Environ. Sci. Technol. 2009, 43, 2254–2261
Multiyear Total and Methyl Mercury Exports from Two Major Sub-Arctic Rivers Draining into Hudson Bay, Canada J A N E L . K I R K * ,† A N D V I N C E N T L . S T . LOUIS Department of Biological Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E9
Received November 6, 2008. Revised manuscript received January 23, 2009. Accepted January 28, 2009.
From 2003 to 2007, concentrations of total mercury and methylmercury (THg and MeHg) were continuously measured in two Canadian sub-Arctic rivers (the Nelson and the Churchill) that drain into western Hudson Bay. THg and MeHg concentrations were low in the Nelson River (mean ( standard deviation, 0.88 ( 0.33 and 0.05 ( 0.03 ng L-1, respectively). The Churchill River, however, had high concentrations of Hg, particularly MeHg (1.96 ( 0.8 and 0.18 ( 0.09 ng L-1, respectively) and hence may be an important source of MeHg to organisms feeding in the Churchill River estuary. A large portion of THg in the Nelson River was particulate-bound (39 ( 23%), while in the Churchill River, most was in the dissolved form (78 ( 15%) and is likely dissolved organic carbon (DOC)bound Hg originating in the surrounding wetlands. In fact, both the Nelson and Churchill Rivers had high DOC concentrations and were therefore large exporters of DOC to Hudson Bay (1480 ( 723 and 392 ( 309 × 103 t year-1, respectively) compared to rivers to the south and east. Despite high Churchill River Hg concentrations, due to large Nelson River flows, average THg and MeHg exports to Hudson Bay from the Churchill River (37 ( 28 and 4 ( 4 kg year-1, respectively) were about onethird and half the Nelson River exports (113 ( 52 and 9 ( 4 kg year-1). Interestingly, combined Hg exports to Hudson Bay from Nelson and Churchill River discharge are comparable to THg inputs from Hudson Bay springtime snowmelt (177 ( 140 kg year-1) but are ∼13 times greater than MeHg snowmelt inputs (1 ( 1 kg year-1). Although Hg inputs from rivers and snowmelt together may account for a large portion of the THg pool in Hudson Bay, these inputs account for a lesser portion of the MeHg pool, thus highlighting the importance of water column Hg(II) methylation as a source of MeHg to Hudson Bay marine food webs.
Introduction Methylmercury (MeHg), a toxic form of Hg that bioaccumulates through food webs, is present in some Canadian high Arctic and Hudson Bay marine mammals at concentrations high enough to pose health risks to northern peoples using these animals as food (1). In the Arviat region of western * Corresponding author phone: (905) 336-4712; fax: (780) 4929234; e-mail:
[email protected]. † Present address: Environment Canada (Canadian Centre for Inland Waters) Burlington, Ontario, Canada. 2254
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Hudson Bay, for example, concentrations of Hg in the muscle, liver, and kidney of beluga whales are generally well above the Canadian fish commercial sale limit of 0.5 ug g-1 wet weight (2). Although it was recently shown that methylation of inorganic Hg(II) in Arctic seawater is an important source of MeHg to Arctic marine food webs (3), inputs of both Hg(II) and MeHg to Arctic marine waters from river discharge may also be substantial, particularly to Hudson Bay. Hudson Bay is large (841 × 103 km2) but shallow (on average ∼103 m deep (4)) and is therefore greatly influenced by river discharge. In fact, Hudson Bay has the largest drainage system in Canada and receives cumulative discharge of ∼710 km3 year-1 from numerous rivers (5, 6). Of the few Arctic rivers that have been examined, several, including the Mackenzie (7, 8) Lena, Ob, and Yenisei (9) Rivers, are known to export large quantities of total Hg (THg; includes Hg(II) and MeHg in a sample) to Arctic seas. The Mackenzie River, for example, located in northwestern Canada, is the largest source of THg (∼2200 kg year-1) and a substantial source of MeHg (∼15 kg year-1) (7) to the Beaufort Sea, where concentrations of MeHg in beluga whales are among the highest in the Canadian Arctic (2). Unfortunately, this is the only Arctic river where MeHg exports have been examined. Rivers draining into Hudson Bay may be of particular interest as several have been altered for hydroelectric power production, which due to the surface flooding often resulting from river diversion and reservoir creation also changes the biogeochemical cycling of Hg (10-13). Flooded soils and vegetation decompose, creating anoxia in the sediments and releasing previously stored carbon and inorganic Hg(II) (12, 13). Together, these conditions stimulate the methylation of Hg(II) to MeHg by sulfatereducing bacteria in surface sediments (14, 15). Newly produced MeHg can then be bioaccumulated through food webs or exported to downstream waterbodies. Since the 1970s, ∼75% of the flow of the Churchill River has been diverted into the Nelson River to increase its hydroelectric power potential (11). Water was diverted at South Indian Lake in northern Manitoba and resulted in the flooding of ∼900 km2 (16). Concentrations of MeHg in fishes of several flooded lakes (e.g., South Indian Lake) dramatically increased (10, 16) and remained elevated above preimpoundment levels for 10-23 years (11). Hg concentrations in fishes of the impacted regions were well studied; however, the input of Hg to Hudson Bay from this system has not been examined in detail, although the Nelson River provides ∼13% of the total discharge to Hudson Bay annually (6). In addition, the lower Churchill River drains vast areas of wetlands, which are known natural sites of MeHg production (12, 17, 18). To determine the quantity of Hg entering Hudson Bay annually from Nelson and Churchill River discharge, we continuously monitored concentrations of unfiltered and filtered THg and MeHg in these two rivers from 2003 to 2007. Exports of nutrients and dissolved organic carbon (DOC) were also determined.
