Evaluation of phytoplankton quality and toxicity risk based on a long-term time series previous to the implementation of a bivalve farm (Basque coast as a case study)

In the last decades there has been a great development in aquaculture worldwide and, on the Basque coast (southeastern Bay of Biscay) in particular, there is a recent interest in implementing bivalve aquaculture in open marine waters. In this context, the study of phytoplankton is essential given that it is the main source of energy for bivalves and, at the same time, a main potential toxicity risk. Bivalves, as filter-feeding organisms, can accumulate phycotoxins and these can be transferred through the food-chain, posing a threat to humans. All this, together with a recently installed pilot-scale bivalve farming, motivated a study of the phytoplankton community. Here, 11-year phytoplankton time series from 16 nearshore and 3 offshore stations off the Basque Country are analyzed, as a preliminary step for evaluating the potential of this region for aquaculture development. Special attention was given to bloom events and potentially toxic taxa. A total of 32 bloom-forming taxa were detected, mostly diatoms. In regard to harmful species, all stations presented many potentially toxic taxa, mostly dinoflagellates. The diatom genus Pseudo-nitzschia was the one blooming in more stations. Pseudo-nitzschia spp. as well as the dinoflagellates Dinophysis spp. and Alexandrium spp., which might be causative of Amnesic, Diarrheic and Paralytic Shellfish Poisoning, respectively, exceeded the abundance limits that would imply toxicity risk in several occasions, mostly during spring and summer. However, it occurred at a low frequency (in average, <15% for Pseudo-nitzschia spp. and <10% for the dinoflagellates). Overall, phytoplankton community composition and abundance, together with the low frequencies for the exceeded alert limits by the three main phycotoxin producing genera, suggest that the area presents appropriate conditions for bivalve aquaculture.