Abstract In marine ecosystems, critical services like fish production, carbon export, or the delivery of nutrients through N 2 -fixation rely heavily on the size spectrum of pelagic organisms, particularly mesoplankton (200-20,000 µ m). However, the linkages between environmental factors and mesoplankton spectral biogeography remain largely unresolved, as so far only limited datasets exist to understand the large-scale shifts in mesoplankton size. Using global compilations of Rhizarian, colonial N 2 -fixer, and Crustacean images, we reveal the role of iron in shaping the size structure and related biogeography of these groups. Our findings underscore the importance of atmospheric sources of iron for N 2 -fixers and Rhizarians while total iron, accounting for organic and inorganic compounds, appeared to explain a high percentage of the variance in Crustacean size structure via apparent recycling. Using environmental explanatory variables, our models reached high R 2 (0.93, 0.88, and 0.79 respectively), providing robust predictions of mesoplankton size structure related to elemental cycling and ecosystem services. Our results suggest that without compensatory mechanisms, future increases in global temperatures could have negative effects on mesoplankton size, possibly limiting carbon export from the productive layers to sequestration depth, that might be offset by expected increases in iron inputs that benefit N 2 -fixers, Rhizarians, and eventually Crustaceans.

Most of the oceanic primary production is lost from surface marine food webs into the deep ocean as waste sinking particles, known as marine snow. Their morphology influences biogeochemical cycles, yet the fate of various types remains poorly understood. We analyzed four annual time-series of in situ images acquired using UVP6 camera systems mounted on robotic floats at different latitudes between 0 to 50°S. Images were objectively classified to define distinct morphotypes within the continuum of observed morphologies. At high latitudes, surface marine snow was seasonally dominated by elongated morphotypes, possibly fresh phytoplankton-chains, whereas at low latitudes more circular morphotypes predominated possibly as a result of biologically mediated coagulation. Transfer efficiency to the mesopelagic varied substantially between morphotypes. Surface-produced marine snow, predominantly transparent, is replaced by spherical and opaque aggregates below the mixed layer. Given the wide latitudinal range, we propose that marine snow aggregation via coagulation in the mixed layer, followed by zooplankton-mediated control of vertical export at depth, represents a general sequence of particle transformations reflected by shifts in marine snow morphologies.

The Sine Saloum inverse estuary (Senegal) supports important local livelihoods, particularly shellfisheries, yet its plankton communities remain poorly documented. This study investigates the spatial and seasonal variability of phytoplankton and zooplankton and their relationship with environmental drivers.Sampling was conducted at (16) stations during the dry and wet season in 2019. Environmental parameters were measured in situ, and plankton samples (50 µm net) were imaged using PlanktoScope and taxonomically validated via Ecotaxa.,Both phytoplankton and zooplankton exhibited strong seasonal and spatial variability, with higher abundance during the wet season. Phytoplankton communities were dominated by diatoms (e.g., Chaetoceros, Bacteriastrum, Odontella), whereas the dry season was characterized by lower overall abundance and increased abundance of taxa such as Cyanobacteria, other Diatoma, and Pleurosigma. Zooplankton communities were dominated by nauplii, whose abundance peaked during the wet season, accounting for up to ~80% of the total abundance. In contrast, the dry season showed a more even taxonomic distribution. Wet conditions favored larger size fractions whereas dry conditions were associated with smaller size fractions. Phytoplankton spatial structuring, with a North-South organization during the wet season and a West-East pattern during the dry season, was driven primarily by salinity and nutrients gradients. In contrast, zooplankton did not show such significant spatial and seasonal variability.

