Nowadays, major environmental issues are the object of large public debates de­ spite the fact that scientific knowledge is often insufficient to draw unequivocal conclusions. Such is the case in the ongoing debate regarding the specific contri­ butions of anthropogenic greenhouse gas emissions and of natural climate changes to global warming. At least 10 to 20 years of additional observations will be re­ quired, before we will be able to conclude, with certainty, on this subject. In the mean time, and as directed by their immediate interests, people will continue to promote contradictory opinions. The media are, in part, responsible for perpetuat­ ing such debates in that they convey indiscriminately the opinion of highly credi­ ble scientists as that of dogmatic researchers, the latter, unfortunately too often expressing working hypotheses as established facts. Naturally, in a similarly mis­ informed manner, pressure groups tend to support the researcher whose opinions most closely represent either their particular ideological battles or their economic interests and, hence, in their own way, add further to the confusion and obscurity of the debate. Only a few years ago, mercury (Hg)contamination in hydroelectric reservoirs was the object of such media and social biases. At the time, analytical data used to support the discourse were themselves uncertain and numerous hypotheses, often times fanciful, were proposed and hastily "delivered" to the public.

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Résumé et synthèse (French summary).- 1 Introduction.- 1.1 The Mercury Issue in Northern Québec.- 1.2 Geographic Setting.- 1.2.1 Drainage Basin and Relief.- 1.2.2 Climate and Hydrology.- 1.2.3 Vegetation and Wildlife.- 1.2.4 Water Quality and Fish.- 1.2.5 Human Settlements.- 1.3 Hydroelectric Developments.- 1.3.1 Reservoirs.- 1.3.2 Other Modified Environments.- 1.4 Evolution of the Biophysical Environment.- 1.4.1 Water Quality.- 1.4.2 Plankton and Benthos.- 1.4.3 Fish.- 1.5 Contents and Rationale.- 2 Analysis of Total Mercury and Methylmercury in Environmental Samples.- Abstract.- 2.1 Introduction.- 2.2 Total Mercury Measurements.- 2.2.1 Total Mercury by Cold Vapor Atomic Absorption Spectrophotometry.- 2.2.1.1 Organisms and Sediment Samples.- 2.2.1.2 Water Samples.- 2.2.2 Total Mercury by Atomic Fluorescence Spectrophotometry.- 2.2.2.1 Organisms and Sediment Samples.- 2.2.2.2 Water Samples.- 2.3 Methylmercury Measurements.- 2.3.1 Organisms and Sediment Samples.- 2.3.2 Water Samples.- 2.4 Conclusions.- Acknowledgements.- Mercury and Methylmercury in Natural Ecosystems of Northern Québec.- 3 Mercury in Natural Lakes and Unperturbed Terrestrial Ecosystems of Northern Québec.- Abstract.- 3.1 Introduction.- 3.2 Materials and Methods.- 3.2.1 Aquatic Systems.- 3.2.2 Terrestrial Systems.- 3.2.3 Analyses.- 3.3 Results and Discussion.- 3.3.1 Mercury in Natural Lakes of Northern Québec.- 3.3.1.1 Presence and Diagenesis of Mercury in Lake Sediments.- 3.3.1.2 Historic Interpretation of the Presence of Mercury in Sediments.- 3.3.1.3 Sedimentary Mercury and Terrigenous Organic Matter.- 3.3.1.4 Mercury in the Water Column of Natural Lakes.- 3.3.1.5 Mercury in Aquatic Macrophytes and in Riparian Plants.- 3.3.2 Mercury in Terrestrial Systems of Northern Québec.- 3.3.2.1 Mercury in Terrestrial Vegetation of Northern Québec.- 3.3.2.2 Distribution of Mercury in Forest Soils and Peatlands of the Boreal Domain.- 3.3.2.3. Anthropogenic Fraction of Mercury in Soils of Northern Québec.- 3.3.3 Patterns of Atmospheric Mercury Deposition throughout Northern Québec.- 3.4 Conclusions.- Acknowledgements.- 4 Bioaccumulation of Mercury and Methylmercury in Invertebrates from Natural Boreal Lakes.- Abstract.- 4.1 Introduction.- 4.2 Materials and Methods.- 4.2.1 Study Areas.- 4.2.2 Sampling.- 4.2.2.1 Insects.- 4.2.2.2 Plankton.- 4.2.3 Analyses.- 4.3 Results and Discussion.- 4.3.1 Total Mercury and Methylmercury Concentrations in Invertebrates.