Denitrification in the water column of an intermittently anoxic fjord

Denitrification was studied in the water column in the Bunnefjord, inner part of the Oslofjord in southern Norway, using a N-15-technique (the isotope pairing method). The fjord is 150 m deep and during our surveys in September-December 1998 hydrogen sulphide was present in the deep water below 80 m. No significant denitrification was found in water samples from the surface layer (4 m depth), but high rates were observed within a deep density gradient between 62 and 78 m depth. Oxygen concentration within this layer was low (<21 mmol m(-3)), and the concentration of NO3 decreased from ca. 15 mmol.m(-3) at 62 m depth to not detectable below 78 m. Pronounced peaks of NO2 up to 4.4 mmol m(-3) were observed at 70-78 m depth. The maximum denitrification rate of 1.5 mmol N m(-3) d(-1) was observed at 70 m depth. Integrated for the whole layer, the denitrification rate was 13 mmol N m(-2) d(-1). A significant linear correlation was found between the denitrification rate and the ambient nitrate concentration which indicated that the rate was primarily controlled by the availability of nitrate in the O-2-poor water. Compared to rates reported for coastal water, denitrification in the water column in the Bunnefjord was high and the process appears to be a major sink of bioavailable nitrogen in the fjord.