Chironomid pupal exuviae as indicators of lake status

Thirty lakes in England and Wales were characterised by the distribution and abundance of chironomid species, collected as pupal exuviae, in correlation with chemical and physical measurements. Alkalinity and lake volume were the best predictors of chironomid species assemblage among lakes. Conductivity, lake volume and altitude were the best variables discriminating lake classes based on species distribution. A biological classification, constrained by conductivity, was used to calculate indicator species scores. A conductivity gradient of indicator species assemblages was produced to demonstrate how temporal or spatial reference conditions could be inferred. Conductivity, lake area and catchment area were the best predictors of profundal species distribution. Potential lake anoxia increased in explanatory importance after exclusion of littoral species. Alkalinity, catchment area and lake area significantly explained distribution of genera. Compared with a classification derived from 208 littoral and profundal species, discriminant analysis of best subsets of indicator taxa found 97 % of lakes were correctly classified using 15 profundal species and 73 % were correct using 7 genera. It was concluded that the pupal exuvial technique was an easy and effective method for characterising large lakes, as required for implementation of the European Water Framework Directive.