Das Zooplankton der Montiggler Seen (Überetsch, Südtirol) in den Jahren 1979–2015

Results of a zooplanktonic rotifer and crustacean monitoring carried out from 1979 to 2015 on two medium altitude lakes located near Bolzano (Italy) are presented. The two adjacent lakes, affected by high natural pollution and rapidly increasing anthropogenic stress, underwent rapid eutrophication requiring restoration since the seventies. Zooplankton species diversity and composition were similar for the two lakes, with the same rotifer species dominating, Filinia terminalis, Keratella cochlearis and Polyarthra dolichoptera, but with Keratella cochlearis as the most frequent species in the Large Lake of Monticolo/Montiggl and Filinia terminalis in the Small Lake of Monticolo/Montiggl. The prevailing cladocerans were small forms (Bosmina spp. and Ceriodaphnia spp.) for both lakes. Among the copepods the calanoid species Eudiaptomus gracilis predominated, while cyclopoid copepods only played a role in the Large Lake.

In both lakes the withdrawal of nutrient-rich deep water, a restoration measure, caused a sharp decrease in the rotifer densities during the first years of the study and remained more or less constant afterwards. The density of the copepods decreased slightly from 1979 to 2015 in the Large Lake, while no significant long-term changes were observed for cladocerans.

The zooplankton-based trophic evaluation yielded a mesotrophic state with tendency to eutrophy for both lakes. When comparing the zooplankton of the two lakes, differences emerged in the quantitative occurrence of individual species, probably due to differences in morphometry and catchment area.