance levels close to in situ conditions. Physiological measurements were performed using incubation chambers in the dark and in the light at different irradiance levels within the range of those measured in situ. Both photosynthesis and calcification rates in L. cabiochae were strongly related to irradiance. Dark respiration averaged 0.20.3 mu molcm(2) thallus h(1) in terms of both O-2 consumption and CO2 release and maximal gross photosynthesis averaged 1.0 mu molcm(2) h(1) in terms of both O-2 production and CO2 uptake. Mean rate of net calcification was 0.1 mu mol CaCO3 cm(2) h(1) in the dark and reached 0.4 mu mol CaCO3 cm(2) h(1) in the light. Diel net and gross organic C productions were estimated to be 3 and 7 mu molC cm(2) thallus d(1,) respectively. Diel net inorganic C production was estimated to be 3 mu mol CaCO3 cm(2) thallus d(1). Despite the low light conditions experienced by the algae at c. 25m depth, L. cabiochae can be considered as a major contributor to primary productivity and calcium carbonate deposition, making coralligenous communities a major carbon and carbonate producer in the Mediterranean Sea.

Global environmental changes, including ocean acidification, have been identified as a major threat to scleractinian corals. General predictions are that ocean acidification will be detrimental to reef growth and that 40 to more than 80 per cent of present-day reefs will decline during the next 50 years. Cold-water corals (CWCs) are thought to be strongly affected by changes in ocean acidification owing to their distribution in deep and/or cold waters, which naturally exhibit a CaCO3 saturation state lower than in shallow/warm waters. Calcification was measured in three species of Mediterranean cold-water scleractinian corals (Lophelia pertusa, Madrepora oculata and Desmophyllum dianthus) on-board research vessels and soon after collection. Incubations were performed in ambient sea water. The species M. oculata was additionally incubated in sea water reduced or enriched in CO2. At ambient conditions, calcification rates ranged between -0.01 and 0.23% d(-1). Calcification rates of M. oculata under variable partial pressure of CO2 (pCO(2)) were the same for ambient and elevated pCO(2) (404 and 867 mu atm) with 0.06+/-0.06% d(-1), while calcification was 0.12+/-0.06% d(-1) when pCO(2) was reduced to its pre-industrial level (285 mu atm). This suggests that present-day CWC calcification in the Mediterranean Sea has already drastically declined (by 50%) as a consequence of anthropogenic-induced ocean acidification.

The production of transparent exopolymeric particles (TEP) by the coccolithophores, Emiliania huxleyi, Calcidiscus leptoporus and Syracosphaera pulchra was investigated in batch cultures. The abundance, size spectra and carbon content of TEP were examined during the exponential growth phase of both haploid and diploid life stages grown under ambient (400 atm) and elevated (760 atm) CO2 partial pressure (pCO(2)) conditions. Results showed species- and life stage-specific differences in TEP production rate (day(1)) derived from abundance and carbon content of TEP. At 400 atm, TEP production rate was the highest in the diploid stage of S. pulchra and E. huxleyi, while TEP carbon content per cell was the highest in the diploid stage of C. leptoporus. At 760 atm, TEP production rate increased in almost all species and was closely related to the cell growth rates (except in the diploid stage of C. leptoporus), while the slope values of the regression lines between TEP size distribution and concentration decreased. This means that the contribution of smaller size TEP was relatively more important than larger TEP in the high pCO(2) treatment. Elevated pCO(2) is potentially able to alter TEP size distribution. TEP-C content cell(1) generally decreased with increasing pCO(2). TEP-C accounted for 124 of the cell particulate organic carbon production and was inversely related to increasing pCO(2). TEP production by C. leptoporus and S. pulchra has not previously been documented. The amount of organic carbon released as TEP by these coccolithophores is comparable to and may even exceed TEP production by some diatoms.

Calcification des otolithes de juvéniles de daurade royale Sparus aurata en réponse à l'acidification des océans

The increasing release of CO2 by human activities leads to ocean acidification and global warming. Both those consequences (i.e., increase in seawater pCO2 and temperature) may drastically affect physiology of marine organisms. The effects of low pH and elevated temperature on the bioaccumulation of radionuclides (241Am, 134Cs) and trace elements (60Co, 54Mn or 75Se) were studied during the embryonic development of the common cuttlefish Sepia officinalis. The lowered accumulation of essential 60Co and 54Mn with decreasing pH was larger at 16°C than at 19°C. Se was not detected in the embryo whereas it penetrated through the eggshell, suggesting that an alternative pathway ensures the supply of this essential metal for the embryo. 241Am was totally retained by the eggshell irrespective of pH and temperature. Finally, the amount of Cs found in the peri-vitelline fluid increased with decreasing pH likely because of an enhanced swelling of the cuttlefish egg under elevated CO2.

Thecosome pteropods play a key role in the food web of various marine ecosystems and they calcify, secreting the unstable CaCO 3 mineral aragonite to form their shell material. Here, we have estimated the effect of ocean acidification on pteropod calcification by exploiting empirical relationships between their gross calcification rates (CaCO 3 precipitation) and aragonite saturation state Ω a , combined with model projections of future Ω a . These were corrected for modern model-data bias and taken over the depth range where pteropods are observed to migrate vertically. Results indicate large reductions in gross calcification at temperate and high latitudes. Over much of the Arctic, the pteropod Limacina helicina will become unable to precipitate CaCO 3 by the end of the century under the IPCC SRES A2 scenario. These results emphasize concerns over the future of shelled pteropods, particularly L. helicina in high latitudes. Shell-less L. helicina are not known to have ever existed nor would we expect them to survive. Declines of pteropod populations could drive dramatic ecological changes in the various pelagic ecosystems in which they play a critical role.

Anthropogenic carbon dioxide (CO2) emissions to the atmosphere and subsequent uptake by the ocean are changing seawater chemistry, a process known as ocean acidification. Research indicates that as ocean acidification continues, reflecting increasing CO2 emissions, it is likely that although some species will be tolerant it will impact many marine organisms and processes, including composition of communities and food webs. Whilst there may be local actions to limit acidification from local sources the root cause of ocean acidification, CO2 emissions, is a global issue requiring global action through United Nations bodies.

Although shelled pteropods are expected to be particularly sensitive to ocean acidification, the few available studies have mostly focused on polar species and have not allowed determining which parameter of the carbonate system controls their calcification. Specimens of the temperate Mediterranean species Creseis acicula were maintained under seven different conditions of the carbonate chemistry, obtained by manipulating pH and total alkalinity, with the goal to disentangle the effects of the pH and the saturation state with respect to aragonite (Ω_a). Our results tend to show that respiration, excretion as well as rates of net and gross calcification were not directly affected by a decrease in pH but decreased significantly with a decrease in Ω_a. Due to the difficulties in maintaining pteropods in the laboratory and the important variability in their abundances in our study site, long-term acclimation as well as replication of the experiment was not possible. However, we strongly believe that these results represent an important step in the mechanistic understanding of the effect of ocean acidification on pteropods physiology.

Ocean acidification is predicted to have significant effects on benthic calcifying invertebrates, in particular on their early developmental stages. Echinoderm larvae could be particularly vulnerable to decreased pH, with major consequences for adult populations. The objective of this study was to understand how ocean acidification would affect the initial life stages of the sea urchin Paracentrotus lividus, a common species that is widely distributed in the Mediterranean Sea and the NE Atlantic. The effects of decreased pH (elevated P-CO2) were investigated through physiological and molecular analyses on both embryonic and larval stages. Eggs and larvae were reared in Mediterranean seawater at six pH levels, i.e. pH(T) 8.1, 7.9, 7.7, 7.5, 7.25 and 7.0. Fertilization success, survival, growth and calcification rates were monitored over a 3. day period. The expression of genes coding for key proteins involved in development and biomineralization was also monitored. Paracentrotus lividus appears to be extremely resistant to low pH, with no effect on fertilization success or larval survival. Larval growth was slowed when exposed to low pH but with no direct impact on relative larval morphology or calcification down to pHT. 7.25. Consequently, at a given time, larvae exposed to low pH were present at a normal but delayed larval stage. More surprisingly, candidate genes involved in development and biomineralization were upregulated by factors of up to 26 at low pH. Our results revealed plasticity at the gene expression level that allows a normal, but delayed, development under low pH conditions.

