Summer Ostreopsis blooms in San Sebastian (South-East Bay of Biscay): The importance of substrate features

During summer 2020 and 2021, harmful episodes of Ostreopsis were first reported in the Bay of Biscay, affecting the Spanish Basque coast, specifically the city of San Sebastian. This led to implement samplings during summer 2022 and 2023 within this region; two close sites distinguished, primarily, by their substrate features were selected. The abundances of Ostreopsis spp. were significantly higher at Ondarreta beach compared to La Concha beach, with the former seemingly favoured by its high water temperature, relatively lower hydrodynamics and great macroalgal coverage of its rocky substrate, observing maximum abundances of 1.86 × 10⁶ cells/g and 1.33 × 10⁵ cells/L in the epiphytic and planktonic samples, respectively. Finally, molecular analyses demonstrated the co-occurrence of Ostreopsis sp. 9 (Ostreopsis cf. siamensis) and Ostreopsis cf. ovata at La Concha Bay, confirming for the first time the presence of the latter on the Spanish coast of the Bay of Biscay.