ORIGINAL PAPER Denitrification and N 2 O effluxes in the Bothnian Bay (northern Baltic Sea) river sediments as affected by temperature under different oxygen concentrations H. Silvennoinen Æ A. Liikanen Æ J. Torssonen Æ C. F. Stange Æ P. J. Martikainen Received: 3 September 2007 / Accepted: 3 March 2008 / Published online: 27 March 2008 Ó Springer Science+Business Media B.V. 2008 Abstract Denitrification rates and nitrous oxide (N 2 O) effluxes were measured at different tempera- tures 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-label- ling 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 treat- ment ( \ 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 denitrifica- tion, 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 denitrifica- tion rates in boreal river sediments but would not increase the amount of N 2 O produced. Keywords Baltic Sea  Denitrification and nitrous oxide effluxes  15 N  Oxygen concentration  River sediments  Temperature Introduction Baltic Sea is suffering from severe eutrophication, mainly caused by human activity, for example agriculture, forestry, and peat mining in the sur- rounding catchments (HELCOM 2003). Lakes, peatlands, rivers, and estuaries within the catchments provide natural filters for the nitrogen load heading to the Baltic Sea (Lepisto ¨ et al. 2006). The northern- most part of the Baltic Sea, the Bothnian Bay, is the most intact part of the Baltic Sea; deep sea areas are in a near-pristine state and primary production is, in contrast with the southern areas (e.g. Gulf of Finland), phosphorus, not nitrogen, limited. The shoreline on the Finnish side, however, shows increased nitrate (NO  3 ) fluxes from several eutro- phied rivers. The rivers entering the Bothnian Bay, in contrast to most of the catchments entering to the Gulf of Finland, are profitable for studying the importance of the river channel in nitrogen removal, as their catchments have low lake coverage. H. Silvennoinen (&)  A. Liikanen  J. Torssonen  P. J. Martikainen Department of Environmental Science, University of Kuopio, P.O. Box 1627, 70211 Kuopio, Finland e-mail: hanna.silvennoinen@uku.fi C. F. Stange Department of Soil Physics, Helmholtz Centre for Environmental Research-UFZ, Theodor-Lieser-Str. 4, 06120 Halle/Saale, Germany 123 Biogeochemistry (2008) 88:63–72 DOI 10.1007/s10533-008-9194-7