Diversity structure of phytoplankton communities and primary productivity in a temperate epicontinental sea

The dynamics of the phytoplankton assemblage, the physical, chemical and biological
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 characteristics
of the water masses, and the annual structure of the phytoplankton assemblage was characteristic
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 community
in the northern part of the Seine Bay in May, and Lepidodinium chlorophorum, which dominated
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 diversity,
when explaining the level of productivity.