Comparison of nitrogen fixation rates determined with different methods: a study in the Baltic Proper

Nitrogen fixation rates were determined at approximately monthly intervals during spring/summer 2001 in the eastern Gotland Sea by means of the N-15(2) 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 N-15(2) 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 434 000 and 792 000 t yr(-1) respectively.