In mainland Portugal, the occurrence of diarrhetic shellfish poisoning toxins (DSTs) from Dinophysis in spring and early summer has been positively related to precipitation. Continental runoff supplies regularly nutrients to the marine food chain. A northwest to the south coast decreasing gradient in DSTs is commonly found, similar to the latitudinal decrease in precipitation and of macronutrients (N, P and Si). A decrease in average precipitation after 2003 led to a reduction in the interannual average concentration of DSTs after 2008 on the NW coast and the S coast. However, the accumulation of DSTs on the NW coast increased in the period 2019-2022. In the last two decades, extensive forest wildfires were recurrent, but the area burned reduced sharply after 2017 following tighter fire control measures. Low levels of DSTs or a low percentage of weeks with samples above the regulatory limit from the NW coast were related to high burned areas in the previous year. The recent increase in 2019-2022 of DSTs, a planktonic biomarker in bivalves, was also coincident with the similar temporal increase in either the recruitment or the biomass of Sardina pilchardus, a small pelagic planktivorous fish in decline after 2005-2006. Low sardine recruitment or low sardine biomass were coincident with low precipitation or high burned areas. Wildfires degrade coastal water quality, as seems reflected both in the bivalve DSTs and the sardine stock time series.
Paulo Vale (Fri,) studied this question.