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

Abstract

Nitrogen fixation rates and related plankton parameters were determined at 8 stations in the Baltic proper and Mecklenburg Bay in 1997 and 1998. Nitrogen fixation was measured with 15N-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 production. 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 fixation 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.