| Summary: | The biomass, abundance of heterocysts and heterocyst frequencies of the N2-fixing cyanobacterium Aphanizomenon flos-aquae and the corresponding environmental parameters were monitored during two separate years near the entrance of the Gulf of Finland. For reference, data on two other N2-fixing cyanobacteria, Anabaena spp. and Nodularia spumigena were collected as well. A. flos-aquae was observed throughout the year, being most abundant in the summer, while Anabaena spp. and N. spumigena were found only during the warmest summer period. A biomass increase of A. flos-aquae was positively associated with temperature and negatively with the DIN:DIP ratio; the highest biomasses were found at temperatures above 10 °C and in molar DIN:DIP ratios less than 10. According to the changes in the heterocyst frequencies, A. flos-aquae had a seasonal N2 fixation strategy in which the maximum frequencies of heterocysts preceded the annual biomass peak.
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