Annual cycle of phytoplankton community through the water column: Study applied to the implementation of bivalve offshore aquaculture in the southeastern Bay of Biscay

This study describes, for the first time, the annual variability of phytoplankton community in different layers of the water column in open waters off the Basque coast (south-eastern Bay of Biscay). Phytoplankton composition, abundance and biomass, together with size-fractionated chlorophyll a, nutrients, and optical and hydrographic conditions, were measured in an experimental bivalve culture area from May 2014 to June 2015. Water column conditions showed the typical dynamics previously described for temperate areas, characterised by winter homogeneity and summer stratification. Phytoplankton temporal variability was studied at depths of 3, 17 and 33 m, and was found to be related to those processes. In particular, temperature and nutrients (mostly nitrate and silicate) were the environmental variables which significantly explained most of the variability of chlorophyll concentration, whereas river flow was the main driver of abundance variability. Total chlorophyll was generally low (0.6 µg L−1 on average). Of the 194 registered taxa, 47.4% belonged to dinoflagellates and 35.1% to diatoms. In addition, diatoms showed the highest biomass values, and haptophytes represented the greatest contribution to cell-abundance. This fact, despite the low chlorophyll values indicating low phytoplankton biomass, could favour mussel growth given the high fatty acid content reported for diatoms and haptophytes.