Predator-induced reduction of freshwater carbon dioxide emissions

Predators can influence the exchange of carbon dioxide between ecosystems and the atmosphere by altering ecosys- tem processes such as decomposition and primary production, according to food web theory1,2. Empirical knowledge of such an effect in freshwater systems is limited, but it has been suggested that predators in odd-numbered food chains sup- press freshwater carbon dioxide emissions, and predators in even-numbered food chains enhance emissions2,3. Here, we report experiments in three-tier food chains in experimental ponds, streams and bromeliads in Canada and Costa Rica in the presence or absence of fish (Gasterosteus aculeatus) and invertebrate (Hesperoperla pacifica and Mecistogaster mod- esta) predators. We monitored carbon dioxide fluxes along with prey and primary producer biomass. We found substan- tially reduced carbon dioxide emissions in the presence of predators in all systems, despite differences in predator type, hydrology, climatic region, ecological zone and level of in situ primary production. We also observed lower amounts of prey biomass and higher amounts of algal and detrital biomass in the presence of predators. We conclude that predators have the potential to markedly influence carbon dioxide dynamics in freshwater systems.