Ten years trends in the oligochaete and chironomid fauna of Lake Neuchâtel (Switzerland)

Abstract

Quantitative surveys of benthic macroinvertebrates (oligochaetes and chironomids) were conducted during 1992, 1997, 2000 and 2002 at a depth of 40 m to monitor the biological quality of sediments in Lake Neuchâtel. Recent declines in frequency of occurrence and abundance of oligochaete species characteristic of oligotrophic conditions (Stylodrilus heringianus, Embolocephalus velutinus) contrasted with the improvement of water quality metrics. Total phosphorus concentrations in lake waters decreased from 63 mg m - 3 in 1980 to 10 mg m - 3 in 2002. Since 1992, significant reductions of total zoobenthic biomass have been recorded and the chironomid community structure reflects typical oligomesotrophic conditions in the upper sediment layer. The population of the oligochaete species Potamothrix vejdovskyi is in clear expansion and indicates that this new species for the lake (1986) has found good conditions for a successful colonization. These divergent responses within the zoobenthic community are discussed according to three main hypotheses: implications of toxic pollutants like heavy metals and organic micropollutants in the sediment, impact of algae on oxygen conditions on the bottom layer and different biological responses of oligochaetes to competition for food and space.