Total mercury and methylmercury accumulation in zooplankton of hydroelectric reservoirs in northern Québec (Canada)

Plankton was sampled in seven reservoirs and in five natural lakes using conical nets of different mesh size. Mean methylmercury (MeHg) concentrations in plankton 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 280 ng g−1 dry wt. to 450 ng g−1 dry wt. 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 dry wt. to 140 ng g−1 dry wt. Our results indicate a biomagnification of the MeHg in the planktonic food web from suspended particulate matter (SPM) to plankton>225-μm mesh and the biomagnification factors are independent of the type of environment (natural lakes or reservoirs). Our results also suggest, in reservoirs, that suspended particulate matter (SPM) eroded from flooded soils by wave and ice action and bacterial production, enhanced by the release of labile carbon and nutrients from the flooded soils, transfer MeHg from flooded soils to plankton. The fluctuations of reservoir water levels gradually erode the flooded soils, thus providing contaminated particles for ingestion by zooplankton: this process stops faster in exposed littoral zones than in protected ones. We propose that these processes increase the bioavailability of MeHg to organisms at the base of the food web in reservoirs, relative to natural lakes, and account for the observed temporal patterns of MeHg levels in planktonic organisms over time.