Geochemistry of Mercury in Two Hydroe ectric Reservoirs in Quebec, Canada'

Patrick Louchouarn, Marc Lucotte, Alfonso Mucci, and Pierre Pichet Universitb dar Quebec B Montreal, Chaire de Recherche en Envisonnement, C.P. 8888, Succ. A, Montreal IQu6bec) H3C 3P8, Canada

Louchouarn, P., M. Lucotte, A. Mucci, and P. Pichet. 5993. Geochemistry sf mercury in two hydroelectric reservoirs in Quebec, Canada. Can. 1. Fish. Aquat. Sci. 50: 269-281.

Carbon, nitrogen, phosphorus, and iron oxyhydroxide profiles were used to explain the retention sf Hg in sed- iments of natural lakes and flooded soils in Quebec. In lake sediments, Hg levels increased from background concentrations of approximately 100 ng/g beneath the 10- to 16-cm depth interval to 150-490 ngig near the surface. This increase was attributed to atmospheric inputs to the environment. in forest soils, the close association of Hg with the surficial organic horizon appears to be a potential source of contamination for the aquatic system through the transport sf particulate matter by surface runoff. High organic matter content observed in the flooded soil of LG-2 Reservoir suggests that postsedimentation biodegradation is a slow process. In contrast, at Cabonga Reservoir, an 80% reduction in C, N, and Hg concentrations was observed between the flooded and the adjacent forest soil. Mechanical erosion processes (marling, waves, ice) rather than biodegradation seem responsible for the observed Boss in C, N, and Hg. We propose that suspension of the humic horizon from flooded soils could constitute a significant source of Hg contamination to the overlying waters and biota until the exposed littoral zone stabilizes.

Bes profils de carbone, azote, phosphore et hydroxydes defer ont servi A expiiquer la retention du mercure dans des sediments lacustres et des sols ennoyes du Quebec. Dans les skdiments lacustres, I'augrnentation des concen- trations de base en Hg, d'environ 100 ng/g 3 18-4 6 crn de profondeur, jusqu'h 150-490 ng/g vers la surface, ?i

kt6 reliee un apport aatmospheriqare de Hg. Bans les sols forestiers, I'association du Wg .3 I'horizon hurrlique de surface represente une source potentielie de contamination du systeme aqktatique lors de I'entrainernent par ruissellement de matiPre particulaire. Les hautes teneurs en rnati&re organique du sol inorad6 du reservoir LG-2 indiquerst que la biodegradation postgdirnentaire est un lent processus. Par contre, dans ie reservoir Cabonga, une reduction de 80 % des concentrations era C, N et Hg a kt$ observee entre le sol inonde et un sol forestier adjacent. Bes processus d'erosion (rnarnage, vagues, glaces), plut8t que la bid6igradation9 semblent &re res- pnsables des pertes observees. La mise en suspension de l'horinon hurnique des sols inondes pourrait seprbsenter une source importante de contamination par le Hg des eaux et des organismes aquatiques jusqu'2 ce que les zones littorales exposees soient stabiliskes.

Received Ianuary 27, 6 992 Accepted August 1 9, 1992 (JB383)

C oncerns over the contamination of aquatic environments by Hg have been raised in the last two decades when high levels of Hg were observed in the biota and sedi-

ments of natural lakes and newly impounded hydroelectric res- ervoirs distant from any point source of contamination (Potter et aB. 1975; Cox et d. 1979; Waite et al. 1980; Ouellet and Jones 1982; Lodenius et al. 1883; Bjorklund et aI. 1884; Bodaly et al. 1984; Evans 1986; Hecky et d. 1987a, 1987b; Messier and Roy 1987; Jackson 1988a, 1888b, 1989; Parks 1988; McMarrty et d. 1989). Long-distance atmospheric trans- port of Hg released by anthropsgenic activities has recently been identified as a significant cause of this contamination (Nri- agu 19'79, 1989; Ouellet md Jones 1982; Bj6rkHund et al. 1984; Evans 1986; Johnson 1987; Ryb& et al. 1989; Schroeder et al. 1989; Verta et al. 1989). The atmospheric input of Hg in lacus- trine sediments involves the deposition of Mg-enriched aerosols on soils, vegetation, and water surfaces as well as its remobil- ization from the watershed though surface runoff (Evans 1986; McMurty et al. 1989; Wybk et al. 1989; Verta et al. 1989). h hydroelectric reservoirs, however, additional amounts of

Hg are directly released into the aquatic system from recently 'A contribution to the Chsake de Recherche Hydro-Qu~becJCRSNW

UQAM.

flooded soils. The degree of Hg contamination in fish from these environments is proportional to the flooded surface area (Abemdthy and Cumbie 197'3; Bmce and Spencer 1979; Lsdenius et al. 1983; Bdaly et al. 1984; Bsucher et aB. 1985; Jones and Saint-Onge 1985; Hecky et d. B987b; Jackson 1988a, 1988b). Abnormally high levels of Hg in the biota in these reservoirs are believed to be due largely to accelerated microbial activity resulting from input of terrestrial organic matter and increased loading of labile Hg (Meister et al. 2979; Bdaly et d. 1984; Boucher et al. 1985; Hecky et al. 1987a; Jackson 1988a, B988b, 1989). Hg occurs under various foms in the aquatie environment,

but it is predominantly as organic or methyl mercury (CH, Hg + ) that it is accumulated in higher aquatic organisms (West60 1966; Jensen and Jeme18v 1969). The production of methyl mercury occurs by methylation of inorganic Hg. It is a process that has been associated with the bacterial degradation of nutrient-rich organic matter in sails and sediments (Jensen and Jernelov 1969; Beijer and Jernelov 1979; Fmtani and Rudd 1980; Rudd et al. 1983; Boddy et d. 1984; Jackson 1988a, 1988b). Methylation of inorganic Hg occurs as a means for certain microorganisms of detoxifying their environment (Pan-Hou and %mu-a 1982) or results from an incorrect synthe-

