Seasonal and multi-annual trends of bivalve toxicity by PSTs in Portuguese marine waters

Temporal and spatial trends of paralytic shellfish toxins (PSTs) in bivalves from Portuguese estuarine and coastal waters, and connectivity of bivalve toxicity among the harvest areas, were examined using long-term data from the national biotoxin monitoring programme. Data from 1994 to 2017 were chosen for commercial bivalve species sensitive to PSTs, and for production areas exhibiting recurrent episodes of bivalve toxicity. Mussels (Mytilus spp.) and cockles (Cerastoderma edule) from the Ria de Aveiro, Mondego estuary, Óbidos lagoon and Ria Formosa, and wedge clams (Donax trunculus) and surf clams (Spisula solida) from the coastal areas Aguda and Olhão were selected. Bivalve toxicity data point to higher incidents of PST episodes in autumn and winter, although in 2008 the toxicity of mussels and cockles in the three estuarine areas was registered in summer. Most likely, favourable oceanographic conditions triggered the bloom formation of Gymnodinium catenatum, which is the species responsible for paralytic shellfish poisoning in Portuguese waters. Episodes in the southern coast of Portugal were less recurrent, although values above the PST regulatory limit displayed also a seasonal signal with a peak between autumn and early winter. On the basis of the number of weeks per month that bivalves showed elevated toxicity values, a connectivity index was defined for the surveyed areas. High connectivity was obtained among Aveiro, Mondego and Óbidos, which are 180 km apart, suggesting that G. catenatum cells are imported from blooms formed or reaching the coastal waters adjacent to these systems. During episodes of elevated toxicity, toxin profiles in contaminated mussels and cockles were dominated by N-sulfocarbamoyl compounds, which are the major toxins produced by the toxic dinoflagellate G. catenatum. The identification of coupled systems relatively to bivalve toxicity has an impact on monitoring programmes and allows improved decision-making on closures of bivalve harvest areas affected by toxic algae.