Abstract The equatorial Atlantic Ocean is characterized by seasonal upwelling and complex nutrient dynamics. These processes shape the temporal and spatial distribution of primary productivity causing a pronounced east-west chlorophyll-a gradient. But whether this surface biomass pattern is reflected in particle production and export remains uncertain. We analyze vertical high-resolution particle size distribution data from Underwater Vision Profilers and physical oceanographic observations collected during two trans-Atlantic cruises conducted in contrasting seasons. During the higher productivity fall season, upwelling and tropical instability waves enhanced local primary production and supported deep export of large particles. In contrast, during the lower productivity boreal spring, unexpectedly high concentrations of small particles (0.1-0.5 mm) were observed in the central basin, concentrated within the Equatorial Undercurrent (EUC), which is supplied by the energetic North Brazil Current retroflection. Backward trajectory analysis and biological and hydrographic evidence suggest that these particles originate from the South American continental margin and are laterally transported eastward along a short advective pathway into the EUC. Estimating Particulate Organic Carbon (POC) flux from the particle size distributions, we find that during the low-productivity period, between 15-35°W, over 70% of the POC flux in the upper 200 m might be attributed to small particles (0.05-0.1mm) carried within the EUC. These findings highlight the importance of lateral subsurface transport for equatorial carbon cycling, thereby influencing deep-sea and benthic ecosystem structure.

Abstract. The Levantine Basin is an ultra-oligotrophic region and the formation site of the Levantine Intermediate Waters. For the first time, a high-resolution 3D coupled hydrodynamic-biogeochemical model, SYMPHONIE-Eco3MS, was used to investigate the seasonal and interannual variability of dissolved oxygen (O2) in the Levantine Basin and estimate its basin-wide budget for the period 2013–2020. Our results show that the simulated O2 concentrations align well with in situ data from research cruises and Argo floats. During winter, the surface layer is undersaturated in oxygen by up to 2 % across the entire basin, leading to atmospheric oxygen absorption. The model shows that on an annual scale, the basin acts as a net sink for atmospheric oxygen, with the Rhodes Gyre exhibiting uptake rates twice as high as the rest of the Levantine Basin. The surface layer also serves as a source of dissolved oxygen for intermediate depths, with 4.2 ± 1.1 mol m-2 year-1 of dissolved oxygen vertically transported. Oxygen is transported laterally into the basin from the Ionian Sea and exported towards the Aegean Sea, with winter heat loss intensity enhancing this lateral export at both surface and intermediate layers. The Levantine Basin alternates between autotrophic and heterotrophic states, depending on the intensity of winter surface heat loss. Spatially, the Rhodes Gyre emerges as a significant oxygen pump, contributing 41 % of the total oxygen production in the surface layer in the Levantine basin. This study highlights the need for further modeling studies on pluri-annual and multi-decadal scales to explore the interannual variability and evolution of the annual oxygen budget across the entire Eastern Basin, particularly in the context of climate change.

Abstract. The marine biological carbon pump (BCP) plays a central role in the global carbon cycle, transporting carbon from the surface to the deep ocean and sequestering it for long periods. Sinking of surface-produced particles, known as the Biological Gravity Pump (BGP) constitutes the main component of the BCP. To study the BGP in the equatorial Atlantic upwelling region, a biogeochemical (BGC) Argo float equipped with an Underwater Vision Profiler 6 (UVP6) camera was deployed from July 2021 to March 2022. The float was recovered after its eastward drift from 23 to 7° W along the equator, during which it conducted profiles to 2000 m depth every 3 d. For the first time in this oceanic region, in situ images and physical and biogeochemical data from a BGC-Argo float were acquired and analyzed in combination with satellite data. During the float trajectory, two blooms were recorded followed by two main export events of sinking aggregates that lasted for over a month, consistently reaching 2000 m depth. A Lagrangian approach was applied to investigate the production, transformation, and deep export of marine particles. Based on the characterization of the morphology of detritus within and outside of the plumes, five particle morphotypes with different sinking properties were detected. Small and dense aggregates were present throughout the water column while porous morphotypes, despite being larger, were predominantly concentrated in the surface layer. Export was driven by small and compact particles with higher particle abundance and flux during upwelling and export events. Our investigation reveals the stability of the equatorial Atlantic BCP system during this period, yielding an export efficiency of 6 %–7 % during and outside of export events. This study highlights the importance of using new technologies on autonomous platforms to characterize the temporal variability in the magnitude and functioning of the BCP.

