Toxic Potential and Long-Term Dynamics of Cyanobacterial Blooms in the Gulf of Gdańsk (Southern Baltic Sea)

Over the past 15 years, Nodularia spumigena blooms in the Gulf of Gdańsk (southern Baltic Sea) have reached exceptional intensity, accounting for up to 90% of phytoplankton biomass during peak summer periods. Our long-term observations revealed oscillations in blooms intensity, with peak nodularin concentrations (up to 45,000 μg/L) recorded in 2012, 2015, and 2018—the highest levels of this toxin documented to date in both the Baltic Sea and worldwide. An extreme cyanobacterial bloom in 2018, caused by unusually high air and water temperatures, covered almost the whole surface of the Gulf of Gdańsk, causing multi-day closures of bathing areas. During this bloom, high levels of microcystins (up to 6640 μg/L MC-LR) were also detected, as well as the presence of 42 cyanopeptides, mainly anabaenopeptides and spumigins, which were present at concentrations 5–10 times higher than hepatotoxins.