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No. 390: Reference values and limit values for eelgrass depth limits for use in the river basin management plans

Timmermann K, Christensen JPA, & Erichsen A. 2020. Referenceværdier og grænseværdier for ålegræsdybdegrænser til brug for vandområdeplanerne. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 28 s. - Videnskabelig rapport nr. 390. http://dce2.au.dk/pub/SR390.pdf



In this report we establish reference and boundary values for the indicator “Depth limit of eelgrass”, which is used for ecological assessment of Danish coastal water bodies according to the Water Framework Directive.


The establishment of reference values is based on a large historical dataset of eelgrass observations collected from 1880-1930. Eelgrass observations from this time period is assumed to reflect a reference condition, which according to the Water Framework Directive is defined as a condition with no, or only very minor anthropogenic alterations from undisturbed conditions.   The historical dataset is used to construct a regression based model, describing eelgrass depth distribution in each water body as a function of the physical parameters: “water exchange”, average water depth” and “stratification”.


Historical eelgrass observations exists for 48 out of 109 coastal water bodies and for these water bodies, water body specific reference values are established based on the observations. For the remaining water bodies, the regression model forced with the physical characteristics for each water body is used to estimate water body specific reference condition for eelgrass depth limit. 


The results show that the reference condition for depth distribution of eelgrass in Danish coastal areas is between 4 meter and 13.3 meter, with lowest values in estuaries and highest values in the more open waters.

Boundary values separating the 5 classes in the classification system of the Water Framework Directive are determined from intercalibrated EQR values. Results show that the boundary between good to moderate status and thus ecological targets (74 % of the reference condition) for eelgrass depth limit is between 3 and 9.8 meters in Danish coastal waters.