Synthetic normalized biomass size spectra (NBSS) comprising non-synoptically sampled phytoplankton, meso- and macrozooplankton, and micronekton including mesopelagic fishes were explored to analyze pelagic community structure in seven regions of the tropical and the subtropical Atlantic representative of different water bodies (NE Brazil shelf, NE Brazil oceanic islands, northern and southern offshore Benguela Upwelling System, northern offshore Canary Current Upwelling System, equatorial region, southern Canary Current Upwelling System oxygen minimum zone). For mesopelagic fishes and micronekton, conversions were applied accounting for sampling biases in relation to other ecosystem components. Three main results were obtained. Firstly, NBSS slopes based on biovolume were significantly shallower than slopes based on carbon contents, as revealed in part through pairwise comparisons of linear models and by ANOVA. The ensemble mean slope for six regions combined (one region omitted due to missing zooplankton data) measured in terms of biovolume was -0.866, and the respective value in terms of carbon biomass was -0.894. Secondly, log ratios of spectral densities, that is, contrasts, between consumers and phytoplankton increased with decreasing primary production. Contrasts for total micronekton and mesopelagic fishes relative to phyto- and zooplankton varied with primary production, indicating that below a primary production of 650 mg C m-2 day-1, their spectral densities were higher than predicted by phytoplankton. Above this level, however, the spectral densities were lower. Thirdly, regional population marginal means were positively correlated with primary production in terms of carbon biomass (p = 0.01) but not biovolume (p = 0.48). Biovolume NBSS slopes were negatively correlated with primary production (p = 0.02), if the northern Benguela Upwelling System was not included (else p = 0.11). The increase in biovolume relative to the carbon biomass of gelatinous organisms is discussed as allometric advantage to enhance trophic transfer efficiency. Trophic transfer efficiencies ranged from 28.9% to 36.8% for open-ocean systems and from 36.8% to 49.7% for coastal and oceanic island habitats. The results suggest that synthetic pelagic size spectra provide a powerful framework to detect regional and functional shifts in Atlantic pelagic communities, offering predictive value for future ocean changes under climate scenarios.

Plankton community structure influences biogeochemical and ecosystem processes, such as sequestration of atmospheric CO 2 , carbon export to the ocean floor, and the productivity of higher trophic levels. One means of analyzing community structure is through the distribution of biovolume across size classes (the size spectrum), since size is a proxy for plankton functional traits. To understand how climate forcing affects plankton communities, we assessed the size spectra in the historical simulations of seven Earth System Models (ESMs) included in the 6th Coupled Model Intercomparison Project and analyzed projected changes under a high emissions scenario (SSP5‐8.5). We compared historical estimates with the Pelagic Size Structure database (PSSdb), a novel size structure dataset from imaging systems. The median slope from models ranged from −1.66 to −1.07, with shallower slopes from this range approximating both the theoretical expectation and PSSdb observations (−1.05), with variations around the median representing differences in the total biovolume distribution across plankton functional groups. Consistent with the observations, most ESMs show steeper slopes and lower biovolume in oligotrophic subtropical gyres compared with productive ocean regions. Historical versus climate change simulations reveal increases in slope and biovolume at high latitudes, associated with greater biomass and productivity, and decreases at lower latitudes, consistent with nutrient limitation from stronger stratification. We emphasize the need for expanded observational data. Despite ESMs not being designed to simulate size, the plankton size spectra from models provide insights on large‐scale ecological and biogeochemical processes, and how climate change could affect these dynamics in the future.

The equatorial upwelling system is characterized by a strong seasonal cycle with relatively cold sea surface temperature (SST) and enhanced primary production in the “cold tongue region” of the eastern basin during boreal summer. During the boreal summer of 2021, the equatorial Atlantic witnessed its most intense warm event since the beginning of satellite observations, which is assumed to have a direct impact on the carbon cycle. Here we use data from a BGC Argo float, deployed in the equatorial upwelling region in order to investigate the production peaks of marine particles during two distinct periods: the decay period of the anomalous weak cold tongue and the period of secondary cooling in boreal winter. In situ images of plankton and particles and physical and biogeochemical data provided by the Underwater Vision Profiler 6 (UVP6) and various sensors mounted on the float were analyzed in conjunction with satellite data (sea surface height, SST, ocean color). The float covered the period between 13 July 2021 and 23 March 2022 drifting eastward from 23-7.4°W along the equator and conducting 2000 m profiles every three days. Our data revealed the occurrence of two blooms with high surface chlorophyll concentrations accompanied by the presence of two carbon export events reaching at least 2000 m depth. Both events exhibited high carbon flux at the mixed layer with a flux of 106±5 mgC.m-2d-1 during the first event compared to 122±17 mgC.m-2d-1 during the second while flux between both events remained below 89 mgC.m-2d-1. However, a distinction in the vertical extent of these events was recorded as there was a slightly higher flux at 2000 m for the winter boreal, 30% higher, suggesting a difference between the carbon attenuation flux export associated with the primary upwelling season with the one observed during the secondary cooling period in the boreal winter. The characterization of the morphology of detritus using in situ imaging and clustering method revealed the presence of five different morpho-types with different sinking properties. Two primary classifications—large and small dense aggregates—emerged as the predominant exported detritus to depths while porous aggregates were more concentrated in the surface layer. Our study revealed a dynamic interaction between various layers, involving carbon production in the surface layer, succeeded by its subsequent export to deeper layers. Finally, this study offers new insights into particle dynamics and the morphology of sinking particles within the equatorial region.

