First Observations on the Effect of a Complete, Exceptional Overturn of Lake Maggiore on Plankton and Primary Productivity

Abstract

This manuscript presents some results of a monitoring of the planktonic communities of Lago Maggiore during the complete overturn of the lake in the winter of 1998/1999. These results are of particular interest not only because holomixy is a rare 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 status. Holomixy can be regarded as a “whole lake experiment” sensu Edmondson (1993): irrespective of the mechanisms underlying the process, it produces an internal enrichment of algal nutrients, and particularly of phosphorus. The increase in biomass and productivity of phytoplankton seems to be driven more by the increase in phosphorus than by a decrease in potential grazers. However, once the luxury amount of nutrients supplied by the overturn was exhausted, the photosynthetic productivity decreased to values more typical of a “normal” spring productivity, in accordance with the present trophic 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 enrichment represent a crucial phase for the storage of the potential energy necessary for the development of the zooplankton population, particularly of cladocera, in summer.