Mathieu Rembauville

Open‐ocean convection process: A driver of the winter nutrient supply and the spring phytoplankton distribution in the Northwestern Mediterranean Sea

This study was a part of the DeWEX project (Deep Water formation EXperiment), designed to better understand the impact of dense water formation on the marine biogeochemical cycles. Here, nutrient and phytoplankton vertical and horizontal distributions were investigated during a deep open-ocean convection event and during the following spring bloom in the Northwestern Mediterranean Sea (NWM). In February 2013, the deep convection event established a surface nutrient gradient from the center of the deep convection patch to the surrounding mixed and stratified areas. In the center of the convection area, a slight but significant difference of nitrate, phosphate and silicate concentrations was observed possibly due to the different volume of deep waters included in the mixing or to the sediment resuspension occurring where the mixing reached the bottom. One of this process, or a combination of both, enriched the water column in silicate and phosphate, and altered significantly the stoichiometry in the center of the deep convection area. This alteration favored the local development of microphytoplankton in spring, whereas nanophytoplankton dominated neighboring locations where the convection reached the deep layer but not the bottom. This study shows that the convection process influences both winter nutrients distribution and spring phytoplankton distribution and community structure. Modifications of the convection spatial scale and intensity (i.e. convective mixing depth) is likely to have strong consequences on phytoplankton community structure and distribution in the NWM, and thus on the marine food web.

Nanoplanktonic diatoms are globally overlooked but play a role in spring blooms and carbon export

Diatoms are one of the major primary producers in the ocean, responsible annually for ~20% of photosynthetically fixed CO2 on Earth. In oceanic models, they are typically represented as large (>20 µm) microphytoplankton. However, many diatoms belong to the nanophytoplankton (2–20 µm) and a few species even overlap with the picoplanktonic size-class (<2 µm). Due to their minute size and difficulty of detection they are poorly characterized. Here we describe a massive spring bloom of the smallest known diatom (Minidiscus) in the northwestern Mediterranean Sea. Analysis of Tara Oceans data, together with literature review, reveal a general oversight of the significance of these small diatoms at the global scale. We further evidence that they can reach the seafloor at high sinking rates, implying the need to revise our classical binary vision of pico- and nanoplanktonic cells fueling the microbial loop, while only microphytoplankton sustain secondary trophic levels and carbon export.Diatoms are major oceanic primary producers, but some species belonging to the nano- and even picoplankton size are poorly characterized. Here the authors describe a massive spring bloom of the smallest known diatom in the Mediterranean Sea and reveal their general oversight at the global scale.

Annual particulate matter and diatom export in a high nutrient, low chlorophyll area of the Southern Ocean

Upper ocean plankton assemblages are known to influence the export of carbon and biominerals from the mixed layer. However, relationships between plankton community structure and the magnitude and stoichiometry of export remain poorly characterized. We present data on biogeochemical and diatom export fluxes from the annual deployment of a sediment trap in a High Nutrient, Low Chlorophyll (HNLC) area upstream of the Kerguelen Plateau (KERFIX station). The weak and tidal-driven circulation provided favorable conditions for a quantitative analysis of export processes. Particulate organic carbon (POC) fluxes were highest in spring and summer. Biogenic silica (BSi) fluxes displayed similar seasonal patterns, although BSi:POC ratios were elevated in winter. Fragilariopsis kerguelensis dominated the annual diatom export assemblage (59.8% of the total valve flux). We identified clusters of diatom species that were positively or negatively correlated to the BSi:POC ratio. Our results indicate that the differential role of certain diatom species for carbon and silicon export, previously identified from iron-fertilized productive areas, is also valid in HNLC regimes. Although annual POC export below the mixed layer of the HNLC site is twofold lower that the one previously reported in a naturally iron-fertilized area of the Kerguelen Plateau, the fraction of seasonal net community production exported is similar at both sites (~1.5%). These findings suggest that natural iron fertilization increases the strength but not the efficiency of carbon export from the mixed layer.

Plankton Assemblage Estimated with BGC‐Argo Floats in the Southern Ocean: Implications for Seasonal Successions and Particle Export

The Southern Ocean (SO) hosts plankton communities that impact the biogeochemical cycles of the global ocean. However, weather conditions in the SO restrict mainly in situ observations of plankton communities to spring and summer, preventing the description of biological successions at an annual scale. Here, we use shipboard observations collected in the Indian sector of the SO to develop a multivariate relationship between physical and bio-optical data, and, the composition and carbon content of the plankton community. Then we apply this multivariate relationship to five biogeochemical Argo (BGC-Argo) floats deployed within the same bio-geographical zone as the ship-board observations to describe spatial and seasonal changes in plankton assemblage. The floats reveal a high contribution of bacteria below the mixed layer, an overall low abundance of picoplankton and a seasonal succession from nano- to microplankton during the spring bloom. Both naturally iron-fertilized waters downstream of the Crozet and Kerguelen Plateaus show elevated phytoplankton biomass in spring and summer but they differ by a nano- or microplankton dominance at Crozet and Kerguelen, respectively. The estimated plankton group successions appear consistent with independent estimations of particle diameter based on the optical signals. Furthermore, the comparison of the plankton community composition in the surface layer with the presence of large mesopelagic particles diagnosed by spikes of optical signals provides insight into the nature and temporal changes of ecological vectors that drive particle export. This study emphasizes the power of BGC-Argo floats for investigating important biogeochemical processes at high temporal and spatial resolution.