Production and Loss of Methylmercury and Loss of Total Mercury from

Environ. Sci. Technol. 1996, 30, 2719-2729

Production and Loss of Methylmercury and Loss of Total Mercury from Boreal Forest Catchments Containing Different Types of Wetlands† V I N C E N T L . S T . L O U I S , * ,‡ JOHN W. M. RUDD,§ CAROL A. KELLY,‡ KEN G. BEATY,§ ROBERT J. FLETT,| AND NIGEL T. ROULET⊥ Department of Microbiology, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada, Department of Fisheries and Oceans, Central and Arctic Region, Freshwater Institute, 501 University Crescent, Winnipeg, Manitoba R3T 2N6, Canada, Flett Research Ltd., 440 DeSalaberry Avenue, Winnipeg, Manitoba R2L 0Y7, Canada, and Department of Geography, McGill University, Burnside Hall, 805 Sherbrooke Street West, Montreal, Quebec H3A 2K6, Canada

Four terrestrial boreal forest catchments containing different types of wetlands were studied to determine their strength as sources or sinks of methylmercury (MeHg) and total mercury (THg) to downstream ecosystems and to determine if patterns seen in one year were consistent over several years. All catchments were sinks for THg. The wetland type, percentage wetland area (0-25%), or annual water yield did not appear to have a consistent effect on the magnitude of this retention. Wetland areas of the catchments were always net sources of MeHg. Unlike for THg, there were large and consistent differences in the source strength among wetland types for MeHg. These differences appeared to be related to differences in the internal hydrology of the wetlands. All types of wetlands were greater sources of MeHg during years of high water yield, but even during years of low flow all wetland types were sources of MeHg. Thus, we conclude that wetlands are important sites of MeHg production in boreal ecosystems on the long term. Upland areas of catchments were consistently sinks for MeHg, and so whole catchment sink/source values were strongly affected by the percentage of wetland areas within a catchment. Mass balance estimates of MeHg input from wetland areas to a lake indicate that the annual input of MeHg from wetlands is larger than the annual uptake of Hg by fish and is similar to the amount of MeHg produced in the lake. Because of the predictable patterns between terrestrial catchments in their strength as sources or sinks of MeHg,

S0013-936X(95)00856-X CCC: $12.00

 1996 American Chemical Society

it is possible to model inputs of MeHg from lake catchments with knowledge of the percentage wetland area in a catchment, the type of wetland contained in a catchment, and the annual water yield of a catchment.

Introduction There are several sources of methylmercury (MeHg) to lakes. These include the atmosphere, runoff from terrestrial catchments, and in-lake production. The relative importance of these sources varies in different situations (1). Here, we focus on the role of terrestrial catchments in adding or removing MeHg from water as it passes through catchments and into lakes. We carried out our study at the Experimental Lakes Area (ELA), northwestern Ontario, which is a site of low atmospheric deposition of both MeHg and total mercury (THg) as compared to other sites in eastern North America and Scandinavia (2). Specifically, we extend results from a previous 1-year study at the ELA, which showed that terrestrial catchments containing wetlands are important sites of MeHg production and important sources of MeHg to lakes, whereas purely upland catchments are sites of demethylation or MeHg retention (3). Catchment type may also influence THg inputs to lakes (4) because catchments remove varying amounts of THg as water flows through them (3, 5-8). The THg may either be stored in soils (6, 9, 10) or volatilized to the atmosphere as Hg0 (11). It is important to know the retention efficiency and the export of THg in terrestrial watersheds because this affects the magnitude of input of THg to lakes, which is one of several factors that controls the rates of in-lake mercury methylation and fish mercury concentration (1, 12). Because of the general importance of catchment type and especially the presence of wetlands in determining MeHg inputs to downstream ecosystems, we undertook this study to answer the following questions: (1) Are all catchments that contain wetlands consistently net MeHg sources, or are some catchments sources in some years and sinks in others? (2) Are some types of wetlands larger per unit area sources of MeHg than other types of wetlands? (3) Is the magnitude of a wetland MeHg source consistent from year to year? We also wanted to know the rate of export and the efficiency of THg retention in the ELA terrestrial catchments, and if this differs from one type of catchment to another.

Site Description and Methods We studied the magnitude of five boreal forest catchments at the ELA as sources or sinks for MeHg and THg (Figure 1, Table 1). A detailed description of these catchments follows. † Contribution No. 7 of the Experimental Lakes Area Reservoir Project (ELARP). * Author to whom correspondence should be addressed; telephone: (204) 983-5226; e-mail address: [email protected]. ‡ University of Manitoba. § Freshwater Institute. | Flett Research Ltd. ⊥ McGill University.

VOL. 30, NO. 9, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Location of the catchments at the Experimental Lakes Area (ELA) in northwestern Ontario. The watershed boundary of each catchment is outlined by a bold line. TABLE 1

Summary of Inputs and Outputs of MeHg and THg to the Whole Catchments and the Wetland Areas Only of the Catchments catchment

area (ha)

