Abstract
The reduction of riverine nutrients inputs is considered the means of choice to improve the eutrophication status of the southern North Sea. With the European Union's Water Framework Directive (WFD) reduction measures presently under debate, two questions arise: (1) What changes in eutrophication indicators can be expected? (2) How do the reductions by the individual member states contribute to these? We combine an element tracing method (TBNT) with a biogeochemical model to analyze the effects of WFD-compliant nitrogen reductions proposed by OSPAR's North Sea member states. We first analyze changes in selected OSPAR assessment parameters relative to a reference simulation. Second, we quantify the source-specific contributions to total nitrogen (TN) in different regions. An overall nitrogen load reduction of 14 % is achieved. However, the response shows significant spatial variations due to strong differences between the countries' load reductions. TN and dissolved inorganic nitrogen reductions up to 60 % and 35 % are simulated near the Bay of Seine (France) and in the German Bight, respectively. Along the Dutch coast, reductions are below 10 %, and no changes occur along the British coast. Reductions in chlorophyll-a are generally lower. The TBNT analysis for the German Exclusive Economic Zone shows a TN reduction in the coastal region comparable to the N reductions in the German rivers (~25 %). In the offshore region, TN is reduced by only 6 % due to the strong influence of riverine sources with only low reductions and non-riverine sources. Our analysis reveals that non-linear responses in the biogeochemistry cause a faster removal of N from rivers with strong reductions by benthic denitrification, which enhances indirectly the removal of N from less reduced sources. Consequently, reductions in remote sources in non-problem areas can have a relevant positive effect on problem areas. This demonstrates that the TBNT method is an ideal tool to put in practice the “source-oriented approach” advocated by OSPAR, and to inform stakeholders about the effects of defined reduction strategies. However, an assessment framework is required to efficiently use it in management and for decision making, either by OSPAR, or in the context of WFD or Marine Strategy Framework Directive.
Highlights
Eutrophication, i.e., the “increase in the rate of supply of organic matter to an ecosystem” (Nixon, 1995), has been and still is an ongoing problem in the North Sea ecosystem, primarily driven by excess riverine nutrient loads
We first provide an overview of the effects of the Water Framework Directive (WFD)-compliant reductions on the actual riverine N loads, and second, how these reduced N loads effect the marine environment with respect to key parameters used within the OSPAR assessment of the eutrophication status
Thereafter, we present the results of the these? We combine an element tracing method (TBNT) analyses of the two simulations with a special focus on the changes in the German Exclusive Economic Zones (EEZ), and on how the individual N reductions affect the N dynamics in the North Sea in general
Summary
Eutrophication, i.e., the “increase in the rate of supply of organic matter to an ecosystem” (Nixon, 1995), has been and still is an ongoing problem in the North Sea ecosystem, primarily driven by excess riverine nutrient loads. As a result of this period of severe eutrophication, the ministers of environment decided on the 2nd International Conference on the Protection of the North Sea (ICNS-2) in 1987 to “reduce the river nutrient loads of phosphorus and nitrogen by 50 %” between 1985 and 1995 to mitigate the negative effects of eutrophication (ICNS-2, 1988) This goal has only been reached for phosphorus (P), but not for nitrogen (N) (Claussen et al, 2009; Lenhart et al, 2010), and several recent studies on oxygen in the North Sea provide evidence that eutrophication still is an important issue (Greenwood et al, 2010; Topcu and Brockmann, 2015; Große et al, 2016, 2017; Queste et al, 2016). It identifies large parts of the southern North Sea between the Belgian and Danish coasts as so-called “problem areas” or “potential problem areas,” while only small regions along the French and British coasts are characterized as such
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