Carstensen, J. & Krause-Jensen, D., 2009: Fastlæggelse af miljømål og indsatsbehov ud fra ålegræs i de indre danske farvande. Danmarks Miljøundersøgelser, Aarhus Universitet. 48 s. – Arbejdsrapport fra DMU nr. 256.
In this report ecological targets and required measures for nutrient reductions in fulfilment of the European Water Framework Directive by 2015 are determined for the open coastal waters of the Danish Straits. Eelgrass depth distribution is, so far, the only biological quality element used for setting ecological targets and determining required measures.
Reference conditions and ecological targets (74% of the reference condition) for the depth of the main eelgrass distribution have been found through a statistical analysis of historical data (from 1883 to 1929). The ecological target for the typologies OW1 and OW2, characterising open coastal waters with a salinity above 18, has been estimated to <st1:metricconverter w:st="on" ProductID="9.0 m">9.0 m</st1:metricconverter> , whereas the environmental target for OW3a and OW3b, characterising open coastal waters with a salinity below 18, was estimated at <st1:metricconverter w:st="on" ProductID="8.1 m">8.1 m</st1:metricconverter> .
Relationships between nitrogen inputs and concentrations as well as between nitrogen concentrations and eelgrass depth distribution have been established. The relationships between nitrogen inputs and concentrations are relatively precise and consistent with other studies, whereas different relationships between nitrogen concentrations and eelgrass depth distribution are obtained depending on the chosen study period. The lack of a universal and consistent relationship between nitrogen concentrations and eelgrass depth distributions is most likely because this relationship is highly dynamical and non-linear, which is reflected in the lack of eelgrass recovery following significant reductions in nitrogen concentrations in later years. There is a need for better understanding and quantitatively describing the dynamics underlying the potential recolonisation of eelgrass.
Scenarios for reducing nitrogen inputs to the Danish straits have shown that fulfilment of the established ecological targets is unlikely. This is due to an apparent lack of eelgrass response to changing nitrogen levels, based on statistical analyses of monitoring data, and that nitrogen concentrations in Danish open coastal waters, to a large extent, are governed by exchanges with the Baltic Sea and <st1:place w:st="on">Skagerrak</st1:place> . Sufficiently low nitrogen levels, that may potentially enable the fulfilment of the ecological targets for eelgrass, can, most likely, only be achieved through measures to also reduce nitrogen levels in both the Baltic Sea and <st1:place w:st="on">Skagerrak.</st1:place>
<st1:place w:st="on">Full report in Danish pdf (2.6MB)</st1:place>
