Climate change effects on denitrification and associated avoidance costs in three Baltic river basin - coastal sea systems

To fight ongoing eutrophication of the Baltic Sea, major international directives and agreements, like the EU Water Framework Directive or the Helcom Baltic Sea Action Plan, aim at a reduction of nutrient loads to the Baltic Sea. These external nutrient load reductions interact with retention and removal processes (ecosystem services) in coastal and marine waters. Beside sedimentation, denitrification is the most important regulation ecosystem service provided by the sea, because it removes nitrogen (N) to the atmosphere and controls eutrophication. Denitrification is a spatially and temporally highly variable process and largely depends on temperature and N concentrations in the water. Climate change will have strong effects on denitrification. Increasing temperatures favour denitrification and changes in precipitation and discharge will alter the availability of N, with strong differences between river basins. We combine a 3D-ecosystem model of the Baltic Sea with economic marginal cost functions for three major Baltic river basins: Odra, Vistula and Neva. Objective is to calculate the monetary consequences of climate change (scenario A1B) for the implementation of water quality objectives, taking into account both, spatial changes in N loads and denitrification. In all regions over 90% of the entering N loads are subject to denitrification mainly in coastal waters. For the climate change scenario (A1B), our approach suggests reduced riverine N-loads of 7690 t/a (Odra), 6410 t/a (Vistula) and increased loads of 3200 t/a for the Neva. At the same time our model results suggest an increased denitrification of N-loads by 9.1% (Odra) and 9.9% (Neva) and minor changes with respect to the Vistula loads. In combination, in all three regions climate change helps to reach policy targets (lower N concentrations in the sea) at lower costs. The monetary benefit of load reduction measures that do not have to be implemented because of climate change are 11,270 million (sic)/a (Odra), 2000 million (sic)/a (Vistula) and 92 million (sic)/a (Neva). The valuations strongly depend on the assumed climate change scenario and the economic approach, but indicate that climate change effects differ much between river basins and should be taken into account in nutrient load reduction management plans.