Multi-scale phytoplankton dynamics in a coastal system of the Eastern English Channel: the Boulogne-sur-Mer coastal area

Abstract. To study changes in phytoplankton community composition at different time scales, an automated flow cytometer (Cytosub, Cytobuoy b.v.) was deployed on the MAREL Carnot automated monitoring station in Boulogne-sur-Mer (France) during spring (2021, 2022) and summer (2022), following an Eulerian approach. Phytoplankton dynamics were recorded every 2 hours, distinguishing 11 Phytoplankton Functional Groups (PFGs) based on optical and fluorescence properties. This enabled detailed characterization of PFG successions, including MicroRED (mostly diatoms) and NanoRED (mostly haptophytes of the genus Phaeocystis globosa) transitions in spring, as well as a summer dominance by PicoORG (pico-cyanobacteria, mostly of the genus Synechococcus) and PicoRED. Four extreme events, including a salinity drop (April 2021), high winds (May 2021 and April 2022), and a marine heatwave (July 2022), caused rapid shifts in phytoplankton community composition. Empirical Mode Decomposition (EMD) and Lomb-Scargle Periodogram (LSP) analyses revealed that 85±10 % of variability in phytoplankton abundance, red fluorescence (proxy of chlorophyll a), and Shannon diversity occurred at relatively short timescales (9 hours to 11 days) for time series of several months, highlighting the value of high frequency monitoring in capturing ecological dynamics under macrotidal conditions in the Eastern English Channel.