First report of Ostreopsis cf. siamensis exceeding alert thresholds in Bay of Plentzia (South-East Bay of Biscay)

First reported in the Basque Country (Spain) in 2007, Ostreopsis spp. have shown increased density and impacts on the tourist industry in recent years, including the first recorded outbreak of respiratory irritation in 2021. A monitoring for benthic HABs was subsequently implemented along the Basque Country coastline. During the late summer 2024, Ostreopsis cf. siamensis blooms exceeded the alert threshold at Plentzia Beach — one of the most popular summer tourist destinations in the south-eastern Bay of Biscay — coinciding with unusually high sea temperatures for the season (max. 23.6°C in late August).