Methods Site Description and Sampling Design. Sampling programs were implemented near the mouths of the Nelson and Churchill Rivers from July 2003 to February 2007. The mouths of the Nelson and Churchill Rivers are located in northern Manitoba, Canada, within the Hudson Plains ecozone (Figure 1). This region is characterized by glaciomarine, glaciolacustrine, and glacial fluvial sediments, vast wetlands which cover over one-third of the land surface, zones of both 10.1021/es803138z CCC: $40.75
2009 American Chemical Society
Published on Web 02/25/2009
FIGURE 1. Sites on the Nelson, Churchill, and North Saskatchewan Rivers where water was sampled for concentrations of unfiltered and filtered THg and MeHg as well as general chemistry. Manitoba Hydro control structures are also shown. “scattered” and “widespread” permafrost, and vegetation similar to that of the Arctic tundra and taiga transitional forest (11). River water was collected near Gillam, Manitoba, at Limestone Generating Station, the largest (1340 MW capacity) and furthest downstream (∼120 km from the river mouth) in a series of run-of-the-river Manitoba Hydro generating stations located on the Nelson River, and at the mouth of the Churchill River near Churchill, Manitoba (Figure 1). At Limestone Generating Station, river water was sampled every two weeks from a “raw” river water tap equipped with a Teflon spout and Teflon Swagelock fittings. Churchill River water was sampled by wading into the river and was therefore collected only during the ice-free season, except for one occasion in April 2004 when samples were obtained by drilling a hole in the ice as described in ref 19. In summers 2005 and 2006 sampling was also conducted immediately downstream of the Missi Falls Manitoba Hydro control structure, which diverts water from the Churchill River into the Nelson River (Figure 1). As at the Churchill River mouth, at Missi Falls water was collected by wading into the river. At each site, water samples for THg and MeHg analyses as well as for general chemistry analysis were collected into acid-washed Teflon and polypropylene bottles, respectively, using the “clean-hands-dirty-hands” Hg sampling protocol (3, 20). Except at Missi Falls, where only unfiltered water samples were collected, a THg sample and a MeHg sample were then filtered through 0.45 µm nitrocellulose membranes in acidwashed Nalgene filter units. THg samples were preserved on-site with trace-metal-grade HCl equal to 0.2% of the sample volume. Within 24 h of collection, all samples were then shipped to the University of Alberta, except in 2003/04 when chemistry samples were sent to the Freshwater Institute
in Winnipeg, Manitoba. Upon arrival, MeHg samples were frozen and chemistry samples submitted for analysis. Between October 2005 and December 2006, a sampling program was also implemented on the North Saskatchewan River, which is also part of the Hudson Bay-Nelson River drainage basin. River water was collected at Edmonton, Alberta, which is located in the Prairies ecozone. This region is characterized by till blanket sediments, 0.45 µm. A high percentage of particulate-bound THg (%pTHg) is common in rivers whose watersheds are composed of easily eroded soils, such as cropland (29, 30), which comprises 47% of the Nelson River catchment (23). Bank erosion due to high postdiversion flows may also contribute to Nelson River sediment loads. Not surprisingly, %pTHg was therefore lowest in winter (32 ( 23%) and spring (33 ( 6%) and highest in summer (50 ( 15%) and fall (54 ( 19%) when catchment erosion is likely highest due to storms and high river flows. Concentrations of filtered MeHg (0.04 ( 0.02 ng L-1) were generally only slightly lower than unfiltered concentrations. Therefore, %pMeHg (27 ( 13%) was lower than %pTHg; this value is only an estimate, however, as %pMeHg could not be calculated when concentrations of MeHg were less than the method