% wetland

inputs to whole catchment

upland valley-bottom wetland 1

5.73 170.3

0 14.0

depositiona

valley-bottom wetland 2

55.3

14.4

depositiona

riverine wetland

98.1

25.2

depositiona inflow from lake 240b

basin wetland

40.2

16.3

depositiona

depositiona

inputs to wetland areas only

output

no wetland areas depositiona runoff from uplandsc depositiona runoff from uplandsc depositiona inflow from lake 240b inflow from valley-bottom 2 catchmentd runoff from uplandsc depositiona runoff from south subcatchmente runoff from west subcatchmente runoff from north subcatchmentf

outflowb outflowb outflowd outflowb

outflowb

a Concentration of MeHg and THg in wet deposition measured; dry deposition of THg estimated as 50% of THg wet deposition. b Concentration of MeHg and THg measured; water volume measured at weir. c MeHg and THg concentrations estimated from upland catchment values; water volume estimated on an areal basis from the valley-bottom wetland 1 catchment. d Concentration of MeHg and THg measured; water volume estimated on an areal basis from the valley-bottom wetland 1 catchment. e MeHg and THg concentrations estimated from concentrations measured in runoff in 1994; water volume estimated on an areal basis from the whole catchment. f MeHg and THg concentrations estimated from concentrations measured in runoff from south subcatchment in 1994; water volume estimated on an areal basis from the whole catchment. For all footnotes, please see text for details.

Upland Catchment. The upland catchment (U in ref 3) has no wetland areas (Figure 1, Table 1). The catchment has a southern exposure and a uniform slope. The area is granodiorite bedrock overlain by thin glacial drift composed largely of sand and gravel (13). Exposed areas of granodiorite bedrock have partial lichen cover. The watershed was clear cut in 1976 (14), resulting in a young forest dominated by dense stands of jack pine (Pinus banksiana Lamb.) and paper birch (Betula papyrifera Marsh.). Runoff was episodic from May to October depending on rainfall and antecedent moisture conditions. In November, flows diminished and remained near zero throughout the winter. Snow accumulated in the uplands until it melted in late March or April. Catchments Containing Valley-Bottom Wetlands. We studied two headwater catchments (U/W 1 and U/W 2 in ref 3) that contained valley-bottom wetland areas that run alongside well-developed streams, which are fed by runoff from upland terrain (Figures 1 and 2, Table 1). The upland

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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 9, 1996

terrain of these two catchments was similar to that of the upland catchment. These upland areas were burned in 1974 and 1980, and forest regeneration was mainly jack pine and paper birch (15, 16). The valley-bottom wetland areas are dominated by Sphagnum spp. with an overstory of alder (Alnus spp.) and black spruce (Picea mariana (Mill.) BSP) (16). Stream flows were more continuous than for the episodic runoff from the upland catchment due to water storage in the valleys. Catchment Containing a Riverine Wetland. The riverine wetland catchment consists of a shallow central pond surrounded by a peatland and an upland area, which drains directly into the peatland (Figure 3). The valley-bottom wetland 2 catchment also drains into the peatland (Figure 1, Table 1). The upland areas of this catchment were burned in 1980. The predominate inflow to the catchment is from lake 240 (L in ref 3; Figure 3). Predominant wetland plants are Sphagnum angustifolium/falax and S. magellanicum, with an overstory of primarily black spruce, tamarack (Larix

FIGURE 2. Annual outputs and inputs of MeHg (mg) to the valley-bottom wetlands (gray shaded areas). Annual water inputs and outputs for the wetlands are in units of 103 m3. Net output of MeHg and % water yields are presented in the boxes under the headings.

laricina), Labrador tea (Ledum groenlandicum Oedes), and leatherleaf (Chamedaphne calyculata (L.) Moench) (17). Water generally flows all year into the catchment from lake 240 and exits by the catchment outflow (Figure 3). Catchment Containing a Basin Wetland. We studied a headwater catchment that contains a basin wetland. The basin wetland catchment (W in ref 3) has three upland subcatchments, which drain into a basin containing a peatland with a central pond (Figures 1 and 4, Table 1). The west subcatchment contains several small wetland areas (Figure 1). The north and south subcatchments are purely uplands. The west and south subcatchments were clear cut during 1973/1974. These areas now contain jack pine, black spruce, balsam fir (Abies balsamea (L.) Mill.), paper birch, and maple (Acer spp.), with some alder understory. The undisturbed north subcatchment is a mature jack pine, black spruce, and paper birch forest. The peatland is composed of two vegetation community types (sedge meadow and open black spruce area; 17). The major peatland plants are Sphagnum angustifolium/falax, with an overstory of primarily black spruce, Labrador tea, leatherleaf, and sweet gale (Myrica gale). Water flowed all year from the catchment. Mercury Sampling Methods and Frequency. Samples for THg and MeHg were taken in Teflon bottles. The bottles

were cleaned, stored, and transported as described in St. Louis et al. (3). The clean-hands, dirty-hands protocol was used for sampling (3). THg samples were preserved with concentrated trace metal grade HCl. MeHg samples were frozen. Both THg and MeHg samples were analyzed usually within 1 month of collection. Flett Research Ltd. analyzed samples for THg using the technique described in ref 18 and for MeHg using the techniques of refs 19 and 20 by cold-vapor atomic fluorescence spectrophotometry. All samples were taken in duplicate, and the concentrations used in this study are averages from the duplicate analyses. If concentrations from duplicate samples deviated from their mean by more than 15%, the analyses were repeated. Blanks and replicate analyses of at least one sample were done with every batch of samples run. The detection limit for THg was 0.2-0.3 ng L-1 at a blank level of 0.3-0.4 ng L-1. MeHg could be detected at levels above 0.01-0.02 ng L-1 at a blank level of 0.05-0.1 ng L-1. Mercury samples were taken at weirs at the outflow of catchments, weekly during high spring runoff, biweekly during summer flow, and monthly during winter when flow occurred. Atmospheric Deposition of MeHg and THg. Volumeweighted mean concentrations of MeHg and THg in wet deposition at the ELA are 0.052 and 4.04 ng L-1, respectively (2). Using these average concentrations and measured