The response of Emiliania huxleyi (Lohmann) W.W. Hay et H. Mohler, Calcidiscus leptoporus (G. Murray et V. H. Blackman) J. Schiller, and Syracosphaera pulchra Lohmann to elevated partial pressure of carbon dioxide (pCO(2)) was investigated in batch cultures. We reported on the response of both haploid and diploid life stages of these three species. Growth rate, cell size, particulate inorganic carbon (PIC), and particulate organic carbon (POC) of both life stages were measured at two different pCO(2) (400 and 760 parts per million [ppm]), and their organic and inorganic carbon production were calculated. The two life stages within the same species generally exhibited a similar response to elevated pCO(2), the response of the haploid stage being often more pronounced than that of the diploid stage. The growth rate was consistently higher at elevated pCO(2), but the response of other processes varied among species. Calcification rate of C. leptoporus and of S. pulchra did not change at elevated pCO(2), whereas it increased in E. huxleyi. POC production and cell size of both life stages of S. pulchra and of the haploid stage of E. huxleyi markedly decreased at elevated pCO(2). It remained unaltered in the diploid stage of E. huxleyi and C. leptoporus and increased in the haploid stage of the latter. The PIC:POC ratio increased in E. huxleyi and was constant in C. leptoporus and S. pulchra. Elevated pCO(2) has a significant effect on these three coccolithophore species, the haploid stage being more sensitive. This effect must be taken into account when predicting the fate of coccolithophores in the future ocean.

Increasing atmospheric carbon dioxide (CO2) concentrations are expected to decrease surface ocean pH by 0.3-0.5 units by 2100 (refs 1,2), lowering the carbonate ion concentration of surface waters. This rapid acidification is predicted to dramatically decrease calcification in many marine organisms(3,4). Reduced skeletal growth under increased CO2 levels has already been shown for corals, molluscs and many other marine organisms(4-9). The impact of acidification on the ability of individual species to calcify has remained elusive, however, as measuring net calcification fails to disentangle the relative contributions of gross calcification and dissolution rates on growth. Here, we show that corals and molluscs transplanted along gradients of carbonate saturation state at Mediterranean CO2 vents are able to calcify and grow at even faster than normal rates when exposed to the high CO2 levels projected for the next 300 years. Calcifiers remain at risk, however, owing to the dissolution of exposed shells and skeletons that occurs as pH levels fall. Our results show that tissues and external organic layers play a major role in protecting shells and skeletons from corrosive sea water, limiting dissolution and allowing organisms to calcify(10,11). Our combined field and laboratory results demonstrate that the adverse effects of global warming are exacerbated when high temperatures coincide with acidification.

Reviews suggest that that the biogeochemical threshold for sustained coral reef growth will be reached during this century due to ocean acidification caused by increased uptake of atmospheric CO2. Projections of ocean acidification, however, are based on air-sea fluxes in the open ocean, and not for shallow-water systems such as coral reefs. Like the open ocean, reef waters are subject to the chemical forcing of increasing atmospheric pCO(2). However, for reefs with long water residence times, we illustrate that benthic carbon fluxes can drive spatial variation in pH, pCO(2) and aragonite saturation state (Omega(a)) that can mask the effects of ocean acidification in some downstream habitats. We use a carbon flux model for photosynthesis, respiration, calcification and dissolution coupled with Lagrangian transport to examine how key groups of calcifiers (zooxanthellate corals) and primary producers (macroalgae) on coral reefs contribute to changes in the seawater carbonate system as a function of water residence time. Analyses based on flume data showed that the carbon fluxes of corals and macroalgae drive Omega(a) in opposing directions. Areas dominated by corals elevate pCO(2) and reduce Omega(a), thereby compounding ocean acidification effects in downstream habitats, whereas algal beds draw CO2 down and elevate Omega(a), potentially offsetting ocean acidification impacts at the local scale. Simulations for two CO2 scenarios (600 and 900 ppm CO2) suggested that a potential shift from coral to algal abundance under ocean acidification can lead to improved conditions for calcification in downstream habitats, depending on reef size, water residence time and circulation patterns. Although the carbon fluxes of benthic reef communities cannot significantly counter changes in carbon chemistry at the scale of oceans, they provide a significant mechanism of buffering ocean acidification impacts at the scale of habitat to reef.

The emergence of ocean acidification as a significant threat to calcifying organisms in marine ecosystems creates a pressing need to understand the physiological and molecular mechanisms by which calcification is affected by environmental parameters. We report here, for the first time, changes in gene expression induced by variations in pH/pCO(2) in the widespread and abundant coccolithophore Emiliania huxleyi. Batch cultures were subjected to increased partial pressure of CO2 (pCO(2); i.e. decreased pH), and the changes in expression of four functional gene classes directly or indirectly related to calcification were investigated. Increased pCO(2) did not affect the calcification rate and only carbonic anhydrase transcripts exhibited a significant down-regulation. Our observation that elevated pCO(2) induces only limited changes in the transcription of several transporters of calcium and bicarbonate gives new significant elements to understand cellular mechanisms underlying the early response of E. huxleyi to CO2-driven ocean acidification.

The anthropogenic release of carbon dioxide (CO2) into the atmosphere leads to an increase in the CO2 partial pressure (pCO2) in the ocean, which may reach 950 µatm by the end of the 21st century. The resulting hypercapnia (high pCO2) and decreasing pH ("ocean acidification") are expected to have appreciable effects on water-breathing organisms, especially on their early-life stages. For organisms like squid that lay their eggs in coastal areas where the embryo and then paralarva are also exposed to metal contamination, there is a need for information on how ocean acidification may influence trace element bioaccumulation during their development. In this study, we investigated the effects of enhanced levels of pCO2 (380, 850 and 1500 µatm corresponding to pHT of 8.1, 7.85 and 7.60) on the accumulation of dissolved 110mAg, 109Cd, 57Co, 203Hg, 54Mn and 65Zn radiotracers in the whole egg strand and in the different compartments of the egg of Loligo vulgaris during the embryonic development and also in hatchlings during their first days of paralarval life. Retention properties of the eggshell for 110mAg, 203Hg and 65Zn were affected by the pCO2 treatments. In the embryo, increasing seawater pCO2 enhanced the uptake of both 110mAg and 65Zn while 203Hg showed a minimum concentration factor (CF) at the intermediate pCO2. 65Zn incorporation in statoliths also increased with increasing pCO2. Conversely, uptake of 109Cd and 54Mn in the embryo decreased as a function of increasing pCO2. Only the accumulation of 57Co in embryos was not affected by increasing pCO2. In paralarvae, the CF of 110mAg increased with increasing pCO2, whereas the 57Co CF was reduced at the highest pCO2 and 203Hg showed a maximal uptake rate at the intermediate pCO2. 54Mn and 65Zn accumulation in paralarvae were not significantly modified by hypercapnic conditions. Our results suggest a combined effect of pH on the adsorption and protective properties of the eggshell and of hypercapnia on the metabolism of embryo and paralarvae, both causing changes to the accumulation of metals in the tissues of L. vulgaris.

