Time series analysis of the toxic dinoflagellate Alexandrium pseudogonyaulax across Northern European waters

The harmful dinoflagellate Alexandrium pseudogonyaulax has been associated with the mortality of marine organisms, including fish. Recent evidence, although based on limited data, suggests an increasing presence across Northern European waters. To confirm this hypothesis, we analysed comprehensive long-term time series data from monitoring stations in Germany, Sweden, Denmark and Norway to assess whether A. pseudogonyaulax has established a sustained presence in the study region and to identify potential environmental drivers of its distribution. The findings show that this species is now a recurrent part of the microalgal community, primarily in the Kattegat, Skagerrak, and southern Baltic Sea. The seasonality of this species was found to be consistent across stations as it primarily occurs from May to October, peaking in July. Logistic regression further revealed increasing trends in occurrence at several sites. No consistent environmental drivers of this expansion were found, although its presence was associated with elevated DIN:PO₄ ratios at some stations, suggesting that other ecological mechanisms, such as top-down processes or species competition, may have facilitated proliferation of A. pseudogonyaulax. The species' salinity tolerance, potential dispersal of resting cysts and climate change-induced warming likely promoted its regional spread, especially into the Baltic Sea. The increasing presence and limited understanding of its toxic effects on marine organisms and humans highlight the need for continued monitoring and further research into its ecological impacts.