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QUEBEC

FIG. 8 . Cabsnga and LO-2 reservoirs: location and sampling stations.

sis of other organic molecules such as amino acids (Tonomura and Kanzdci 1969; Jemel6v 1975). Microbial methylation of Hg is enhanced by anoxic conditions resulting from degradation of organic-rich sediments. However, under anoxic conditions, inorganic Hg is apt to be immobilized by snlphides (Lodenius et al. 1983; Thanabalasingarra and Pickering 1985; Barks 1988; Hyland et al. 1990), reducing its availability to methylation (Jenerlijv 1975). Because SO: - concentrations in freshwaters are generally low, gulphides should contribute only little to the immobilizatisn of Hg(II) in most Iacustrine sediments. The inorganic Hg, liberated under aerobic conditions and escaping the anoxic zone, can be absorbed by fresh organic matter or iron and manganese oxyhydroxides in the oxic zone near the sediment-water interface (Thanabalasingam and Pickring B 985).

Jackson (1989) reported that in humus-rich sediments the presence sf Fe oxyhydroxides may enhance methylation, despite their ability tto bind inorganic Hg reducing the biological uptake of this element by phytoplankton and benthic organisms (Tessier et 211. 1984; Jackson B 9888s). Furthemore, he proposed (1989) that the presence of hurnic acid coatings on clays of both

nutrient-rich and nutrient-poor sediments depresses methyla- tion and enhances demethgrlaticsn. The production of srgano- mercuric complexes would appear to hinge on a series s f bisgesckremical factors that affect the dparnic balance between methylation and demethylatisn processes. In turn, this balance determines methyl mercury levels in the interstitial waters and its flux out of the sediments. Up to wow, the levels of Hg found in the biota of hydroelectric reservoirs have thus been explained mainly as a function sf backrial methylation and demethylation activities in flooded soils and sediments. In turn, these activities seem to depend on the quality and quantity of organic matter, bisavailable nutrients, and sedimentary colloids such as oxides, clay minerals, md humic matter (Jackson 1989).

Reported Hg contibanination of fish in hydroe%eetric reser- voirs of northern Quebec and Manitoba generally attained max- imum levels after 2-7 yr of impoundment in anonpiscivorcsus species (Strange 1985; Messier and Roy 198%; Verdm et al. '1991) but was still increasing after 7-9 yr of impoundment in piscivorous species (Messier and Roy 1987; Verdon et al. 1991). Field observations reveal that these high Hg levels decreased and stabilized to near background values after 1 5-

278 Can. J . Fish. Aqua. Sci., VQE. 90, 199.3

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TABLE 1 . Description s f sampling sites with data on redox potential at and below the sediment-water interface s f sediments and Wooded soils.

Station 4 E h

Skpl ing Station Station depth surface depth site (No) description (mV) mv)"

-

Cabonga Reservoir

Laporte Lake Cabonga Resewoir Laporte Lake LG-2 Wesewsir

Ancient lacust~nae 6 26 - 15 sediment

Lacustrine sediment 12.5 70 - 1 1 3 Flooded forest soil 1.5 43 - 76 Forest soil Ancient lacustrine

sediment Lacustrine sediment 24 - - Mossy bed 1.8 193 -3263 Flooded soil Forest soil - - -

"Mean redox potential along the stabilized E, profile measured with respect to a AgIAgCI reference electrode (usually from the 2 - to 2-cm interval below the sediment-water interface.).

28 yr in Finnish reservoirs (Verta et al . 1986) and Cedar Lake, Manitoba (Hecky et a%. 1987a), or 20-30 yr in Southern Indian Lake, Hsett Lake, Manitoba (Strange %985), and various res- ervoirs in Quebec (Messier a d Roy 1987: Verdon et al. 199 1). The period required for recovery depends on such factors as the harvest of the fi shergr resources and the annual discharges of the reservoirs (Verdon et al. 1991). While the rapid bioac- cumulation of Hg has been well documented in fish from North American md European reservoirs (Knight and HeKing 1972; Potter et al. 1975; Cox ef al. 1979; Kent and Johnson 1979; Meister et al. 1979; Waite et al. 1980; Lodenius et al. 1983; Vegaa et al. 1986; Messier md Roy 1987; Verdon et al. 1991), the reason for stabilization sf Hg levels has not yet been clearly identified. It appears that the reduction and stabilization of Hg bioaccumulation in older hydroelectric resewsirs may be due in part to a new steady state reached in the flooded soils, although the sequestration of remobilized Hg by new organic matter and/or losses to the atmosphere may also account for the observed stabilization. In the present study, different sedirnen- txy and soil environments are compaed to illustrate the 'appaent9 ' steady state of lacustrine sediments and forest soils,

the transient state of a recently flooded soil, and the new "apparent" steady state reached in an old flooded soil. These comparisons will be used to explain the observed Hg csncen- trations in the v ~ o u s environments studied and to support the proposal of a potentially significant but SO far neglected mech- anism for the active release of Hg from recently Wooded soils to the reservoir environments.

Locations in western Quebec (Fig. 1) that included hydro- electric reservoirs and natural lakes were chosen to study the geochemistry of Hg in lacustrine sediments, forest soils, and differently aged flooded soils. Two reservoirs, Cabsnga and La Grande-2 (LG-2), and their respective reference Bakes, Laporte and Detcheven-y, are separated by approximately 8" of latitude. Cahnga Reservoir (47"N, 76"W) is located in the east-central region of La VC~wdrye Wildlife Natural Park, approximately 356 krn northwest of Montreal. The bedrock consists sf h e - cambrim igneous md mefamophic rocks covered with a mixed

forest of birch (Betuba papyr~fera), maple (Acer saccharurn), fir (Abl'es baBsamea), and black spruce (Picea mariana). LG-2 Reservoir (53"N, 77"W) belongs to the hydroelectric complex of the La Grande River in the James Bay territories, a region located on the granitic bedrock of the P r e c m b ~ a n shield. The region is characterized by a low relief, a moist subarctic con- tinental climate, and a Boreal forest dominated by black spruce (Picea map-iana) and grey pine (Pisaus banksiana). Resemoir impoundment occaamed during the late l"a0's for Cabonga and during 1978-79 for LG-2. The creation of LG-2 resulted in the inundation of 2638 h2 of land (92% of the total surface area of the reservoir). Data on flooded mudace area are not available for Cabonga swing to the lack of studies undertaken at the time of impoundment. Maximal water level fluctuations are 7.7 rn in LG-2 and 4.7 m in Cabonga.

