The role of chironomids (Diptera) in the profundal macrozoobenthos in Finnish reservoirs

The proundal macrozoobenthos of 12 Finnish inland polyhumic reservoirs was studied by quantitative sampling in 1984 and 1986. Additionally, the succession of the macrozoobenthos in one young reservoir (Lake Kyrkösjärvi) was studied during its first six years. The zoobenthos material was analyzed using the DCA (detrended correspondence analysis) method, and the ordination of sites (i.e. reservoirs), was related to associated environmental data. The role of chironomids was, in general, important in the profundal of the reservoirs. Because of the coarse bottom quality at many sampling sites, many littoral taxa were found in the profundal zone. The genusChironomus dominated (>50% of the total zoobenthic biomass) especially in the acidic, upper-reach reservoirs.Chironomus larvae also dominated in the zoobenthos of Lake Kyrkösjärvi during its first three years. Some of the reservoirs showed similarities with the dystrophic lake type of the lake typology system (Brundin, 1956). Because of the initially eutrophic character of the bottom, many reservoirs belonged to eutrophic lake types. Finnish reservoirs appear to contain a zoobenthic fauna intermediate between natural lakes and bog pools. This theme was discussed from the ecological and evolutionary point of view ofChironomus species.