Cyanobacterial blooms in the Gulf of Gdańsk (southern Baltic): the main effect of eutrophication

The most striking aspect of eutrophication in the Baltic are the summer blooms of cyanobacteria. Some of the blooms are toxic to marine organisms and poisonous to people. Our studies, conducted over the last three decades, report on mass occurrences of Aphanizomenon flos-aquae (L.) Ralphs and Nodularia spumigena Mertens. It is generally assumed that cyanobacterial blooms in the Baltic Sea are stimulated by the low ratio of N:P and initiated by high water temperatures. The mean annual value of this ratio in the Gulf of Gdańsk, since 1981, is 7:1-8:1, with the lowest values, of about 4, being seen in July when the cyanobacterial blooms begin. During three years of observations (1992-1994) the smallest number of recorded taxa was reported in 1993, concurrent with the lowest water temperature (16°C in summer). In August 1994, when the temperature increased to 22°C, a huge bloom was seen. Nodularia spumigena was very abundant in that bloom, and nodularin concentration of 2.59 mg toxin per g dry weight of bloom sample was recorded. A similar situation has also been observed in the years 2003-2006.