Bioaccumulation of Mercury and Methylmercury in Invertebrates from Natural Boreal Lakes

Abstract

Plankton was sampled in 13 natural lakes using conical nets of different mesh size, aquatic insect larvae were sampled in 11 lakes by dragging 250 μm mesh framed nylon bags and adult insects were collected by emergence floating traps. Total mercury (Hg) concentrations in insects (larvae and adults) and zooplankton ranged from 31 ng g^-1 dry weight (dw) to 793 ng g^-1 dw. Methylmercury (MeHg) levels were lower but showed similar variation from 25 ng g^-1 to 575 ng g^-1 dw. The mean proportion of MeHg to total Hg concentrations depended on the feeding behavior of the animals, increasing from 35–45% in detritivore insect larvae (dipterans, ephemeropterans, trichopterans) to 70–85% in predators (heteropterans, coleopterans, odonates). Similarly, the mean proportion of MeHg increased in the planktonic food web from 20–25% in the 20–75 μm mesh plankton to 60–70% in the 225 μm mesh plankton. These differences are attributed to the biomagnification of MeHg in the food web with the biomagnification factor between two adjacent trophic levels being about 3. Two principal factors control the contamination of predators at the top of the food chain: (1) the bioavailability of Hg to lower trophic levels (influenced by the quality and the Hg concentration of the ingested food) and (2) the feeding behavior of the animals.