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. Fine-scale physical structures and ocean dynamics strongly influence andregulate biogeochemical and ecological processes. Theseprocesses are particularly challenging to describe and understand because of their ephemeral nature. The OSCAHR (ObservingSubmesoscale Coupling At High Resolution) campaign was conducted in fall 2015in which a fine-scale structure (1–10  km ∕ 1–10 days) in thenorthwestern Mediterranean Ligurian subbasin was pre-identified using bothsatelliteand numerical modeling data. Along the ship track, various variables were measured at the surface (temperature, salinity,chlorophyll  a and nutrient concentrations) with ADCP current velocity. Wealso deployed a new model of the CytoSense automated flow cytometer (AFCM)optimized for small and dim cells, for near real-time characterization of thesurface phytoplankton community structure of surface waters with a spatialresolution of a few kilometers and an hourly temporal resolution. For thefirst time with this optimizedversion of the AFCM, we were able to fully resolve Prochlorococcus picocyanobacteria in addition to the easilydistinguishable Synechococcus. The vertical physical dynamics and biogeochemical properties of the studied area wereinvestigated by continuous high-resolution CTD profiles thanks to a moving vessel profiler (MVP) during the vessel underwayassociated with a high-resolution pumping system deployed during fixedstations allowing sampling of the water column at a fine resolution(below 1  m ). The observed fine-scale feature presented a cyclonic structure with a relatively cold core surrounded by warmerwaters. Surface waters were totally depleted in nitrate and phosphate. In addition to the doming of the isopycnals by the cycloniccirculation, an intense wind event induced Ekman pumping. The upwelled subsurface cold nutrient-rich water fertilized surface watersand was marked by an increase in Chl  a concentration.Prochlorococcus and pico- and nano-eukaryotes were more abundant incold core waters, while Synechococcus dominated in warm boundarywaters. Nanoeukaryotes were the main contributors ( >50  %)in terms of pigment content (red fluorescence) and biomass. Biological observations based on the mean cell's red fluorescencerecorded by AFCM combined with physical properties of surface waters suggest a distinct origin for two warm boundary waters.Finally, the application of a matrix growth population model based on high-frequency AFCM measurements in warm boundary surfacewaters provides estimates of in situ growth rate and apparent net primary production for Prochlorococcus ( μ=0.21 d−1 , NPP  =0.11 mg C m - 3 d - 1 ) and Synechococcus ( μ=0.72 d−1 , NPP  =2.68 mg C m - 3 d - 1 ), which corroborate their opposite surface distribution pattern. The innovative adaptive strategy appliedduring OSCAHR with a combination of several multidisciplinary and complementary approaches involving high-resolution in situobservations and sampling, remote-sensing and model simulations provided a deeper understanding of the marine biogeochemical dynamicsthrough the first trophic levels.

biogeosciences

Coupling physics and biogeochemistry thanks to high-resolution observations of the phytoplankton community structure in the northwestern Mediterranean Sea