Zooplankton biodiversity in the lakes of South Tyrol (Italy)

Abstract
The results of a survey on the planktic rotifers and crustaceans in 70 South Tyrolean lakes at altitudes ranging from 215–2892 m a.s.l., carried out between 1979 and 2015, are presented. The samples were collected both as part of lake monitoring efforts and in support of various scientific projects. The composition of zooplankton communities in different lakes is compared and the influence of environmental factors on biodiversity is examined. The total number of taxa amounted to 99, with 57 rotifers, 26 cladocerans and 16 copepods. The spectrum of taxa and their frequency of occurrence differed between lakes located above and below the treeline. Polyarthra dolichoptera was the most frequent species in all lakes, followed by Kellicottia longispina, Keratella cochlearis and Synchaeta spp. in the lakes below tree line and Keratella cochlearis and Keratella gr. quadrata in the lakes above tree line (alpine lakes). The cladocerans were mainly represented by species of Daphnia, Bosmina and Ceriodaphnia in the lakes below treeline and by Chydorus sphaericus above it. Among the copepods, Cyclops abyssorum tatricus played a prominent role in the alpine lakes. The rotifers Anuraeopsis fissa, Brachionus calyciflorus, Collotheca pelagica, Pompholyx sulcata, as well as the cladocerans Daphnia ambigua and Daphnia pulex found in this study of South Tyrolean lakes are presently not recorded in the national species lists. The analysis of the taxonomic composition of the zooplankton communities identified three groups of lakes, mainly differing in terms of altitude and trophic level. In the natural lakes below the treeline, the number of taxa per lake was significantly higher than in the alpine lakes and higher than in reservoirs. The number of taxa decreased significantly with increasing altitude but also correlated with the area of the lake and with parameters related to lake primary production and to the geology of the catchment area. The differences in the Shannon index values of the individual lakes were small, and the index correlated significantly only with altitude.