There were four sampling sites at each of the two locations (Fig. 1): (1) the lacustrine sediments of the Bake existing prior to the impoundment of the reservoir (deepest part of the basin; stations 1 and B 8 for Cabonga and LG-2, respectively), (2) the offshore zone of a natural lake situated within 12 km of each reservoir but not affected by its creation (Laporte Lake, station 14; Detcheveq Lake, station 22), (3) a Wooded soil in each reservoir (stations 2 and 20 for Cabonga and LG-2, respec- tively), and (4) an adjacent natural forest soil (stations 9 and 23 for Cabonga and LG-2, respectively). A brief description of sampling sites, their depth, and redox potential in surficiaI sed- iments md at depth is given in Table 1.

The sampling sites within the reservoirs were a small basin in Cabonga and Chapus Bay (also known as Toto Lake) in LG-2. Laporte and Detchevemy Bakes were sampled and used as "reference" sites to represent preimpoundrnent background conditions in lacustrine sediments and adjacent natural forest soils in the Cabonga and LG-2 regions, respectively. In the LG-2 region, Chapus Bay and Detchevemy Lake are similar in size and maximum depth. In the southern sampling site, how- ever, Laporte Lake is deeper and nmswer than the basin sam- pled in Cabonga, and its sumounding slspes are steeper. Thus, Laporte Lake may act as a funnel, trapping and concentrating allwhthowous inputs derived Rom the steep catchment area, while in the Cabrmga basin, the sunounding gentle slopes and

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the constant flat md shallow depth do not favour deposition of dlochthsnous matekd in any particular area.

Materials and Methods

TABLE 2. Totd Hg concentrations (pph) for NRC's standad sediment and fish tissue samples.

H!? Hg Stmdahd ( P P ~ ) ~ (ppWb (ppb)'

Sample Collection

The deepest pat sf each basin was detemined with a portable echosounder, while the flooded forest zones were identified visually (presence of stumps still in place, dead trees, etc.) by scuba divers. Sampling was conducted in late August m&or early September 1990. Relatively undisturbed sediment cores were collected with a miniature stainless steel box core (23 x 23 cm). The box cores were subsmpled with a IS-cm- diameter BVC core taking care to avoid perturbation of the sediment-water interface. In the flooded soil zones, sediments were sampled manually by scuba divers using 15-cm-diameter PVC cores. In addition, in these flooded zones, surficid flocculated sediments were sampled in situ with sterilized 60-mL syringes.

At each offshore and flooded forest station, cores were collected in duplicate. One of the two cores (on average 18- 25 cm long) was immediately transferred to a wltrogen-filled glove box where redox potentials (E,) were measured with Agl AgCl and platinum electrodes within a few hours of sampling (Table I). The second core was sectioned at I-cm intervals over its entire length as was extruded manually. Each sediment section was divided into two parts, sealed in individual plastic bags, and immediately frozen.

Laboratory Analyses

Analytical data for the sediments are reported on a dry weight basis. Water content was detemined from weight loss after freeze-drying. Prior to analysis of total Mg concentrations in the sediments, the smples were spun down for 15 min at 2000 q m to remove excess water. The analyses were then performed on wet sediments, since this procedure minimizes loss of Hg through volatilization. C and N concentrations, '%c/'~C ratios, Fe and Mn oxyhydroxides, and associated P concentrations were determined on freeze-dried and finely ground sediments.

Approximately 1 g of centrifuged sediments was digested in 10 mL of concentrated nitric acid (HNO,) with a low level of Hg plus 1 mL of quartz-distilled 6 M hydrochloric acid (HCI). This oxydatiorm procedure was conducted to dissolve both organornercuric compounds and Hg sulphides (SPEQ 1979) in the f o m of Hg(1I). The digestion was pepformed at a temper- ature sf 120°C for 6 h in 250-mm-long test tubes. The upper part of each tube was cooled by a ventilating system to induce condensation and thus prevent loss through volatilization . Blank solutions were carried through the whole procedure to test for possible contamination during open test-tube digestion or from reagents and acids. Two to four subsamples from each depth interval were digested to evaluate the reproducibility of the ana- lytical method.

Hg concentrations of the digested sediment solutions were determined by cold vapour atomic fluorescence spectrometry using a modification of the method of Bloom md Fitzgerald (1988) dso see Ndahara et al. 1978; Godden and Stockwell 1989). Dissolved Hg(1II) is reduced to elemental Hg(0) vapour by tin(1H) chloride and rapidly carried by a stream s f Hg-free argon into a quartz cell, whex it is excited into fluorescence by UV radiation. This method is less susceptible to interfer- ences (NAahara et al. 1978) and 10-100 times more sensitive than atomic absorption (Blwm a d Fitzgerald 1988). Although

BCSS-1 129 2 12 146 9 6.1 (25) 153 9 8 1 (34) MESS- 1 171 & 14 184 r 9.1 (24) 199 2 14(38) DORM-1 798 -+ 74 - 763 9 34 (13)

"Stmdmd values provided by NRC. 'Experimental values obtained after 1 h of digestion (numbers of

analyses performed in parentheses). "Experimental values obtained after 6 h of digestion (numbers of

analyses performed in parentheses).

the determination of Hg in natural samples at the subnanogram level can be achieved using a preconcenf9ation device (usually a gold trap), this tedious method is more sensitive than required for the present study. Consequently, we measured Hg flusres- cence directly from the argon stream at an optimal flow of 100- 200 mLlmin, allowing the analysis of 10-28 samples per hour.

