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Nitrogen fixation rates and related plankton parameters were determined at 8 sta- tions in the Baltic proper and Mecklenburg Bay in 1997 and 1998. Nitrogen fixation was mea- sured with 15 N-tracer method in unenriched samples. Measurable nitrogen fixation rates were found from July to October, with highest rates in August when heterocystous cyanobacteria formed blooms. Nitrogen fixation rates measured during a moderate bloom in 1998 were related to biomass of heterocystous and coccoid (non-heterocystous) cyanobacteria and primary produc- tion. The size fraction <10 µm contributed significantly to total nitrogen fixation both during day and night. It is discussed whether this may be due to small, non-heterocystous cyanobacteria, which were abundant in summer and autumn. They may separate photosynthesis and nitrogen fixation temporally (day and night) and may be especially responsible for the high nitrogen fix- ation rates observed in the dark. As the fraction of pico- and nanoplankton was not considered in earlier studies, a new budget of nitrogen fixation in the Baltic proper has been estimated. On average, daily nitrogen fixation rates of 2.5 mmol N m -2 d -1 (in July/August 1997/1998) and mean annual nitrogen fixation of 125 mmol N m -2 yr -1 were estimated for the Baltic proper. The high variability is discussed. For summer 1998, a budget of the nitrogen cycle at the main station in the Gotland Sea is given.

Evidence of nitrogen fixation by non-heterocystous cyanobacteria in the Baltic Sea and re-calculation of a budget of nitrogen fixation