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FIGURE 3. Annual outputs and inputs of MeHg (mg) to the central riverine wetland (gray shaded areas). Annual water inputs and outputs for the wetland are in units of 103 m3. Net output of MeHg and % water yields are presented in the box under the heading.

precipitation volumes (Table 2), we estimated that annual wet deposition for 1990/1991, 1991/1992, and 1992/1993 was respectively 0.39, 0.45, and 0.34 mg ha-1 for MeHg and 31, 35, and 27 mg ha-1 for THg. Several studies have found that dry deposition of THg is about 50% of wet deposition (21-23). We used this percentage plus measured wet deposition to estimate total deposition inputs of THg at the ELA (Table 3). Present understanding is that there is little MeHg in dry deposition (24, 25). Therefore, we used wet deposition measurements as deposition inputs for our mass balance MeHg calculations. Mass Balance Budget Calculations. The basic equation used in the input-output budget calculations for each catchment or wetland area within a catchment was

NetHg ) OHg -

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∑I

Hg

(1)


where for either THg or MeHg, ∑IHg is the annual sum of all inputs (mg) to the catchment. Inputs to whole catchments or wetland areas only may include wet deposition (plus dry deposition for THg), stream inflows, and direct runoff from any upland catchments if present. OHg is the annual output (mg) from the catchment outflow. Annual budgets were calculated for the period November 1-October 31, because snowfall following freezup in early November contributed to runoff in the subsequent spring. At the whole catchment level, a negative value for NetHg indicates that the catchment is a net sink for MeHg or THg, while a positive value indicates that the catchment is a net source. In the case of MeHg, the term “sink” means that incoming MeHg is either destroyed through demethylation or stored within the catchment. The term “source” means there is a net production of MeHg within the catchment.

FIGURE 4. Annual outputs and inputs of MeHg (mg) to the central basin wetland (gray shaded areas). Annual water inputs and outputs for the wetland are in units of 103 m3. Net output of MeHg and % water yields are presented in the box under the heading. TABLE 2

Seasonal Average Temperatures and Precipitation at the Experimental Lakes Area Meteorological Site av temp (°C)

precipitation (mm)

seasona

1990/1991

1991/1992

1992/1993

1990/1991

1991/1992

1992/1993

Nov 1-Mar 31 Apr 1-May 31 Jun 1-Aug 31 Sep 1-Oct 31 annual mean/totalb

-10.6 9.9 19.5 6.7 3.3

-9.6 6.2 14.7 7.8 2.1

-10.3 7.0 16.5 5.3 2.0

131.2 167.4 197.6 259.1 755.3

182.0 135.4 400.3 149.7 867.4

96.9 83.3 392.8 87.0 660.0

a Snow usually accumulates November-March. Spring runoff occurs during April and May, and water temperatures tend to be cool during this period. This is also the time when the forest begins to green. June-August is considered the summer period. Air and water temperatures begin to drop in September until freezup in early November. b Annual mean temperature; annual total precipitation.

A summary of all of the inputs and outputs for the budget calculations for each catchment and for the wetland area within each catchment is given in Table 1. Detailed descriptions of these inputs and outputs are given under each catchment heading below. Upland Catchment. Whole Catchment. Budget calculations for the upland catchment were made by eq 1. OHg was calculated from measurements of concentration and flow between September 1991 and November 1993. There was no flow during the summer of 1991. Mercury concentrations in the spring of 1991 were not measured. Instead, they were estimated from the average spring concentrations measured at the upland catchment in 1992. OHg was calculated for each period between sampling dates using

OHg )

[Hgt1] + [Hgt2] V 2

(2)

where [Hgt1] and [Hgt2] are mean concentrations of either THg or MeHg in water at two consecutive sampling times,

and V is the total volume of water that flowed from the catchment between those times. Annual outputs of THg and MeHg (mg yr-1) were calculated by summing outputs for each sample period. Valley-Bottom Wetland Catchments. Whole Catchment. Mercury budgets for both catchments were calculated using eqs 1 and 2, with inputs from deposition only (Table 1). The outflow of the valley-bottom wetland 1 catchment was hydrologically gauged. The outflow of the valley-bottom wetland 2 catchment could not be gauged, so water yields were estimated on an areal basis from the nearby valley-bottom wetland 1 catchment, which was very similar with respect to geology, basin physiography, and vegetation cover (3). Mercury concentrations, however, were measured at the outflow of both valley-bottom wetland catchments. Wetland Areas Only. The area of valley-bottom wetland within a catchment was calculated from digitized tracings of aerial photos (1:16 000). Inputs to the valley-bottom wetlands were direct deposition and runoff from the


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a Deposition volumes in 1990/1991, 1991/1992, and 1992/1993 were 755.3, 867.4, and 660.0 mm, respectively. b Annual budgets for MeHg and THg are graphically represented in Figure 5. c Catchment areas are listed in Table 1. d A positive result indicates the catchment is a source of MeHg. e % export was calculated as (∑inputs/output) × 100. f A negative result indicates that the catchment is a sink for THg. g Total watershed area above the riverine wetland outflow is 821.1 ha (lake 240 watershed of 723 ha + riverine wetland watershed of 98.1 ha).