detection limit (MDL; 0.02 ng L-1). Average concentrations of unfiltered THg were also quite low at the mouth of the Churchill River (1.96 ( 0.8 ng L-1) (Figure 3A). The Churchill River, however, had high MeHg concentrations (0.18 ( 0.09 ng L-1) and is therefore likely an important source of MeHg to Hudson Bay organisms feeding in the Churchill River estuary (Figure 3C,D). Concentrations VOL. 43, NO. 7, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Average concentrations of unfiltered THg and MeHg (ng L-1) and DOC (mg L-1) in Churchill River water collected immediately downstream of the Churchill River diversion at Missi Falls and at the river mouth during summer 2005 and 2006. Error bars represent the standard deviation, and an asterisk indicates significantly different (p < 0.001) concentrations at the two above locations. of unfiltered THg and MeHg were lowest during winter (0.68 and 0.03 ng L-1, respectively) (Figure S3, Supporting Information). Although we were only able to sample Churchill River water once during the ice-covered season, the above values likely represent “baseline” Hg concentrations during low-flow conditions as Hg concentrations varied little throughout winter in the Nelson and North Saskatchewan Rivers (Figure S3). Although wintertime concentrations were similar in the Nelson and Churchill Rivers, in all other seasons, concentrations of THg and MeHg at the mouth of the Churchill River were significantly higher than those in the Nelson River (p < 0.01). Hg concentration also followed different seasonal patterns on the Churchill River than on the Nelson River. Churchill River Hg concentrations were greatest in spring (2.60 ( 0.79 and 0.24 ( 0.14 ng L-1, for THg and MeHg, respectively) and then decreased slightly throughout summer (2.04 ( 0.68 and 0.21 ( 0.09 ng L-1) and fall (1.78 ( 0.83 and 0.14 ( 0.06 ng L-1). Also in contrast to the Nelson River, in the Churchill River, %MeHg was high (10 ( 6%, with values >20% sometimes observed), which is indicative of high Hg(II) methylation rates (31) and suggests that active methylation occurs within the Churchill River and/or its watershed. Interestingly, during the summers of 2005 and 2006, average concentrations of unfiltered Hg at Missi Falls (0.79 ( 0.14 and 0.04 ( 0.02 ng L-1), located immediately downstream of the diversion (Figure 1), were significantly lower than at the Churchill River mouth during the same period (2.19 ( 0.77 and 0.24 ( 0.10 ng L-1, respectively, p < 0.01) (Figure 4). During this period, %MeHg was 5 ( 2% at Missi Falls but 13 ( 7% downstream at the river mouth. We therefore hypothesize that the vast wetlands along the lower Churchill River export large quantities of both THg and MeHg to the lower Churchill River during the ice-free season. Freshwater wetlands often possess ideal conditions for the growth of sulfate-reducing bacteria, including the presence of labile DOC, sulfate, and anaerobic surface sediments (14, 32), and are therefore known Hg(II) methylation “hot spots” (17, 18). Furthermore, studies of Hg exports from rivers with differing watershed compositions have demonstrated that MeHg yields increase with the fraction of wetland area in the watershed (29, 30). At the mouth of the Churchill River, concentrations of filtered Hg (1.71 ( 0.74 and 0.14 ( 0.07 pg L-1 for THg and MeHg, respectively) were only slightly lower than unfiltered concentrations, suggesting that the majority of Hg entering Hudson Bay in Churchill River discharge is bound to DOC rather than to particulates. In fact, DOC concentrations at the Churchill River mouth were greater than those measured upstream at Missi Falls (Figure 4). Furthermore, %pHg was highest in winter (41% of THg, for example) and fall (22 ( 23%) and lowest in spring (15 ( 7%) 2258
