TOTAL MERCURY AND METHYLMERCURY ACCUMULATION IN ZOOPLANKTON OF HYDROELECTRIC RESERVOIRS IN NORTHERN QUEBEC (CANADA)

Plankton was sampled in seven reservoirs and in five natural lakes usi ng conical nets of different mesh size. Mean methylmercury (MeHg) conc entrations in plankton increased rapidly in the first years of floodin g and remained high in non-eroded littoral zones of reservoirs 16 year s 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 lak es with values ranging from 20 ng g(-1) dry wt. to 140 ng g-l dry wt. Our results indicate a biomagnification of the MeHg in the planktonic food web from suspended particulate matter (SPM) to plankton > 225-mu m 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 flo oded soils by wave and ice action and bacterial production, enhanced b y the release of labile carbon and nutrients from the flooded soils, t ransfer MeHg from flooded soils to plankton. The fluctuations of reser voir water levels gradually erode the flooded soils, thus providing co ntaminated particles for ingestion by zooplankton: this process stops faster in exposed littoral zones than in protected ones. We propose th at these processes increase the bioavailability of MeHg to organisms a t the base of the food web in reservoirs, relative to natural lakes, a nd account for the observed temporal patterns of MeHg levels in plankt onic organisms over time. (C) 1998 Elsevier Science B.V.