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Large, deep, subalpine Lago Maggiore has been regarded as an example of stability and resilience: while changes in zooplankton taxa composition were mainly recorded with introduction of allochthonous fish in the 1950s, eutrophication and reoligotrophication mainly affected abundance and biomass. As part of a monitoring project on pelagic zooplankton, samples were routinely collected in the open-water at least monthly over a 30 year period. During this time, copepods' numerical dominance resulted from the same species assemblage, including large Mixodiaptomus laciniatus (W. Lilljeborg 1889) and Cyclops abyssorum (G. O. Sars 1863), and smaller Eudiaptomus padanus (G. Burckhardt 1900) and Mesocyclops leuckarti (C. Claus 1857); large Megacyclops viridis (L. Jurine 1820) were recorded only occasionally in the open-water. Eudiaptomus gracilis (G.O. Sars 1863) invaded the lake in October 2006. Adult body length was typical of oligotrophic freshwater environments, and largely overlapped that of M. laciniatus, a species rarer than con-generic and numerically-dominant E. padanus. In the two years following E. gracilis invasion, M. laciniatus was not found in Lago Maggiore, suggesting that it disappeared from the lake. This is the second important change in Lago Maggiore copepod community, following the disappearance of Heterocope saliens (W. Lilljeborg 1863), which was recorded in the 1950s attendant with the introduction of fishes to the lake. The replacement of M. laciniatus by E. gracilis in Lago Maggiore is regarded as an opportunity for discussing the importance of size-mediated, rather than taxonomic-based, interactions between invasive and indigenous species, given the species' overlapping body size and similar level of abundance.
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