Information value of chironomid remains in the uppermost sediment layers of a complex lake basin

Abstract

The subfossil remains of chironomids were analyzed from four short (35 – 50 cm) sediment cores in eutrophicated Lake Vanajavesi, southern Finland. The chironomid analysis was found to be a useful palaeolimnological technique for indicating the following aspects: (1) The pollution history of the lake during the last 150 years. A succession from an oligotrophic Micropsectra-Monodiamesa community, through eutrophic Chironomus communities, to the disappearance of the chironomid fauna, was detected. (2) The bottom dynamic conditions at the sampling site. (3) The water level changes of the lake. (4) The significance of bioturbation in the core chronology and stratigraphy. The disappearance of the chironomid remains in the cores was correlated with the occurrence of annually laminated sediment.

The most important factor disturbing the interpretation of the cores was the redeposition of head capsules by erosion and transportation. Detailed identification of the remains is necessary to avoid the misinterpretations caused by redeposition. Bioturbation reduces the time resolution of the cores. It is necessary to use dating methods which take the effect of bioturbation into account.