59.8 82.1 22.7 60.6 42.8 49.9 72.4 19.7 43.0 25.8 2 869 8 273 362 908 415 1 497 1 719 613 704 536 741 000 851 000 304 000 349 000 265 000 2 993 3 438 1 227 1 409 1 072 39 44 16 18 14 1990/1991 1991/1992 1990/1991 1991/1992 1992/1993

737 365 000 1 475 19 1992/1993

969 480 000 1 938 25 1991/1992

844 418 000 1 687 22 1990/1991

87 100 76 2 598 2.3 2.6 2.0 67 1990/1991 1991/1992 1992/1993 1990/1991

upland upland upland valley-bottom wetland 1 valley-bottom wetland 2 valley-bottom wetland 2 valley-bottom wetland 2 riverine wetland riverine wetland basin wetland basin wetland basin wetland

175 43 300 201 49 700 153 37 800 5 196 1 290 000

32 172

1 258 816 000 92 6 268 3 100 000 263 34 90 99

0.11g 0.32g 0.85 2.2 2.5

3.5g 10g 9.0 23 10

930 000 3 240 000 69 000 211 000 114 000

0.21 0.48 0.45 1.80 2.13

129.6 121.7 213.4 499.0 719.7

-29.35 -32.13 -36.77 -29.97 -29.68

25.8 34.9 772 30

0.54

14

94 000

0.20

158.3

-26.04

48.5 63.8 50

0.90

1 856

34

233 000

0.45

200.3

-19.01

18.9 33.6 73.8 0.29

851

15

79 000

-0.10

-30.39

27.0 25.6 16.1 19.2 44.0 35.7 25.5 38.6 -25.62 -33.81 -29.81 -28.09 18.2 18.6 18.3 63.8 0.07 0.09 0.07 0.25

115 107 58 3 010

20 19 10 18

11 700 12 700 6 100 247 000

-0.32 -0.37 -0.28 -0.14

16

MeHg (mg) H2O (m3) MeHg THg (mg) (mg) THg (mg) H2O (m3) MeHg THg (mg) (mg) year catchment

stream inputs dry deposition

0.4 0.5 0.4 43

NetMeHg % of MeHg NetTHg % of THg % H2O (mg ha-1)d inputs exportede (mg ha-1)f inputs exportede Yield

output - ∑inputsb

H2O (m3) THg (mg)

THgc (mg ha-1) MeHgc (mg ha-1)

catchment exports 9

wet depositiona

Whole Catchment Mass Balance Budgets of MeHg, THg, and H2O

TABLE 3

2724


surrounding uplands (Figure 2, Table 1). Runoff from uplands into wetland areas could not be measured directly and was estimated as follows. Water yields from the uplands were estimated using the water yields from the whole valleybottom wetland 1 catchment (wetland area was 14% of the total catchment area; Table 1). For upland MeHg concentration, we used the geometric mean concentration of MeHg measured from September 1991 to November 1993 at the nearby upland catchment. Physiography of the upland catchment and the uplands of both catchments containing valley-bottom wetlands is very similar (see above). Riverine Wetland Catchment. Whole Catchment. MeHg and THg budgets for the catchment containing the riverine wetland were calculated using eq 1, with inputs from deposition and lake 240 (Table 1). Inputs from lake 240 were calculated using eq 2. Wetland Area Only. There were four inputs to the wetland: wet deposition directly onto the wetland, inflow from lake 240, runoff from the east subcatchment (valleybottom wetland 2), and direct runoff from the uplands (Figure 3, Table 1). The strength of the riverine wetland as a source of MeHg was calculated from eq 1 with MeHg inputs from lake 240 and outputs from the wetland being the same as for the whole catchment. Input of MeHg from the east subcatchment is the outflow from the valley-bottom wetland 2 catchment. Input of MeHg from upland runoff was estimated in the same way as was upland runoff in catchments with valley-bottom wetlands (see previous section). Basin Wetland Catchment. Whole Catchment. Mercury budgets for the catchment containing the basin wetland were calculated using eqs 1 and 2, with inputs from deposition only (Table 1). Wetland Area Only. MeHg budgets for the basin wetland were calculated from eq 1. There were four inputs of MeHg to the wetland area of the catchment: wet deposition directly onto the wetland surface, and runoff from the three upland subcatchments (Figure 4, Table 1). To calculate the input of MeHg from runoff from each subcatchment, we first estimated runoff volume because the upland subcatchments were not hydrologically gauged. These uplands are a large portion of the whole catchment (89.4%), and the whole catchment outflow is gauged. Thus, after runoff leaves the uplands, its volume is altered only by direct precipitation onto the wetland and by evaporation from the wetland, and so

Quplands ) Qoutflow + Evaporationwetland Wet depositionwetland (3) where Quplands is the upland water yield, Qoutflow is the whole catchment outflow volume, Wet depositionwetland is the deposition volume onto the wetland surface, and Evaporationwetland is the volume of water that evaporates from the wetland, assuming zero storage. The only unmeasured parameter was Evaporationwetland. We estimated seasonal total evaporation from the pond area of the wetland using evaporation pan data (measured at the ELA meteorological site), multiplied by a pan coefficient of 0.7 to account for temperature differences between the pan and the pond surface (26). Evaporation from the vegetated surface of the wetland was estimated to be 75% of pond evaporation (27).

