Rising temperatures, declining predator populations and eutrophication are three pervasive global changes that alter communities in aquatic ecosystems. Aquatic systems are linked to adjacent terrestrial environments by the reciprocal exchange of resources, but the effects of global change drivers on these cross-ecosystem subsidies are poorly understood. We used a year-long experiment in 1000L mesocosms to investigate the combined effects of warming (3°C), elevated nutrient loads and predaceous fish (threespine sticklebacks, Gasterosteus aculeatus) on the flux and uptake of aquatic-terrestrial resource subsidies. We monitored the export of aquatic production to terrestrial environment by qualifying the emergence of aquatic insects, and the processing of terrestrial subsidies into aquatic food webs as the decomposition rates of terrestrial detritus.
Results/Conclusions
Predation, nutrient enrichment and warming altered the exchange of cross-ecosystem subsidies in different ways. Fish predation on benthic invertebrates dramatically reduced the emergence of aquatic invertebrate biomass and also decreased the potential uptake of terrestrial resources by reducing leaf litter breakdown rates. Conversely, nutrient enrichment increased aquatic emergence, but did not alter the rate of leaf litter breakdown. Warming altered the phenology of aquatic subsidies by stimulating the earlier emergence of aquatic insects in spring, and enhanced the potential uptake of terrestrial resources in aquatic food webs by accelerating the breakdown of leaf litter. Our results indicate that global change drivers can alter the reciprocal exchange of resource subsidies to and from adjacent ecosystems. Strong top-down control may act to decouple resource transfer between aquatic and terrestrial ecosystems, whereas nutrient enrichment and warming appear to enhance the exchange of resources between these ecosystems.