Bottom-Up Nutrient and Top-Down Fish Impacts On Methylmercury Flux From Aquatic Ecosystems: A Synthesis of Recent Studies

Insects emerging from aquatic environments provide a critical energy subsidy to consumers in terrestrial ecosystems, but emergent insects may also expose terrestrial consumers to aquatic contaminants like methylmercury (MeHg), the so-called “dark side of subsidies.”  We used a series of field surveys and experiments to explore how predatory fish and nutrients influence the biomass and MeHg concentration of aquatic macroinvertebrates.  We first surveyed macroinvertebrates in small man-made ponds in North Texas.  Ponds without fish had a higher biomass of macroinvertebrates and taxa with higher concentrations of MeHg, which resulted in a larger pool of MeHg in the macroinvertebrate community.  These results suggested that emergent insect-mediated MeHg flux could be greater from ponds without fish than from ponds with fish.  We tested this hypothesis in mesocosm and pond experiments and found that the flux of MeHg via emergent insects was strongly suppressed by the top-down effects of fish and enhanced by the bottom-up effects of nutrients.  These studies suggest that the potential for insect-mediated MeHg flux increases with Hg contamination levels of the environment, but that the realized MeHg flux of individual insect taxa is determined by fish predation and aquatic nutrient concentration.