The effects of elevated partial pressure of CO2 (pCO(2)) and temperature on the coccolithophore Syracosphaera pulchra were investigated in isolation and in combination. Both the diploid and the haploid life stages were studied. Batch cultures were grown under 4 conditions: 400 mu atm and 19 degrees C; 400 mu atm and 22 degrees C; 740 mu atm and 19 degrees C; and 740 mu atm and 22 degrees C. The growth rate (mu) significantly increased under elevated pCO(2) only in the haploid stage and showed a different pattern with respect to temperature: it was higher at an elevated temperature in the haploid stage at 400 mu atm whereas it decreased in the diploid stage at 740 mu atm. Increasing both parameters together increased the growth rate by 11 % in the haploid stage only. Elevated pCO(2) had a negative impact on the content of particulate organic carbon (POC), production and cell size in both life stages at 19 degrees C, while no significant effect was observed at 22 degrees C. Increasing temperature significantly increased the content of POC and production in the diploid stage at 740 mu atm, while at 400 mu atm it significantly decreased both the content of POC and production in the haploid stage. A simultaneous increase in pCO(2) and temperature had a negative effect on the content of POC and production in the haploid stage only. Neither the rate of calcification (production of particulate inorganic carbon, PIC) nor the PIC:POC ratio were significantly affected by elevated pCO(2), temperature or their interaction. These results showed a strong interactive effect between pCO(2) and temperature in affecting the physiology of S. pulchra, an effect that was often more pronounced in the haploid life stage. Elevated pCO(2) had a stronger effect than temperature.

The paired chemical reactions, Ca2+ + 2HCO(3) (-) a dagger'' CaCO3 + CO2 + H2O, overestimate the ratio of CO2 flux to CaCO3 flux during the precipitation or dissolution of CaCO3 in seawater. This ratio, which has been termed psi, is about 0.6 in surface seawater at 25A degrees C and at equilibrium with contemporary atmospheric CO2 and increases towards 1.0 as seawater cools and pCO(2) increases. These conclusions are based on field observations, laboratory experiments, and equilibrium calculations for the seawater carbonate system. Yet global geochemical modeling indicates that small departures of I from 1.0 would cause dramatic, rapid, and unrealistic change in atmospheric CO2. I can be meaningfully calculated for a water sample whether or not it is in equilibrium with the atmosphere. The analysis presented here demonstrates that the atmospheric CO2 balance can be maintained constant with respect to seawater CaCO3 reactions if one considers the difference between CaCO3 precipitation and burial and differing values for psi (both < 1.0) in regions of precipitation and dissolution within the ocean.

Changes in the carbonate chemistry of coral reef waters are driven by carbon fluxes from two sources: concentrations of CO2 in the atmospheric and source water, and the primary production/respiration and calcification/dissolution of the benthic community. Recent model analyses have shown that, depending on the composition of the reef community, the air-sea flux of CO2 driven by benthic community processes can exceed that due to increases in atmospheric CO2 (ocean acidification). We field test this model and examine the role of three key members of benthic reef communities in modifying the chemistry of the ocean source water: corals, macroalgae, and sand. Building on data from previous carbon flux studies along a reef-flat transect in Moorea (French Polynesia), we illustrate that the drawdown of total dissolved inorganic carbon (C-T) due to photosynthesis and calcification of reef communities can exceed the draw down of total alkalinity (A(T)) due to calcification of corals and calcifying algae, leading to a net increase in aragonite saturation state (Omega(a)). We use the model to test how changes in atmospheric CO2 forcing and benthic community structure affect the overall calcification rates on the reef flat. Results show that between the preindustrial period and 1992, ocean acidification caused reef flat calcification rates to decline by an estimated 15%, but loss of coral cover caused calcification rates to decline by at least three times that amount. The results also show that the upstream-downstream patterns of carbonate chemistry were affected by the spatial patterns of benthic community structure. Changes in the ratio of photosynthesis to calcification can thus partially compensate for ocean acidification, at least on shallow reef flats. With no change in benthic community structure, however, ocean acidification depressed net calcification of the reef flat consistent with findings of previous studies.

Larvae of the Mediterranean pteropod Cavolinia inflexa were maintained at controlled pH(T) values of 8.1, 7.82 and 7.51, equivalent, respectively, to pCO(2) levels of 380, 857 and 1,713 mu atm. At pH(T) 7.82, larvae exhibited malformations and lower shell growth, compared to the control condition. At pH(T) 7.51, the larvae did not make shells but were viable and showed a normal development. However, smaller shells or no shells will have both ecological (food web) and biogeochemical (export of carbon and carbonate) consequences. These results suggest that pteropod larvae, as well as the species dependent upon them or upon adults as a food resource, might be significantly impacted by ocean acidification.

Abstract. Several experiments have shown a decrease of growth and calcification of organisms at decreased pH levels. There is a growing interest to focus on early life stages that are believed to be more sensitive to environmental disturbances such as hypercapnia. Here, we present experimental data, acquired in a commercial hatchery, demonstrating that the growth of planktonic mussel (Mytilus edulis) larvae is significantly affected by a decrease of pH to a level expected for the end of the century. Even though there was no significant effect of a 0.25–0.34 pH unit decrease on hatching and mortality rates during the first 2 days of development nor during the following 13-day period prior to settlement, final shells were respectively 4.5±1.3 and 6.0±2.3% smaller at pHNBS~7.8 (pCO2~1100–1200 μatm) than at a control pHNBS of ~8.1 (pCO2~460–640 μatm). Moreover, a decrease of 12.0±5.4% of shell thickness was observed after 15d of development. More severe impacts were found with a decrease of ~0.5 pHNBS unit during the first 2 days of development which could be attributed to a decrease of calcification due to a slight undersaturation of seawater with respect to aragonite. Indeed, important effects on both hatching and D-veliger shell growth were found. Hatching rates were 24±4% lower while D-veliger shells were 12.7±0.9% smaller at pHNBS~7.6 (pCO2~1900 μatm) than at a control pHNBS of ~8.1 (pCO2~540 μatm). Although these results show that blue mussel larvae are still able to develop a shell in seawater undersaturated with respect to aragonite, the observed decreases of hatching rates and shell growth could lead to a significant decrease of the settlement success. As the environmental conditions considered in this study do not necessarily reflect the natural conditions experienced by this species at the time of spawning, future studies will need to consider the whole larval cycle (from fertilization to settlement) under environmentally relevant conditions in order to investigate the potential ecological and economical losses of a decrease of this species fitness in the field.

The ocean absorbs about 25% of anthropogenic CO2 emissions, which alters its chemistry. Among the changes of the carbonate system are an increase in the partial pressure of CO2 (pCO(2)) and a decline of pH; hence, the whole process is often referred to as `ocean acidification'. Many microbial processes can be affected either directly or indirectly via a cascade of effects through the response of non-microbial groups and/or through changes in seawater chemistry. We briefly review the current understanding of the impact of ocean acidification on microbial diversity and processes, and highlight the gaps that need to be addressed in future research. The focus is on Bacteria, Archaea, viruses and protistan grazers but also includes total primary production of phytoplankton as well as species composition of eukaryotic phytoplankton. Some species and communities exhibit increased primary production at elevated pCO(2). In contrast to their heterocystous counterparts, nitrogen fixation by non-heterocystous cyanobacteria is stimulated by elevated pCO(2). The experimental data on the response of prokaryotic production to ocean acidification are not consistent. Very few other microbial processes have been investigated at environmentally relevant pH levels. The potential for microbes to adapt to ocean acidification, at either the species level by genetic change or at the community level through the replacement of sensitive species or groups by non-or less sensitive ones, is completely unknown. Consequently, the impact of ocean acidification on keystone species and microbial diversity needs to be elucidated. Most experiments used a short-term perturbation approach by using cultured organisms; few were conducted in mesocosms and none in situ. There is likely a lot to be learned from observations in areas naturally enriched with CO2, such as vents, upwelling and near-shore areas.

