Denitrification in estuarine sediments in the eastern Gulf of Finland, Baltic Sea

The Gulf of Finland receives annually ca. 140 000 t of nitrogen of which 70% enters to the easternmost part. Mass balance calculations indicate that on average 5 g N m(-2) a(-1) is denitrified in this area. In August 1995, accumulation areas were studied at eight stations along the eutrophication gradient from the inner Neva estuary to the open sea area. Rates of direct denitrification were measured using N-15 isotope pairing technique in sediment. Denitrification rates ranged from 0.4 to 50 mu mol N m(-2) h(-1). The lowest rates were measured in the inner Neva estuary (< 1 mu mol N m(-2) h(-1)). In the outer Neva estuary, in more organic sediments, denitrification rates varied from 2 to 10 mu mol N m(-2) h(-1). Highest denitrification rates (30-50 mu mol N m(-2) h(-1)) were measured in the outer estuary and in the open Gulf where bioturbating macrofauna were present in high numbers (3500-4700 ind m(-2)) and dominated by Monoporeia affinis and Pontoporeia femorata. Also, nitrification activity was highest in these areas. The water above the sediment was well oxygenated (7.2-9.2 mg l(-1)) in all stations. In some areas dissimilatory NO3- reduction to NH4+ seemed to be the main pathway in sediment nitrogen dynamics. These results are in agreement with existing mass balance calculations. As strong seasonality in denitrification rates is evident, the calculation of annual denitrification level would require sampling frequencies, up to at least 10 per year. The stimulatory effect of bioturbating infauna is of major importance in the annual denitrification budget.