TABLE 4

Annual Mean Concentrations ((SD) of MeHg and THg in Water Flowing from the Various Catchments, Inflows, and Runoff Subcatchmentsa MeHg (ng L-1)

catchment

1990/1991

upland valley-bottom wetland 1 valley-bottom wetland 2 riverine wetland basin wetland lake 240 inflow to riverine wetland runoff from south subcatchment of basin wetland runoff from west subcatchment of basin wetland

0.031 ( 0.117 ( 0.046

0.038 ( 0.012 0.055 ( 0.028

13.45 ( 11.40 ( 2.76

8.82 ( 2.48 9.19 ( 1.87

0.249 ( 0.090

0.258 ( 0.139 0.355 ( 0.138

11.35 ( 2.63

8.52 ( 2.18 6.26 ( 1.71

0.145 ( 0.080 0.727 ( 0.567 0.041 ( 0.010

0.081 ( 0.024 0.456 ( 0.311 0.674 ( 0.521 0.054 ( 0.018

3.43 ( 1.44 4.83 ( 1.50 1.39 ( 0.93

2.62 ( 1.04 4.32 ( 1.16 3.50 ( 1.09 2.07 ( 0.71

0.019b

1991/1992

1992/1993

THg (ng L-1)

1994

1990/1991 1.72b

Results and Discussion Weather conditions differed markedly during each of the 3 years of this study. We found that in examining the effects of weather on hydrology and mercury biogeochemistry, it was important to consider not only average annual conditions but also specific conditions occurring during each summer period. This was because water yield, which is an important factor in controlling Hg transport, is determined by precipitation and evaporation which varies widely from summer to summer. On an annual basis, 1991/1992 was the wettest year, followed by 1990/1991 and 1992/1993 (Table 2). However, the summer of 1990/1991 was the

1992/1993

1994

0.045 ( 0.022c

5.43 ( 1.72c

0.321 ( 0.105c

7.36 ( 1.95c

a Means were calculated for the period November 1-October 31 unless otherwise stated. 1991. c Mean of eight samples taken between April 20 and July 20, 1994.

To calculate inputs of MeHg from runoff from each subcatchment, we also needed MeHg concentrations. For the west and south subcatchments, geometric mean concentrations were obtained from eight measured spring melt and storm samples taken between April 20 and July 20, 1994. No sampling site was available in the north subcatchment, and runoff MeHg concentration was assumed to be the same as for the south subcatchment because both subcatchments contained no wetland areas. We did not calculate THg mass balance budgets for wetland areas of the catchments as was done for MeHg. This was because rates of THg retention in the upland and wetland areas were very similar. Errors associated with our budget calculations are as follows. In an average year, 28% of annual precipitation at the ELA occurred as snow and 72% occurred as rainfall. The Nipher gauge used at the ELA to measure snowfall is within 10% of true (28). Errors in measurement of precipitation are about 5% or less for annual estimates and are within 5% for stream discharge (29). It is common for most water balance studies to calculate one or more terms of the budget as the residual. In our budgets, evaporation was usually the only residual. In general, errors associated with water budgets are lowest for long durations (i.e., annual), which is why we present annual budgets in this study. Analytical precision was (5% for THg and (0.015 ng L-1 for MeHg (Flett Research Ltd.; 30). These values were within 9% and 12% of a consensus value obtained at a recent international interlab comparison for THg and MeHg, respectively (31).

1991/1992

b

Samples were not taken during the period April-June

hottest and driest of the three summers (Table 2), and streamflows stopped for a period of 6 weeks. As a result, even though 1990/1991 was not the driest year in terms of annual wet deposition, water yields for that year were the lowest for any of the years of our study (Table 3). The summer of 1991/1992 was, on average, cool and wet (Table 2), and as a result streams flowed for most of the year and water yield was the highest of the three years (Table 3). Summer temperature, precipitation, and water yields in the summer of 1992/1993 were intermediate between the other two years (Tables 2 and 3). Total Mercury. There was a consistent pattern in average annual concentrations of THg in runoff from the five catchments. Runoff from the upland catchment had the highest concentrations of THg, followed by the catchments containing the valley-bottom wetlands, basin wetland, and riverine wetland (Table 4). High evaporation (i.e., low water yields) in upland areas was probably responsible for high concentrations of THg in the outflows of the upland catchment and catchments containing valley-bottom wetlands. THg concentrations in the outflow of the riverine and basin wetland catchments were much lower (Table 4), and this was probably related to the much higher water yields from these catchments (Table 3). Export of THg (i.e., mass output) from the riverine wetland catchment watershed, which includes a large lake just upstream of the wetland (3), averaged 6.8 mg ha-1 yr-1 over a 2-yr period (Table 3). Because lakes are large sinks for THg (3), this export was much smaller than the export from the other four catchments, which were headwater catchments (i.e., had no upstream inflow). THg exports from the four headwater catchments were remarkably similar on an annual basis, averaging between 14 and 21 mg ha-1 yr-1 (Table 3). The similarity of THg export from various types of terrestrial watersheds to lakes at the ELA suggests that lake-to-lake differences in fish mercury concentration, which can be substantial in boreal forest lakes (e.g., ref 32), are probably not explained by differences in THg input to the lakes. Other factors such as inputs of MeHg from wetlands (e.g., Table 3; 3, 33) or differences in lake temperature due to lake size, which affects in-lake MeHg production (32), may explain the observed differences in fish MeHg concentration in neighboring lakes.


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FIGURE 5. Annual net retention of THg (a) and net output of MeHg (b) for the purely upland catchment and for four catchments containing three different types of wetland. Details of the inputs and outputs to the catchments are presented in Tables 1 and 3.

