An attempt to trace eutrophication in a shallow lake (Balaton,Hungary) using chironomids

Lake Balaton, the largest shallow lake in Central Europe, is about 20 000 years old. An enormous increase in tourism and the disproportionate building development of the last few decades has resulted in the acceleration of eutrophication in the lake. Widespread research to reveal the causes of water-quality deterioration and possible ways of protection against it have recently started. The investigation of the larvae of non-biting midges (Diptera: Chironomidae) in the sediment of the open-water zone has also begun. The contemporary faunal composition strongly correlates with the trophic gradient along the longitudinal axis of the lake. We therefore supposed that the eutrophication process should be identifiable from the analysis of subfossil chironomid head capsules from the upper (15 cm thick) layer of the sediment. We found that quantitative results could only be obtained when fragments as well as relatively intact head capsules are considered. Our data verify that the originally oligo-mesotrophic community has been gradually replaced by eutrophic species in a west to east direction. Large-bodied larvae belonging to the Chironomus plumosus group mix the sediment down to 15 cm as they build their tubes and consequently alter the original proportions of head capsules at the different levels. So the sequence of communities through the sediment-layers is not quite reliable.