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and summer (15 ( 11%), suggesting that in spring and summer the majority of Hg in the lower Churchill River is DOC-bound Hg originating in the surrounding wetlands while in fall and winter a larger portion is particulate-bound Hg originating further upstream. Lastly, wetlands surrounding the Nelson River estuary may also contribute Hg to the lower Nelson River downstream of our sampling site at Limestone Generating Station; therefore, exports of Hg to Hudson Bay from Nelson River discharge reported in this study may be conservative. Average concentrations of unfiltered THg and MeHg as well as %MeHg in the North Saskatchewan River at Edmonton, Alberta, were low and similar to values measured in the Nelson River (1.54 ( 2.63 and 0.06 ( 0.05 ng L-1, 5 ( 2%, respectively). Furthermore, concentrations of Hg on the North Saskatchewan River followed a seasonal pattern similar to that observed on the Nelson River (Figure S3, Supporting Information). Unfiltered THg and MeHg concentrations were more variable than in either of the other rivers, however, ranging from 0.45 to 14.33 ng L-1 and from 0.02 to 0.19 ng L-1, respectively (Figure 3A,C). Erratic Hg concentrations have been previously observed in rivers with predominantly urban watersheds (30) and, although we sampled near the city limits, likely reflect some anthropogenic Hg inputs from Edmonton. Similar to the Nelson River, a large portion of THg in the North Saskatchewan River was particulate-bound (55 ( 21%). This is not surprising since 67% of the North Saskatchewan River watershed is cropland (23), and like other rivers traversing the prairies, the North Saskatchewan River carries a heavy silt load (23). In fact, as in the Nelson River, %pTHg was highest in the summer (63 ( 18%), likely due to cropland runoff. July snowmelt in the Rocky Mountains (23) releases glacial flour which also likely contributes to summertime North Saskatchewan River particulate loads. Hg concentrations in both the Nelson and Churchill Rivers varied among years, mostly due to hydrological changes. In 2003-2004 and most of 2005, low precipitation across most of Canada, including the Hudson Plains (459 and 368 mm year-1 at Gillam and Churchill, respectively, 2003-2004), resulted in low average flows in the Nelson and Churchill rivers (2495 ( 876 and 140 ( 96 m3 s-1, respectively) (Figure 2). In 2005-2006 precipitation increased greatly (573 and 549 mm year-1 at Gillam and Churchill, respectively), resulting in high river flows (4602 ( 1177 and 1016 ( 865 m3 s-1). In fact, in 2005, the wettest year, flows in the Churchill River were near prediversion levels (1972-1976 flows were 1266 ( 317 m3 s-1). In the Nelson and Churchill Rivers, average concentrations of unfiltered THg were significantly higher during high-flow periods (1.01 ( 0.29 and 2.20 ( 0.73 ng L-1, respectively, summer 2005 to winter 2007) than in low-flow
TABLE 1. Average Annual Exports of Hg and Nutrients to Hudson Bay from Nelson and Churchill River Dischargea unfiltered THg (kg)
filtered THg (kg)
unfiltered MeHg (kg)
filtered MeHg (kg)
TN (103 t)
TP (t)
DOC (103 t)
Nelson River 2003/2004 2004/2005 2005/2006
60 164 114
39 83 64
12 6
9 5
37 83 55
2910 6830 5840
684 2090 1670
average annual
113 ( 52
62 ( 21
9(4
7(3
58 ( 23
5190 ( 2040
1480 ( 723
Churchill River 2003/2004 2004/2005 2005/2006
7 63 41
6 49 34
1 8 2
0.5 6 2
2 20 13
146 651 629
67 682 427
average annual
37 ( 28
30 ( 22
4(4
3(3
12 ( 9
475 ( 286
392 ( 309
spring snowmelt
177 ( 140
total pool in Hudson Bay
30800 ( 23900c
b
1(1
b
3400 ( 800c
a
Inputs of Hg to Hudson Bay from spring snowmelt as well as the total pools of Hg in Hudson Bay seawater are also shown. b Calculated from Hg concentrations in springtime Hudson Bay snowmelt and snow water equivalence values provided in ref 20 and from the surface area of Hudson Bay (4). c Calculated from Hg concentrations in water masses of Hudson Bay provided in ref 3, the thickness of each water mass (37, 38), and the average depth and surface area of Hudson Bay (4).
