High spatial variability of phytoplankton assessed by flow cytometry, in a dynamic productive coastal area, in spring: The eastern English Channel

The distribution of phytoplankton (from pico-to microphytoplankton) was investigated, at single-cell level and at high spatial resolution, during an oceanographic cruise across the eastern English Channel (EEC) between April 27 and 29, 2012. Seawater was continuously collected from surface waters and analysed on board at high frequency (one sample every 10 min), by using a new generation of pulse-shape recording scanning flow cytometer (CytoSense, Cytobuoy©). A Bray–Curtis matrix analysis based on phytoplankton composition allowed the discrimination of 4 communities. Within these communities, abundance, cell size as well as single cell and total red fluorescence of 8 phytoplankton groups were measured. Picoeukaryotes and Synechococcus spp cells dominated the mid Channel and most of the English waters monitored, whereas waters off Eastbourne as well as French coastal waters (under remote and direct estuarine influence) were characterized by the dominance of Phaeocystis globosa haploid and diploid cells. Most of the total red fluorescence signal, which correlated with chlorophyll a concentrations, was attributable to P. globosa and, to a lesser extent, to diatoms. In addition to sub-mesoscale variation within phytoplankton communities, the single-cell features within each phytoplankton group gave information about the physiological status of individual phytoplankton cells.