May Climate Change Help Nitrogen Management In River Basin – Coastal Sea Systems?

Most marine and coastal water ecosystems worldwide suffer from eutrophication. It is the result of an enrichment of nutrients in the water body, like phosphorus and nitrogen.  Most common consequences are a proliferation of algae, reduced water transparency, and oxygen depletion. Fighting eutrophication is a major societal task and many international directives and agreements on reducing nutrient loads into the sea exist. To implement nutrient load reduction measures is very expensive and often causes conflicts.

Our question is how does ongoing climate change affect eutrophication and its management?  We focus on nitrogen because its availability mainly determines the primary productivity in seas. Beside sedimentation, denitrification is the most important regulating 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 nitrogen concentrations in the water. In general, it is assumed that climate change and increasing temperatures favor denitrification and improve the cleaning capacity of our seas.

However, these internal processes interact with the external supply of nitrogen, entering with large rivers.  In river basins, the consequences of climate change on nitrogen loads are much harder to predict and differ between regions. Major driving factors are changes in nitrogen concentrations and water budgets, resulting from changes of precipitation pattern in river basins.

We apply an ecosystem model for the Baltic Sea and simulate denitrification rates for the present situation and for a future climate (scenario A1B). We focus on three major, contrasting river basins, namely Odra (Germany/Poland), Vistula (Poland) and Neva (Russia). By linking the ecological model with regional economic calculations, we assess how changing river loads in a future climate interact with changing denitrification in the sea, calculate the monetary consequences for river basin management approaches and discuss the management implications.

In all our Baltic case studies over 90% of the entering nitrogen 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 nitrogen loads by 9.1% (Odra) and 9.9% (Neva) and minor changes with respect to the Vistula loads.

In combination, in all three river basins, climate change helps to reach policy targets, namely lower nitrogen concentrations in the adjacent coastal sea, at lower costs. The monetary benefit of load reduction measures that do not have to be implemented because of climate change are estimated 11,270 million €/a for the Odra, 2,000 million €/a for the Vistula and 92 million €/a  for the 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 need to be taken into account in nutrient load reduction management plans.

These findings were described in the article entitled Climate change effects on denitrification and associated avoidance costs in three Baltic river basin – coastal sea systems, published in the Journal of Coastal Conservation. This work was led by Gerald Schernewski from the Leibniz Institute for Baltic Research.