Ecological classification of large lakes in Finland: comparison of classification approaches using multiple quality elements

Ecological classifications according to the Water Framework Directive (WFD) are presented for a set of 32 large (surface area > 75 km(2)) Finnish lakes. We compared three different approaches: classification according to the strictest biological quality element (One-out, All-out approach, OoAo); numerical integration of biological quality elements (BQE) to determine median scores; and the national classification based on weight-of-evidence (WoE) framework. We also examined the sensitivity of eutrophication metrics to phosphorus concentrations. The WoE based classification proposed that the ecological status in most lakes is high (12/32) or good (17), whereas the integration of BQEs ranked more lakes (18) to high status. Of the biological elements, macrophytes and phytoplankton indicated the status closest to those given by the national WoE classification whereas generally fish indicated higher and macroinvertebrates lower status. Compared to the OoAo classification, the national WoE approach gave similar, downgraded or upgraded status classes. Downgrading was due to the higher weight given to water quality and eutrophication pressures. Upgrading was due to the lower weight given to a single macroinvertebrate metric with poorly represented data. In our opinion, the classification based on the WoE approach produces more realistic status estimates than the OoAo classification. Nevertheless, in practical lake management the evidence from the strictest classification metrics still needs to be thoroughly considered.