Nitrogen fixation by diazotrophic cyanobacteria in the Baltic Sea and transfer of the newly fixed nitrogen to picoplankton organisms

Nitrogen fixation (15N2-tracer method) by filamentous cyanobacteria and the flux of the newly fixed nitrogen into picoplanktonic organisms were investigated during three cruises in the central Baltic Sea in July and August 1995 and July 1996. The diazotrophic cyanobacteria consisted mainly of Aphanizomenon and Nodularia and the picoplankton fraction mainly of Synechococcus, a coccoid nondiazotrophic cyanobacterium. The average N2-fixation rate measured during the two July studies (3.9 nmol l−1 h−1 in 1995 and 2.4 nmol l−1 h−1 in 1996), was about 13 times higher than in August 1995 (0.3 nmol N2 l−1 h−1). During the July studies, 5–10% of the total N2-fixation was found in the picoplankton fraction, but none was found in August. The study shows that in the early stage of a cyanobacterial bloom new nitrogen is provided for the pelagic foodweb through release of recently fixed nitrogen by the diazotrophs, whereas in the late stage the input of new nitrogen occurs mainly through lysing of decaying filamentous cyanobacteria cells. Considerable temporal and local variations in nitrogen fixation found in this study indicate that broader studies in time and location are needed to assess the contribution of N2-fixation to the nitrogen budget of the Baltic Sea.