Temperature and N:P ratio as factors causing blooms of blue-green algae in the Gulf of Gdańsk

The bloom dynamics of two planktonic, nitrogen-fixing species of blue-green algae Aphanizomenon flos-aquae (L.) Ralfs and Nodularia spumigenaMertens in the Gulf of Gdańsk were studied. The Gulf of Gdańsk is a shallow part of the Baltic Sea where the water has a mean salinity of ca 8 PSU. Increased riverine nutrient input and/or changes in the N:P ratio are indicated as factors causing blue-green algae blooms. A low N:P ratio appears to trigger blooms. The mean annual value of N:P in the Gulf of Gdańsk since 1981 is 6.5 with a minimum of ca 3. There could be a link between temperature and the year-to-year differences in bloom intensities. A huge bloom of toxic N. spumigena was reported in July 1994 when the water temperature rose to 22◦C.