Calculated Fluxes of Mercury to Fish in the Robert-Bourassa Reservoir

Abstract

A detailed analysis of existing fish and plankton data allowed estimation of biomass and mercury (Hg) fluxes in fish during the years 1978–1984 immediately following the flooding of the reservoir. Fish were divided into two arbitrary groups: piscivores and prey (non-piscivores) accounting globally for more than 90% of all fish biomass captured during the 7 years period. Extensive catch-per-unit-effort data available over time was used to estimate fish yield based on productivity and to calculate standing stocks allocated to both groups. A strictly pelagic food chain (from plankton to non-piscivorous fish to piscivores) was assumed for the computation of the biomass and Hg fluxes from one trophic level to another. Using a verified plankton model for the reservoir, “available” biomass and Hg fluxes to prey fish (non-piscivores) were calculated. From the field data and calculated standing stocks, the “available” biomass and Hg fluxes from the prey fish to the piscivorous fish were also calculated. Conversely, from field data and the calculated standing stocks for both groups of fish, fluxes “required” to explain the fish biomass and Hg concentration of fish, were also computed. The magnitude of the deficits between the “available” and “required” fluxes showed that an entirely pelagic food chain could not account for observed Hg in fish, but that the greatest vector of Hg transfer to fish must have been the benthos. This conclusion was supported independently by analyses of fish stomach contents and recent data on Hg concentrations in benthos. The fluxes of total Hg, as dissolved Hg but more likely Hg associated with particulates, from decomposing vegetation and soil to the water column were also calculated in order to estimate the global fluxes of Hg within several compartments of the reservoir in the first few years after flooding.