Carstensen J & Dahl K. 2019. Macroalgal indicators for Danish Natura 2000 habitats. Aarhus University, DCE – Danish Centre for Environment and Energy, 45 pp. Technical Report No. 142. http://dce2.au.dk/pub/TR142.pdf
The applicability of using macroalgal indicators for assessing the ecological potential according to the European Habitats Directive was investigated. Three indicators have been developed and applied to macroalgal monitoring data from 12 stone reefs and 21 coastal habitats. Ecological indicators must reflect ecological status as affected by human activities. Therefore, a key component of indicator development is to separate natural variations from variations caused by anthropogenic pressures. Observations of macroalgal cover, distilled into three indicators (cumulative cover, proportion of opportunists, and number of perennial species), are influenced by physical exposure, salinity, grazing from sea urchins and light limitation, with light limitation partly reflecting nutrient pressure from land.
These different sources of variability affect the indicators to varying extent in different types of areas. Physical exposure is most pronounced at shallow depth (less than 2-4 m). Sea urchins can completely graze down the entire macroalgal community, stressing the importance of recording sea urchins in the monitoring programme. The macroalgal community structure is strongly controlled by salinity, limiting the number of species in brackish water and shifting the composition towards more opportunistic species. In deeper waters, the macroalgal community changes from light-saturated to light-limited growth, and this change is linked to the attenuation of light in the water column, which varies between sites and over time, primarily reflecting differences in eutrophication. This report presents statistical models that can separate these differences sources of variation and provide a ´”cleansed measure” (indicator) of anthropogenic disturbance.
Using Secchi depth as a proxy for light attenuation, we show that the macroalgal indicators respond to variations in Secchi depth over time at the open-water stone reefs, and that the spatial variation in macroalgal indicators in coastal habitats is correlated with mean Secchi depths for the habitats. Historical data are not available for guiding a target setting of the macroalgal indicators, but ecological targets for Secchi depth from the Baltic Sea Action Plan and the implementation of the Water Framework Directive (WFD) can be translated into targets for the macroalgal indicators.
