ABSTRACT Aim Assessing the influence of diel vertical migration (DVM) on biogeographic patterns to improve the macroecological characterisation of the structure and function of zooplankton communities. Location North Atlantic Ocean and adjacent seas. Taxon Marine copepod species. Methods We base our bioregionalisation on Continuous Plankton Recorder (CPR) data of copepod species abundances from 1966 to 2021. We separate day and night samples using the solar elevation corresponding to civil twilight. For each condition, we interpolate abundances onto a grid adapted to the irregular sampling effort. We then generate a bipartite network (geographical cells—species) on which we apply the Map Equation clustering algorithm to delineate bioregions and identify their underlying copepod communities. We use canonical correspondence analyses to characterise the resulting bioregions in terms of environmental forcings, species composition and community‐weighted mean traits. Results We identify four bioregions for both day and night partitions, with dynamic transitions and changes in spatial patterns as well as in community composition between day and night. While environmental forcings seem to transcend diel variations, ecological features of bioregions show day/night discrepancies: higher copepod diversity at night is driven by changes in species composition mediated by diel vertical migration. Main Conclusions We highlight how day/night variations driven by highly migratory copepod species shape community spatial patterns and species composition. We uncover distinct levels of functional diversity across bioregions, suggesting different responses of copepod communities to environmental changes. Transition zones emerge as crucial markers of pelagic bioregion connectivity, emphasising their dynamic nature. Embracing a partitioning approach that better captures these dynamics is essential for understanding how ecosystems function and will evolve in response to climate changes.

Abstract. In marine ecosystems, most physiological, ecological, or physical processes are size dependent. These include metabolic rates, the uptake of carbon and other nutrients, swimming and sinking velocities, and trophic interactions, which eventually determine the stocks of commercial species, as well as biogeochemical cycles and carbon sequestration. As such, broad-scale observations of plankton size distribution are important indicators of the general functioning and state of pelagic ecosystems under anthropogenic pressures. Here, we present the first global datasets of the Pelagic Size Structure database (PSSdb), generated from plankton imaging devices. This release includes the bulk particle normalized biovolume size spectrum (NBSS) and the bulk particle size distribution (PSD), along with their related parameters (slope, intercept, and R2) measured within the epipelagic layer (0–200 m) by three imaging sensors: the Imaging FlowCytobot (IFCB), the Underwater Vision Profiler (UVP), and benchtop scanners. Collectively, these instruments effectively image organisms and detrital material in the 7–10 000 µm size range. A total of 92 472 IFCB samples, 3068 UVP profiles, and 2411 scans passed our quality control and were standardized to produce consistent instrument-specific size spectra averaged to 1° × 1° latitude and longitude and by year and month. Our instrument-specific datasets span most major ocean basins, except for the IFCB datasets we have ingested, which were exclusively collected in northern latitudes, and cover decadal time periods (2013–2022 for IFCB, 2008–2021 for UVP, and 1996–2022 for scanners), allowing for a further assessment of the pelagic size spectrum in space and time. The datasets that constitute PSSdb's first release are available at https://doi.org/10.5281/zenodo.11050013 (Dugenne et al., 2024b). In addition, future updates to these data products can be accessed at https://doi.org/10.5281/zenodo.7998799.

The objective of the TONGA oceanographic expedition was to study the control of productivity and carbon sequestration by micronutrients of shallow hydrothermal origin in the Western Tropical South Pacific (WTSP) Ocean. The 37-day oceanographic survey took place on board the R/V L’Atalante in 2019 between Oct. 31 to Dec. 6 (Nouméa-Nouméa). Over a large area of the WTSP the team acquired numerous results on both the entire water column (up to the sediment) and the atmosphere. Specific task are represented on figure 1: (task 1) to characterize chemically and optically shallow hydrothermal fluids and to compare the source from below (shallow hydrothermal fluids) with the source from above (atmospheric deposition); (task 2) to quantify the dynamical dispersion of the fluids at small and regional scale; (task 3) to investigate the impact of the shallow hydrothermal sources on the biological activity and diversity, and the feedback to the atmosphere via the oceanic emissions of primary and secondary aerosols. (Task 4) to communicate about the campaign (see for example our Tweeter account (https://twitter.com/tongaproject) and the movie (26’) both in French (https://www.youtube.com/watch?v=e5kAd0i6Dck) and English (https://www.youtube.com/watch?v=UeABf-cVR-k). A long west to east (up to the blue waters of the gyre) transect allowed to characterize the different biogeochemical provinces crossed and a focus in the region of the Lau Basin allowed to investigate the impact of shallow hydrothermal sources. A series of short and long stations allowed to fully characterize the stocks and the fluxes in the different provinces. Short-term (up to 10 days) processes studies have been conducted (drifting moorings and minicosms experiments). Part of these results will feed into important modeling work. A fixed mooring line launched at the end of the campaign and recovered in Nov. 2020 as well as the 7 ARGO floats and 20 drifting buoys that were dropped during the campaign provide a broader temporal context of the acquisitions done during the campaign. An important focus of the campaign was the trace metal characterization of the entire water column. For this, TONGA has been labeled by the international program GEOTRACES (https://www.geotraces.org/). The impact on biological communities of fluids is supported by the international IMBER program (https://imber.info/). The TONGA project is also part of the LEFE program (funding by LEFE-CYBER and LEFE-GMMC), the ANR (Appel à projets génériques) and the Fondation A-MIDeX of the Aix-Marseille Université.

