STUDIES ON ZOOPLANKTON IN 2 ACIDIFIED HIGH-MOUNTAIN LAKES IN THE ALPS

Being located in remote areas, alpine lakes are good indicators of reg ional and global pollution, and are particularly sensitive to atmosphe ric depositions. When situated in areas where acidic rocks dominate, t hey are sensitive to acidification. In the framework of an internation al project partially funded by the EU, a two-year study on zooplankton was carried out on two lakes, Lake Paione Inferiore (LPI) and Lake Pa ione Superiore (LPS), selected because of their susceptibility to acid ification. In particular, LPS is permanently acidified, with pH rangin g between 5.3 and 6.2, and LPI is acidified during the ice melt, when pH drops to 6.1 units. In addition, LPI is subjected to further anthro pogenic interference, since fish (Onchorynchus mykiss) have been repea tedly introduced into the lake during the last thirty years. Literatur e information is available on the species composition and the seasonal dynamics of the zooplankton communities of the two lakes before these anthropogenic disturbances took place. Previously, the two lakes had a similar species composition and abundant zooplankton. An analysis of the present-day situation reveals significant changes compared to the past. Both acidification and the introduction of fish are responsible for the marked alterations observed. By reconstructing of the past cl adocera assemblages and by analysing the literature on lakes with diff erent impacts of the two factors (pH and fish), we can estimate the re lative importance of the two different anthropogenic disturbances on s pecies composition and abundance of the zooplankton communities of the two lakes.