Baltic Sea nitrogen fixation estimated from the summer increase in upper mixed layer total nitrogen
Ulf Larsson, Susanna Hajdu, Jakob Walve, and Ragnar Elmgren
Limnol. Oceanogr., 46(4), 2001, 811–820

We estimated nitrogen fixation from the increase in total nitrogen (N₂ 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.