Invasive trout increase the climatic sensitivity of zooplankton communities in naturally fishless lakes

1. Widespread stocking of fish into naturally fishless montane lakes has restructured their food webs by excluding invertebrate top predators and releasing smaller zooplankton from predation and competition. The persistence of non-native trout in these lakes may also compound the effects of global warming, such that current efforts to manage these two stressors independently may be ineffective. 2. We conducted a survey of 22 naturally fishless and stocked lakes positioned along an altitudinal (and thus climatic) gradient and a two-factor experiment using outdoor mesocosms. We hypothesised that stocked rainbow trout (Oncorhynchus mykiss) increase the effects of higher temperature on zooplankton production in naturally fishless lakes by suppressing large invertebrates (e.g. Chaoborus), which then releases from predation smaller species that are more sensitive to warming. We also expected that introduced trout would stimulate primary production by suppressing larger grazers and increasing nutrient availability. 3. The survey confirmed that non-native trout and temperature are significant explanatory variables of zooplankton species turnover. Both lines of evidence agreed that the positive influence of higher temperature on total zooplankton biomass occurred only in the presence of non-native trout, probably because warming stimulated reproduction of smaller, herbivorous species that are favoured in lakes with stocked fish. Otherwise, the direct effects of higher temperatures on fishless communities were negligible. The positive effect of fish on primary production also probably provided the extra food to support greater reproduction by these small herbivorous species under warmed conditions. 4. Stocking trout into naturally fishless montane lakes increases the sensitivity of their zooplankton to the otherwise subtle direct effects of higher temperature. The likely effect of global warming on food webs in montane lakes may depend on their trophic structure, and the destabilizing influence of introduced fish.