We also calculated net retention of THg by the catchments (OTHg - ∑ITHg). With the exception of the riverine wetland catchment, which had an upstream inflow, THg inputs to the catchments were from wet and dry deposition (Table 1). We did not include the possible contribution of weathering as an input. There are no data on Hg weathering rates in boreal ecosystems. We do, however, have data on rates of weathering of other elements such as Si, Ca, and Na calculated from long-term ELA deposition and watershed outflow records. Si weathering, for example, in the ELA watersheds is typically 5600 g ha -1 yr -1 (34). The average concentration of Si in granodiorite bedrock at the ELA is 0.34 g g-1 (35), whereas Hg concentrations in granite elsewhere are reported to be only 9-200 ng g-1 (36). If weathering of Hg occurred at the same rate as Si, weathering would contribute 0.2-3 mg of Hg ha-1 yr-1 to the catchments. This weathering input would be small relative to the average input of 47 mg ha-1 yr-1 from wet + dry deposition (see Site Description and Methods). The actual weathering rate could be quite different, however, depending on which particular minerals the Hg in the ELA bedrock and tills are associated. Therefore, it is not possible in this paper to include the weathering input in the calculation of Hg retention in the ELA catchments. The retention numbers reported are therefore underestimates of true retention, but inclusion of weathering would not change the finding that all catchment types were sinks for THg. The range of net THg retention in catchments was small (19-37 mg ha-1 yr-1) for all the 12 annual measurements (Table 3, Figure 5a). There was no obvious pattern among

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catchments or years in the size of the THg sink (Figure 5a), even though catchments differed in hydrology, vegetation, and wetland type. For example, in 1990/1991, which was a low water yield year, the basin wetland catchment was the largest sink per unit area for THg, and the upland catchment was the smallest. The reverse was true for 1991/ 1992 (Figure 5a), which was a high water yield year. St. Louis et al. (3) concluded, based only on 1 year of study (1990/1991), that the catchment containing the basin wetland was a larger sink for THg than the upland catchment or the catchments containing valley-bottom wetlands. Based on 3 years of data, we now conclude that the presence or absence of wetlands in these catchments had no discernible effect on THg retention. Two other studies (8, 37) concluded that the presence of wetlands in a catchment decreased retention of THg. The difference between our study and theirs is probably explained by the comparatively low percentages of wetland area in our catchments [1425% at the ELA versus 44% in Wisconsin (37) and 30% in eastern Ontario (38)]. Methylmercury. There was a definite pattern among the five terrestrial catchments with respect to water yields, MeHg concentrations, and net output of MeHg to lakes. This pattern and the possible reasons will be discussed for each catchment. Upland Catchment. Water yields from the upland catchment were generally low (16-27%; Table 3) probably because this catchment was on a south-facing slope that resulted in high rates of evaporation. MeHg concentrations in the catchment outflow (0.03-0.06 ng L-1; Table 4) were very low and similar to the average MeHg concentration in wet deposition at the ELA (0.05 ng L-1). An average of 2.3 mg yr-1 of MeHg entered the upland catchment in wet deposition, whereas only about 0.4 mg yr-1 exited (Table 3). Consequently, over the 3-yr period, the upland catchment was consistently a sink for MeHg with 80-82% of the MeHg that entered the catchment being either stored or demethylated (Table 3). Catchments Containing Valley-Bottom Wetlands. Water yields from the two catchments containing valley-bottom wetlands were very low in 1990/1991 (19%; Table 3) due to the very dry, hot summer (Table 2) as compared to the following 2 years when the valley-bottom wetland 2 catchment water yields were 49 and 26% (Table 3). Average concentration of MeHg in water flowing from the valley-bottom wetland catchments was always 2-7 times higher (0.12-0.36 ng L-1; Table 4) than for the upland catchment and for wet deposition. The valley-bottom wetland 2 catchment was a sink for MeHg in the year of low water yield but was a downstream source of MeHg during the 2 years of relatively high water yields (Table 3, Figure 5b). The valley-bottom wetland 1 catchment was also a sink for MeHg during the low water yield year (Table 3, Figure 5b). Even though both catchments containing valley-bottom wetlands were sinks for MeHg in 1990/1991, they were much smaller (by 3-fold) sinks per unit area than was the upland catchment (Table 3, Figure 5b). This was because the wetland areas of these catchments were small sources of MeHg even in the year of low water yield (Figure 2). The wetland areas of the valley-bottom wetland 2 catchment were large sources of MeHg during the high water yield years (1991/1992 and 1992/1993; Figures 2 and 6), and as a result, the catchment as a whole was also a net source of MeHg during those years (Table 3, Figure 5b).

FIGURE 6. Annual net output of MeHg for three types of wetlands at the ELA. Details of the inputs and outputs to wetland areas of the catchments are given in Table 1 and Figures 2-4.