Abstract. Plankton and detritus are essential components of the Earth’s oceans influencing biogeochemical cycles and carbon sequestration. Climate change impacts their composition and marine ecosystems as a whole. To improve our understanding of these changes, standardized observation methods and integrated global datasets are needed to enhance the accuracy of ecological and climate models. Here, we present a global dataset for plankton and detritus obtained by two versions of the Underwater Vision Profiler 5 (UVP5). This release contains the images classified in 33 homogenized categories, as well as the metadata associated with them, reaching 3,114 profiles and ca. 8 million objects acquired between 2008–2018 at global scale. The geographical distribution of the dataset is unbalanced, with the Equatorial region (30° S – 30° N) being the most represented, followed by the high latitudes in the northern hemisphere and lastly the high latitudes in the Southern Hemisphere. Detritus is the most abundant category in terms of concentration (90 %) and biovolume (95 %), although its classification in different morphotypes is still not well established. Copepoda was the most abundant taxa within the plankton, with Trichodesmium colonies being the second most abundant. The two versions of UVP5 (SD and HD) have different imagers, resulting in a different effective size range to analyse plankton and detritus from the images (HD objects >600 µm, SD objects >1 mm) and morphological properties (grey levels, etc.) presenting similar patterns, although the ranges may differ. Therefore, recommendations are provided for the appropriate use of this data when conducting studies. A large number of images of plankton and detritus will be collected in the future by the UVP5, and the public availability of this dataset will help it being utilized as a training set for machine learning and being improved by the scientific community. This will reduce uncertainty by classifying previously unclassified objects and expand the classification categories, ultimately enhancing biodiversity quantification. The dataset that constitutes this first release is available at SEANOE.

Simultaneous detection and sizing of plankton and marine particles is now possible at global scale with the Underwater Vision Profiler 6 (UVP6) mounted on BGC-Argo floats. Combined with other biogeochemical sensors, the UVP6 delivers Essential Ocean Variables (EOVs), from nutrients to plankton and detritus, critical for monitoring and modeling. To date, over a hundred of UVP6 have been deployed by different laboratories across all oceans. When deployed on BGC-Argo floats, particle size distribution or taxa counts -obtained through embedded recognition, are typically the only available data, as the floats are generally not recovered. Here we report multi-year patterns of plankton and particles obtained from four successful deployments and recoveries at different latitudes, ranging from the equator to 50° South and depths down to 2000 m. Objects larger than 0.6 mm were classified using machine learning recognition (for plankton and particle) and k-means clustering (only for particles) methods. To date, five morphological categories of marine snow (particles > 500µm) were defined, based on shape, darkness, and structural heterogeneity, while plankton images were validated by experts in 20 broad categories. We show how these results can be used to assess plankton diversity, detritus composition, carbon vertical flux, and attenuation down to the bathypelagic layers in a wide range of environmental conditions. In cases of low mesoscale activity, results show that different phytoplankton blooms produce different marine snow morphotypes having different fates. Dense marine snow is found to be the most exported and also the deepest (down to 2000 m depth). Other morphotypes, such as filaments or porous marine snow, were generally not exported below the surface layer. Size and morphology were important to determine marine snow sinking speed. In high mesoscale activity, the steady marine snowfall is disrupted by ocean horizontal and vertical circulations and intermittent export events are observed down to 600m depth. When fully integrated in a global network of BGC-Argo floats, underwater cameras will complement existing global observations of biogeochemical variables and small planktonic organisms, detected by optical sensors, by also capturing data on larger organisms and particles.

