Denitrification in sediments is of major importance to the Baltic Sea nitrogen (N) budget. However, little is known about denitrification in the water column in the Baltic. We tested whether denitrification could be active in pelagic cyanobacterial aggregates, commonly found in the Baltic Sea during warm summer months. In these aggregates, anoxic microniches may form. Such microzones suggest a possibility for anaerobic processes, such as denitrification by the heterotrophic bacteria associated

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... ith the cyanobacteria. Denitrification and nitrogen (N 2 ) fixation in a cyanobacterial bloom were measured on a 3 wk cruise in the Baltic Sea in order to determine whether the Baltic Sea cyanobacterial blooms act as sources or sinks of N. Experimental conditions analogous to formation of anoxic microniches within cyanobacterial aggregates did not activate the denitrification process, even when anoxic conditions prevailed for several hours. Only in 3 cases was denitrification, measured using the 15 N-isotope pairing method, detectable, giving rates of 0.8 to 1.8 nmol N 2 l -1 h -1 . Nitrogen fixation, determined using the acetylene reduction assay, varied from 0.03 to 1.85 µmol N 2 l -1 h -1 . According to this study, the blooms of N 2 -fixing cyanobacteria in the Baltic Sea must be seen solely as sources, not sinks for N in the marine pelagic environment.