- 4.3.2 Bioavailability of Total Mercury and Methylmercury for Insect Larvae.- 4.3.3 Environmental Factors Influencing Mercury Bioavailability.- 4.3.4 Biomagnification of Methylmercury along the Food Web.- 4.3.5 Diversity, Biomass and Mercury Burdens.- 4.4 Conclusions.- Acknowledgements.- 5 Mercury in Fish of Natural Lakes of Northern Québec.- Abstract.- 5.1 Introduction.- 5.1.1 Study Area.- 5.2 Materials and Methods.- 5.3 Results.- 5.4 Discussion.- 5.5 Conclusions.- Acknowledgements.- 6 Mercury in Birds and Mammals.- Abstract.- 6.1 Introduction.- 6.2 Study Area.- 6.3 Materials and Methods.- 6.4 Results and Discussion.- 6.4.1 Mercury in Birds.- 6.4.2 Relationship with Geographical and Habitat Distribution..- 6.4.3 Diet Types.- 6.4.4 Eggs.- 6.4.5 Terrestrial Mammals.- 6.4.6 Marine Mammals.- 6.4.7 Freshwater Seals.- 6.5 Conclusions.- Acknowledgements.- Mercury Dynamics at the Flooded Soil-Water Interface in the Reservoirs.- 7 In Vitro Release of Mercury and Methylmercury from Flooded Organic Matter.- Abstract.- 7.1 Introduction.- 7.2 Materials and Methods.- 7.2.1 Sample Collection.- 7.2.2 Experimental Set up.- 7.2.3 Analytical Methods.- 7.2.3.1 Total Hg.- 7.2.3.2 MeHg.- 7.3 Results.- 7.4 Discussion.- 7.5 Conclusions.- Acknowledgements.- 8 Mercury Dynamics at the Flooded Soil-Water Interface in Reservoirs of Northern Québec: in Situ Observations.- Abstract.- 8.1 Introduction.- 8.2 Materials and Methods.- 8.2.1 Study Sites.- 8.2.2 Sampling.- 8.2.3 Analyses.- 8.3 Results and Discussion.- 8.3.1 Temporal Evolution of Mercury in Flooded Soils.- 8.3.2 Mercury in the Water Column of Flooded Systems.- 8.3.2.1 Dissolved Fraction.- 8.3.2.2 Particulate Fraction.- 8.3.3 Influence of Flooded Soils on the Biogeochemical Cycle of Mercury in Reservoirs.- 8.3.3.1 Diffusion of Mercury from Flooded Soils.- 8.3.3.2 Burrowing Organisms and Erosion.- 8.3.3.3 Enhanced Biological Production and Methylation of Mercury.- 8.4 Conclusions.- Acknowledgements.- Evolution of Mercury Concentrations in Aquatic Organisms from Hydroelectric Reservoirs.- 9 Bioaccumulation of Methylmercury in Invertebrates from Boreal Hydroelectric Reservoirs.- Abstract.- 9.1 Introduction.- 9.2 Materials and Methods.- 9.2.1 Study Area.- 9.2.2 Sampling.- 9.2.2.1 Insects.- 9.2.2.2 Plankton.- 9.2.2.3 Flooded Soils and Lake Sediments.- 9.2.3 Analysis.- 9.3 Results and Discussion.- 9.3.1 Methylmercury Concentrations in Invertebrates.- 9.3.2 Biomagnification of Methylmercury Along the Invertebrate Food Chain.- 9.3.3 Bioavailability of Methylmercury to Insects.- 9.3.4 Bioavailability of Methylmercury to Plankton.- 9.3.5 Biomass and MeHg Burdens.- 9.4 Conclusions.- Acknowledgements.- 10 Mercury Accumulation in Fish from the La Grande Complex: Influence of Feeding Habits and Concentrations of Mercury in Ingested Prey.- Abstract.- 10.1 Introduction.- 10.2 Materials and Methods.- 10.3 Results and Discussion.- 10.3.1 Non-Piscivorous Fish.- 10.3.2 Piscivorous Fish.- 10.4 Conclusions.- Acknowledgements.- 11 Post-Impoundment Evolution of Fish Mercury Levels at the La Grande Complex, Québec, Canada (from 1978 to 1996).- Abstract.- 11.1 Introduction.- 11.2 Materials and Methods.- 11.3 Results.- 11.3.1 Reservoirs.- 11.3.2 Dwarf Lake Whitefish.- 11.3.3 Rivers with Reduced Flow.- 11.3.4 Diversion Routes.- 11.3.5 Downstream from Reservoirs.- 11.3.5.1 The Caniapiscau River.- 11.3.5.2 The La Grande River.- 11.3.5.3 The Coast of James Bay.- 11.4 Discussion.- 11.4.1 Reservoirs.- 11.4.1.1 Increases of Fish Mercury Levels in Reservoirs.- 11.4.1.2 Factors Explaining Differences Observed Between Reservoirs.- 11.4.1.3 Duration of the Phenomenon in Reservoirs.- 11.4.2 Downstream from Reservoirs.- 11.4.3 Rivers with Reduced Flow.- 11.5 Conclusions.- Acknowledgements.- 12 Calculated Fluxes of Mercury to Fish in the Robert-Bourassa Reservoir.- Abstract.- 12.1 Introduction.- 12.2 Materials and Methods.- 12.2.1 Estimation of Fish Standing Stocks.- 12.2.2 Estimation of Biomass Fluxes.- 12.2.3 Estimation of Hg Fluxes.- 12.2.4 Calculation of Hg Fluxes from Vegetation.…