Deutschland-Nederland Wasserqualität - Waterkwaliteit

Harmonisation of the phytoplankton assessment in the German-Dutch Wadden Sea

The undesirable accumulation of nutrients (eutrophication) remains a problematic issue in many European coastal waters, although measures taken in recent decades have already led to a decrease in nutrient river loads and coastal nutrient concentrations and thus to a reduction of impacts on coastal ecosystems such as the Wadden Sea UNESCO World Heritage Site.

Currently, phytoplankton biomass serves as a key indicator of the extent of eutrophication. Since the previous assessment of phytoplankton in the German-Dutch coastal waters in the course of the Water Framework Directive (WFD) did not provide satisfactory results, this transboundary research project provided further insights that contribute to a more comprehensive understanding of the Wadden Sea system and a harmonized assessment of phytoplankton.

This project took an innovative approach by adopting a multi-causal research approach and considered different parameters affecting phytoplankton and eutrophication in a transboundary ecosystem modelling. The latest scientific findings and existing long-term monitoring data were incorporated and analysed to achieve the most realistic representation of the Wadden Sea system in the different ecosystem models.
The data analyses and modelling results show that there are no major differences in chlorophyll concentrations between the German and Dutch Wadden Sea, and thus different levels of chlorophyll thresholds currently in place in Germany and the Netherlands are not supported by natural conditions and scientific knowledge. Further modelling results show that the required nitrogen reductions in the rivers to achieve the WFD management target of 2.8 mg TN/l in the coastal waters may not result in the coastal water bodies achieving good ecological status according to the WFD for the parameter chlorophyll, even if the management target of 2.8 mg TN/l is met in the rivers. This, and the data analyses carried out, indicate that chlorophyll does not react linearly to nitrogen reductions and that nitrogen is not the sole factor determining phytoplankton biomass in the coastal waters of the Wadden Sea.

The various analyses of the plankton data have led to the new insight that the visible changes in the phytoplankton community are not exclusively based on changes in the eutrophication situation in the Wadden Sea, but also reflect a continuous, natural shift towards new communities. Changes in individual plankton parameters are very closely related to changes in environmental conditions, but since environmental conditions are in a constant state of flux (changes in nutrient levels, climate change), it is difficult to describe and assess a stable status quo for phytoplankton. The lack of a stable status quo therefore makes it very difficult to set thresholds for different plankton parameters to assess its ecological status. However, parameters that describe dynamics (such as various biodiversity parameters) can be used to measure the influence of environmental factors. These should be taken into account in an extended and holistic approach to phytoplankton assessment within the WFD.

Furthermore, this project has strengthened the cooperation and exchange between the German and Dutch authorities and research institutions and promoted a common understanding of the cross-border issue of eutrophication. Beyond the activities of the project duration, the results of this project contribute important findings to current assessment procedures on phytoplankton and eutrophication as well as further developments and discussions in the context of the WFD, MSFD and OSPAR and provide important long-term foundations for a scientifically based system understanding of the Wadden Sea.