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Baltic Sea nitrogen fixation estimated from the summer increase in upper mixed layer total nitrogen
Author(s) -
Larsson Ulf,
Hajdu Susanna,
Walve Jakob,
Elmgren Ragnar
Publication year - 2001
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.2001.46.4.0811
Subject(s) - redfield ratio , nitrogen fixation , nitrogen , diazotroph , cyanobacteria , biomass (ecology) , zoology , biology , environmental science , phytoplankton , nutrient , chemistry , ecology , bacteria , genetics , organic chemistry
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.