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No. 170: Macroalgae indicators for assessing ecological status in Danish WFD water bodies

Carstensen J. 2020. Macroalgae indicators for assessing ecological status in Danish WFD water bodies. Aarhus University, DCE – Danish Centre for Environment and Energy, 74 pp. Technical Report No. 170. 


The ecological status of macroalgae is an important component of the EU Water Framework Directive biological quality element ‘Macroalgae and angiosperms’. In this report, a comprehensive macroalgae data set from the Danish monitoring programme has been compiled to produce three macroalgae indices (indices represent aggregated observations from the raw monitoring data): 1) cumulative cover of macroalgae, 2) number of perennial macroalgae species, and 3) relative cover of opportunistic macroalgae species. The data set spans 58 water bodies distributed over 23 different types with depth ranges down to 25 m. These data were combined with monitoring data describing environmental conditions in the analysed water bodies.

The three macroalgae indices were analysed using non-linear statistical models, which partitioned natural variations from the effects of human disturbance. Changes in the macroalgae indices with depth were described by physical exposure, grazing by sea urchins, salinity and light conditions. Cumulative cover and the number of perennial species typically exhibit three distinct phases over the depth gradient from regulation by physical exposure near the surface, maximum levels of these macroalgae indices at intermediate depths, and attenuation at deeper depths due to light limitation. Parameter estimates for the attenuation of cumulative cover and number of perennial species with depth are suitable macroalgae indicators, because they show clear responses to light attenuation and therefore constitute sentinels of eutrophication. On the other hand, the relative cover of opportunists was primarily controlled by salinity and did not express variations in response to changing light or nutrient conditions. Consequently, the relative cover of opportunists is not suitable as operational indicator (based on current monitoring data) for ecological status assessment in relation to the WFD, as it appears more driven by natural variations than eutrophication.

Reference conditions and class boundaries for the attenuation of cumulative cover and number of perennial species with depth can be computed using existing reference conditions and class boundaries for light attenuation (based on historical eelgrass depths), translating these by means of the established linear relationship between attenuation of macroalgae indicators and light attenuation. Reference conditions and class boundaries are proposed for all water bodies with light attenuation reference values, except for Limfjorden. Further analyses are required for developing a macroalgae assessment method for Limfjorden.

For assessing ecological status of ‘Macroalgae and angiosperms’, the two macroalgae indicators (attenuation of cumulative cover and number of perennial species) should be combined with the existing indicator for eelgrass main depth limit. A method for combining these indicators is proposed that first combines the two macroalgae indicators before combining these with the eelgrass indicator. For combining such indicators, it is important that they are first transformed into a common scale, which is proposed to be a standardized EQR scale obtained through a piecewise linear transformation. The two macro­algae indicators are combined by averaging their EQR standardised values before this average is combined with the EQR standardised value for eelgrass main depth limit by averaging. The combination method is illustrated step-by-step with examples from two water bodies and two periods. It is recommended to use this approach for assessing the ecological status of ‘Macroalgae and angiosperms’ and to quantify the confidence in the classification.