METHYL MERCURY IN AQUATIC INSECTS FROM AN EXPERIMENTAL RESERVOIR

Our objective was to study the effects of experimental flooding of a s mall wetland lake on the methyl mercury (MeHg) concentrations in aquat ic insects and to compare MeHg concentrations in insects with those in water and fish from the same system. Insects were collected from the shorelines of the experimental reservoir before and after flooding, an undisturbed wetland lake, and an oligotrophic lake, all in the Experi mental Lakes Area in northwestern Ontario. Samples were identified to the lowest possible taxon and categorized into functional feeding grou ps (FFGs; predators or collector/shredders). The insects were analyzed for MeHg and total Hg using clean techniques. Contamination was not a problem because levels of MeHg in insects were much higher than backg round concentrations. Odonata, Corixidae, Gerridae, Gyrinidae, and Phr yganeidae/Polycentropodidae exhibited increases in MeHg concentrations in response to flooding. When data were grouped into FFGs, increases were observed in predators. There were insufficient numbers of collect or/shredders collected to make a definitive conclusion on MeHg increas es. Predators exhibited an approximately threefold increase in MeHg co ncentrations after flooding compared with a 20-fold increase in water concentrations and a four- to five-fold increase in fish concentration s. Trends in MeHg concentrations in aquatic insects from reservoirs an d natural lakes in Finland and northern Quebec were similar to ours. E vidence of an increase in MeHg concentrations in the lower food web he lps explain increases in MeHg concentrations in fish from reservoirs b ecause food is the major pathway of MeHg uptake in fish tissue.