Mussel farming is a key economic activity in the northwestern Adriatic Sea yet challenged from harmful algal blooms (HABs) caused by toxin-producing Dinophysis species. These mixotrophic dinoflagellates produce lipophilic toxins that accumulate in shellfish, leading to diarrhetic shellfish poisoning (DSP) and posing risks to human health. In this study, we analysed 26 years (1998–2023) of environmental and biological data from the Emilia-Romagna and Marche coastal regions to investigate long-term trends in Dinophysis and toxicity dynamics, and their relationships with environmental variables amid ongoing climate and ecosystem changes.

Clear seasonal patterns were identified in Dinophysis species composition: D. acuminata and D. sacculus were predominant in spring and summer, followed by D. caudata, while D. fortii and D. tripos were more prevalent in autumn and winter. DSP events primarily occurred during colder months and were correlated with the presence of D. caudata (rs = 0.84), D. fortii (rs = 0.83), and D. tripos (rs = 0.66). Each species displayed distinct thermal preferences, highlighting temperature as a key ecological driver influencing bloom dynamics and toxicity patterns.

We observed a significant increase in Dinophysis absence (+5.35% per year) and a concurrent decline in shellfish toxicity (-3.31% per year), alongside reductions in dissolved inorganic nitrogen (DIN), phosphate, total phosphorus, and chlorophyll-a concentrations, suggesting a shift in nutrient regimes and coastal productivity. Nonetheless, anomalous years such as 2015 and 2022, marked by prolonged toxicity under cooler conditions, illustrated how short-term climate variability can override longer-term trends.

Our findings underlined the complex and evolving interplay between climate forcing, nutrient dynamics and HABs occurrence. As climate change continues to alter coastal environments, sustained monitoring and the development of robust ecological forecasting tools will be essential to track ecosystem responses over time, to support adaptive management strategies, and safeguard the sustainability of mussel aquaculture in the Adriatic Sea.