These observations are consistent with the findings of Branfireun et al. (39), who concluded that most of the MeHg that is produced in wetlands during dry periods is stored in the peat and then transported out of the wetland during periods of high water flow. Catchment Containing a Riverine Wetland. The hydrology and the mass balance budgets of this catchment were dominated by the large water input from upstream lake 240, which receives water from a 723-ha watershed. Concentrations of MeHg in the inflowing lake water were very low (0.04-0.05 ng L-1; Table 4) and similar to concentrations of MeHg in wet deposition. Concentrations of MeHg in water leaving this catchment were 2-3 times higher than the lake and precipitation inputs (Table 4). MeHg produced in this catchment came from both the valley-bottom wetland 2 (which was contained within this catchment) and the riverine wetland (Figures 1 and 3). In 1990/1991, the low water yield year (Table 3), approximately equal amounts of MeHg entered the whole catchment from wet deposition (39 mg) and from the upstream lake (32 mg, Table 3). In 1991/1992 (a high water yield year), inputs from the lake were 5 times higher than from atmospheric deposition (Table 3). This large increase from the lake source was because the watershed area of the lake is much greater than that of the riverine wetland catchment. The catchment as a whole was a net source of MeHg in both years (0.21 and 0.48 mg ha-1 yr-1; Table 3). We also estimated mass balance MeHg budgets for the riverine wetland area of this catchment separately from the remainder of the catchment (Figure 3). Inputs of MeHg to the wetland were largest from the lake inflow, followed by inputs from the valley-bottom wetland 2 catchment (Figure 3). Wet deposition and direct runoff from the upland areas of the catchment were relatively minor inputs. The riverine wetland was a net source of MeHg in both years (2.14 and 1.79 mg ha-1 yr-1; Figure 3). The strength of this source was similar to the valley-bottom wetlands (Figure 2) and was 10.2 and 3.7 times greater per unit area than the catchment as a whole in 1990/1991 and 1991/ 1992, respectively (Table 3, Figures 5b and 6). Catchment Containing a Basin Wetland. This catchment was consistently the largest source of MeHg per unit area of all the catchments studied (Table 3, Figure 5b). Concentrations of MeHg in the outflow of the basin wetland catchment were always at least 10 times higher than average

concentrations in wet deposition over a 3-yr period (Table 4). The mass of MeHg leaving the catchment was 2-7 times greater than the mass input of MeHg from atmospheric deposition (Table 3). Thus, this catchment was a consistent source of MeHg in all 3 years, ranging from 0.45 to 2.13 mg ha-1 yr-1 (Table 3, Figure 5b). We also estimated mass balance budgets for the wetland area of this catchment separately from the entire catchment (Figure 4). The wetland area received MeHg directly from wet deposition and from three upland subcatchments. Two of these subcatchments (north and south; Figure 4) contained no wetland areas, and concentrations and mass inputs of MeHg from these subcatchments were low (Table 4, Figure 4). The third upland subcatchment (west; Figure 4) contained some wetland areas (34), and concentrations and inputs from this subcatchment were much higher than from the other subcatchments (Table 4, Figure 4, (34)). The basin wetland was the strongest source of MeHg per unit area compared with all other wetlands types in this study (Figure 6). This source was about 3-10 times larger than that from the valley-bottom wetlands and about 7 times larger than that from the riverine wetland (Figures 2-6). General Discussion. There was no relationship between the MeHg and THg mass balance budgets of these catchments (Figure 5a,b). While all catchments retained similar amounts of THg (Table 3, Figure 5a), there was wide, consistent variation in the strength of catchments as sources of MeHg (Table 3, Figure 5b). This lack of relationship between THg and MeHg has been shown for a variety of other boreal systems (e.g., refs 3 and 30). Thus, in studying Hg biogeochemistry, it is important to emphasize that the production and transport of MeHg cannot be predicted from the behavior of THg. MeHg is the most toxic form of mercury and is the most efficiently bioaccumulated. With respect to MeHg, our results clearly showed that upland areas without wetlands were consistently sinks for MeHg. This was true in all 3 years for the upland catchment (Figure 5b) even though water yields varied widely (Table 3). It was also true for the south subcatchment of the basin wetland catchment, which also contained no wetland areas and always had very low MeHg concentrations in runoff (Table 4). Microbial methylation of mercury often occurs in anaerobic environments, whereas microbial demethylation of MeHg can occur aerobically (12). The thin layers of upland soil at the ELA tend to be aerobic and, therefore, may favor demethylating bacteria. Wetland areas were always net sources of MeHg, but there were large consistent differences among wetland types. The basin wetland was a consistently greater source of MeHg than the other two wetland types, even in 1990/ 1991, which was the lowest water yield year (Table 3, Figures 4 and 6). The percent wetland area in all the catchments that contained wetlands was similar (14-25%, Table 1). Thus, the type of wetland in a catchment was important in determining differences in the importance of a catchment as a source of MeHg to lakes (Figures 5b and 6). The differences in MeHg production rate among the wetland types may be related to differences in hydrology within the wetlands. Hydrology is an important factor in determining wetland vegetation and biogeochemical processes (e.g., ref 40), and this may also be true for mercury methylation. For example, in the basin wetland, Branfireun et al. (39) found that MeHg concentrations in the peat and porewater were highest at discrete locations where ground-


9

2727

TABLE 5

Average Annual MeHg Inputs to Lake 239 at the Experimental Lakes Area and Estimates of Hg Bioaccumulation in Fish in the Lake inputs to lake 239 (area of subcatchment or lake in ha)a

annual MeHg inputs (mg)b

valley-bottom wetland 1 (170.3)

96f

NW subcatchment (56.4)

32g

NE subcatchment (12.4)

7g

upland direct runoff (98.2)

8h

deposition onto lake surface (56.1) total total (mg ha-1 of lake 239)

fish species northern pike (Esox luscius) lake trout (Salvelinus namaycush) white sucker (Catostomus commersoni) yellow perch (Perca flavescens)

22i 165 2.9

average annual Hg fish biomass Hg concn fish age bioaccumulation in fish in lake in lake in fish in lake 239 (kg)c (µg g-1)d 239 (yr)c 239 (mg)e 97