Thecosome pteropods (pelagic mollusks) can play a key role in the food web of various marine ecosystems. They are a food source for zooplankton or higher predators such as fishes, whales and birds that is particularly important in high latitude areas. Since they harbor a highly soluble aragonitic shell, they could be very sensitive to ocean acidification driven by the increase of anthropogenic CO2 emissions. The effect of changes in the seawater chemistry was investigated on Limacina helicina, a key species of Arctic pelagic ecosystems. Individuals were kept in the laboratory under controlled pCO(2) levels of 280, 380, 550, 760 and 1020 mu atm and at control (0 degrees C) and elevated (4 degrees C) temperatures. The respiration rate was unaffected by pCO(2) at control temperature, but significantly increased as a function of the pCO(2) level at elevated temperature. pCO(2) had no effect on the gut clearance rate at either temperature. Precipitation of CaCO3, measured as the incorporation of Ca-45, significantly declined as a function of pCO(2) at both temperatures. The decrease in calcium carbonate precipitation was highly correlated to the aragonite saturation state. Even though this study demonstrates that pteropods are able to precipitate calcium carbonate at low aragonite saturation state, the results support the current concern for the future of Arctic pteropods, as the production of their shell appears to be very sensitive to decreased pH. A decline of pteropod populations would likely cause dramatic changes to various pelagic ecosystems.

Abstract. Atmospheric CO2 partial pressure (pCO2) is expected to increase to 700 μatm or more by the end of the present century. Anthropogenic CO2 is absorbed by the oceans, leading to decreases in pH and the CaCO3 saturation state (Ω) of the seawater. Elevated pCO2 was shown to drastically decrease calcification rates in tropical zooxanthellate corals. Here we show, using the Mediterranean zooxanthellate coral Cladocora caespitosa, that an increase in pCO2, in the range predicted for 2100, does not reduce its calcification rate. Therefore, the conventional belief that calcification rates will be affected by ocean acidification may not be widespread in temperate corals. Seasonal change in temperature is the predominant factor controlling photosynthesis, respiration, calcification and symbiont density. An increase in pCO2, alone or in combination with elevated temperature, had no significant effect on photosynthesis, photosynthetic efficiency and calcification. The lack of sensitivity C. caespitosa to elevated pCO2 might be due to its slow growth rates, which seem to be more dependent on temperature than on the saturation state of calcium carbonate in the range projected for the end of the century.

Growth rate, C:N ratio, phospholipid-derived fatty acids (PLEA) distribution and organic carbon isotope composition (partial derivative C-13) of both life stages of the Prymnesiophyceae Emiliania huxleyi (Lohmann), Calcidiscus leptoporus (Murray and Blackman) and Syracosphaera pulchra (Lohman) were analyzed. Cultures were grown at two different CO2 partial pressures (pCO(2), 400 and 760 mu atm) under nutrient-replete conditions. Growth rates (mu) increased at 760 mu atm in all species. C:N ratios were unaffected by pCO(2) in C leptoporus and in the diploid stage of E. huxleyi; they significantly decreased in the haploid stage of E. huxleyi and in both life stages of S. pulchra At 400 mu atm, high similarity was found between the lipid composition of C. leptoporus and S. pulchra differing from that of E. huxleyi. The haploid and diploid life stages showed significant differences in mono- and polyunsaturated C18 fatty acids (MUFA and PUFA), which were more abundant in either stage depending on the species. Except for palmitoleic, vaccenic acids and docosahexaeonic acid (DHA). C leptoporus and S. pulchra fatty acids were lower compared to those of E. huxleyi. Differences in partial derivative C-13 between particulate organic carbon (POC) and PLFA (Delta partial derivative C-13(lipid-POC)) showed common trends at both pCO(2): palmitoleic, oleic acids and DHA were systematically depleted compared to total cell biomass, C18 PUFA were enriched while saturated fatty acids (SAFA) were enriched in C leptoporus and S. pulchra and depleted in E huxleyi. Elevated pCO2 influenced PLFA abundance, Pc of lipids and POC in a highly species-specific way. Fatty acids were generally more depleted at 760 mu atm than at 400 mu atm but the effect was variable among PLEA classes. The relationship between coccolithophore isotopic and PLFA composition and CO2 concentration are analyzed and consequent variations in the future web-chain related to changes in carbonate chemistry are discussed. (C) 2010 Elsevier B.V. All rights reserved.

The effect of carbonate ion concentration ([CO 2− 3 ]) on calcification rates estimated from shell size and weight was investigated in the planktonic foraminifera Orbulina universa and Globigerinoides sacculifer. Experiments on G. sacculifer were conducted under two irradiance levels (35 and 335 µmol photons m −2 s −1). Calcification was ca. 30% lower under low light than under high light, irrespective of the [CO 2− 3 ]. Both O. universa and G. sacculifer exhibited reduced final shell weight and calcification rate under low [CO 2− 3 ]. For the [CO 2− 3 ] expected at the end of the century, the calcification rates of these two species are projected to be 6 to 13% lower than the present conditions, while the final shell weights are reduced by 20 to 27% for O. universa and by 4 to 6% for G. sacculifer. These results indicate that ocean acidification would impact on calcite production by foraminifera and may decrease the calcite flux contribution from these organisms.

The first symposium on ``The Ocean in a High-CO2 World'' in 2004 proved to be a landmark event in our understanding of the seriousness of ocean acidification, as reported in Oceanography (Cicerone et al., 2004). The scientific community reunited in 2008 for a second symposium on ``The Ocean in a High-CO2 World.'' During the four years between the two symposia, more scientific papers were published on the topic of ocean acidification than during the preceding 55 years. Ocean acidification is now widely cited in the press and is familiar to many nonscientists. Participants at the 2008 symposium identified new research priorities and stressed the importance of improving international coordination to facilitate agreements on protocols, methods, and data reporting in order to optimize limited resources by greater sharing of materials, facilities, expertise, and data. Despite major uncertainties, the research community must find ways to scale up understanding of individual organisms' responses to provide meaningful predictions of ocean acidification's effects on food webs, fisheries, marine ecosystems, coastal erosion, and tourism. Easy-to-understand information, such as simple indicators of change and of thresholds beyond which marine ecosystems will not recover, is also needed for management and policymaking.

Abstract. The cold-water coral Lophelia pertusa is one of the few species able to build reef-like structures and a 3-dimensional coral framework in the deep oceans. Furthermore, deep cold-water coral bioherms may be among the first marine ecosystems to be affected by ocean acidification. Colonies of L. pertusa were collected during a cruise in 2006 to cold-water coral bioherms of the Mingulay reef complex (Hebrides, North Atlantic). Shortly after sample collection onboard these corals were labelled with calcium-45. The same experimental approach was used to assess calcification rates and how those changed due to reduced pH during a cruise to the Skagerrak (North Sea) in 2007. The highest calcification rates were found in youngest polyps with up to 1% d−1 new skeletal growth and average rates of 0.11±0.02% d−1±S.E.). Lowering pH by 0.15 and 0.3 units relative to the ambient level resulted in calcification being reduced by 30 and 56%. Lower pH reduced calcification more in fast growing, young polyps (59% reduction) than in older polyps (40% reduction). Thus skeletal growth of young and fast calcifying corallites suffered more from ocean acidification. Nevertheless, L. pertusa exhibited positive net calcification (as measured by 45Ca incorporation) even at an aragonite saturation state (Ωa) below 1.

The effects of elevated partial pressure of CO2 (pCO(2)) and temperature, alone and in combination, on survival, calcification and dissolution were investigated in the crustose coralline alga Lithophyllum cabiochae. Algae were maintained in aquaria during 1 year at near-ambient conditions of irradiance, at ambient or elevated temperature (+3 degrees C) and at ambient [ca. 400 parts per million (ppm)] or elevated pCO(2) (ca. 700 ppm). Algal necroses appeared at the end of summer under elevated temperature first at 700 ppm (60% of the thallus surface) and then at 400 ppm (30%). The death of algae was observed only under elevated temperature and was two- to threefold higher under elevated pCO(2). During the first month of the experiment, net calcification was significantly reduced under elevated pCO(2). At the end of the summer period, net calcification decreased by 50% when both temperature and pCO(2) were elevated while no effect was found under elevated temperature and elevated pCO(2) alone. In autumn and winter, net calcification in healthy algae increased with increasing temperature, independently of the pCO(2) level, while necroses and death in the algal population caused a net dissolution at elevated temperature and pCO(2). The dissolution of dead algal thalli was affected by elevated pCO(2), being two- to fourfold higher than under ambient pCO(2). These results suggest that net dissolution is likely to exceed net calcification in L. cabiochae by the end of this century. This could have major consequences in terms of biodiversity and biogeochemistry in coralligenous communities dominated by these algae.

