Toxic Nodularia spumigena blooms in the coastal waters of the Gulf of Gdańsk: a ten-year survey

In the Baltic Sea, summer blooms of the filamentous, nitrogen-fixing cyanobacterium Nodularia spumigena are favoured by high P concentrations at low N:P ratios and a salinity range of 5–13 PSU. The blooms are initiated by calm and sunny weather, an elevated surface water temperature and thermal stratification. The mass occurrence of N. spumigena in coastal waters is a matter of special concern, as the cyanobacterium produces nodularin, a potent pentapeptide hepatotoxin. In the Gulf of Gdańsk, the large-scale occurrence of N. spumigena was recorded for the first time in 1994. Blooms of a similar intensity occurred in 2001, 2003 and 2004. Nodularin concentrations in freeze-dried bloom samples varied from 0.01 to 4.01 mg g−1 d.w. In the coastal waters of the Gulf of Gdańsk, cell-bound nodularin concentrations in 2004 and 2005 attained maxima of 25 852±107 μg dm−3 and 3964±125 μg dm−3, respectively. Microscopic analysis revealed the presence of diverse Nodularia forms, with the dominance of curved filaments in bloom samples. The results of in situ studies and remote sensing measurements indicate a high frequency and intensity of cyanobacterial blooms in the Gulf of Gdańsk in the last ten years.