Baltic Sea nitrogen fixation estimated from the summer increase in upper mixed layer total nitrogen

We estimated nitrogen fixation from the increase in total nitrogen (N 2 gas excluded) in the upper 20 m during the summer biomass increase of heterocystous filamentous cyanobacteria at the off‐shore Landsort Deep station (BY31, 5 yr) and at 10 more stations in all major basins of the Baltic Sea proper. Estimated fixation rates were 2.3–5.9 mmol N m −2 d −1 , within the range of reported direct measurements. Estimated total fixation in the Baltic Sea proper, 180–430 Gg N yr 21 taking nitrogen settling loss and atmospheric deposition into account, was sufficient to sustain 30–90% of the June‐August pelagic net community production. Filamentous cyanobacteria (mostly Aphanizomenon sp.) had low C: N and C: P ratios in spring 1998, indicating internal storage of both N and P. From early June, when their biomass growth started, ratios rose gradually to the biomass peak in August and early September, when the C: N ratio (6.5 mol/mol) was close to the Redfield ratio, but the C: P ratio reached 420, almost four times Redfield. The C: N ratio of the peak biomass was 1.5 times that in spring, and the C: P ratio was 13 times higher. The high C: P ratio indicates a smaller P demand by filamentous diazotrophs than expected from Redfield ratios. Only a few percent of the P mineralized daily is needed for filamentous cyanobacterial growth in summer. Filamentous cyanobacteria incorporated 16–41 mmol N m −2 into biomass (C : N 5 6.2) at BY31 in summer 1998. This was less than the estimated nitrogen fixation, suggesting fixed N leaks from growing diazotrophs.