A dilution and size fractionation approach was used to study the separate and combined effects of viruses and flagellates on prokaryotic production ([H-3] leucine incorporation) and community composition (16S rRNA gene PCR and denaturing gradient gel electrophoresis [DGGE]) in the upper mixed layer and the deep chlorophyll maximum in the offshore Mediterranean Sea. Four experiments were established using differential filtration: a resource control without predators (C treatment), treatment in the presence of viruses (V treatment), treatment in the presence of flagellates (F treatment), and treatment in the presence of both predators (VF treatment). The V and VF treatments increased prokaryotic abundance (1.4- to 2.3-fold) and the number of DGGE bands (by up to 43%) and decreased prokaryotic production compared to the level for the C treatment (by 22 to 99%). For the F treatment, significant differences compared to the level for the C treatment were found as well, but trends were not consistent across experiments. The relative abundances of the high-nucleic-acid subgroups of prokaryotes with high scatter (HNAhs) in flow cytometer settings were lower in the V and VF treatments than in the C and F treatments. These differences were probably due to lysis of very active HNA prokaryotes in the V and VF treatments. Our results indicate that the presence of viruses or viruses plus flagellates sustains prokaryotic diversity and controls prokaryotic production by regulating the proportion of the highly active members of the community. Our data also suggest that lysis and grazing control influences the relationship between bacterial community composition and prokaryotic production.

Increasing atmospheric CO2 concentration affects calcification in most planktonic calcifiers. Both reduced or stimulated calcification under high CO2 have been reported in the widespread coccolithophore Emiliania huxleyi. This might affect the response of cells to photosynthetically active radiation ( PAR; 400-700 nm) and ultraviolet radiation (UVR; 280-400 nm) by altering the thickness of the coccolith layer. Here we show that in the absence of UVR, the calcification rates in E. huxleyi decrease under lowered pH levels (pH(NBS) of 7.9 and 7.6; pCO(2) of 81 and 178 Pa or 804 and 1759 ppmv, respectively) leading to thinned coccolith layers, whereas photosynthetic carbon fixation was slightly enhanced at pH 7.9 but remained unaffected at pH 7.6. Exposure to UVR (UV-A 19.5 W m(-2), UV-B 0.67 W m(-2)) in addition to PAR (88.5 W m(-2)), however, results in significant inhibition of both photosynthesis and calcification, and these rates are further inhibited with increasing acidification. The combined effects of UVR and seawater acidification resulted in the inhibition of calcification rates by 96% and 99% and that of photosynthesis by 6% and 15%, at pH 7.9 and 7.6, respectively. This differential inhibition of calcification and photosynthesis leads to significant reduction of the ratio of calcification to photosynthesis. Seawater acidification enhanced the transmission of harmful UVR by about 26% through a reduction of the coccolith layer of 31%. Our data indicate that the effect of a high-CO2 and low-pH ocean on E. huxleyi ( because of reduced calcification associated with changes in the carbonate system) enhances the detrimental effects of UVR on the main pelagic calcifier.

The lithium isotope compositions ((7)Li/(6)Li) and Li/Ca ratios of shallow-water and deep-sea corals (Porites lutea, Cladocora caespitosa, Lophelia pertusa and Desmophyllum cristagalli) were measured using a Cameca ims 1270 ion microprobe. The two C. caespitosa samples were grown under controlled conditions at CO(2) partial pressures (pCO(2)) of 416 +/- 29 mu atm and 729 +/- 30 mu atm, respectively. In situ analyses show that all samples are isotopically homogeneous (within analytical precision, +/- 1.1 parts per thousand, 1 sigma) and display significantly lower delta(7)Li values relative to seawater. indicating a significant isotope fractionation during aragonite formation. In contrast to all other elements analysed so far, there is no relationship between the Li isotopic compositions and the skeletal ultrastructure. However, Li/Ca does show variation correlated with ultrastructure, albeit with significant differences between species. This implies that the bio mineralization mechanisms, which are supposed to be different for the different skeletal components, do not influence the Li isotopic composition in corals. In particular, the model of Rayleigh fractionation in a semi-enclosed calcifying fluid is incompatible with the homogeneity of the Li isotope compositions at the micrometer scale We also. show that changes in pCO(2) (and pH) do not significantly affect the Li isotope signature. Nevertheless, a small but significant and systematic difference in Li isotopic composition is observed between deep-sea azooxanthellate and shallow-water zooxanthellate corals. The lack of dependence on pH and pCO(2) and on skeletal ultrastructure indicates that the U isotopic signature of corals could be used as a proxy for reconstructing the paleo-delta(7)Li of seawater and, potentially, for deconvolving past continental weathering rates. (C) 2009 Elsevier B.V. All rights reserved.

Abstract. Although future changes in the seawater carbonate chemistry are well constrained, their impact on marine organisms and ecosystems remains poorly known. The biological response to ocean acidification is a recent field of research as most purposeful experiments have only been carried out in the late 1990s. The potentially dire consequences of ocean acidification attract scientists and students with a limited knowledge of the carbonate chemistry and its experimental manipulation. Hence, some guidelines on carbonate chemistry manipulations may be helpful for the growing ocean acidification community to maintain comparability. Perturbation experiments are one of the key approaches used to investigate the biological response to elevated pCO2. They are based on measurements of physiological or metabolic processes in organisms and communities exposed to seawater with normal or altered carbonate chemistry. Seawater chemistry can be manipulated in different ways depending on the facilities available and on the question being addressed. The goal of this paper is (1) to examine the benefits and drawbacks of various manipulation techniques and (2) to describe a new version of the R software package seacarb which includes new functions aimed at assisting the design of ocean acidification perturbation experiments. Three approaches closely mimic the on-going and future changes in the seawater carbonate chemistry: gas bubbling, addition of high-CO2 seawater as well as combined additions of acid and bicarbonate and/or carbonate.

Abstract. Thecosome pteropods (shelled pelagic molluscs) can play an important role in the food web of various ecosystems and play a key role in the cycling of carbon and carbonate. Since they harbor an aragonitic shell, they could be very sensitive to ocean acidification driven by the increase of anthropogenic CO2 emissions. The impact of changes in the carbonate chemistry was investigated on Limacina helicina, a key species of Arctic ecosystems. Pteropods were kept in culture under controlled pH conditions corresponding to pCO2 levels of 350 and 760 μatm. Calcification was estimated using a fluorochrome and the radioisotope 45Ca. It exhibits a 28% decrease at the pH value expected for 2100 compared to the present pH value. This result supports the concern for the future of pteropods in a high-CO2 world, as well as of those species dependent upon them as a food resource. A decline of their populations would likely cause dramatic changes to the structure, function and services of polar ecosystems.

Dilution experiments were carried out to investigate the community composition and the metabolic response of seawater and freshwater bacteria to cross-transplantation, and the effects of nor.-indigenous bacterial hosts on viral dynamics. Changes in viral and bacterial abundance and production, as Well as bacterial respiration, carbon demand and diversity were regularly monitored over a 6 d period. Bacterial production in the transplanted seawater (SB-t) and freshwater (FB-t) bacteria treatments was stimulated up to 256 and 221 %, respectively, compared to controls. The stimulation of bacterial production and carbon demand was accompanied by a decrease in bacterial richness. Net viral production was stimulated by 81% in SB-t and repressed by 75% in FB-t. Transplantation increased the virus-induced mortality of marine bacteria, but decreased it for freshwater bacteria. These results suggest that (1) marine bacteria can readily oxidize freshwater dissolved organic matter, and (2) freshwater viruses might be able to infect marine hosts, thus highlighting their potential role in fueling bacterial growth under resource stress or nutrient-depleted conditions.

The expression of genes of biogeochemical interest in calcifying and noncalcifying life stages of the coccolithophore Emiliania huxleyi was investigated. Transcripts potentially involved in calcification were tested through a light-dark cycle. These transcripts were more abundant in calcifying cells and were upregulated in the light. Their application as potential candidates for in situ biogeochemical proxies is also suggested.

