There is much evidence that climate is rapidly changing at a global scale, especially regarding mean annual temperatures, precipitations and evaporation. The consequences of this rapid environmental change on freshwater biota are still not clear, but undoubtedly they could be severe. Among the main effects of climate change, we can individuate the enhancement of water temperatures, particularly important for poikilothermic organisms, with the consequent diminution of dissolved oxygen, and the reduction of available habitats for most stenothermal organisms. Another consequence of climate change is the alteration of hydrologic cycles, with increasing intensity and frequency of extreme events such as droughts, especially in Southern Europe. This scenario could severely affect freshwater biota, especially in mid-latitude regions, such as the Italian peninsula: shifts in phenology, life cycles and distribution ranges are likely to be expected for many organisms, with the extinction of many sensitive species. In particular, species adapted to perennial and cold waters are likely to suffer reductions in their distribution range and also local extinctions, while more tolerant organisms may enlarge their distribution ranges. Global climate change may also promote and enhance invasions of alien species. In this work, concepts and hypotheses about the presumable impacts of climate change upon freshwater biota are reported, with examples and predictions related to the Italian situation.

In the rivers of northern Italy, the presence of water is typically continuous all year long, although in the last five decades there has been a conspicuous increase in drought periods, mainly caused by human impacts and climate change. The aim of this study was to assess the impact of the length of the drought periods on macroinvertebrate assemblages. We collected invertebrates in four reaches of the Po river, characterised by different periods of absence of surface water. We found significant differences among the stations in invertebrate abundance and taxa richness, with a decrease in the more drought‐affected stream reaches. Collector‐gatherers significantly increased as the drought period lengthened, while the opposite occurred for scrapers and shredders. The areas with a discontinuous presence of water were mainly colonised by small, fast‐growing, plurivoltine organisms. A main result of our study is that only a few taxa appear to be able to survive in reaches with intermittent flow, underlining the great ecological difference between perennial and naturally intermittent streams. Our results suggest that the recent increase of droughts will likely cause an impoverishment of benthic communities in prealpine rivers.