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Denitnfication in the sediments of the open northern Baltic Sea was measured using the isotope palling method The highest denitnfication actlvity was found In the central Gulf of Flnland, where denitriflcation vaned between 150 and 650 pm01 N m d ' The bulk of the denitnfication was coupled to the No3-production by nitrification The mass of benthic fauna was found to b e the most significant factor affecting the rate of denitnfication. Denitrification rate was highest in late summer and early autumn. Calculated as an average ior the Gulf of Finland, the denitrification rate obtained in the present study is 45 kt N yr-l. This gives a release of ca 30% of the external N input by denitrification.

Dilute formaldehyde was the most suitable treatment to inhibit sediment bacteria, since bacterial activity remained low during long-term incubations and the chemical changes in the sediment were minimal. The inhibiting effects of HgCl2, autoclaving, and gamma radiation were diminished during longer incubations; these treatments also caused increases in dissolved nutrients.

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