An experiment was conducted during summer, 1992, in a protected bay of the La Grande-2 reservoir (northern Quebec) to evaluate the role of erosional processes in the transfer of mercury from a flooded soil to the water column. Up to 57% of the organic carbon, approximately 4% of the inorganic mercury, and nearly 71% of the methylmercury from the humic horizon of a flooded podzol were transferred to the water column and transported outside the experimental perimeter in the form of particulate matter after one resuspension event. In contrast, total dissolved mercury concentrations in the water column decreased during the resuspension event as a result of scavenging by suspended particles. We postulate that most of the methylmercury produced in the submerged soils is associated with fine-grained organic particles that remain in suspension for some time before flocculating and accumulating at the sediment–water interface. We suspect that these particles are a potential food source and can be readily ingested by pelagic and benthic detritus-feeders as well as trapped by periphyton. Resuspension and erosion processes along the exposed shores of a recently impounded reservoir may thus accelerate the transfer of mercury, primarily as methylmercury, from flooded soils to predatory fish.

Suspended particulate matter (SPM) retained by continuous-flow centrifugation, suspended matter of 20-150 mu m diameter, and zooplankton exceeding 150 mu m were sampled in the LG-2 and LA-1 reservoirs and in four natural lakes in northern Quebec during June, August, and September 1992 and 1993. Inorganic Hg concentrations in the three compartments did not differ significantly between the reservoirs and natural lakes, but methyl mercury (MeHg) concentrations in SPM (mean of 0.05 ppm) and zooplankton (up to 0.84 ppm) from LG-2 were 7 and 5 times higher, respectively, than in the natural lakes. The MeHg bioamplification factors between SPM and zooplankton ranged from 4 to 9 in the reservoirs and natural lakes. MeHg concentrations in SPM reached a maximum during August and were positively correlated with organic matter content. High MeHg concentrations were observed in the SPM and zooplankton sampled in August in a small impoundment separated from LA-1 by the LA-40 dike, which had been flooded for about 1 year. These data suggest that methylation and transfer of MeHg from flooded soils to SPM and zooplankton are rapidly accomplished. The process involves the bioaccumulation of MeHg by phytoplankton and (or) the ingestion of suspended soil-derived organic particles by zooplankton.

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