The European Project on Ocean Acidification (EPOCA) is Europe's first large-scale research initiative devoted to studying the impacts and consequences of ocean acidification. More than 100 scientists from 27 institutes and nine countries bring their expertise to the project, resulting in a multidisciplinary and versatile consortium. The project is funded for four years (2008 to 2012) by the European Commission within its Seventh Framework Programme. This article describes EPOCA and explains its different aspects, objectives, and products. Following a general introduction, six boxes highlight outcomes, techniques, and scientific results from each of the project's core themes.

Humans utilise about 40% of the earth's net primary production (NPP) but the products of this NPP are often managed by different sectors, with timber and forest products managed by the forestry sector and food and fibre products from croplands and grasslands managed by the agricultural sector. Other significant anthropogenic impacts on the global carbon cycle include human utilization of fossil fuels and impacts on less intensively managed systems such as peatlands, wetlands and permafrost. A great deal of knowledge, expertise and data is available within each sector. We describe the contribution of sectoral carbon budgets to our understanding of the global carbon cycle. Whilst many sectors exhibit similarities for carbon budgeting, some key differences arise due to differences in goods and services provided, ecology, management practices used, land-management personnel responsible, policies affecting land management, data types and availability, and the drivers of change. We review the methods and data sources available for assessing sectoral carbon budgets, and describe some of key data limitations and uncertainties for each sector in different regions of the world. We identify the main gaps in our knowledge/data, show that coverage is better for the developed world for most sectors, and suggest how sectoral carbon budgets could be improved in the future. Research priorities include the development of shared protocols through site networks, a move to full carbon accounting within sectors, and the assessment of full greenhouse gas budgets.

A dynamic model has been developed to represent biogeochemical variables and processes observed during a bloom of <i>Emiliania huxleyi</i> coccolithophore. This bloom was induced in a mesocosm experiment during which the ecosystem development was followed over a period of 23-days through changes in various biogeochemical parameters such as inorganic nutrients (nitrate, ammonium and phosphate), total alkalinity (TA), dissolved inorganic carbon (DIC), partial pressure of CO₂ (pCO₂), dissolved oxygen (O₂), photosynthetic pigments, particulate organic carbon (POC), dissolved organic carbon (DOC), Transparent Exopolymer Particles (TEP), primary production, and calcification. This dynamic model is based on unbalanced algal growth and balanced bacterial growth. In order to adequately reproduce the observations, the model includes an explicit description of phosphorus cycling, calcification, TEP production and an enhanced mortality due to viral lysis. The model represented carbon, nitrogen and phosphorus fluxes observed in the mesocosms. Modelled profiles of algal biomass and final concentrations of DIC and nutrients are in agreement with the experimental observations.

Surface ocean pH is likely to decrease by up to 0.4 units by 2100 due to the uptake of anthropogenic CO2 from the atmosphere. Short-term experiments have revealed that this degree of seawater acidification can alter calcification rates in certain planktonic and benthic organisms, although the effects recorded may be shock responses and the long-term ecological effects are unknown. Here, we show the response of calcareous seagrass epibionts to elevated CO2 partial pressure in aquaria and at a volcanic vent area where seagrass habitat has been exposed to high CO2 levels for decades. Coralline algae were the dominant contributors to calcium carbonate mass on seagrass blades at normal pH but were absent from the system at mean pH 7.7 and were dissolved in aquaria enriched with CO2. In the field, bryozoans were the only calcifiers present on seagrass blades at mean pH 7.7 where the total mass of epiphytic calcium carbonate was 90 per cent lower than that at pH 8.2. These findings suggest that ocean acidification may have dramatic effects on the diversity of seagrass habitats and lead to a shift in the biogeochemical cycling of both carbon and carbonate in coastal ecosystems dominated by seagrass beds.

Planktonic primary production and respiration in 2 coastal oligotrophic sites of the Northwest Mediterranean Sea were examined. Primary production was quantified using 3 methods (light and dark changes in dissolved O-2, O-18-labeling and C-14 uptake technique) using in situ bottle incubations. Gross primary production (GPP) based on the O-2 light-dark technique was not significantly different from that using the O-18-labeling technique, indicating that the former technique provides accurate estimates of GPP in these environments. Respiration in the dark was not significantly different frorn respiration in the light. Total C-14 uptake (including the dissolved and particulate organic carbon fractions) during the whole duration of the light period approached GPP and consequently overestimated net primary production.

Rates of primary production were measured in 2 estuaries (Randers Fjord, Denmark, and the Scheldt estuary, Belgium/The Netherlands) using 3 different incubation methods: (1) the oxygen light-dark method (O2-LD), (2) 14C incorporation and (3) 18O labeling. Estimates based on the 14C incorporation technique were not significantly different from those obtained using the O2-LD technique. The 18O approach provided rates significantly lower than the 2 other techniques. Ratios of O2-LD to 18O-based rates (range: 0.99 to 3.54) were often statistically significantly higher than 1 and increased with decreasing salinities and/or lower oxygen concentrations. The underestimation of gross primary production by the 18O method may be due to an intracellular recycling of labeled oxygen which increased in magnitude with decreasing external oxygen conditions. These results suggest that the 18O method must be used with extreme care in nutrient-rich, low oxygen systems.

The community metabolism of a shallow infralittoral ecosystem dominated by the calcareous macroalgae Corallina elongata was investigated in Marseilles (NW Mediterranean), by monitoring hourly changes of seawater pH and total alkalinity over 6 d in February 2000. Fair weather conditions prevailed over the study period as indicated by oceanographic (temperature, salinity, and current velocity and direction) and meteorological variables, which validated the standing water hypothesis. This temperate ecosystem exhibited high community gross primary production (GPP = 519 +/- 106 mmol C m(-2) d(-1), n = 6) and also supported high rates of community respiration (R). As a result, the system was slightly autotrophic (net community production, NCP = 20 mmol C m(-2) d(-1)), with a GPP/R ratio of 1.06. NCP exhibited circadian variations with 2- to 3-fold changes in community respiration, both in the light and in the dark. Rates of net community calcification also exhibited circadian variations, with positive rates (up to 24 mmol CaCO3 m(-2) h(-1)) for irradiance values > 300 W m(-2) (about 1380 mu mol photon m(-2) s(-1)). Below this irradiance threshold, net community dissolution prevailed. Daily net calcification (G) was on average 8 mmol CaCO3 m(-2) d(-1). CO2 fluxes generated by primary production, respiration, and calcification suggest that the study site was a potential atmospheric CO2 sink of 15 mmol CO2 m(-2) d(-1) at the time of measurement.

One of the major features of the coastal zone is that part of its sea floor receives a significant amount of sunlight and can therefore sustain benthic primary production by seagrasses, macroalgae, microphytobenthos and corals. However, the contribution of benthic communities to the primary production of the global coastal ocean is not known, partly because the surface area where benthic primary production can proceed is poorly quantified. Here, we use a new analysis of satellite (SeaWiFS) data collected between 1998 and 2003 to estimate, for the first time at a nearly global scale, the irradiance reaching the bottom of the coastal ocean. The following cumulative functions provide the percentage of the surface (<i>S</i>) of the coastal zone receiving an irradiance greater than <i>E_z</i> (in mol photons m^-2 d^-1): <i>S</i>_Non-polar = 29.61 - 17.92 log₁₀(<i>E_z</i>) + 0.72 log₁₀²(<i>E_z</i>) + 0.90 log₁₀³(<i>E_z</i>) <i>S</i>_Arctic = 15.99 - 13.56 log₁₀(<i>E_z</i>) + 1.49 log₁₀²(<i>E_z</i>) + 0.70 log₁₀³(<i>E_z</i>) Data on the constraint of light availability on the major benthic primary producers and net community production are reviewed. Some photosynthetic organisms can grow deeper than the nominal bottom limit of the coastal ocean (200 m). The minimum irradiance required varies from 0.4 to 5.1 mol photons m^-2 d^-1 depending on the group considered. The daily compensation irradiance of benthic communities ranges from 0.24 to 4.4 mol photons m^-2 d^-1. Data on benthic irradiance and light requirements are combined to estimate the surface area of the coastal ocean where (1) light does not limit the distribution of primary producers and (2) net community production (<i>NCP</i>, the balance between gross primary production and community respiration) is positive. Positive benthic <i>NCP</i> can occur over 33% of the global shelf area. The limitations of this approach, related to the spatial resolution of the satellite data, the parameterization used to convert reflectance data to irradiance, the lack of global information on the benthic nepheloid layer, and the relatively limited biological information available, are discussed.

