Contrasting Patterns in Chironomid (Chironomidae) Communities of Shallow and Deep Boreal Lakes Since the 1960s

Chironomids have been widely used as indicators of trophic conditions of lakes due to their species-specific environmental requirements. In order to understand resilience and deterioration of aquatic ecosystems due to increases and/or decreases in external loading, recent chironomid community and water chemistry data of seven southern Finnish lakes with varying anthropogenic pressures and bathymetric properties were compared with similar data from the 1960s. Altogether 64 taxa were found. At present, the most numerous taxa are Procladius spp., Chironomus f.l. plumosus, Chironomus f.l. salinarius, Stictochironomus f.l. psammophilus, Benthalia spp., Tanytarsus spp. and Cladotanytarsus. In four lakes, current abundances of chironomid larvae are clearly lower, and in two lakes abundances are higher than during the 1960s. In two shallow, previously highly eutrophic lakes, the chironomid community composition revealed an improvement in the trophic status, while in three deep, previously moderately oligotrophic lakes, a slight deterioration was found. The most dramatic increase in chironomid density and diversity was found in a lake which was most heavily polluted in 1960s. In deep lakes, a slow gradual eutrophication seemed to be responsible for the observed replacement of the previous oligotrophic communities, dominated by Monodiamesa bathyphila, Stictochironomus spp., Polypedilum f.l. breviantennatum and Heterotanytarsus apicalis by eutrophic taxa such as Chironomus f.l. salinarius, Chironomus f.l. plumosus, Einfeldia spp., Sergentia coracina and Microtendipes spp. Among the key factors responsible for changes were dissolved oxygen concentration, food availability, sediment quality, and in one lake, toxic effluents from industry.