Under conditions of global warming, organisms are expected to track their thermal preferences, invading new habitats at higher latitudes and altitudes and altering the structure of local communities. To fend off potential invaders, indigenous communities/populations will have to rapidly adapt to the increase in temperature. In this study, we tested if decades of artificial water heating changed the structure of communities and populations of the Daphnia longispina species complex. We compared the species composition of contemporary Daphnia communities inhabiting five lakes heated by power plants and four non-heated control lakes. The heated lakes are ca. 3-4 °C warmer, as all lakes are expected to be by 2100 according to climate change forecasts. We also genotyped subfossil resting eggs to describe past shifts in Daphnia community structure that were induced by lake heating. Both approaches revealed a rapid replacement of indigenous D. longispina and D. cucullata by invader D. galeata immediately after the onset of heating, followed by a gradual recovery of the D. cucullata population. Our findings clearly indicate that, in response to global warming, community restructuring may occur faster than evolutionary adaptation. The eventual recolonisation by D. cucullata indicates that adaptation to novel conditions can be time-lagged, and suggests that the long-term consequences of ecosystem disturbance may differ from short-term observations. Temperature increase, a major component of climate change 1 , is expected to exert a strong direct impact on the functioning of freshwater organisms, e.g., modifying their physiology, development, and/or fitness 2-5. Rising temperatures will also trigger a multitude of indirect effects on aquatic habitats, including alteration of abiotic parameters (e.g., oxygen saturation and ice cover duration) 6,7 and biotic interactions (e.g., changes in primary production, predation intensity, mismatches in the occurrence of interdependent species, and altered hostparasite interaction dynamics) 6,8,9. These changes will have an impact on the performance of individuals, and consequently on the structure and functioning of populations and communities 10,11. Based on the current effects of climate change and models of further warming, it is predicted that many species could be threatened with extinction 12,13. Thus, forecasting species' responses to the warming climate is a timely issue in ecology, evolutionary biology, and environmental protection. According to Bellard and coauthors 14 , there are three non-exclusive types of organismal responses to climate change, namely, alterations in (1) time of occurrence (e.g., shifts in phenology), (2) space (e.g., altered distribution range), and (3) self (i.e., changes in organisms' physiology, not related to spatial or temporal changes). A change