Suitability of two areas of the Basque coast to sustain shellfish aquaculture according to both the presence of potentially toxic phytoplankton and the biotoxins regulated by the European Union

Abstract

The composition and dynamics of microalgae play an important role in shellfish aquaculture, since phytoplankton is the main source of energy for filter-feeding bivalves as well as the main potential toxicity risk. Together with the increase in world aquaculture production in the last decades, there is recent interest in the implementation of shellfish aquaculture on the Basque coast (southeastern Bay of Biscay). In this context, the study of the potentially toxic phytoplankton abundance and dynamics has become essential, since the viability of shellfish aquaculture in the area could be compromised by biotoxins. In the present study, two euhaline sites of the Basque shelf, one inshore (Mutriku) and the other offshore (Mendexa), were compared during a one-year period. The main aim was to determine which site was more suitable for the development of shellfish aquaculture, from the perspective of exposure to toxic phytoplankton, by comparing the composition and abundance of the potentially toxic phytoplankton community and the concentrations of toxins in mussel flesh. The mussels that grew offshore presented a higher amount of okadaic acid (OA), in accordance with the fact that this site (Mendexa) also presented a higher cell abundance of Dinophysis acuminata, a potential producer of OA. In addition, although Dinophysis spp., Pseudo-nitzschia spp. and Alexandrium spp. exceeded their cell alert thresholds several times at both studied sites, the dinoflagellates presented a higher frequency of exceedance at Mendexa. Moreover, the percentage of samples with toxin concentrations that exceeded quantification limits was higher at Mendexa as well. Therefore, from the perspective of the currently regulated biotoxins, in the Basque Country, inshore euhaline waters seem to be more suitable for mussel aquaculture than offshore waters.