One of the major features of the coastal zone is that part of its sea floor receives a significant amount of sunlight and can therefore sustain benthic primary production by seagrasses, macroalgae, microphytobenthos and corals. However, the contribution of benthic communities to the primary production of the global coastal ocean is not known, partly because the surface area where benthic primary production can proceed is poorly quantified. Here, we use a new analysis of satellite (SeaWiFS) data collected between 1998 and 2003 to estimate, for the first time at a nearly global scale, the irradiance reaching the bottom of the coastal ocean. The following cumulative functions provide the percentage of the surface of the coastal zone receiving an irradiance greater than <i>E_z</i>: <P style="line-height: 20px;"></P> <i>P_a</i>^Non-polar=28.80-16.69 log₁₀(<i>E_z</i>)+0.84 log₁₀²(<i>E_z</i>)+0.83 log₁₀³(<i>E_z</i>) <P style="line-height: 20px;"></P> <P style="line-height: 20px;"></P> <i>P_a</i>^Arctic=16.01-15.67 log₁₀(<i>E_z</i>)+2.03 log₁₀²(<i>E_z</i>)+1.00 log₁₀³(<i>E_z</i>) <P style="line-height: 20px;"></P> Data on the constraint of light availability on the major benthic primary producers and net primary production are reviewed. Some photosynthetic organisms can grow deeper than the nominal bottom limit of the coastal ocean (200 m). The minimum irradiance required varies from 0.4 to 5.1 mol photons m^-2 d^-1 depending on the group considered. The daily compensation irradiance of benthic communities ranges from 0.24 to 4.4 mol photons m^-2 d^-1. Data on benthic irradiance and light requirements are combined to estimate the surface area of the coastal ocean where (1) light does not limit the distribution of primary producers and (2) net community production (NCP, the balance between gross primary production and respiration) is positive. Positive benthic NCP can occur over 37% of the global shelf area. The limitations of this approach, related to the spatial resolution of the satellite data, the parameterization used to convert reflectance data to irradiance, and the relatively limited biological information available, are discussed.

Planktonic and benthic incubations (bare and <I>Posidonia oceanica</I> vegetated sediments) were performed at monthly intervals from March 2001 to October 2002 in a seagrass vegetated area of the Bay of Palma (Mallorca, Spain). Results showed a contrast between the planktonic compartment, which was on average near metabolic balance (-4.6±5.9 mmol O₂ m^-2 d^-1) and the benthic compartment, which was autotrophic (17.6±8.5 mmol O₂ m^-2 d^-1). During two cruises in March and June 2002, planktonic and benthic incubations were performed at several stations in the bay to estimate the whole-system metabolism and to examine its relationship with partial pressure of CO₂ (pCO₂) and apparent oxygen utilisation (AOU) spatial patterns. Moreover, during the second cruise, when the residence time of water was long enough, net ecosystem production (NEP) estimates based on incubations were compared, over the <I>Posidonia oceanica</I> meadow, to rates derived from dissolved inorganic carbon (DIC) and oxygen (O₂) mass balance budgets. These budgets provided NEP estimates in fair agreement with those derived from direct metabolic estimates based on incubated samples over the <I>Posidonia oceanica</I> meadow. Whereas the seagrass community was autotrophic, the excess organic carbon production therein could only balance the planktonic heterotrophy in shallow waters relative to the maximum depth of the bay (55 m). This generated a horizontal gradient from autotrophic or balanced communities in the shallow seagrass-covered areas, to strongly heterotrophic communities in deeper areas of the bay. It seems therefore that, on an annual scale in the whole bay, the organic matter production by the <I>Posidonia oceanica</I> may not be sufficient to fully compensate the heterotrophy of the planktonic compartment, which may require external organic carbon inputs, most likely from land.

We studied the direct effects of CO2 and related changes in seawater carbonate chemistry on marine planktonic organisms in a mesocosm experiment. In nine outdoor enclosures (;11 m3 each), the partial pressure of CO2 (pCO2) in the seawater was modified by an aeration system. The triplicate mesocosm treatments represented low (;190 parts per million by volume (ppmV) CO2), present (;410 ppmV CO2), and high (;710 ppmV CO2) pCO2 conditions. After initial fertilization with nitrate and phosphate a bloom dominated by the coccolithophorid Emiliania huxleyioccurred simultaneously in all of the nine mesocosms; it was monitored over a 19-day period. The three CO2 treatments assimilated nitrate and phosphate similarly. The concentration of particulate constituents was highly variable among the replicate mesocosms, disguising direct CO2-related effects. Normalization of production rates within each treatment, however, indicated that the net specific growth rate of E. huxleyi, the rate of calcification per cell, and the elemental stoichiometry of uptake and production processes were sensitive to changes in pCO2. This broad influence of CO2 on the E. huxleyi bloom suggests that changes in CO2 concentration directly affect cell physiology with likely effects on the marine biogeochemistry.

We studied the direct effects of CO, and related changes in seawater carbonate chemistry on marine planktonic organisms in a mesocosm experiment. In nine outdoor enclosures (similar to 11 m(3) each), the partial pressure of CO2 (pCO(2)) in the seawater was modified by an aeration system. The triplicate mesocosm treatments represented low (similar to 190 parts per million by volume (ppmV) CO2), present (similar to 410 ppmV CO2), and high (similar to 710 ppmV CO2) pCO(2) conditions. After initial fertilization with nitrate and phosphate a bloom dominated by the coccolithophorid Emiliania huxleyi occurred simultaneously in all of the nine mesocosms; it was monitored over a 19-day period, The three CO2 treatments assimilated nitrate and phosphate similarly. The concentration of particulate constituents was highly variable among the replicate mesocosms, disguising direct CO2-related effects. Normalization of production rates within each treatment, however, indicated that the net specific growth rate of E. huxleyi, the rate of calcification per cell, and the elemental stoichiometry of uptake and production processes were sensitive to changes in pCO(2). This broad influence of CO2 on the E huxleyi bloom suggests that changes in CO2 concentration directly affect cell physiology with likely effects on the marine biogeochemistry.

We studied the direct effects of CO, and related changes in seawater carbonate chemistry on marine planktonic organisms in a mesocosm experiment. In nine outdoor enclosures (similar to 11 m(3) each), the partial pressure of CO2 (pCO(2)) in the seawater was modified by an aeration system. The triplicate mesocosm treatments represented low (similar to 190 parts per million by volume (ppmV) CO2), present (similar to 410 ppmV CO2), and high (similar to 710 ppmV CO2) pCO(2) conditions. After initial fertilization with nitrate and phosphate a bloom dominated by the coccolithophorid Emiliania huxleyi occurred simultaneously in all of the nine mesocosms; it was monitored over a 19-day period, The three CO2 treatments assimilated nitrate and phosphate similarly. The concentration of particulate constituents was highly variable among the replicate mesocosms, disguising direct CO2-related effects. Normalization of production rates within each treatment, however, indicated that the net specific growth rate of E. huxleyi, the rate of calcification per cell, and the elemental stoichiometry of uptake and production processes were sensitive to changes in pCO(2). This broad influence of CO2 on the E huxleyi bloom suggests that changes in CO2 concentration directly affect cell physiology with likely effects on the marine biogeochemistry.

