Eutrophication modelling of the North Sea: two different approaches

Abstract Two different eutrophication models as applied to the North Sea are discussed in this paper. The ERSEM (European Regional Seas Ecosystem Model) model, being developed in the framework of the EU Marine Science and Technology program, is a complex (70 state variables) ecosystem model which dynamically simulates the large-scale cycling of carbon, oxygen and the macronutrients N, P and Si over the seasonal cycle. The model consists of an interlinked set of modules, describing the biological and chemical processes in the (stratified or non-stratified) water column and in the (vertically layered) benthic system, as forced by light and temperature. Advective and diffusive transport is included by driving the model with the output of the 3D General Circulation Model of the North Sea by Backhaus, aggregated into daily exchange volumes between the ICES (International Council for the Exploration of the Sea) boxes, which form the spatial compartments of the model. The other model, MIKE21 EU, consists of an advanced 2D circulation model in combination with modules describing advection-dispersion processes, oxygen dynamics and carbon/nutrient cycling in the lower trophic levels. The carbon and nutrient dynamics (N and P) are aggregated into 12 state variables. The horizontal spatial resolution of the model is very high, with a cell size of 18.5 × 18.5 km. Both models include the freshwater inflows and the associated organic and inorganic nutrient loadings as well as cross-boundary flows as time series of flow rates and concentrations. The suitability of either model to address eutrophication-related questions is exemplified.