MEPS 242:73-82 (2002)  -  doi:10.3354/meps242073

No sign of denitrification in a Baltic Sea cyanobacterial bloom

Susanna Hietanen1,*, Pia H. Moisander1,2, Jorma Kuparinen1,**, Liisa Tuominen1

1Finnish Institute of Marine Research, PO Box 33, 00931 Helsinki, Finland
2Institute of Marine Sciences, University of North Carolina at Chapel Hill, 3431 Arendell Street, Morehead City, North Carolina 28557, USA
*E-mail: **Present address: Department of Ecology and Systematics, Division of Hydrobiology, University of Helsinki, PO Box 65, 00014 Helsingin Yliopisto, Finland

ABSTRACT: 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 with the cyanobacteria. Denitrification and nitrogen (N2) 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 15N-isotope pairing method, detectable, giving rates of 0.8 to 1.8 nmolN2 l-1 h-1. Nitrogen fixation, determined using the acetylene reduction assay, varied from 0.03 to 1.85 µmolN2 l-1 h-1. According to this study, the blooms of N2-fixing cyanobacteria in the Baltic Sea must be seen solely as sources, not sinks for N in the marine pelagic environment.

KEY WORDS: Denitrification · Cyanobacteria · Nitrogen fixation · Baltic Sea

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