Impact of Drought and Wildfires in Recent Trends of Diarrhetic Shellfish Toxins in Cockles from Northwest Portugal and Its Similarities with Sardine Stock Trends in the Period 2001–2022

Abstract

In Portugal, high levels of diarrhetic shellfish toxins (DSTs) originating from Dinophysis spp. are accumulated in bivalves, mainly on the northwest coast (NWC). The occurrence of DSTs in spring and early summer is positively related to precipitation. A decrease in average precipitation after 2003 led to a reduction in DSTs after 2008. However, the accumulation of DSTs in the NWC increased in the period 2020–2022. The hydrological year of 2022 was marked by extreme drought. In cockles from the endemic area of Ria de Aveiro, 37.3% of the weeks tested presented samples above the regulatory limit (RL). The previous record was 31.9% in 2005, also an extremely dry year. The average percentage of weeks above the RL in severe drought years surpassed both the low and high precipitation years. In severe dry years, toxicity was dominated mainly by D. acuta toxins. While D. acuminata grows after the abundant river discharges in spring, D. acuta is capable of growing in summer during upwelling favourable conditions. In the last two decades, extensive forest wildfires were recurrent, but the area burnt reduced sharply after 2017 following tighter fire control measures. Low levels of DSTs or low percentage of weeks with cockles above the RL were related to high burnt areas in the previous year. The recent increase in 2020–2022 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 planktivorous fish in decline after 2005/2006. Both low sardine recruitment and biomass were coincident with low precipitation or high burnt areas. Wildfires degrade coastal water quality, as seems reflected both in the cockle’s DST and the sardine stock time series.