OOS 46-2 - Trout introductions alter catchment nutrient retention via mediation of cross-habitat subsidies

In aquatic ecosystems, introduced predators greatly reduce the abundance of emerging insects which provide a crucial nutrient subsidy to terrestrial systems. However, we do not know either how changing climate affects these food web disruptions, or what any such changes will mean for dynamics of terrestrial recipient systems. To address these questions, we conducted comprehensive zoogeochemical surveys of lakes and nearshore meadows in California’s Sierra Nevada. We surveyed lake sites both with and without established populations of predatory trout across a climate gradient. At each site, we quantified aquatic-terrestrial insect subsidies via floating emergence traps in the littoral zone and flypaper cards deployed from 0 to 50 meters inland. We also measured soil characteristics at each site, as well as nitrogen content and delta-15N in soils and plant tissues, in order to trace the contribution of aquatic-derived nutrients to terrestrial systems. Despite significant reductions in insect subsidy provision, perennial plants growing around lakes with fish were richer in nitrogen and had heavier delta-15N signatures than those at fishless lakes. This effect persisted up to 50 meters from the lakeshore, and effect size differed across the climate gradient in our study. In addition, fish presence and climate were significantly associated with differences in a broad panel of soil nutrients. Our results provide a uniquely complete assessment of anthropogenic disruption effects across multiple ecosystem types and trophic levels. By integrating food web analysis, spatial and temporal variability, and nutrient cycling, our work demonstrates the importance of a zoogeochemical perspective in the study of ecosystem ecology.