Carstensen, J. 2008: Estimation of nutrient reductions to achieve phytoplankton ecological targets in the Wadden Sea. National Environmental Research Institute, University of Aarhus. 28 pp. Research Notes from NERI No. 246.

 

Summary

The European Water Framework Directive aims at establishing good ecological status in all European waters by 2015. The objective of the intercalibration process is to harmonise ecological target setting between countries sharing similar water types. In the North East Atlantic Geographical Intercalibration Group, the 90-percentile of the chlorophyll a concentration (March-September) was chosen as the indicator for the biological element phytoplankton. A value of 7.5 µg l-1 has been selected as the boundary between good and moderate ecological status for the Danish Wadden Sea. The objective of this report is to assess if this target value is met with the present input of nitrogen and if not fulfilled, calculate the required reductions from local sources considering projected future reductions from the Elbe River. In 2001-2006 the 90-percentile of the chlorophyll a distribution was above 15 µg l-1 for the three different deeps comprising the Danish part of the Wadden Sea. Clearly, good ecological status is not achieved at present.

 

In this report, cause-effect relationships have been established between nitrogen inputs from local sources and the Elbe River to total nitrogen concentrations in the outer and inner Wadden Sea. The cause-effect relationships were supplemented by relationships between total nitrogen concentrations and chlorophyll a (means and 90-percentile). From these relationships scenarios for nutrient reductions from local sources (land and direct atmospheric deposition) were calculated and combined with scenarios for reduced nitrogen inputs from the Elbe River. Even for the largest considered reductions of nitrogen inputs, including a complete removal of local sources to the three subareas of the Wadden Sea, it is unlikely to achieve good ecological status. This suggests that the proposed target value for good ecological status from the intercalibration process cannot be applied to the Danish Wadden Sea.

 

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