Use case of biomass-based benthic invertebrate index for brackish waters in connection to climate and eutrophication

Biological water quality indices are increasingly used for coastal water quality assessment. It is not clear, how are anthropogenic disturbances and climate variability captured in the dynamics of different indices. In this study, we evaluate simultaneous contributions of both eutrophication- and climate-related factors to the variability of a benthic invertebrate index ZKI. The index is adopted to classify the coastal brackish waters in the NE Baltic Sea. We found quite similar contributions of climate and eutrophication, which together described on average about 65% of the variability of the ZKI in the study area. These interactions are likely affected by local geomorphology, with the index values being less related to eutrophication on coarse bottoms and more to climate patterns on steep slopes. The relative contribution of eutrophication to index variability did not change across the pressure gradient according to the Baltic Sea Pressure Index. The results indicate that index assessments may easily become misinterpreted if the knowledge about the other sources of variability is missing and advise to account for climatic and hydrographic factors.