Roger Schetagne

Export of mercury downstream from reservoirs

Environmental effects monitoring at the La Grande hydroelectric complex (Québec, Canada) revealed important increases in mercury levels in fish caught downstream from reservoirs. A study was carried out in 1997 immediately below the Caniapiscau Reservoir to identify by which components mercury is transported downstream from reservoirs and to assess the amount of mercury being exported. Stomach contents of lake whitefish (Coregonus clupeaformis) captured immediately below the Caniapiscau Reservoir were examined to determine which components transfer mercury from lower trophic levels to fish. The analyses of water samples and drifting organisms collected below the reservoir indicate that the dissolved fraction (< 0.45 microm) and the suspended particulate matter (0.45-50 microm) are the major components by which methylmercury is transferred downstream of reservoirs, accounting for 64 and 33%, respectively, of the total amounts exported. Drifting organisms such as plant debris, benthic invertebrates, fish, phytoplankton and zooplankton are much less important pathways for mercury export because of their very low biomass per water volume coming out of the generating station, as opposed to the high biomass of suspended particulate matter. However, zooplankton is the major component by which methylmercury is directly transferred to non-piscivorous fish downstream.

Different mercury bioaccumulation rates between sympatric populations of dwarf and normal lake whitefish (Coregonus clupeaformis) in the La Grande complex watershed, James Bay, Québec

Fish mercury concentrations were monitored before and after the construction of the La Grande hydroelectric complex, located in the James Bay region, Québec, Canada. Monitoring results revealed that sympatric populations of dwarf and normal lake whitefish (Coregonus clupeaformis) occur in reservoirs and natural lakes of the area. Dwarf specimens bioaccumulate mercury more rapidly than normal individuals. In natural Sérigny Lake, 6–7 year old dwarf specimens show mean total mercury levels 1.5 times higher than normal individuals of the same age (0.33 vs 0.21 mg kg-1 wet weight). Eleven years after the impoundment of the Caniapiscau reservoir, 6–7 year old dwarf specimens show total mercury concentrations 3 times higher than normal specimens of similar ages (0.74 vs 0.27 mg kg-1). Stomach content analysis showed that although large normal individuals feed on different prey than do dwarf individuals, similar methyl mercury levels were found in prey of both forms. We suggest that dwarf individuals bioaccumulate mercury more rapidly than normal specimens because they begin to mature at an earlier age (usually at age 2 to 3 compared to age 6 for normal specimens) and thus produce proportionally less flesh to dilute the mercury assimilated. Total mercury analysis indicating higher concentrations in flesh than in gonads support this hypothesis. Our results suggest that when monitoring mercury levels in fish in areas where populations of dwarf and normal lake whitefish coexist, dwarf and normal specimens must be distinguished because of their different bioaccumulation rates.

The use of polynomial regression analysis with indicator variables for interpretation of mercury in fish data

Mercury levels in fish in reservoirs and natural lakes have been monitored on a regular basis since 1978 at the La Grande hydroelectric complex located in the James Bay region of Québec, Canada. The main analytical tools historically used were analysis of covariance (ANCOVA), linear regression of the mercury-to-length relationship and Student-Newman-Keuls (SNK) multiple comparisons of mean mercury levels. Inadequacy of linear regression (mercury-to-length relationships are often curvilinear) and difficulties in comparing mean mercury levels when regressions differ lead us to use polynomial regression with indicator variables.For comparisons between years, polynomial regression models relate mercury levels to length (L), length squared (L2), binary (dummy) indicator variables (Bn), each representing a sampled year, and the products of each of these explanatory variables (L × B1, L2 × B1, L × B2, etc.). Optimal transformations of the mercury levels (for normality and homogeneity) were found by the Box-Cox procedure. The models so obtained formed a partially nested series corresponding to four situations: (a) all years are well represented by a single polynomial model; (b) the year-models are of the same shape, but the means may differ; (c) the means are the same, but the year-models differ in shape; (d) both the means and shapes may differ among years. Since year-specific models came from the general one, rigorous statistical comparisons are possible between models.Polynomial regression with indicator variables allows rigorous statistical comparisons of mercury-to-length relationships among years, even when the shape of the relationships differ. It is simple to obtain accurate estimates of mercury levels at standardized length, and multiple comparisons of these estimations are simple to perform. The method can also be applied to spatial analysis (comparison of sampling stations), or to the comparison of different biological forms of the same species (dwarf and normal lake whitefish).

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

Abstract 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.

Post-Impoundment Evolution of Fish Mercury Levels at the La Grande Complex, Québec, Canada (from 1978 to 1996)

At the La Grande hydroelectric complex, the evolution of fish mercury (Hg) levels was monitored in reservoirs, along river diversion routes, and in rivers with modified flow, from 1978 to 1996. Five fish species were considered: two non-piscivorous, lake whitefish (Coregonus clupeaformis) and longnose sucker (Catostomus catostomus), and three piscivorous, northern pike (Esox lucius), walleye (Stizostedion vitreum) and lake trout (Salvelinus namaycush). Total Hg concentrations were measured by standard cold vapour atomic absorption spectrophotometry, and expressed in mg kg-1, wet weight. In reservoirs, concentrations in all species increased rapidly after impoundment, peaking after 5 to 9 years in non-piscivorous fish, and after 10 to 13 years in piscivorous species, at levels 3 to 7 times those measured in surrounding natural lakes, then significantly and gradually declined. Data from the La Grande complex strongly suggest that concentrations return to natural levels after 10 to 25 years for non-piscivorous species, and after 20 to 30 years for piscivorous ones. This duration of the phenomenon is corroborated by results from other reservoirs in Canada and Finland. Monitoring of Hg levels in fish and studies of drifting organisms also show that Hg is exported downstream from reservoirs, probably mostly by reservoir Hg-rich organic debris as well as by plankton, aquatic insects or small fish. This export may affect Hg levels in fish over long distances downstream from reservoirs in the absence of large deep bodies of water allowing sedimentation of organic debris or biological uptake of Hg-rich organisms originating from reservoirs. Along a series of 4 large reservoirs emptying into one another, the highest Hg levels were systematically measured in fish caught immediately downstream from reservoir outputs, but no cumulative effect in fish Hg levels was observed from the first to the last reservoir. In a river which sustained a reduction of flow of over 90%, fish Hg levels remained within the range of concentrations measured in fish collected from natural surrounding lakes.

