Global warming may lower thermal barriers against invasive species in freshwater ecosystems - A study from Lake Constance.

European freshwater ecosystems are increasingly invaded by exotic animal and plant species. Apart from increased connectivity between previously separated watersheds, the increasing temperature of the hydrosystems favors the spread of exotic species. The freshwater fauna of Central Europe is still shaped by the cold-adapted animal assemblages resulting from the last glaciation. It is less diverse, and the species are putatively less performant competitors, compared to the warm-adapted, species-rich fauna of the Ponto-Caspian realm, from which many current aquatic invaders are coming. Our study analyses potential mechanisms explaining the coexistence between one of the most impacting aquatic invaders of the past decades, the 'killer shrimp' Dikerogammarus villosus and the previously dominating amphipod Gammarus roeselii in Lake Constance, using laboratory predation experiments and field surveys. Our results indicate two key drivers for coexistence: low winter temperatures and the substrate structure of the alga Chara sp. At temperatures below 6 °C, the predation pressure on G. roeselii was strongly reduced; G. roeselii can therefore disperse throughout the littoral in winter, avoiding predation by D. villosus. Artificial heating of a section of the lake shore, however, resulted in local extinction of G. roeselii by D. villosus. The macroalga Chara sp. completely inhibited predation by D. villosus on G. roeselii. Climate change scenarios indicate that global warming might destroy this thermal refuge during winter until 2085. For the survival of G. roeselii it will then be crucial, which part of the Chara population will maintain epigeic plant parts during winter. The complex interplay between thermal and physical refuges for native species in the context of climate change and changing trophic status of freshwater systems, as disentangled by our study, shows that ecosystem management and restoration strategies need to better consider multiple stressors (and their rather complex mitigation strategies).