Lake Orta: an overview on recent zooplankton research

Our recent Lake Orta work has sought to improve our paleolimnological inferences of differing effects of past pollution on rotifer vs. the previous Cladoceran proxies, and, quite separately, to determine if contemporary vs. resurrected Daphnia galeata lineages from the lake differ in Cu sensitivity. The sediment cores ORTA 07/1A and ORTA 07/2A have been the starting point of these studies. In both cores, we found clear and consistent responses of Rotifera resting eggs to environmental changes, similar to those previously observed for Cladocera. Perhaps surprisingly, the abundance of resting eggs increased during times of heavy pollution, and new morphotypes were added to those already present. The fine time resolution provided by core ORTA 07/2A allowed us to compare sediment-based analyses with contemporary hydrochemistry data generated during periods of industrial pollution. The presence of apparently viable resting eggs along with empty (i.e. hatched) egg cases during this period suggests that, unlike other organisms, some rotifers survived the periods of heaviest pollution in the lake. We used life table experiments to compare the sensitivity of contemporary vs. resurrected Daphnia galeata lineages to copper concentrations of 10 and 40 µg/L. The contemporary Daphnia were all dead by day 10 and failed to reproduce. In contrast, the Daphnia cohorts from the past actually performed better in low Cu(II) concentrations than in the copper-free, controls. Finally, to help us explain the success of D. galeata in Lake Orta only after reductions in its acidity, we tested the acid and Cu sensitivity of contemporary newborn Daphnia galeata. These animals were quite tolerant of Cu, but very sensitive to acidity, consistent with the timing of their re-appearance in the lake.