Transboundary nutrient transports in the North Sea : model study

Eutrophication in the North Sea is caused by excess nutrient inputs, mainly through rivers. Besides river inputs atmospheric deposition plays a role. Due to transboundary transports of Nutrients in the North Sea it is not easy to trace back the contribution of different nutrient sources to eutrophication problems in coastal areas. The Water Framework Directive (WFD) requires that mitigating measures are planned per catchment area. To facilitate discussions on required mitigating measures for marine eutrophication a better insight is required in the sources and transboundary transports of nutrients. The Dutch contribution to eutrophication within and outside Dutch continental waters as well as outside contributions to Dutch waters are of interest. This report describes the results of a model study aiming to get a better insight in the sources and transboundary nutrient transports in the southern North Sea. The model study is done using the GEM southern North Sea model. The relative contributions by nitrogen and phosphorus sources to different regions in the southern North Sea have been estimated with two methods: 1. total nitrogen and total phosphorus as semi-conservative tracers; 2. labelled nutrients within a regular GEM model, according to the nutrient tracer method.