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Global mean surface temperatures are increasing. All ecosystems are likely to be affected and there is much interest at present in predicting the effects. In freshwater environments, we expect to observe, among other things, effects similar to those observed under eutrophication, such as increases in zooplankton population density and biomass as a result of enhanced population growth rates. Lago Maggiore underwent rapid eutrophication during the 60s and 70s, with a return to oligotrophy during the 80s and the 90s. Thus, it provides a case study to test the hypothesized eutrophication-like effects of recent climate warming. More specifically, we compare zooplankton biomass and density during the exceptionally warm years of the recent oligotrophic phase with values during the non-warm years of oligotrophy, and during years of the mesotrophic phase. This permits an analysis of zooplankton biomass and density with respect to temperature increase compared with the effects of eutrophication. Zooplankton population density and biomass sharply increased in 2003, the warmest year of the last century, as a result of Cladocera, particularly Daphnia, attaining values typical of the mesotrophic phase. These values were exceptional compared to typical values and were strongly different from those attained during cooler years since re-oligotrophication. Mean annual values of zooplankton density and biomass recorded in 2003 were fully comparable to typical values during the mesotrophic period. This observation confirms the hypothesis of an overall eutrophication-like effect of climate warming. Seasonal trends, characterized by an earlier start of population growth, are consistent with the effects of an increase in water temperature, as observed in laboratory experiments on the reproductive and growth strategies of Daphnia.
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