Integrated ecological assessment of Danish Baltic Sea coastal areas by means of phytoplankton and macrophytobenthos

The Water Framework Directive (WFD) demands an integrated assessment of ecological quality based on biological parameters. In this context combined macrophytobenthos and phytoplankton data sets along the Danish Baltic Sea coast were analysed for similarities and differences in their response to abiotic variables. Zostera marinaʹs depth limits showed a significantly negative correlation with concentrations of total-nitrogen, total phosphorus and chlorophyll a as well as with Myrionecta rubra biomass and a strongly positive correlation with Secchi depth. The results documented that selected phytobenthos and phytoplankton indicators show correlated responses to water quality. All biotic and abiotic parameters clustered in two groups, indicating two trophic states but, at the same time, also two distinct salinity classes. One class was characterised by low nutrient levels and low salinity while the other class was characterised by high nutrient levels and high salinity, indicating that the mixing of relatively nutrient poor brackish Baltic water with more nutrient rich North Sea water overruled traditional estuarine gradients in the investigated area. The results therefore allow an analysis of the eutrophication state regarding the additional influence of decreased salinity on euryhaline marine species. The consequences of the results are discussed in relation to classification systems for brackish water ecosystems.