Immission targets for nutrients (N and P) in catchments and coastal zones: a North Sea assessment

Concern about the rising concentrations of nutrients and their adverse effects on freshwater, estuarine and marine organisms led to political action in 1970s and 1980s. Emission targets, aimed at reducing the load to the aquatic environment, were set. Also, immission targets for the concentration of different nitrogen and phosphorus compounds were set in various countries. Immission targets are the background concentration and the quality objective of nutrients. The background concentration is defined as the concentration that could be found in the environment in the absence of any human activity. The quality objective is based on eco-physiological results and adopted by policymakers in order to minimise the risk of environmental damage. The criteria and nomenclature for targets are ambiguous and inconsistent; different names are used for the targets. Setting target values for nutrients in water bodies that are interconnected in a catchment area is a perilous affair. A wide range in the background concentration and quality objectives for different nitrogen and phosphorus compounds in river and sea water is found between 10 European Union countries around the North Sea. For instance for nitrate the minimum versus maximum difference for the objective is 255 fold. For the coastal waters, a method is proposed that makes it possible to calculate the target at a given salinity. It is recommended that further development of eutrophication targets, should be based on ecological science. Links must be made between the desired ecological target (e.g. chlorophyll concentration) and the chemical target at a catchment level, taking into account that water flows across borders and flows from one place to another.