Influence of carbon availability on dentrification in the central Baltic Sea

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Denitrification was investigated in the Baltic proper at two stations with different conditions in the deep water. The Gotland Deep was examined as an example of a basin with anoxic, H,Scontaining deep water and station T was taken as an example of low-oxygen (CO.2 ml liter I), sulfide-free deep water. Denitrification was measured by the acetylene blockage method; in addition, N,O reduction was followed in samples without acetylene. To shed light on the factors limiting denitrification, we compared in situ rates to denitrification after adding nitrate or electron donors. Denitrification was restricted to the layer of the oxic-anoxic interface in the Gotland Deep and to the water layer near the sediment of station T. For both stations it could be shown that denitrification was not limited by nitrate availability. A lack of available organic C seemed to limit denitrification rates and growth of denitrificrs. As a result of C limitation in the water column, denitrification was restricted to energy-rich interfaces. In the low-oxygen water away from energyrich interfaces, the less C-demanding nitrification-denitrification coupling (NH,’ + N,O + N2) seemed to be favored. Denitrification in the water of the central Baltic seems to be subjected to strong variability due to changing C supply during the course of the year. However, limitation by C availabilitv can be assumed for most of the year and should be taken into account in calculating the N budget of the Baltic. The Baltic Sea as a whole can be described as a big estuary. The Baltic proper represents the largest and southernmost part of it. The Baltic Sea is strongly influenced by human activity insofar as loads of N and P are concerned (Elmgren 1989). The Baltic proper is an ecosystem whose primary production is controlled by N availability (Wulff and Rahm 1988; Graneli et al. 1990). Denitrification, defined as the bacterially mediated process of dissimilatory reduction of ionic nitrogen oxides (N03and NOz-) to gaseous nitrogen compounds (NO, N20, and N2), can be regarded as a major process of elimination of available N (Goering 198 5). Thus, denitrification is an important process in counteracting eutrophication (Ronner 1985). The Baltic proper is characterized by a permanent halocline (60-90 m) that inhibits ’ Current address: NIWAR, Water Quality Centre, P.O. Box 1 l115, Hamilton, New Zealand. Acknowledgments Assistance during the cruises by J. Wesnigk and R. Lilischkis and the support by the crews of RV Aranda and RV Poseidon are acknowledged. We thank E. L. Poutanen for support on the cruise in 1987. The project was financed by the Bundesministerium fti Forschung und Technologie (project MFU 05471). seasonal vertical mixing deeper than 70 m. The renewal of water masses below the halocline relies on horizontal exchange processes that are discontinuous and, especially below 130 m, rare events. Stagnation periods between deep-water renewal may last up to 10 yr, as was the case before our investigation. During these stagnation periods, oxygen level decreases in the water masses below the halocline and, especially in basins like the Gotland Deep, H2S from the sediment accumulates in the deep water (Stigebrandt and Wulff 1987). This investigation was aimed at understanding the factors limiting denitrification in the central Baltic. It is generally accepted that oxygen concentration, the availability of organic C (as electron donor), and NO,(as electron acceptor) are the most important factors controlling the occurrence and rate of denitrification (Hattori 1983). Although N03and the more reduced nitrogen oxides can all serve as electron acceptors for denitrification, it is assumed that N03is preferred to the more reduced intermediates of the denitrification path (Goering 1985). The importance of the availability of organic C for the rate of denitrification was pointed out by Liu and Kaplan (1984), who