Control tests perfomed on National Research Council of Canada's (NRC) standard dried sediments (MESS-1, BCSS-1) on fish muscle (DORM-1) showed that there were ns losses of Hg from the open reactive test tubes md that the digestion s f methyl mercury was complete (Table 2). The reproducibility of the analytical method was 7% for the digestion of sediment samples and 4.5% for the fish tissue. Values obtained in this study for the standard sediments are higher than values reported by NRC but are in g o d agreement for the fish tissue. Table 2 also shows Hg concentrations measured for the two standard sediment smples after 1 h of digestion. For both standards, the reproducibility is 4-5%, while the mean values are 8 3- 15 ppb lower than those obtained after 6-h digestions. It would appear that the increased period of digestion carried out in this study releases a fraction of Hg not readily extracted with the shorter digestion period used in the NWC method. In the ease of the fish tissue, however, similar Hg levels obtained by the NRC method md the 6-h digestion indicate that Hg present in bio- logical tissues is fully oxidized after a short digestion period. Further details on the analytical and extraction methods are PIT- sented elsewhere (Chaire de Recherche en Envimnnement HQI CRSNGIUQAM 1992).

Organic C and total N concentrations were measured with a Carlo-Erba CHI% analyzer. Stable C isotope ratios (83Cl'2@) of particulate organic C (BOC) were detemined with a SZRA-12 mass spectrometer. POC from freeze-dried sediment samples was converted to C 0 2 by combustion at 450°C for B h in sealed tubes containing precombusted Cu8. After the usual correc- tions for 178 md peak overlap (Hillaire-Marcel 1976), the results were calculated and reported as 6I3C (%a) values relative to the Pee Dee Belemnite limestone standard (PDB). Differ- ences in the l3C/"C ratios of POC from lacustrine sediments and forest soils were considered significant only when they were greater than 1%. Smaller variations were attributed to sample heterogeneity and thus are not discussed. The Fe oxyhydrox- ides md associated reactive phosphates were extracted using the citrate-dithionite-bicarbnate (CDB) method of Lucotte md d' Awglejan (1985). The precision of the method is 9 7% at the 95% confidence level. In this study, the Fe oxyhydrox- ides and associated orthophosphates are lak1ld ""cb" (Fe,,, and PC,). The organic P (Po,) was also extracted with a 1 N HCH leach on precombusted samples, following a preliminary acid leach to remove detrital B.

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Results

The sediment concentration-depth profiles of Hg in the offshore stations of Caboarga and lCCf-2 reservoirs (stations 1 and 18, respectively) and their reference lakes (Laporte M e , station 14; Betcheveq Lake, station 22) dl showed relatively stable Mg levels at depth and a gradual increase towards the surface in the top 18- 16 em (Fig. 2). Similar profiles were reported by Evms (1 986) and Verta et al. (1989) and are usually interpreted as a reflection of increased mthropogenic production and atmospheric release of Hg in the Bast few decades (Forstner and Wittmann 1981; Ouellet and Jones 1982; Bodaly et al. 1984; SaIomons and Forstner 1984; Evms 1986; McMurty et d. 1989; Ryb& et al. 1989; Verta et al. 1989).

With the exception of Laporte Lake (station 14), the profiles of Hg in the offshore zones varied between a minimum of 50- 180 ngig in the deepest part of the core and a maximum sf 120-200 ngig in the surface sediments. The very high concentrations of Hg found in the surface sediments of Laporte Lake (exceeding 460 nglg) might be attributed to the

geomorphological specificity of its basin, which may act as an efficient trap for organic material from the surrounding wooded area. At this station, the high Mg concentrations were associated with exceptionally high G and N concentrations (240 a d 9 mglg, ~spectively) as compared with the other offshore stations (stations I , B 8, and 22; Fig. 3).

The concentrations of Hg in terrestrial forest soils from both Iscations were similar in trend but different in absolute concentrations (Fig. 2). The forest soil collected near Lapode Lake (station 9) showed a maximum of 300-410 nglg at 2- 6 cm of depth, which corresponds to the C- md N-rich humic "0" horizon (Nadelhoffer and Fry 1988; Fig. 3). The Hg concentrations decreased sharply down the soil profile attaining values close to the analytical detection limit (5 nglg) throughout a sandy horizon at a depth of 10-21 cm. The slight increase in Hg concentrations below 22 cm corresponded to the top of the mineral accumulation "B" horizon where Fe oxyhydroxides accumulate (Fig. 4). In the forest soil from Detcheveq Lake (station 23), the maximum Mg concentrations were two to three times lower than at station 9 and again were restricted to the humic horizon (4-6 crn depth).

Depth (cm) FIG. 2. Wg concentrations (dry weight) at the various sampling stations. (a) Southern Iacusthiaae sediments: @, Cabonga Reservoir (Sta 1); 0, Laporte M e (Sta 14); (b) worthern lacustrine sediments: a, LG-2 Reservoir (Sta 118); 0, Detcheveq Lake (Sta 22); (c) southern f m s t soils: 8, flooded (Sta 2); 0, nanflooded (Sta 9); (d) worthern forest soils: 8, flooded (Sta 20); 0, nonflooded (Sta 23).

Can. J . Fish. Aquat. Sci., Vob. 50, 1993 273

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Sta 2 N +Sla 9 N +Sta 20 N +Sta 23 N

Depth (cm) FIG. 3. C and N concentrations ( d q weight) at the various sampling stations. (a) Southern kicustrine sediments: em, Cab~nga hservoir (Sta 1); On, Lapfie Lake (Sta 14); (b) northern Isacustri-ine sediments: em, EG-2 Reservoir (Sta 18); 00, Detchevev Lake (Sta 22); (c) southern forest soils: em, flooded (Sta 2); 063, nonflooded (Sta 9); (d) northern forest soils: em, flooded (Sta 20); Om, nonflooded (Sta 23).