periods (0.66 ( 0.26 and 1.62 ( 0.73 ng L-1, respectively, summer 2003 to spring 2005, p < 0.05). Although concentrations of unfiltered Hg increased during wet years on the Nelson River, average concentrations of filtered THg did not change throughout the study period, demonstrating that precipitation-induced river discharge increases released particulate-bound Hg from the Nelson River watershed. On the Churchill River, however, concentrations of unfiltered and filtered THg increased simultaneously during wet years (Figure 3A,B), demonstrating that precipitation increases resulted in release of dissolved Hg, likely due to flooding of surrounding wetlands. On the Nelson River, average concentrations of unfiltered MeHg were also significantly higher during wet years (0.06 ( 0.04 ng L-1) than dry years (0.03 ( 0.01 ng L-1, p < 0.001). Concentrations of MeHg also increased greatly on the Churchill River in 2005, but by 2006, concentrations were comparable to those observed in 2003-2004 (Figure 3C,D). We therefore hypothesize that large rain events in 2005 increased the connectivity of the Churchill River to the surrounding wetlands, where MeHg had been previously produced and stored. Similar processes have been observed in the Mackenzie River system, where annual spring flooding of the numerous delta lakes and wetlands releases large quantities of MeHg to the main river channel (8). Reflooding of wetland soils following periods of water table drawdown is also known to release oxidized sulfur (33), labile DOC, and nutrients (34, 35), all of which may temporarily increase Hg methylation rates. Although Hg concentrations were higher during highflow years on both rivers, significant positive relationships between flow and concentrations of unfiltered THg and MeHg were observed on the Nelson River (r2 ) 0.22 and 0.66, respectively, p < 0.01) but not on the Churchill River (r2 < 0.1 and 0.11, respectively, p > 0.05) (Figure S4A-D, Supporting Information). Significant relationships between flow and Hg concentrations were also observed on the North Saskatchewan River (Figure S4E,F). On the Nelson and North Saskatchewan Rivers, flow explained a smaller percentage of the variation in filtered than in unfiltered Hg concentrations, suggesting that flow controls transport of particulate-bound Hg more than dissolved Hg. THg in the Arctic Red River, the largest Mackenzie River tributary, is also predominantly particulatebound (∼72%) and strongly related to flow (r2 ) 0.73) (7). The low concentrations of filtered THg observed in the Nelson and North Saskatchewan Rivers are comparable to
those previously measured during fall in the Lena, Ob, and Yenisei Rivers (1.0 ( 0.12, 0.56 ( 0.12, and 0.30 ( 0.14 ng L-1, respectively (9)), which are the three largest rivers in the Arctic in terms of river discharge (36). In the Mackenzie River, however, the fourth largest river in the Arctic, average summertime concentrations of THg are higher and more variable (7.18 ( 4.31 and 2.77 ( 2.11 ng L-1, for unfiltered and filtered Hg, respectively (7)) than those reported here. Unlike our study systems, in the Mackenzie River, concentrations of both unfiltered and filtered THg are also ∼7 times higher during freshet than during summer, likely due to springtime surface inundation and bank erosion (7). Our study rivers do not typically experience surface flooding during freshet, and due to hydroelectric power development, peak Nelson River flows do not always occur during spring (23). Interestingly, average spring and summertime concentrations of filtered MeHg in the Mackenzie River (0.08 ( 0.04 ng L-1 (7)) were higher than those measured in the Nelson and North Saskatchewan Rivers but were only about half those observed in the Churchill River. Exports of TN, TP, DOC, and Hg to Hudson Bay from Nelson and Churchill River Discharge. Annual exports of DOC, TN, TP, and Hg to Hudson Bay from Nelson and Churchill River discharge were calculated for the 2003/ 04-2005/06 hydrological years (defined as Dec 1 to Nov 30) (Table 1). Although concentrations of TN, DOC, and Hg were higher in the Churchill River than in the Nelson, due to high Nelson River flows, annual Nelson River exports were higher. On average, for example, the Nelson River exported ∼5 times more TN ((58 ( 23) × 103 t year-1) to Hudson Bay than the Churchill River ((12 ( 9) × 103 t year-1). Due to the high TP concentrations in the Nelson River, Nelson River TP exports (5190 ( 2040 t year-1) were ∼11 times greater than those from the Churchill River (475 ( 286 t year-1). DOC exports from the Nelson and Churchill Rivers were high (1480 ( 723 and 392 ( 309 × 103 t year-1, respectively) compared to those from Hudson Bay rivers to the south and east, which predominantly drain areas of taiga shield or tundra with continuous permafrost (25) ((16-89) × 103 t year-1 from the Nastapoka, Little Whale, and Great Whale Rivers (28)). Furthermore, large portions of Nelson and Churchill River DOC exports may be colored dissolved organic matter (CDOM), which is known to affect ultraviolet radiation penetration, photic depth, and stratification in Hudson Bay (25). In fact, it was recently shown that rivers draining into VOL. 43, NO. 7, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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western Hudson Bay, including the Nelson and Churchill Rivers, have higher levels of CDOM than rivers further south or east (25). On average, the Nelson River exported 113 ( 52 kg of THg and 9 ( 4 kg of MeHg to Hudson Bay per year (Table 1). Unfortunately, 2003/04 MeHg exports could not be determined because most samples collected that year were
