Warming modifies trophic impacts of invading shrimp with impact varying among primary producers

Environmental change drivers, such as climate change and species invasions, can disrupt biological networks and weaken ecosystem stability. Acting together, these drivers may interact and alter the effects on the ecosystem. However, few studies have examined the interactive effects of climate change and species invasions, particularly in aquatic systems. The Baltic Sea is vulnerable to species invasions given its young age, low salinity and low native biodiversity, compounded by rapid warming trends. The shrimp Palaemon elegans has recently invaded the more northern parts of the sea, facilitated by its high tolerance for low salinity and warm conditions. We investigated the combined effect of warming and invasion of P. elegans on trophic dynamics in the Baltic Sea. We manipulated temperature (14 or 19°C) and shrimp presence (present or absent) in a 30‐day mesocosm experiment using a full factorial design. We found the shrimp to reduce amphipod and zooplankton abundance independent of temperature, while warming increased zooplankton abundance and tended to increase amphipod abundance independent of shrimp presence. However, the impact of the shrimp on primary production depended on temperature. Shrimp presence increased algal biomass under the higher temperature but not under the ambient temperature, while it increased phytoplankton abundance under the ambient temperature but not the higher temperature, where its impact was dampened. Thus, rather than uniformly strengthening the top–down effect of the shrimp, warming rebalanced the biotic control, with benthic algae and pelagic phytoplankton responding differently, causing structural changes to the trophic dynamics. Our results underscore the importance of assessing interactions among stressors, as their combined effects on ecosystems may not be inferred from individual stressor effects, as well as the importance of considering species‐specific responses, as these may differ, which can result in complex changes to the ecosystem.