In Cabonga, the Hg content of the flooded soil (station 2) was much lower than that of the adjacent nonflooded forest soil and was even slightly lower than that observed in offshore sediments (station I). These low Hg levels were associated with low C md N concentrations (Fig. 3) compared with those sf the adjacent forest soil (station 9). In LG-2, the Hg concentrations in flooded soil (station 20) were up to three times higher on average than those found in the nearby unperturbed forest soil (station 23) and twice those of the preimporandment sediments (station 18). At station 20, however, Hg distHibution was associated with two distinct pedologic layers. In the top BO cm, high Hg concentrations (up to 320 wgig) were associated with a mossy bed. Below this saarFacial organic layer, Hg concentrations showed a second but lower maximum (up to 250 ng/g) that magi be associated with the hurnic horizon of a forest soil (Fig. 3).

Geochemical Environment

In the offshore stations of Cabonga Reservoir and Detch- e v e q Lake (stations 1 md 22, respectively), the POC and total N concentrations remained relatively constant below the inter- face (Fig. 3). This may indicate that below the sudicial sedi- mentary layer the rate of biodegradation is almost nil either as a result of the refractory nature of the buried organic matter or the inefficiency of the fermentation reactions. At the offshore stations sf Laporte M e md LG-2 Reservoir (stations 14 a d 18, respectively), a small but significant decrease in C and N concentrations with depth was sbsewed in the first 4-5 cm of the core. With the exception of the offshore station at Laporte Lake, the maximum and minimum concentrations of C and N at the three other stations were spa the order of 68-80 and 2.5- 4 mg/g, respectively. At all four offshore stations (1, 14, 18,

2'74 Can. J . Fish. A q ~ a t . Sci., VQE. 50, 6993

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Depth (cm) FIG. 4. Fe,, concentrations (dry weight) at the various sampling stations. (a) Southern lacustrine sediments: @, Cabonga Reservoir (Sta I); 0, Laprte Lake (Sta 149; (b) northern lacustrine sediments: a, EG-2 Reservoir (Sta 18); 0, Detcheverry Cake (Sea 22); (c) southern forest soils: @, flooded (Sta 2); 0, maonflooded (Sta 9); (d) northern forest soils: @, flooded (Sta 20); 0, nonflooded (Sta 23).

and 22), the distribution of Fe oxyhydroxides did not follow the regula increase in Wg concentrations from the 18- to I 6-cm interval to the sediment-water interface (Fig. 4). Except for the remobilisation of Fe,,, to the oxic surface under oxic conditions (e.g. at stations 14 and 22; Fig. 4), Fe oxyhydrox- ides did not show a pattern of enrichment or depletion with sediment depth. The reactive phosphate distribution (PC,; Fig. 5) followed that of the Fe,,, the reactive P being easily adsorbed onto metal oxides such as FeOOH (Manning and Jones 1982). The behaviour of Po,, however, seems more closely related to that of the organic matter, as its distribution paralleled the C and N profiles (Fig. 5).

In the flooded soil of EC-2 (station 20), the high C and N csncentrations were still similar, after 12 yr of impoundment, to the concentrations found in the humic horizon of the ad~a~en t forest soil (station 23). This suggests that the degradation of the organic matter in this flooded soil must be a slow process. In contrast, in the 66)-yr-old inundated soils of Cabonga (station 2), the C sand N concentrations (30-80 and 1.5-2 mgBg, respec- tively) were four to five times lower than those observed in the reference forest soil (station 9). This suggests that either an

exceptionally efficient biodegradation or same mechanical ero- sion process is responsible for the major loss of organic matter. At stations 2 and 20 (Cabonga and EG-2, respectively), the increasingly reduced conditions with depth (Table 1) are prok- ably responsible for the observed remobilization of Fe,,, and associated PC,, from the anoxic layer and their precipitation near the oxic surface layer (Fig. 4). P,,,, however, initially associated with the humic horizon in terrestrial soils (stations 9 and 23; Fig. 5 ) , has completely disappeared at station 2 (Cabonga) where the C and N concentrations were extremely low. At station 20 (LG-2), P,,, still remains associated with the humic horizon underlying the surficial mossy bed.

The 6°C profiles in the lacustrine sediments were generally constant with depth except when there was a significant decrease in C and N concentrations (Fig. 3 and 6). This was the ease in the offshore stations of both Laporte and LG-2 (sta- tions 14 and 18, respectively) where a "C enrichment ( - 38) to - 29 and - 29 to - 27%0, respectively) over the first 8 cm (Fig. 6) paralleled the small decrease in C and N csncentrations described earlier (Fig. 3). The two other lacustrine stations (stations 1 and 22) showed slightly higher 6°C values (Fig. 6).

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+Sta 1 Pcdb +Sta 18 Pcdb 8

+ Sta 1 Porg b +Sta 18 Porg

4 Sta 14 Pcdb +8a 22 Pcdb

t +Sta 14 Porg +Sta 22 Porg

C +Sta 2 Pcdb + Sla 2 Porg +Sta 9 Pcdb

+ Sta 26 Pcdb + Sta 20 Porg +Sta 23 Pcdb -s- Sta 23 Porg

Depth (em) FIG. 5. Pcdb and Berg koncentrations (dry weight) at the various sampling stations. (a) Southern lacustrine sediments: a m , Cabnga Resewroir (Sta 1); 80, Lapone Lake (Sta 2114); (b) northern lacustrine sediments: EG-2 Reservoir (Sta 18); Oi, Detcheverry Lake (Sta 22); (c) southern forest soils: @W, flooded (Sta 2); 067, nsnflooded (Sta 9); (d) northern forest soils: a m , flooded (Sta 20); OD, nonflocsded (Sta 23).

