Intact sediment cores from rivers of the Bothnian Bay (Baltic Sea) were studied for denitrification based on benthic fluxes of molecular nitrogen (N 2 ) and nitrous oxide (N 2 O) in a temperature controlled continuous water flow laboratory microcosm under 10, 30, 100, and 300 lM of 15 N enriched nitrate (NO 3 -, *98 at. %). Effluxes of both N 2 and N 2 O from sediment to the overlying water increased with increasing NO 3 -load. Although the ratio of N 2 O to N 2 increased with increasing NO 3 -load, it remained below 0.04, N 2 always being the main product. At the NO 3 -concentrations most frequently found in the studied river water (10-100 lM), up to 8% of the NO 3 -was removed in denitrification, whereas with the highest concentration (300 lM), the removal by denitrification was less than 2%. However, overall up to 42% of the NO 3 -was removed by mechanisms other than denitrification. As the microbial activity was simultaneously enhanced by the NO 3 -load, shown as increased oxygen consumption and dissolved inorganic carbom efflux, it is likely that a majority of the NO 3 -was assimilated by microbes during their growth. The 15 N content in ammonium (NH 4 ? ) in the efflux was low, suggesting that reduction of NO 3 -to NH 4 ? was not the reason for the NO 3 -removal. This study provides the first published information on denitrification and N 2 O fluxes and their regulation by NO 3 -load in eutrophic high latitude rivers.

Denitrification rates and nitrous oxide (N 2 O) effluxes were measured at different temperatures and for different oxygen concentrations in the sediments of a eutrophied river entering the Bothnian Bay. The experiments were made in a laboratory microcosm with intact sediment samples. 15 N-labelling was used to measure denitrification rates (Dw). The rates were measured at four temperatures (5, 10, 15 and 20°C) and with three oxygen inputs (\0.2, 5, and 10 mg O 2 l -1 ). The temperature response was highly affected by oxygen concentration. At higher O 2 concentrations (5 and 10 mg O 2 l -1 ) a saturation over 10°C was observed, whereas the anoxic treatment (\0.2 mg O 2 l -1 ) showed an exponential increase in the temperature interval with a Q 10 value of 3.1. The result is described with a combined statistical model. In contrast with overall denitrification, the N 2 O effluxes from sediments decreased with increasing temperature. The N 2 O effluxes had a lower response to oxygen than denitrification rates. The N 2 O/N 2 ratio was always below 0.02. Increased temperatures in the future could enhance denitrification rates in boreal river sediments but would not increase the amount of N 2 O produced.