The relationship between whole-system metabolism estimates based on planktonic and benthic incubations (bare sediments and seagrass, <I>Posidonia oceanica</I> meadows), and CO₂ fluxes across the air-sea interface were examined in the Bay of Palma (Mallorca, Spain) during two cruises in March and June 2002. Moreover, planktonic and benthic incubations were performed at monthly intervals from March 2001 to October 2002 in a seagrass vegetated area of the bay. From the annual study, results showed a contrast between the planktonic compartment, which was heterotrophic during most of the year, except for occasional bloom episodes, and the benthic compartment, which was slightly autotrophic. Whereas the seagrass community was autotrophic, the excess organic carbon production therein could only balance the excess respiration of the planktonic compartment in shallow waters (<10 m) relative to the maximum depth of the bay (55 m). This generated a horizontal gradient from autotrophic or balanced communities in the shallow, seagrass-covered areas of the bay, to strongly heterotrophic communities in deeper areas, consistent with the patterns of CO₂ fields and fluxes across the bay observed during the two extensive cruises in 2002. Finally, dissolved inorganic carbon and oxygen budgets provided NEP estimates in fair agreement with those derived from direct metabolic estimates based on incubated samples over the <I>Posidonia oceanica</I> meadow.

The functional response of a seawater bacterial community transplanted into freshwater dissolved organic matter (DOM) was investigated together with the response of natural populations of bacteria to size-fractioned natural source water. Seawater bacteria were incubated over a period of 8 d in size-fractionated, freshwater DOM collected from Randers Fjord, Denmark, during spring (April) and summer (August) of 2001. Three fractions were used: 0.2 mum filtered (<0.2 mum-DOM), >1 kDa (high molecular weight, HMW-DOM) and <1 kDa (low molecular weight, LMW-DOM). The results were compared with parallel control incubations of freshwater bacteria in size-fractionated freshwater DOM and seawater bacteria in size-fractionated seawater DOM. There were few differences in bacterial abundance (BA) and production (BP) within each incubation type in spring, but this was not the case in summer. While the seawater bacteria transplanted into freshwater HMW-DOM performed similarly to those in seawater HMW-DOM, freshwater bacteria in freshwater HMW-DOM exhibited higher BA, BP and bacterial respiration (BR), indicating a difference in the physiological abilities of the seawater bacterial assemblage compared to that of the natural freshwater assemblage. Bacterial growth efficiency (BGE) varied between 11 and 41% and the highest values were generally in the HMW-DOM size fraction. Comparison of the `bioavailability' of the DOM predicted from BGE, the amino acid degradation index (DI), dissolved organic carbon degradation rates and the bioavailability index showed that net change in DI of dissolved combined amino acids over the course of an incubation can be a good predictor of most of the other indices. The separation of DOM into molecular weight size fractions resulted in different estimates of bioavailability than would have been predicted from the rates observed in the <0.2 mum-DOM fraction. These results further demonstrate the flexibility of bacteria in their ability to utilize different sources of DOM, and highlight the variability that can be observed when different indices are used to determine the bioavailability of organic matter to heterotrophic bacteria.

The respiration rates of a pelagic community and of its microbial fraction (<1.2 μm) were measured at two depths in the oxic layer of a meromictic alpine lake (Cadagno, Switzerland) using the oxygen technique. The duration of the incubations were 12, 24 and 55 h. Bacterioplankton abundance (DAPI counts) and composition (whole cell hybridization using 11 group-specific rRNA-targeted oligonucleotide probes) were measured during the incubations. Respiration generally increased with time, especially in the microbial fraction, or remained similar. This result was not always consistent with changes in bacterial abundance and cell volume. The composition of the community also changed during the incubations. The abundance of β-Proteobacteria increased during the course of all the experiments. These results extend the previous conclusions drawn in marine environments to fresh waters and demonstrate that, in addition to changes in bacterial abundance, cell volume and biomass, changes in the taxonomic composition of the bacterial community can occur during discrete incubations of freshwater planktonic communities.

The effect of feeding on the carbon isotopic composition of zooxanthellae, animal tissue and skeleton was investigated in the scleractinian coral Stylophora pistillata. Two sets of corals were grown with filtered seawater under controlled conditions. One group of colonies was fed $Artemia$ sp. nauplii and compared to a control group that was starved. Fed corals exhibited higher concentrations of chlorophyll (60% more), soluble protein (4 times more) and calcification rates (29% more) than starved colonies. The net photosynthetic rate was higher in starved than in fed corals (18.53 $\pm$ 6.99 and 6.78 $\pm$ 2.06 $\mu$mol O$_2$ cm$^{-2}$ h$^{-1}$) respectively), whereas dark respiration was not significantly different (8.74 $\pm$ 2.27 and 6.66 $\pm$ 0.40 $\mu$mol $_2$ cm$^{-2}$ h$^{-1}$). The average $\delta^{13}$C value of $Artemia$ sp. nauplii used for feeding was -12 $\delta^{13}$C was significantly heavier in zooxanthellae than in animal tissues, for both fed (-10.1 vs -11.7‰) and starved colonies (-10.9 vs -13.2‰). Artemia sp. carbon was incorporated into the coral tissue as shown by the heavier $\delta^{13}$C in fed than in starved colonies (- 11.7 to -13.2‰, respectively), although there was no difference in the $\delta^{13}$C of the zooxanthellae fraction. Skeletal $\delta^{13}$C was similar in fed and starved colonies (mean -4.6‰). Skeletal $\delta^{18}$O composition was, however, significantly different between the 2 treatments (-4.24 to -4.05‰ for fed and starved colonies, respectively), which may have been due to differences in the calcification rates of fed and starved corals. These data are used to establish a conceptual model of the carbon flow between the various compartments of a symbiotic coral. It suggests that the skeletal $\delta^{13}$C is not sensitive to heterotrophic food supply.

novel experimental protocol is described that assists investigations of the effect of environmental parameters on records from the carbonate skeletons of scleractinian corals. It involves the culture of coral colonies on glass slides so as to time the skeletal deposition and environmental records precisely. The value of the technique is demonstrated via calibration of the relationship between skeletal δ18O and seawater temperature in 2 species of coral obtained from the Gulf of Aqaba. Colonies were grown at 5 temperatures between 21 and 29°C. For Acropora sp. this relationship gave a slope of -0.27o/oo °C-1, a value close to previous estimates. The δ18O signature of Stylophora pistillata displayed a high variability between colonies and gave an average slope much lower than previous estimates (-0.13o/oo °C-1). These data may indicate a taxonomic difference and the need to re-examine the systematics of this genus. Nevertheless, such variability in colonies of a single species or of a set of closely related species may have implications for the use of coral skeleton as proxy records.

Oxygen isotope ratios were measured along the growth axis of branches of the scleractinian coral Acropora formosa collected at 2 and 12 metre depths at Yonge reef (Northern Great Barrier Reef, Australia). Measurements were made between two reference points separated by a distance corresponding to a growth period of six months, from mid-winter to mid-summer. For each of the two reference points, information concerning the environmental parameters controlling the development of the coral colonies was collected in situ. The variability of the isotopic values recorded from coral skeletons grown in identical physical and chemical conditions cannot be ascribed to technical problems, but may rather be induced by calcification mechanisms. In order to define the influence of calcification processes on oxygen isotopic composition, a simple mathematical model is developed, simulating the behaviour of oxygen isotopes. According to Gladfelter's studies (1982, 1983, 1984), we infer that the main factors affecting the isotopic ratio of skeletal aragonite in the scleractinian coral Acropora genus are on the one hand the initial quantity of aragonite deposited at the apical part of the branch and consequently the relative amount of primary and secondary aragonite infilling residual pores during coral growth, and on the other hand, the duration of the secondary aragonite infilling. Comparisons between the measured and the calculated isotopic profiles reveal that differences in calcification processes account for isotopic discrepancies encountered in the different colonies analysed. This study stresses that care must be taken when using the oxygen isotope composition of coral skeleton as a paleoenvironmental proxy.