The Western Tropical South Pacific (WTSP) basin has been identified as a hotspot of atmospheric dinitrogen fixation due to the high dissolved iron ([DFe]) concentrations (up to 66 nM) in the photic layer linked with the release of shallow hydrothermal fluids along the Tonga-Kermadec arc. Yet, the effect of such hydrothermal fluids in structuring the plankton community remains poorly studied. During the TONGA cruise (November-December 2019), we collected micro- (20-200 μm) and meso-plankton (>200 μm) samples in the photic layer (0-200 m) along a west to east zonal transect crossing the Tonga volcanic arc, in particular two volcanoes associated with shallow hydrothermal vents (< 500 m) in the Lau Basin, and both sides of the arc represented by Melanesian waters and the South Pacific Gyre. Samples were analyzed by quantitative imaging (FlowCam and ZooScan) and then coupled with acoustic observations, allowing us to study the potential transfer of phytoplankton blooms to higher planktonic trophic levels. We show that micro- and meso-plankton exhibit high abundances and biomasses in the Lau Basin and, to some extent, in Melanesian waters, suggesting that shallow hydrothermal inputs sustain the planktonic food web, creating productive waters in this otherwise oligotrophic region. In terms of planktonic community structure, we identified major changes with high [DFe] inputs, promoting the development of a low diversity planktonic community dominated by diazotrophic cyanobacteria. Furthermore, in order to quantify the effect of the shallow hydrothermal vents on chlorophyll a concentrations, we used Lagrangian dispersal models. We show that chlorophyll a concentrations were significantly higher inside the Lagrangian plume, which came into contact with the two hydrothermal sites, confirming the profound impact of shallow hydrothermal vents on plankton production.

L’évaluation du Bon Etat Ecologique du descripteur 1 - Biodiversité « Habitats pélagiques » de la Directive-Cadre Stratégie pour le Milieu Marin (DCSMM) repose sur le seul critère D1C6 dans les 4 sous-régions marines françaises (Manche - Mer du Nord, Mers Celtiques, Golfe de Gascogne et Méditerranée Occidentale). Les 3 indicateurs utilisés actuellement (PH1, PH2 et PH3) ont été développés par les groupes de travail de la convention OSPAR pour l’Atlantique Nord-Est. Ils ciblent l’analyse des changements de la biomasse ou d’abondance, de la composition (i.e. rapports entre groupes fonctionnels) et de la diversité des communautés phyto- et zooplanctoniques. La synthèse bibliographique de Goffart et al. (2022a) au titre des habitats pélagiques de la DCSMM a montré qu’une caractéristique majeure du phytoplancton méditerranéen est la présence d’un maximum profond de chlorophylle (« Deep Chlorophyll Maximum », DCM) pendant toute la période de stratification thermique. En Méditerranée Occidentale, le DCM est essentiellement le résultat d’une accumulation de biomasse active. Il constitue une source de nourriture importante pour le zooplancton et une partie de l’ichtyoplancton qui s’y concentrent 6 à 8 mois par an. Lors du cycle 2 de l’évaluation du Bon Etat Ecologique (BEE) des Habitats pélagiques, l’indicateur PH2 OSPAR - Phytoplancton a été adapté aux eaux françaises par Duflos et al. (2018) et appliqué aux eaux de surface des 4 sous-régions marines, sans que les spécificités du phytoplancton méditerranéen (e.g. dynamique ; variabilité spatio-temporelle ; composition et groupes fonctionnels) ne soient prises en considération. Cette note synthétise un jeu de données méditerranéennes pour proposer des adaptations méthodologiques de l’indicateur PH2 DUFLOS - Phytoplancton basé sur les changements de biomasse (concentration en chlorophylle a). Elle a pour objectif d’aboutir à un indicateur PH2 MEDIT - Phytoplancton qui intègre les spécificités des communautés phytoplanctoniques méditerranéennes, en particulier en ce qui concerne leur variabilité spatio-temporelle.