First observations on the effect of a complete, exceptional overturn of Lake Maggiore on plankton and primary productivity

This manuscript presents some results of a monitoring of the planktonic com munities of Lago Maggiore during the complete overturn of the lake in the w inter of 1998/1999. These results are of particular interest not only because holomixy is a rar e event in this well-known water body, but also because these are the first such results obtained after a significant change in the lake's trophic sta tus. Holomixy can be regarded as a "whole lake experiment" sensu EDMONDSON (1993 ): irrespective of the mechanisms underlying the process, it produces an in ternal enrichment of algal nutrients, and particularly of phosphorus. The increase in biomass and productivity of phytoplankton seems to be drive n more by the increase in phosphorus than by a decrease in potential grazer s. However, once the luxury amount of nutrients supplied by the overturn wa s exhausted, the photosynthetic productivity decreased to values more typic al of a "normal" spring productivity, in accordance with the present trophi c condition of the lake. Similarly, the enhancement of phytoplankton growth seems likely to have stimulated the numerical increase of zooplankton, and especially of small, fast reproducing rotifer species. Moreover, the spring pulse of primary productivity and the phosphorus enric hment represent a crucial phase for the storage of the potential energy nec essary for the development of the zooplankton population, particularly of c ladocera, in summer.