Seasonal variations of phytoplankton community in relation to environmental factors in an oligotrophic area of the European Atlantic coast (southeastern Bay of Biscay)

In the present study, seasonal variability of physico-chemical variables and phytoplankton community as well as their relationships were studied for oligotrophic coastal waters of southeastern Bay of Biscay. During a 4-year period (2012-2015), a total of 265 phytoplankton taxa were identified, mainly represented by dinoflagellates and diatoms. The highest cell abundances were usually found in spring, mainly attributed to diatoms. Similarly, the biggest contribution to total biomass was given by diatoms: highest values (geometric mean) were found in winter and spring. Although phytoplankton abundance was mostly composed of small cells (2-20μm), biomass was similarly represented in the 2-20μm and >20μm size ranges. Between 21 and 29% of total species variability was significantly explained by different physico-chemical variables. However, this percentage was notably lower at the level of major taxonomic groups. In general, nutrients (mainly ammonium and phosphate) and temperature explained the highest percentage of species variability, whilst salinity played an important role in the summer months. Among the potentially toxic taxa, Dinophysis and Phalacroma species in summer and autumn appeared associated with relatively high ammonium concentrations.