Spatio-temporal patterns in phytoplankton assemblages in inshore–offshore gradients using flow cytometry: A case study in the eastern English Channel

•Phytoplankton was addressed by flow cytometry along an inshore–offshore transect.•Thirteen periods were considered during the winter–spring–summer transition in 2012.•Picoeukaryotes, Synechococcus spp. and Phaeocystis globosa were the most abundant.•Some phytoplankton groups showed inshore–offshore abundance gradients.•Seasonal variability accounted for most of the changes in phytoplankton abundance.

A pulse-shape recording flow cytometer (CytoSense©) was applied to the monitoring of changes in phytoplankton distribution along an inshore–offshore transect across the eastern English Channel (EEC), on 13 occasions during the main productive period of the year. Amongst the eight phytoplankton groups discriminated, picophytoplankton (picoeukaryotes and Synechococcus spp.) and Phaeocystis globosa nanoflagellates were the main contributors to total phytoplankton abundance, while Diatoms-like, Coccolithophores, and Cryptophytes represented each one less than 5%. High spatial resolution revealed important changes on relatively short distances. Moreover, a general decrease of Diatoms-like, P. globosa haploid cells, Coccolithophores, and picoeukaryote abundance was evidenced from inshore to offshore waters, associated with an increase of Synechococcus spp. abundance. Seasonal variability accounted for 71% of phytoplankton abundance changes. Compared to previous studies in the area the CytoSense allowed highlighting new players during the winter–spring–summer phytoplankton succession: (i) high abundance of Synechococcus spp. and picoeukaryotes I in winter and of Synechococcus spp. also in the summer, (ii) a transient abundance peak of picoeukaryotes II, and (iii) high abundance of Coccolithophores and Cryptophytes during the wax of P. globosa bloom and in the summer. The relationships between environmental variables and phytoplankton assemblages indicated that nutrients and the daily light intensity were the most important parameters in structuring the winter–spring–summer transitions.