Abstract
Abstract. Rates of denitrification in sediments were measured with the isotope pairing technique at different sites in the southern and central Baltic Sea. The rates varied between 0.5 μmol N m−2 h−1 in sands and 28.7 μmol N m−2 h−1 in muddy sediments and showed a good correlation to the organic carbon contents of the surface sediments. N-removal rates via sedimentary denitrification were estimated for the entire Baltic Sea calculating sediment specific denitrification rates and interpolating them to the whole Baltic Sea area. Another approach was carried out by using the relationship between the organic carbon content and the rate of denitrification. The N-removal by denitrification in sediments varied between 426–652 kt N a−1, which is around 48–73% of the external N inputs delivered via rivers, coastal point sources, and atmospheric deposition. Moreover, an expansion of the anoxic bottom areas was considered under the assumption of a rising oxycline from 100 to 80 m water depth. This leads to an increase of the area with anoxic conditions and an overall decrease in sedimentary denitrification by 14%. Overall, we show here that this type of data extrapolation is a powerful tool to estimate the nitrogen losses for a whole coastal sea and may be applicable to other coastal regions and enclosed seas.
Highlights
Dealing with large quantities of anthropogenic nitrogen inputs is one of the biggest challenges for marine ecosystems and coastal states throughout the world
Due to its specific structure with several deep basins separated by shallow sills, a shallow connection to the North Sea, and a high freshwater input, the Baltic Sea suffers from either permanent or periodic anoxia caused by water column stratification in their central parts (Schinke and Matthaus, 1998)
It could be shown that our way of combining one-point in situ measurements at different sediment stations with the application of sediment or Corg distribution maps is a powerful tool to estimate N-removal via sedimentary denitrification and – if present – via Anammox for an ecosystem
Summary
Dealing with large quantities of anthropogenic nitrogen inputs is one of the biggest challenges for marine ecosystems and coastal states throughout the world. Due to its specific structure with several deep basins separated by shallow sills, a shallow connection to the North Sea, and a high freshwater input, the Baltic Sea suffers from either permanent or periodic anoxia caused by water column stratification in their central parts (Schinke and Matthaus, 1998). This “natural anoxia” is enhanced by excessive nutrient inputs (Conley et al, 2009a). A rise of the oxycline to a depth of 80 m has been well documented in the past (Baltic Environmental Database)
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