Long-term phytoplankton community dynamics in the Western English Channel

Over a 15-year period (1992-2007), weekly water samples were collected from the L4 time-series station in the Western English Channel and analysed for phytoplankton community structure and abundance. The data produced have been analysed to identify seasonal patterns, inter-annual variability and long-term trends in the composition of the seven main functional phytoplankton groups. Phyto-flagellates numerically dominated accounting for on average ca. 87% of the phytoplankton abundance while diatoms, Phaeocystis, coccolithophorids, dinoflagellates and ciliates contributed 13% of abundance. Distinct seasonal and inter-annual changes in the abundance and floristic composition of the functional groups were observed. Significant long-term changes in abundance showed that, over the study period, diatoms and Phaeocystis decreased while coccolithophorids, the dinoflagellate Prorocentrum minimum and some heterotrophic dinoflagellate and ciliates increased in abundance. These changes highlight the importance of long-term observations for the understanding of natural temporal variability in plankton communities. Such shifts in the community composition at L4 could have important consequences for ecosystem function.