Simultaneous measurements of marine snow (particles larger than 600 µm) morphologies, estimates of their in situ sinking speeds, and midwater attenuation in export plumes were performed for the first time using a biogeochemical (BGC)-Argo float equipped with optical and imaging sensors. The float was deployed and recovered after drifting for 1 year in the sluggish-flow regime of the Angola Basin. Six consecutive chlorophyll a and particulate matter accumulation events were recorded at the surface, each followed by an export plume of sinking aggregates. Objects larger than 600 µm were classified using machine learning recognition and clustered into four morphological categories of marine aggregates. Plankton images were validated by an expert in a few broad categories. Results show that different types of aggregates were produced and exported from the different blooms. The different morphological categories of marine snow had different sinking speeds and attenuation for a similar size, indicating the effect of morphology on sinking speed. However, a typical size-to-sinking relationship for two of the categories and over the larger observed size range (100 µm to a few millimeters) was also observed, indicating the importance of size for sinking. Surprisingly, in situ-calculated sinking speeds were constantly in the lower range of known values usually assessed ex situ, suggesting a methodological effect, which is discussed. Moving away from purely size-based velocity relationships and incorporating these additional morphological aggregate properties will help to improve the mechanistic understanding of particle sinking and provide more accurate flux estimates. When used from autonomous platforms at high frequency, they will also provide increased spatio-temporal resolution for the observation of intermittent export events naturally occurring or induced by human activities.

The Rhodes Gyre is a cyclonic persistent feature of the general circulation of the Levantine Basin in the eastern Mediterranean Sea. Although it is located in the most oligotrophic basin of the Mediterranean Sea, it is a relatively high primary production area due to strong winter nutrient supply associated with the formation of Levantine Intermediate Water. In this study, a 3D coupled hydrodynamic-biogeochemical model (SYMPHONIE/Eco3M-S) was used to characterize the seasonal and interannual variability of the Rhodes Gyre's ecosystem and to estimate an annual organic carbon budget over the 2013-2020 period. Comparisons of model outputs with satellite data and compiled in situ data from cruises and Biogeochemical-Argo floats revealed the ability of the model to reconstruct the main seasonal and spatial biogeochemical dynamics of the Levantine Basin. The model results indicated that during the winter mixing period, phytoplankton first progressively grow sustained by nutrient supply. Then, short episodes of convection driven by heat loss and wind events, favoring nutrient injections, organic carbon export, and inducing light limitation on primary production, alternate with short episodes of phytoplankton growth. The estimate of the annual organic carbon budget indicated that the Rhodes Gyre is an autotrophic area, with a positive net community production in the upper layer (0-150 m) amounting to 31.2 ± 6.9 gCm^-2yr^-1. Net community production in the upper layer is almost balanced over the 7-year period by physical transfers, (1) via downward export (16.8 ± 6.2 gCm^-2yr^-1) and (2) through lateral transport towards the surrounding regions (14.1 ± 2.1 gCm^-2yr^-1). The intermediate layer (150-400 m) also appears to be a source of organic carbon for the surrounding Levantine Sea (7.5 ± 2.8 gCm^-2yr^-1) mostly through the subduction of Levantine Intermediate Water following winter mixing. The Rhodes Gyre shows high interannual variability with enhanced primary production, net community production, and exports during years marked by intense heat losses and deep mixed layers. However, annual primary production appears to be only partially driven by winter vertical mixing. Based on our results, we can speculate that future increase of temperature and stratification could strongly impact the carbon fluxes in this region.

The Mediterranean is an oligotrophic sea where dynamics is driven in part by dense water formations. Levantine Intermediate Water (LIW), formed mostly in the Rhodes Gyre, a persistent cyclonic circulation in the eastern Mediterranean Sea, plays a key role in the thermohaline circulation of the entire eastern and western basins. This Ph.D. thesis work, carried out as part of the PERLE (Pelagic Ecosystem Response to deep water formation in the Levant Experiment) project, aims to study the variability of the biogeochemical cycles and pelagic ecosystem in the Levantine Basin, and in particular in response to the LIW formation, over a multi-annual period, using a high-resolution 3D coupled modeling approach. For that purpose, a 3D coupled hydrodynamic-biogeochemical model, SYMPHONIE-Eco3M-S, was implemented over the Mediterranean Sea and the period from 2011 to 2020. It was calibrated and assessed using cruises, satellite, and BGC-Argo float observations. The assessment of the model shows that it reproduces reasonably well the main spatial and temporal variability observed. Based on the coupled model, first we investigate organic carbon dynamics in the Rhodes Gyre. On an annual scale, the Rhodes Gyre acts as an autotrophic ecosystem and a source of organic carbon for the surrounding areas. Winter severity markedly influences the phytoplankton ecosystem functioning and organic carbon variability of the area: years characterized by strong heat loss at the sea surface and deep mixing show in the upper layer enhanced nutrient supply, phytoplankton growth, and exports of organic carbon towards intermediate depths and the surrounding Levantine Basin. Second, the coupled model was used to quantify the contribution of air-sea flux, biological processes, and exchange fluxes with the neighboring seas in the variability of the dissolved oxygen dynamics in the Levantine Basin, and at a lower scale, the Rhodes Gyre. The Levantine acts as an oxygen sink for the atmosphere, with maximum rates of uptake in the Rhodes Gyre during winter. The upper layer of the Levantine Basin was found to be a source of oxygen for the intermediate layer and the Aegean Sea.