Predictors of mercury and pesticide fluxes by emerging adult aquatic insects from Prairie Pothole wetlands

Aquatic and terrestrial food webs are linked by movements of adult aquatic insects from aquatic to terrestrial ecosystems. Contaminants can affect these linkages by reducing insect survival and increasing tissue concentrations through adult metamorphosis, thus changing contaminant flux from water to land. Most anthropogenically influenced freshwater ecosystems are exposed to multiple contaminants. To better understand the combined effects of contaminants on aquatic-terrestrial linkages, we investigated how contaminants affect these linkages, focusing on the combined effects of mercury and pesticides on insect-mediated contaminant flux from wetlands. In a field survey of 15 wetlands in the Prairie Pothole Region, an important agricultural region of North America, we found that insect-mediated mercury flux was 3.5 times more strongly predicted by emergence biomass than tissue mercury. Thus, factors that were previously found to influence aquatic insect emergence biomass in this system, including insecticide tissue concentrations in adult insects, open water surface area, and agricultural land use, were the most likely to drive insect-mediated mercury flux. Pesticide flux, on the other hand, was 3.3 times more strongly predicted by tissue concentration of pesticides than by emergence. Thus, factors that influence pesticide exposure, and to a smaller extent emergence biomass, were the most likely to drive pesticide flux. Our results show how factors driving contaminant fluxes by adult aquatic insects differ by contaminant class and suggest one mechanism by which toxic effects of one contaminant (pesticides) could influence the flux of another (mercury). Predicting contaminant fluxes in areas where different mixtures of contaminants are present can aid in identifying risk to insectivores.