Bottom-up nutrient and top-down fish impacts on mercury flux from aquatic ecosystems

Methyl mercury (MeHg) is one of the most hazardous contaminants in the environment, adversely affecting both human and wildlife health. Recent studies have demonstrated that aquatic insects biotransport MeHg and other contaminants to terrestrial consumers, but the factors that regulate the flux of MeHg out of aquatic ecosystems via emergent insects have not been studied. I used experimental mesocosms to test the hypothesis that insect emergence and the associated flux of MeHg from aquatic to terrestrial ecosystems is affected by both bottom-up nutrient effects and top-down fish consumer effects. Nutrient addition led to an increase in MeHg flux primarily by enhancing the biomass of emerging MeHg-contaminated insects, while fish decreased MeHg flux primarily by reducing the biomass of emerging insects. These factors were interdependent such that the nutrients effects were more pronounced when fish were absent and the fish effects were more pronounced when nutrients concentrations were high.