Evaluation of eutrophication variables and thresholds in the Dutch North Sea in a historical context — A model analysis

• We compare Dutch marine eutrophication parameters for 1930, 1960, 1985, and 2007, by means of an ecosystem model analysis.
• Current dissolved inorganic nitrogen concentrations along the Dutch coast have been reduced to levels similar as in 1960.
• Current dissolved inorganic phosphate concentrations in the Dutch North Sea are similar to levels in 1930.
• Reducing nitrogen will cause a shift from a P-limited coastal ecosystem towards a more pristine, N-limited coastal ecosystem.
• Several recommendations are given to improve assessment parameters and threshold levels.

This study presents an evaluation of eutrophication variables and thresholds in the Dutch North Sea in a historical context by means of a model analysis. An ecosystem model (Delft3D-GEM for the North Sea) was used to simulate four different years, representing periods with little anthropogenic input (1930), medium anthropogenic input (1960), and high anthropogenic input (1985), as well as current (2007) conditions. Model runs differed with respect to nutrient inputs (riverine input and atmospheric deposition) but physical and hydrodynamic conditions were kept unchanged, to enable a clear assessment of the impact of changes in nutrient inputs. Model results showed that current (2007) dissolved inorganic phosphate (DIP) concentrations along the Dutch North Sea coast are similar to levels in 1930, while dissolved inorganic nitrogen (DIN) concentrations have been reduced to levels that were present in 1960. Chl a concentrations did not show clear differences between 2007, 1930 and 1960, but Phaeocystis levels were higher in 2007 than in 1930 and 1960 in all areas, although these differences were not statistically significant. Furthermore, model results indicate that DIN (rather than DIP) should be the focus of nutrient reduction efforts, since reducing nitrogen will cause a shift from a P-limited coastal ecosystem towards a more pristine, N-limited coastal ecosystem. Finally, it was found that assessment parameters and threshold levels should be revised and the harmonization between them should be improved, for which several recommendations are given.