In normal forest soils (stations 9 and 231, although the @ and N concentrati~sns showed significant changes through the dif- ferent horizons (Fig. 3), the 613C values remained nearly constant throughout the core length (Fig. 6)- While this obser- vation contrasts with the "3C enrichment observed by Nadel- hoffer and Fry (1 988) in the first 5 cm of a forest soil, it was in agreement with a more recent study (Vitorello et al. 1989) which reported constant 813C values through the first 30 cm of a forest soil- The average 6I3C values of - 26%0 obtained in this study are wela within the range of 614C values for forest typical soil organic matter ( - 25 to - 2'7%0, EaZerte 1983; NdeHhoffer and Fry 1988; Vitorello et al. 1989). In the flooded soils of Cabonga and LG-2 (stations 2 and 20, respectively; Fig. $), subsurface 6'3C d u e s were very similar to those of the terrestrial forest soils (stations 9 and 23; - 24 to - 26700, respectively). These values were found beneath a much lighter isotopic signature with minimal 8l3C values in the sudicial sed- imentary layer ( - 26.5 and - 3 I%o, respectively). The sudden change in C isotopic compo~iti(~n ~cearred in the first 4 cm sf

sediments with a magnitude of 2.5%0 in Cabsnga mQ 4 % ~ in EG-2.

The concentrations of Hg in surficial sediments of the natural lakes a d reservoirs studied are well above the "background" value of 108 angig (dry weight) estimated for the sediments of North American natural lakes (Forstner and Wittrnann 198 1 ; @ahill and Shirnp 1984; Evans 11986)- The ubiquitous increase in Hg concentrations from 50-1433 ngig at a depth of 10- 16 cm to 150-490 ng/g at the surface suggests either remobilization of Hg from bottom to surface sediments or recent Hg loading. Although vertical mobilization of Hg in the sediment column could influence seadace concentrations, similar Hg profiles have been attributed to an increase in atmospheric inputs of trace metals since the onset of the industrial era (Ouellet and Jones 2982; Bodaly et al. 1984; Evms 1986; McMurty et al. 1989; Wyb& et al. 1989; Verta et A. 1989). Using a mean sedimew-

276 Can. J . ~ i s h . Aqsdar. Sci., V01. 50, 1993

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Depth (cm) FIG. 6. 813C values (versus PDB) of POC at the various sampling stations. (a) Southern lacustrine sediments: e, Cabonga Reservoir (Sta 1); 8, Laporte Lake (Sta 14); (b) northern lacustrine sediments: @, LG-2 Reservoir (Sta IS); Q, %)etchevemy Lake (Sta 22); (c) southern forest soils: 8, flooded (Sta 2); 0, wonflooded (Sta 9); (8) northern forest soils: 9, flooded (Sta 28); 8, nonflmded ( S h 23).

tatiow sate of 2-3 m d y r for dl lakes studied (obtained from 2'"Pb measurements by (y. spectrometry; Chaire de Recherche en Environnement HQICRSNGIUQAM 1992), we have been able to date the onset of Hg enrichment (intersection of the baseline and the slope sf the gradient) in the lacustrine sedi- ments at the beginning of the twentieth century (1920 & 10), a period of industrial expansion in North America.

Aerosols carrying adsorbed Mg have been shown to disperse over wide areas and settle on vegetation, soils, and water sur- faces far from their source (Nriagu 1979, 1989; Barnellet and Jones 1982). In soils, the Hg is then bound by organic matter and remains associated with the karmic horizon (Kre&el 1943; Bodaly et al. 1984). This is clearly apparent in the two forest soils analyzed in this study (stations 9 and 23). The Hg profile is closely related to the organic mattes of the humic horizon (C, N, and Po,, concentrations; Fig. 2, 3, and 5) whereas it is m t affected by the leaching mechanisms whereby Fe oxyhy- droxides (Fe,,) migrate down to the horizon of mineral accurnulatptisna (Fig. 4).

Surface mnoff from soils of a drainage basin transport Hg- enriched organic matter into lakes, thereby promoting accu- mulation of Hg in lacustrine sediments (Evms 1986; McMurty

et al. 1989). The hypothesis that Bakes receiving greater amcpunts of temigensus organic matter have higher loadings sf Hg (Mchaurty et al. 1989) is supported by the C, N, and Hg profiles in the offshore sediments sf Laporte Lake (station 14). At that station, high organic matter content is associated with exceptionally high Hg concentrations unmatched in any of the other lacustrine sediments sampled in this study (stations 1, 18, and 22). This is probably due to the high organic matter and Mg concentrations found in the nearby forest soil (station 9; Fig. 2 and 3). In a more recent study on lacustrine sediments from 12 natural lakes spread nearly equally over 9" of latitude, a strong correlation was found (r = 0.94) between surface organic matter content (C and N) and surface Hg concentrations (Lucotte et al. 1B2), showing that Hg loads are strongly asso- ciated with organic matter inputs to these environments.

Surface mnoff of Hg-enriched humic matter may then explain the high rates of Mg deposition observed in these natural lakes and reservoirs. However, this process cannot explain the dif- ference in the level of Hg contamination from recently created hydroelectric reservoirs and the adjacent natural lakes (Bodaly and Hecky 1979; Bodaly et al. 1984; Jackson 1988a; Verdon et al. 1991). The observation that Hg accumulation by fish of

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hydroelectric reservoirs is directly proportional to the flooded surface area (Abemalthy and Curnbie 1977; Bmce md Spencer 19'99; BodaHy et al. 1984; Hecky et al. 198Tb) may indicate that inundated soils provide a new and important source of Hg to the system. However, physical, chemical, md biological properties of the flooded soils determine the release rate sf bio- available Hg (Potter et al. 1975; Rudd et aH. 11983; Jackson 1988a9 1988b, 1989).

