Defining the ecological status of small forest lakes using multiple biological quality elements and palaeolimnological analysis

A new classification system compliant with the EU's Water Framework Directive (WFD) was employed to assess the ecological status Of Six small forest lakes differently affected by catchment forestry. The classification is based on metrics involving four biological quality elements (phytoplankton, aquatic macrophytes, benthic macroinvertebrates and fish) and their nationally defined reference and class boundary values. The classification evidence obtained from single metrics and quality elements, on the "One-out, All-out" principle, which establishes status according to the weakest measure, was compared with the integrated classification system used in the Finnish national status assessments. The national classification system harmonizes the individual measures by scoring and expresses overall Status class as a median score across quality elements. In addition, because the amount of reference data for this lake type was limited, palaeolimnological analyses were conducted to determine lake-specific changes during past decades. The results demonstrated that individual metrics and quality elements may result in highly variable classifications, and consequently pessimistic ecological estimates, if the One-out, All-out principle is applied. It is concluded that the integration of evidence from multiple quality elements evens out their variability and results in more realistic status assessments in small humic boreal forest lakes.