Lake acidification assessed using chironomid pupal exuviae

The European Union Water Framework Directive (WFD) aims to achieve good ecological status of its waterbodies by 2015 through the determination of reference state to provide a measure of perturbation by human impacts based on taxonomic composition and abundance of aquatic species. Collections of floating pupal exuviae discarded by emerging adult midges (Diptera: Chironomidae) provide a cost-effective method for obtaining representative ecological data from lakes. Chironomid and environmental data were collected from 203 lake surveys of all WFD types found in Britain. Canonical Correspondence Analysis provided species optima and niche breadths in relation to acidity measures. An acidification score derived from species optima was produced for each lake survey and compared with the lake's reference score modelled on physical characteristics. The Ecological Quality Ratio (EQR) of observed to reference scores were classified into WFD quality bands based on the relative frequency of sensitive to tolerant chironomid species. Sampling variation of EQR was used to determine the confidence of each lake survey classification. Species-based EQR provided a sensitive measure of lake perturbation from high ecological status. Analyses were repeated with generic data obtained in much less time. Precision obtaining generic EQR was lower but still acceptable for providing a high confidence of classification at High, Good and Moderate status.