In the sediments of natural lakes, as represented by "refer- ence' ' sites (stations 14 and 22) and preimpouwdmerat reservoir sites (stations 1 md 181, the biodegradation of buried organic matter could only be detected in Laporte Lake and LG-2 (sta- tions 14 and 18). A slight decrease with depth in C md N (Fig. 3) and a significant 1-2% enrichment in I3C of POC (Fig. 6) could be attributed to represent the extent of bacterial activity in the first few centimetres of the sediments. Such a marked shift in the 6'" of organic matter has previously been attributed to the degradation sf organic matter by rnethano- genesis in sediments (Turner and Fritz 1983; Herczeg 1988). In contrast, differential preservation of ligninlike material dur- ing organic matter decay does not occur in these systems because it should lead to 13C depletion rather than the observed "3C enrichment (Tan and Strain 11983; Spiker and Hatcher 1984; Benner et al. 1987). The activity of methanogenic bacteria may thus be responsible for the moderate degradation sf the organic matter observed at both stations 14 md 18. Moreover, the bio- degradation of organic matter is only observed in lacustrine sediments with a light 8I3C sudicial signature ( - 29 to - 3 1%; stations 18 and 14), probably indicating a high fraction of autochthonous material (characteristic 6' C values of - 32 to - 34%; Lac* 1983). It seems that an isotopically heavier organic matter in lacustrine sediments ( - 25 -5 to - 2% .5%0, stations 22 and 1) reflects a greater contribution sf terrestrially derived refractory organic (1 (typical 8'" vdues of - 2 5 to -27%; L a ~ e 1983). The contribution of autochthonous material to lake sediments may be influenced by inputs of nutrients fmm a particular drainage basin such as Laporte Lake (station 24) or from newly impounded forest soils at LG-2 (station 18).

ABI four sets of lacustrine sediment samples show constant organic matter and 6'" profiles starting either from the surface (stations 1 and 22; Fig. 3 and 6) or from beneath the suficial biodegradation zone (stations BS md 14; Fig. 3 and 6). The various lacushne sedimentary environments thus appear in steady state in terns of organic activity, as they are not affected, or only very moderately, by biotic processes of degradation. Similarly , in these lacustrine sediments, the Fe,,, profiles show either a remobilization just below the surface (stations 14 and 22; Fig. 4) or constant concentrations produced under anoxic conditions (stations B and 18; Fig. 4), indicating that steady- state geochemical conditions are also prevalent for remobiliz- able metals such as Fe. In comparison, the Hg concentrations in the same cores decrease regularly from the surface down to 10- B 6 crn depth in Iacustrine sediments. The resulting Hg pro- files appear totally unrelated to the Fe,,, distribution. Hence, we propose that the Hg enrichment nem the surface of lacustrine sediments is mainly caused by increased inputs of anthropo- genic airborne Hg rather than diagenetic remobilization. An evaluation of the potential for Hg methylation in these sedi- ments would require a more accurate estimate of bacterial activ- ity and other factors which determine the relative efficiency of methylation/demethyXatid~n reactions.

The extremely low C, N, and P,, concentrations in the old floded soil of Cabonga (station 2), as opposed to the forest

soil near Laporte Lake (station 9; Fig. 3 and 5) , cannot be solely attributed to the maturation of the organic matter since the time of impoundment. AS can be appreciated from the high C and N concentrations in the flooded ssil s f LG-2 (station 20) after 12 yr of impoundment, decay of organic matter is a slow pro- cess. %f we assume that Cabonga's Wooded soil originally had a C and N content similar to that of the nearby forest soil (station 9), it is doubtful that bacterial activity could have degraded up to 80% of all organic matter, including refractory material 4e.g. humic matter, lignin). The exceptionally low C, N, and Po,, concentrations seem to support the field observations that the humic horizon in this shallow flooded area has been completely washed away by mechanical erosion (e.g. ice, waves, marling). The extremely low Hg concentrations at this station (Pig. 29 and at a nearby marling zone (Chaire de Recherche en Envi- rornnernenmt HQICRSNGIUQAM 1982) thus suggest that most sf the Hg associated with the humic horizon (approximately O- 8 cm) in the original forest soil has been resuspended in the water column along with the organic material. The remaining Hg maxima are associated with the overlapping peaks of both Fe,, md P,, profiles close to what remains now as the new surface of the soil (Fig. 4 and 5). The 60-yr period since the time of impomdment has probably allowed for dI the leached Fe,, in the original mineral accumulation horizon sf the forest soil to be remobilized to the oxidized surface of the core. Although there is a certain relationship between Mg and Fe,,, in this ancient flooded soil ( r = 9.691, there is as yet no evi- dence for a strong absorption of Hg on Fe-oxyhydroxides, However, the Bow concentrations of organic matter and Hg in this soil, its stable biogeochemical conditions, and the presence of FeOOH all suggest that the remaining potential for Hg meth- ylation in Cabnga's flooded soil is now very limited. This may be supposed by the low Hg levels found in whitefish (0.14- 0.30 m a g ; st = 6) and northern pike (1 -34- 1 .50 mgikg; n = 5) from the adjacent 40-yr-old Bozois Reservoir (Messier and Roy 198'7; Verdsn et al. 2 89 1).

In the recently flooded soil of LG-2 (station 201, there seems to be an ongoing remobilization of Fe oxyhydroxides and asso- ciated reactive phosphates from the lower moxic horizons to the upper more oxidized surface layers (Fig. 4 and 5; Table 1). This would indicate that the geochemical conditions existing in a terrestrial environment, where Fe,,, and PC,, are associated with the mineral accumulation horizon, shifted after flooding towards those of an aquatic environment such that Fe,,, was remobilized m d migrated with the redox b o u n d q close to the sediment-water interface. The high C and N concentrations at this station (Fig. 31, similar to those observed in the humic horizon of the adjacent forest soil (station 231, and the 6 1 ' ~ value of - 26966, characteristic of regular forest ssil organic C ( L a f i e 1983; Nadeihoffer and Fry 1988; Vitoxllo et al. 19891, suggest that the organic matter in the northern flooded forest soil has not k e n significantly biodegraded or eroded by mechanical processes. Therefore9 in the flooded soil of LG-& the organic C and N content remains sufficiently high to sustain a significant microbial Hg methyfating activity. This would be supported by the high Hg levels in northern pike (=3 mgikg; n = 5) of this reservoir, still increasing after 9 yr of in~pound- ment (Verdon et al. X 99 1) .