The biological carbon pump (BCP) comprises a wide variety of processes involved in transferring organic carbon from the surface to the deep ocean. This results in long-term carbon sequestration. Without the BCP, atmospheric CO2 concentrations would be around 200 ppm higher. This study reveals that ocean dynamics at the mesoscale and submesoscale could have a major impact on particulate organic matter (POM) vertical distribution. Our results indicate that intense submesoscale frontal regions, such as those between mesoscale eddies, could lead to an important accumulation and transport of POM from the mixed-layer depth (MLD) down to the mesopelagic zone. To reach these conclusions, a multifaceted approach was applied. It included in situ measurements and marine snow images from a BGC-Argo float equipped with an Underwater Vision Profiler (UVP6), satellite altimetry data, and Lagrangian diagnostics. We focused our study on three intense features in marine snow distribution, observed during the 17-month-long float mission in the Cape Basin in the southwest of Africa. These features were located in the frontal region between mesoscale eddies. Our study suggests that a particle injection pump induced by a frontogenesis-driven mechanism has the potential to enhance the effectiveness of the biological pump by increasing the depth at which carbon is injected into the water column. This work also emphasizes the importance of establishing repeated sampling campaigns targeting the interface zones between eddies. This could improve our understanding of the mechanisms involved in the deep accumulation of marine snow observed at eddy interfaces.

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.

Quantifying the ocean's ability to sequester atmospheric carbon is essential in a climate change context. Measurements of gravitational carbon export to the mesopelagic seldom balance the carbon demand or the oxygen consumption there, suggesting the potential presence of other mechanisms of carbon export. We deployed a biogeochemical Argo float in a cyclone in the Benguela upwelling system for five months, and estimated vertical carbon export and respiration in the eddy via particle imagery with an underwater vision profiler 6 in a quasi Lagrangian way. A sensitivity analysis shows that, under certain assumptions, oxygen consumption rates could match the carbon supply and carbon demand. We furthermore identified a mechanism of vertical particulate carbon export, the full eddy core submergence pump. Our analysis suggests that at 450 m depth, within this eddy, this pump exports about one fourth to half of the total carbon compared to the biological gravitational pump.

The use of trait-based approaches and trait data in zooplankton ecology is rapidly growing to better understand and predict the patterns of zooplankton distributions and their role in aquatic ecosystems and biogeochemical cycles. Although the number of zooplankton trait-based studies and available trait datasets is increasing, several challenges remain for the findability, accessibility, interoperability, and reusability (FAIR) in trait-based approaches that, if unaddressed, may stifle progress in this research area. Here, we review recent applications of trait-based approaches in zooplankton research and summarize the currently available trait data resources. To realize the potential of trait-based approaches to resolve ecological roles of zooplankton, datasets and approaches must adhere to FAIR principles. We provide recommendations and pathways forward to ensure FAIRness while highlighting the importance of collaborative efforts. These practical and easily implementable strategies will enhance the FAIRness of trait data, ultimately advancing zooplankton ecological research and connecting these findings to aquatic ecosystem functioning.

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.

AbstractThriving in both epipelagic and mesopelagic layers, Rhizaria are biomineralizing protists, mixotrophs or flux-feeders, often reaching gigantic sizes. In situ imaging showed their contribution to oceanic carbon stock, but left their contribution to element cycling unquantified. Here, we compile a global dataset of 167,551 Underwater Vision Profiler 5 Rhizaria images, and apply machine learning models to predict their organic carbon and biogenic silica biomasses in the uppermost 1000 m. We estimate that Rhizaria represent up to 1.7% of mesozooplankton carbon biomass in the top 500 m. Rhizaria biomass, dominated by Phaeodaria, is more than twice as high in the mesopelagic than in the epipelagic layer. Globally, the carbon demand of mesopelagic, flux-feeding Phaeodaria reaches 0.46 Pg C y−1, representing 3.8 to 9.2% of gravitational carbon export. Furthermore, we show that Rhizaria are a unique source of biogenic silica production in the mesopelagic layer, where no other silicifiers are present. Our global census further highlights the importance of Rhizaria for ocean biogeochemistry.

