INFLUENCE OF WATER-COLUMN STABILITY ON PHYTOPLANKTON SIZE AND BIOMASS SUCCESSION PATTERNS IN THE CENTRAL CANTABRIAN SEA (BAY OF BISCAY)

The seasonal cycle of phytoplankton biomass in the Cantabrian Sea (Bay of Biscay) is characterized by peaks produced during the spring blooms, as in most temperate seas. Although stratification and light intensity variabilities were high throughout the year, only a small range of low water-column stability and light intensity was associated with the biomass maximum. Out of this range, accumulations of biomass would be limited primarily by high turbulence and low light intensities in winter, and low nutrient concentrations, sedimentation and grazing in summer. Associated with these periods six seasonal stages have been identified using particle size distributions (PSD). These stages corresponded to three different spring types, and summer, fall and winter types. Most of the variability of the PSD was associated with the maximum of biomass during the spring blooms. During these blooms the distributions were dominated either by small, large or by several sizes of particles. In summer the PSD were polymodal, while in winter the biomass was uniformly distributed within the considered particle size range. The transition between PSD types was paralleled by similar changes in phytoplankton species composition. The seasonal cycle of phytoplankton species abundance was characterized by the alternance of diatoms in water-mixing periods and dinoflagellates in stratification periods. However, microflagellates were abundant in diverse conditions throughout the year. As a result of microflagellate abundance and the formation of cell chains and aggregates, particle size composition during the bloom period resulted largely unpredictable using water-column stability data.