Distribution patterns of benthic crustaceans in a formerly meromictic lake with changing trophic conditions (Lake Piburg, Tyrol, Austria)

The change of the crustacean fauna in the soft sediment of an Alpine lake (Lake Piburg) due to reoligotrophication is investigated. Samples were taken with a Core-Sampler (Kajak-type) along two transects from 5 m to the maximum depth of the lake (24 m) in November 1994 and May 1995, and compared to a study from 20 years ago. The crustacean fauna typically consisted of Cladocera, Copepoda, and Ostracoda with benthic, planktonic and diapausing representatives. Density and biomass of crustaceans showed high variation with water depth and season. The average density of crustaceans in the soft sediment reached 105,289 ind. m(-2) with a biomass of 409.9 mg DW m(-2) in November; in May these values were much lower (25,333 ind. m(-2) and 66.1 mgDWm(-2)). Comparisons with an earlier study, when the hypolimnion was anoxic, suggest that changes in distribution patterns, densities, and species composition were due to a change in trophic state. The recovery of the hypolimnion reflected a trend towards oligotrophy. In contrast, the crustacean community in the hypolimnion still indicates eutrophy.