Abstract. The open ocean plays a critical role in mitigating climate change by sequestering carbon dioxide (CO2) from the atmosphere for long periods of time. This carbon storage occurs over decades to millennia and relies on the physical pump that transports cold, dense, and DIC-rich waters to the deep ocean, as part of the ocean’s overturning circulation, and the biological carbon pump (BCP). The BCP encompasses a wide range of processes, from the fixation of atmospheric CO2 by phytoplankton activity to carbon sequestration in the deep ocean. Atmospheric CO2 concentrations would be about 200 ppm higher than in a world without biology, and the global climate would be much warmer by default. This study highlights the idea that BCP efficiency is enhanced by the ocean dynamics at mesoscale and submesoscale. In fact, our results suggest that frontal regions, such as those between mesoscale eddies, could lead to an important accumulation and transport of particulate organic matter (POM) from the mixed layer depth (MLD) down to depths of about 600 meters. To reach these conclusions, a multifaceted approach was applied. It included in-situ measurements and marine snow images from a BGC Argo float equipped with an Underwater Vision Profiler (UVP6), satellite altimetry data, and Lagrangian physics diagnostics. We focused our study on three intense features in marine snow distribution observed during the 17-month long float mission in the Cape Basin, southwest of Africa. These features were located in the frontal region between mesoscale eddies. Our study suggests that a particle injection pump induced by a frontogenesis-driven mechanism has the potential to enhance the effectiveness of the biological pump by increasing the depth at which carbon is injected into the water column. This work also emphasizes the importance of establishing repeated sampling campaigns targeting the interface zones between eddies. This could improve our understanding of the mechanisms involved in the deep accumulation of marine snow observed at eddy interfaces.

Summary paragraph Plankton are essential in marine ecosystems. However, our knowledge of overall community structure is sparse due to inconsistent sampling across their very large organismal size range. Here we use diverse imaging methods to establish complete plankton inventories of organisms spanning five orders of magnitude in size. Plankton community size and trophic structure variation validate a long-held theoretical link between organism size-spectra and ecosystem trophic structures. We found that predator/grazer biomass and biovolume unexpectedly exceed that of primary producers at most (55%) locations, likely due to our better quantification of gelatinous organisms. Bottom- heavy ecosystems (the norm on land) appear to be rare in the ocean. Collectively, gelatinous organisms represent 30% of the total biovolume (8-9% of carbon) of marine plankton communities from tropical to polar ecosystems. Communities can be split into three extreme typologies: diatom/copepod-dominated in eutrophic blooms, rhizarian/chaetognath-dominated in oligotrophic tropical oceans, and gelatinous-dominated elsewhere. While plankton taxonomic composition changes with latitude, functional and trophic structures mostly depend on the amount of prey available for each trophic level. Given future projections of oligotrophication of marine ecosystems, our findings suggest that rhizarian and gelatinous organisms will increasingly dominate the apex position of planktonic ecosystems, leading to significant changes in the ocean’s carbon cycle.

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.

Plankton size spectra are important indicators of the ecosystem state, yet such measurements are typically biased by the available sampling methods. Here, we combined individual size measurement from two zooplankton imaging approaches-in situ observation by the Underwater Vision Profiler5 and Multinet-collection supplemented by ex situ imaging via Zooscan-obtained in the global ocean, to calculate zooplankton normalized biovolume size spectra (NBSS) for all organisms larger than 1 mm. The reconstructed NBSS combining both datasets resulted in increased biomass estimates by adding organisms poorly sampled by either of the methods. The optimal values measured by both methods are used to reconstruct the zooplankton biovolume and biomass distributions. The reconstructed slopes appeared steeper and closer to those measured only by the UVP5 (+7.6%) and flatter than those obtained only from the Multinet (-20%), particularly in tropical and temperate latitudes. The main difference in tropical and temperate NBSS from the two devices is due to the fragile rhizarians that were not accounted for when using net data. When possible, we suggest using in situ and ex situ technologies together, and we provide potential indications on how to correct for missing components of the community when only one method is available.