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Abstract
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.
Highlights
Despite representing only 0.2 % of the global photosynthetically active carbon (C) biomass, phytoplankton accounts for about half of the global primary productivity on Earth (Falkowski et al, 1998; Field et al, 1998)
The main cyclonic circulation was divided into two parts: a small recirculation centered on (8.75◦ W, 43.80◦ N) and a second one in the southwest separated by a local minimum in current intensity, both observed in AVISO and ADCP data
The cruise strategy utilized an adaptive approach based on both satellite and numerical modeling data to identify a dynamical feature of interest and to track its evolution
Summary
Despite representing only 0.2 % of the global photosynthetically active carbon (C) biomass, phytoplankton accounts for about half of the global primary productivity on Earth (Falkowski et al, 1998; Field et al, 1998). It forms the basis of the marine food web and exerts a major control on global biogeochemical cycles. The heterogeneity and the fine-scale variability of phytoplankton abundance have been observed and described from the 1970s (Platt, 1972; Denman et al, 1976), but the community structure variability on this scale remained uncharted at this time. While on a basin scale the phytoplankton community structure is relatively well constrained, on smaller scales both modeling (Lévy et al, 2001; Clayton et al, 2013; Lévy et al, 2014; d’Ovidio et al, 2015) and observation (Claustre et al, 1994; d’Ovidio et al, 2010; Clayton et al, 2014; Martin et al, 2015; Cotti-Rausch et al, 2016) studies have revealed during the last decades that phytoplankton community structure exhibits strong variability (Levy et al, 2015)
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