MEPS 297:23-31 (2005)  -  doi:10.3354/meps297023

ABSTRACT: Nitrogen fixation rates were determined at approximately monthly intervals during spring/summer 2001 in the eastern Gotland Sea by means of the ¹⁵N₂ method. Additionally, a comprehensive set of variables were measured in order to apply 3 different budget approaches for the determination of integrated nitrogen fixation: (1) the nutrient concept based on excess PO₄ owing to the low DIN/PO₄ ratios (6.8) in the winter surface water, (2) the CO₂ concept using a combined carbon/nitrogen mass balance and (3) the biomass concept based on the increase in total nitrogen during the cyanobacteria bloom. For the spring season (March to May) the budget approaches gave rise to postulating an additional nitrogen input of about 100 to 200 mmol m^–2. However, this was not attributed to nitrogen fixation because ¹⁵N₂ rate measurements from several years did not show any significant nitrogen fixation during spring. Rate measurements integrated from May to August yielded a nitrogen fixation of 138 mmol m^–2, which was consistent with the estimate based on the biomass concept. The CO₂ concept resulted in a nitrogen input of 268 mmol m^–2 from May to August. The nutrient concept was questionable because the excess PO₄ was consumed during spring and not transferred to the summer bloom. High PON/POP ratios in summer indicated that PO₄ release from particulate organic matter provided some of the PO₄ necessary for the primary production based on nitrogen fixation. The estimates obtained from the rate measurements and from the CO₂ concept are considered to represent the lower and upper limit for the nitrogen fixation in the Baltic Proper and correspond to a total nitrogen input of 434000 and 792000 t yr^–1 respectively.

KEY WORDS: Nitrogen fixation · Cyanobacteria bloom · Nutrient · Redfield ratio · Baltic Sea