Bioaccumulation of Methylmercury in Invertebrates from Boreal Hydroelectric Reservoirs

Abstract

Methylmercury (MeHg) concentrations in invertebrates (insects and zooplankton) increased rapidly in the first years of flooding and remained high in non-eroded littoral zones of reservoirs 16 years after flooding with values ranging from 45 ngg^-1 dry weight (dw) to 650 ng g^-1 dw. In general, both biomass and MeHg content in invertebrates from the reservoirs were 3–5 times (up to 10 times) higher than in their counterparts from natural lakes. Plankton collected from pelagic zones 8 years after impoundment showed MeHg concentrations similar to those of natural lakes with values ranging from 20 ng g^-1 dw to 140 ng g^-1 dw. The results indicate a biomagnification of MeHg along the invertebrate food chain, since MeHg concentrations increase when moving from suspended particulate matter (SPM) to > 150 μm-mesh plankton for the pelagic food chain and from the substrates to the predators insects for the benthic food chain. The biomagnification factors are independent of the type of environment (natural lake or reservoir). The results also suggest, in reservoirs, that SPM eroded from flooded soils by wave and ice action, and the biofilm at the soil-water interface may constitute the active route for the transfer of MeHg from flooded soils and vegetation to the invertebrates. In the long term, the fluctuation of reservoir water levels, which gradually erodes the flooded soils, results in a loss of habitat for insects, a decrease of the amount of SPM and a reduction of the overall invertebrate biomass and MeHg burden. These processes probably explain the decrease of Hg levels observed in non-piscivorous fish, as both the food and the MeHg sources provided by the invertebrates decrease over time.