Mercury in Fish of Natural Lakes of Northern Québec

Mercury (Hg) concentrations in fish have been documented for over 180 sampling stations located in natural lakes and rivers of northern Quebec. Mean Hg concentrations were estimated for fish of standardized length and of the following species: longnose sucker (Catostomus catostomus), white sucker (Catostomus commersoni), lake whitefish (Coregonus clupeaformis), northern pike (Esox lucius), walleye (Stizostedion vitreum), and lake trout (Salvelinus namaycush). Mean concentrations for 400-mm non-piscivorous fish of various lakes, ranging from 0.05 to 0.30 mg kg-1, were always well below the Canadian marketing standard of 0.5 mg kg-1 wet weight. For piscivorous species, concentrations often exceeded this standard, with mean levels ranging from 0.30 to 1.41 mg kg-1 for standardized lengths of 400 to 700 mm (depending on the species). Inter-lake variability within the same region is important for all fish species, as estimated mean concentrations often vary by factors of 3 to 4 for neighboring bodies of water. In the Nottaway-Broadback-Rupert region, where lakes and rivers display a wide range of physical and chemical properties, higher fish Hg concentrations were usually found in bodies of water with high organic content as described by color and concentrations of tannins, as well as total and dissolved organic carbon.

Evaluation of a Questionnaire-Based Method for the Estimation of Methylmercury Exposure of Recreational Anglers in the James Bay Territory (Québec, Canada)

The impoundment of reservoirs temporarily increases the methylation of mercury bound to flooded soils and vegetation and the transfer of methylmercury (MeHg) to fish. MeHg levels in various fish species of hydroelectric reservoirs located in the James Bay territory increased by factors of 3 to 7, then gradually declined toward initial concentrations 10 to 20 years after flooding, depending on reservoir characteristics. The potential risk of increased MeHg exposure for recreational anglers who consume fish from these reservoirs had not been assessed previously. A less invasive method than systematic measurement of Hg levels in hair was developed to determine MeHg exposure of recreational anglers. A fish consumption questionnaire-based approach was combined with a toxicokinetic model to estimate the corresponding hair MeHg concentrations. The results were compared with actual analytical determinations of hair Hg levels for the 94 recreational anglers recruited for the study. The values predicted by the model based on self-reporting consumption overestimated actual hair Hg levels by an average factor greater than 6. The mean hair level predicted for the most recent period (September-October) was 23.3 µg.g−1 compared to 3.6  µg.g−1 for the measured value. Although the questionnaire protocol may certainly be improved to increase the precision of estimations, direct hair Hg measurement remains the more effective means to assess Hg exposure.

Intensity and duration of effects of impoundment on mercury levels in fishes of hydroelectric reservoirs in northern Québec (Canada)

Abstract At the La Grande Hydroelectric Complex (Québec, Canada), total mercury (THg) levels in fish were monitored from 1978 to 2012 in more than 37 000 fish comprising 5 species: lake whitefish (Coregonus clupeaformis), longnose sucker (Catostomus catostomus), northern pike (Esox lucius), walleye (Sander vitreus), and lake trout (Salvelinus namaycush). In reservoirs, concentrations of all species increased rapidly after impoundment, peaking after 4–11 yr in nonpiscivorous species and after 9–14 yr in piscivorous species, at levels 2–8 times higher than those measured in surrounding natural lakes. In fish of standardized length, maximum levels reached 0.33–0.72 μg g−1 in nonpiscivorous species and 1.65–4.66 μg g−1 in piscivorous species. Depending on the reservoir, the return to levels equivalent (p < 0.05) to those found in fish in surrounding natural lakes was completed after 10–20 yr for all nonpiscivorous species and after 20– 31 yr in most piscivorous species, if no additional flooding occurred. These results tend to confirm the findings of other authors suggesting that the following reservoir characteristics play a major role in determining the intensity and duration of after-impoundment THg increases in fish: flooded area, annual volume of water flowing through the reservoir, filling period, water temperature, and percentage of flooded area located in the drawdown zone.

Résumé et synthèse

Dans cet ouvrage, nous presentons une synthese des etudes portant sur la problematique du mercure (Hg) dans les milieux aquatiques naturels et les reservoirs hydroelectriques du Nord du Quebec. Cette synthese est basee sur plus de vingt ans de suivi environnemental au complexe La Grande realise par Hydro-Quebec, ainsi que sur les resultats d’etudes menees pendant plus de dix ans par des equipes de recherches de l’Universite du Quebec a Montreal, de l’Universite de Sherbrooke, de la Faculte de Medecine veterinaire de l’Universite de Montreal, du Service canadien de la faune et d’Hydro-Quebec.