A decade-long study on harmful dinoflagellate blooms and biotoxin contamination in mussels from the north-western Adriatic Sea (Mediterranean Sea).

The north-western Adriatic Sea is one of the most eutrophic areas of the Mediterranean Sea and an important aquaculture region, accounting for about 90% of Italian shellfish production. Since the 1970s, this area has experienced frequent harmful algal blooms, posing risks to consumers and coastal ecosystems. Despite the regular monitoring required by European legislation, long-term studies integrating taxonomic analysis of phytoplankton species, associated biotoxins, and environmental data remain limited. This study examines an 11-year dataset (2012-2022) from the Emilia-Romagna region coast, focusing on timing and occurrence trends of toxic dinoflagellates related to the shellfish toxicity. Phytoplankton species distribution and biotoxins content in farmed mussels were analysed, with particular attention to the presence of analogues. From 2012 to 2015, the most prevalent toxins were yessotoxins (YTXs, up to 8.44 mg kg), primarily homo-YTX and 45OH homo-YTX, which were associated with autumn blooms of Gonyaulax spinifera and Lingulaulax polyedra. From 2015, okadaic acids and derivatives (OAs, up to 941 µg kg) became the most prevalent phycotoxin group, associated to Dinophysis fortii and Dinophysis acuminata presence during autumn. This temporal shift was likely driven by increasing surface water temperature and salinity, which contributed to reduced water column mixing favouring oligotrophic species. Notably, low levels of YTXs persisted in mussels despite decreased abundances of producing species. Additionally, Alexandrium spp. cells were consistently detected, even in the absence of saxitoxins (STXs) in bivalves. These findings highlighted the need for integrated studies on strain-specific toxicity, phycotoxins accumulation in bivalves, and field monitoring.