The interactive effects of warming, predation and eutrophication on the exchange of resources between aquatic and terrestrial environments

Rising temperatures, altered predator guilds and eutrophication are three pervasive global changes that alter freshwater communities. However, freshwater systems are linked to adjacent terrestrial environments by the reciprocal exchange of resources, and the effect of global change drivers on these cross-ecosystem subsidies are poorly understood. We used a year-long mesocosm experiment to investigate the interactive effects of warming (3°C), elevated nutrient loads and predaceous fish (threespine sticklebacks, Gasterosteus aculeatus) on the export of adult aquatic insects to terrestrial environment, and the import of terrestrial subsidies into aquatic food webs through the breakdown of leaf detritus. Predation, nutrient enrichment and warming had contrasting effects on the exchange of these cross-ecosystem subsidies. Fish predation on benthic insects dramatically reduced the biomass of aquatic insect emergence and also decreased the potential uptake of terrestrial resources by reducing leaf litter breakdown. Conversely, nutrient enrichment increased the production of adult insects, but had little effect on leaf litter breakdown. Warming advanced the phenology of aquatic emergence, and also accelerated litter breakdown. Our results indicate that human impacts on freshwaters can alter the reciprocal exchange of resources across the land-water interface. Strong top-down control may decouple aquatic and terrestrial food webs, whereas nutrient enrichment and warming appear to enhance the exchange of resources between these ecosystems.