Benthic Macroinvertebrates in the Big Eau Pleine, a Fluctuating, Central Wisconsin Reservoir

Abstract

Benthic macroinvertebrate distribution, abundance, and composition were observed in a fluctuating {7.7 m) central Wisconsin reservoir during 1973-1974. Chironomidae and Oligochaeta represented 98 percent of the total fauna by number. The chironomid, Chironomus plumosus, and oligochaete, Limnodrilus sp., each averaged 36 percent of the total benthic biomass. Annual mean numbers and biomass in areas exposed to atmosphere, exposed to ice cover, or remaining inundated were 3025/m^2 (1.8 g/m^2), 4311/m^2 (4.5 g/m^2), and 8558/m^2 (16.0 g/m^2 ) respectively. A substantial portion of the benthic fauna was stranded and subsequently decreased rapidly in drying and frozen substrates exposed to atmosphere. Total benthic numbers and biomass were greatest immediately below the drawdown limit. Recolonization required three months {mid-March to mid-June, 1974) to attain pre-drawdown values for numbers and biomass, and subsequently both were greater after reinundation than before the substrate was exposed. Recolonization of areas exposed to the atmosphere was greater in substrates containing large amounts of organic matter than in sandy areas containing little organic matter. Sorting and transportation of sediments redistributed organic materials from the regulated zone to below the drawdown limit, and macrophytes were eliminated from the regulated zone. When compared to other reservoirs and lakes, the density of benthos in the Big Eau Pleine Reservoir can be neither regarded as high nor low. However, when compared with non-fluctuating reservoirs the number of Ephemeroptera, Plecoptera, Trichoptera, Amphipoda, and Gastropoda was low.