Local and regional processes driving Dinophysis acuminata abundance and okadaic acid detection in offshore farmed mussels in the southeastern Bay of Biscay

Abstract

The marine biotoxins of the okadaic acid (OA) group (i.e. dinophysistoxins (DTXs), OA and their derivatives) are a major concern for bivalve aquaculture worldwide, causing frequent harvest closures and sales bans due to the risk of diarrheic shellfish poisoning (DSP). This study aims to better understand the ecological dynamics of microalgae associated with DSP toxins in the southeastern Bay of Biscay, which is crucial for the management of shellfish production in Basque coastal waters (Northern Spain); its ultimate goal is to assist in the development of an early warning system for this area. Our results show that during 2017-2022 the concentration of DTXs in mussels growing in long-lines off the Basque coast was negligible, but OA was over the European Union regulatory limit every year between March and June. These episodes of contamination with OA were related to Dinophysis acuminata and explained by a succession of processes involving nutrification followed by increased irradiance and water column stabilization, which could occur locally or in water masses transported to the production area. Therefore, local scale monitoring alone seems insufficient to predict the OA accumulation in mussels with the necessary accuracy and promptness in this highly dynamic offshore area. In this regard, transport models that assimilate regional wind and river runoff data, in combination with remote sensing and near-real time in situ monitoring, are recommended to forecast the arrival of harmful blooms originated eastward on the French shelf.