Dynamics of phytoplankton diversity structure and primary productivity in the English Channel

Abstract

The dynamics of the phytoplankton assemblage, the physical, chemical and biologi- cal parameters, and primary productivity and production were monitored in the central English Channel along a transect between Ouistreham and Portsmouth from January to December 2010. The spatial patterns of the phytoplankton assemblage were controlled by the hydrological charac- teristics of the water masses, and the annual structure of the phytoplankton assemblage was char- acteristic of the central English Channel and was controlled by seasonality. The spring bloom was dominated by a single species, Chaetoceros socialis, and associated with low microphytoplankton evenness and Shannon-Wiener indices, whereas the evenness index was high from late spring to winter and associated with the proliferation of pico- and nanophytoplankton cells. We identified 2 species responsible for harmful algal blooms, Phaeocystis globosa, which dominated the commu- nity in the northern part of the Seine Bay in May, and Lepidodinium chlorophorum, which domi- nated the community near the French coast in September. We examined the relationship between microphytoplankton diversity and maximum primary production and productivity. We found a negative parabolic relationship between the diversity indices (evenness and Shannon-Wiener) and maximum primary production, and a positive parabolic relationship between the number of taxa (richness) and maximum primary production. However, we found no relationship between maximum productivity and the evenness or richness indices. High levels of productivity were measured during the increasing abundance of pico and nanophytoplankton cells, highlighting the importance of taking the dominant functional group into account, rather than the degree of diver- sity, when explaining the level of productivity.