Hw the first 2 cm of this flooded soil, we observed signifi- cantly lower C concentrations and 6°C values compared with those sf the organic matter at depth (Fig. 3 and 6). The decrease in C consentrations and the depletion of 613&3 in the surface POC could be attributed to two different processes: (1) a sig- nificant bacterial degradation restricted to the top 2 cm of the

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organic horizon with preservation of ligninlike material depleted in '" or (2) the accurnu8ation of an autochthonous sedimentary layer with lower C c~ncentrations and a much lighter & I 3 @ sig- nature than the forest hramic matter. In the first case, since bioI- imiting nutrients such as Pa, (Bowden 1984) and recactive N (Jackson 1986) are present in LG-2's surficial submerged soil (Fig. 3 and 5) and Hg, POC, md FeOOH concentrations are significant (Fig. 2, 3, and 4), the potential for Hg methylation in this environment seems to remain high. Hn the second case, considering that the studied flooded soils are within the photic zone, it seems likely that autochthonous material has accu- mulated om the surface s f the soil under these conditions. The accumulation sf new material would be supported by the very negative & I 3 @ values ( - 319470) in the saarficial sediments. These 6°C. values are comparable with those of colonial blue-green bacteria ( - 32 to - 34%0) observed in late August in a southern Quebec lake (EaZerte 1983).

Conclusions

The relationships between Hg concentrations, organic con- tent, and Fe oxyhydroxide concentrations in vertical lacustrine sediment profiles determined in this study allowed us to relate the recent increase of Hg concentrations in lake sediment cores to the atmospheric transport of anthropogenic Hg since the early 19W9s. The effects of this transport are observed over distances as far as 2800 h away from the industrial belt of the northern United States and southern Canada.

In all four lacustrine sediments studied, there is apparently no significant biodegradation of the organic matter below the immediate surface layer. The distribution of Fe oxyhydroxides and associated reactive pphsphates indicates apparent steady- state geschemical conditions over the entire sediment profile. Ns evidence was found for a strong association of Hg to Fe,,,, indicating either a lack of diagenetic remobilization of Hg or a strong competition for binding Hg(I1) released to the interstitial waters with other components of the solid phase be.$. fresh organic matter).

In forest soils, high Hg concentrations are confined to the harmic "8 "orizon, suggesting a strong association between Hg and organic matter. In these soils, Hg is not affected by the leaching processes that result in the migration of Fe to the min- eral accumulation 'B" horizon. The surface weathering of for- est soils and subsequent transport of Hg-rich terrestrial organic matter to adjacent lakes most likely serve as a major source of Mg to aquatic environments (Evans 1986; McMu~ty et al. 1989).

The high organic C and N content of the Wooded soil in LG-2 suggests that the biodegradation of organic matter in sub- merged soils is a slow process at northern latitudes. Microbial activity in this soil is apparently restricted to the sudicial sed- imentary layer and is probably stimulated by the presence of organic P, a limiting factor for bacterial activity ((Bowden 1984). The concentrations of organic nutrients and Hg remain high in LG-2's flooded soils more than 12 yr after flooding, md thus the potential for Hg methylation and release to the overlying biota remains high in this environment.

The apparent 80% reduction in organic matter and Hg con- tent between an unperturbed forest soil and a soil flooded 60 yr ago at Cabonga must result from mechanical erosion processes rather than fmm a rapid biodegradation of the humic horizon. Resuspension of organic and %&-rich particulate matter in a reservoir just after flooding probably increases the rate of trans- fer of Hg to the biota. The biodegradation activity, instead of

being restricted to the sediment-water interface in flooded soils, is transferred to suspended organic matter in the water column. In fact, Jackson (1988b) noted that enhanced biodegradation of organic matter in the water column, as opposed to the sedi- ments. increases the bioavaiiability of Mg .

The perturbation of terrestrial systems through flooding can cause release of Hg into reservoir waters by two processes: ( f ) biodegradation of organic matter mainly at the sedirnent- water interface, whereby the highly toxic, kisavailable methyl mercury is generated as a by-product (Beijer and Jernelbv 1979; Fumtani and Rudd 1988; Rudd et al. 1983; Bodaly et al. 1984; Jackson 1986, 1988a, l988b, 19891, and (2) mechanicaB ero- sion processes that may be responsible for the rapid release of inorganic Hg associated with humic matter of flooded soils to the water column. Based solely on PrnaethylatioddemethyIation ratios measured in hydroelectric reservoirs of Manitoba, Warnsey (1998) estimated that Hg levels in fish would decrease and stabilize to natural values only after 150 yr. Various studies agree, however, that the stabilization of Hg levels in fish from reservoirs reaches natural values within 15-38 yr after impoundment (Strange 1985; Verta et al. 1986; Hecky et al. 1987a; Messier and Wsy 1984; Verdon et al. 1991). The mag- nitude of the meghylatisn/demethylatiow rates and their evcmlu- tion in time within the flooded soils may thus be overestimated. The present study seems to indicate that mechanics! processes of erosion in the first years of impoundment may accelerate methylation reactions and act as a significant source of Hg con- tamination until the exposed littoral zones stabilize.

Acknowledgments

This research was supported by a grant from Hydro-Quebec, the Conseil de la Recherche en Sciences Naturelles et en GCnie (CRSNGJ NSEWC), and the Universitk du Qutbec to the Chaire de Recherche en Environnement HQICRSNGJUQAM attributed to Claude Hillaire- hfacel. Additional financial support was provided by Hydro-Quebec in the form of a summer bursary to the first author as well as by NSEWC (operating grants) and FCAR (team grant) to M.L. and A.M. Many people assisted in field coHection, and they are all gratefully acknowl- edged. More sspecificalIy, we would like to thank Pierre Ferland for the organization and direction of the field work. We also thank Louise Coumoyer, Benoit Fortin, Constance Cuignad, and Sophie Tran for their help in the laboratorye This paper beraefitted from the insightfuj and critical comments s f two anonymous reviewers. Their efforts are greatly appreciated.

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