Coupling physics and biogeochemistry thanks to high-resolution observations of the phytoplankton community structure in the northwestern Mediterranean Sea
Authors: Marrec Pierre , Grégori Gérald , Doglioli Andrea M. , Dugenne Mathilde , Della Penna Alice , Bhairy Nagib , Cariou Thierry , Hélias Nunige Sandra , Lahbib Soumaya , Rougier Gilles , Wagener Thibaut , Thyssen Melilotus 
Published: March 2018

Abstract. Fine-scale physical structures and ocean dynamics strongly influence and
regulate biogeochemical and ecological processes. These
processes are particularly challenging to describe and understand because of their ephemeral nature. The OSCAHR (Observing
Submesoscale Coupling At High Resolution) campaign was conducted in fall 2015
in which a fine-scale structure (1–10 km∕1–10 days) in the
northwestern Mediterranean Ligurian subbasin was pre-identified using both
satellite
and 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. We
also deployed a new model of the CytoSense automated flow cytometer (AFCM)
optimized for small and dim cells, for near real-time characterization of the
surface phytoplankton community structure of surface waters with a spatial
resolution of a few kilometers and an hourly temporal resolution. For the
first time with this optimized
version of the AFCM, we were able to fully resolve Prochlorococcus picocyanobacteria in addition to the easily
distinguishable Synechococcus. The vertical physical dynamics and biogeochemical properties of the studied area were
investigated by continuous high-resolution CTD profiles thanks to a moving vessel profiler (MVP) during the vessel underway
associated with a high-resolution pumping system deployed during fixed
stations 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 warmer
waters. Surface waters were totally depleted in nitrate and phosphate. In addition to the doming of the isopycnals by the cyclonic
circulation, an intense wind event induced Ekman pumping. The upwelled subsurface cold nutrient-rich water fertilized surface waters
and was marked by an increase in Chl a concentration.
Prochlorococcus and pico- and nano-eukaryotes were more abundant in
cold core waters, while Synechococcus dominated in warm boundary
waters. Nanoeukaryotes were the main contributors (>50 %)
in terms of pigment content (red fluorescence) and biomass. Biological observations based on the mean cell's red fluorescence
recorded 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 surface
waters provides estimates of in situ growth rate and apparent net primary production for Prochlorococcus (μ=0.21 d−1, NPP =0.11 mgCm-3d-1) and Synechococcus (μ=0.72 d−1, NPP =2.68
mgCm-3d-1), which corroborate their opposite surface distribution pattern. The innovative adaptive strategy applied
during OSCAHR with a combination of several multidisciplinary and complementary approaches involving high-resolution in situ
observations and sampling, remote-sensing and model simulations provided a deeper understanding of the marine biogeochemical dynamics
through the first trophic levels.