0.39

12

3

89

0.39

15

2

131

0.17

6

4

200

0.17

1

34

43 0.8

a Inputs to lake 239 from Beaty and Lyng (14). b Average for the budget years 1990/1991-1992/1993. c Estimates of fish biomass and average fish age from catch/release surveys in lake 239 (42). d Mercury concentration measured in lake trout and white sucker from various lakes at the ELA (43), whereas concentrations in northern pike and yellow perch assumed to be similar to those in lake trout and white sucker, respectively, from surveys in lakes near the ELA in which these species coexist (44). The majority of the Hg is presumed to be MeHg (45). e Species biomass multiplied by average Hg concentration in species and divided by average age of species in the lake. f Measured in 1990/1991 and estimated on an areal basis in 1991/1992 and 1992/1993 from the valley-bottom wetland 2 catchment. g Both these subcatchments contain wetlands. We therefore estimated MeHg inputs on an areal basis in 1990/1991 from the valley-bottom wetland 1 catchment and in 1991/1992 and 1992/1993 from the valley-bottom wetland 2 catchment. h Estimated on an areal basis from the upland catchment. i Lake surface area multiplied by annual MeHg input (mg ha-1) from deposition (see Site Description and Methods).

water upwelled from the underlying mineral substrate (i.e., poor fen areas). These areas in the wetland were apparently sites of high rates of MeHg production. Water flows in the valley-bottom and riverine wetlands were restricted to the peat layer, with little interaction with the underlying mineral substrate. Thus, poor fen areas were not present in the valley-bottom wetlands or in the riverine wetland, and this may be the reason that net MeHg production was lower than in the basin wetland. Because hydrology also affects plant community structure within wetlands (40), identifying patterns in vegetation communities could prove to be a valuable tool in identifying and therefore modeling the strength of sites as MeHg sources. In addition to the type of wetland, water yield also appears to exert control over the timing of MeHg export from the wetlands. For example, export of MeHg from the wetlands was relatively small during low water yield years and high during high water yield years (Figures 2-4), and during periods of high flow, MeHg concentrations remained high in outflow water (39). The maintenance of high concentrations suggests that there is a large store of MeHg on the solid peat resulting from methylation in the wetland (39). This large store of MeHg is then available for exchange with the porewater and transport out of the wetlands during periods of high flow. Although all wetland types were always sources of MeHg, even during years with low water yields, this does not mean that every catchment that contains a wetland is a net MeHg source on an annual basis. Because upland areas are sinks for MeHg, the whole catchment sink/source budget will depend on the relative proportions of uplands and wetlands within the catchment. Year-to-year variability may also play a role. For example, in the catchments that contained the valley-bottom wetlands, the source of MeHg from the wetland areas in the low water yield year was not large enough to offset the upland sink (Figures 2, 5b, and 6). Because all types of wetlands within the catchments were sources of MeHg in every year, we conclude that wetlands are important sites of MeHg production on the

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9


long term. However, although we can quantify the strength of the source, we cannot yet quantify the annual gross rate of production of MeHg in a wetland. This is because change in storage is not included in our mass balance budgets, and the amount of MeHg stored on the solid peat is large and may be increasing or decreasing in any given year (39, 41). To put into perspective the possible importance of MeHg loadings from wetlands in a lake catchment, we estimated the annual Hg bioaccumulation in fish in an ELA lake and inputs of MeHg from wet deposition and its watershed containing wetlands and uplands (Table 5). We used lake 239 because the valley-bottom wetland 1 catchment drains into it (Figure 1). We estimated that for the 3-yr period of this study, on average 165 mg of MeHg annually entered lake 239 from watershed runoff and deposition directly onto the lake surface (Table 5). This input was in excess of the estimated 43 mg of Hg annually accumulated in the fish population of lake 239 (Table 5). In addition to inputs from the catchments, in-lake MeHg production must also be an important source of MeHg to aquatic biota (46). A recent study of another nearby ELA lake (lake 240; Figure 1) concluded that in-lake and external sources of MeHg were of about equal importance (46). Sellers et al. (46) also concluded that on an annual basis, photodegradation of MeHg destroyed an amount of MeHg approximately equal to inputs from external sources. Together these data emphasize that MeHg production, transport, and destruction in ecosystems is very dynamic, and each of these processes should be clearly understood. When photodegradation of MeHg was considered in the mass balance MeHg budget of lake 240, Sellers et al. (46) estimated that approximately 450 mg of MeHg was produced annually within the lake or 10 mg ha-1 surface area of the lake (44.1 ha; 14). This MeHg production rate is similar to the net annual production rates we estimated for the wetland areas of our catchments (Figure 6). Even though MeHg is produced in both wetlands and lakes, wetlands are sources of MeHg and lakes are net sinks (3, 46). The reason for this could be that the photodegradation

of MeHg is significant in lakes, but is unlikely to be important in wetlands because water entering lakes from wetlands is usually not exposed to sunlight for long periods of time. The cycling of MeHg in the environment resulting in its bioaccumulation in fish and other organisms is too complex for understanding based only on empirical results. As a result, the cycle is being subjected to analysis by mathematical models (e.g., refs 47-49). These models are currently attempting to estimate the input of MeHg to lakes from catchments as well as in-lake production of MeHg. Our study has identified three important factors that need to be included in predictions of MeHg inputs from terrestrial watersheds. These are the percentage of wetland area in a watershed, the type of wetland, and the annual water yield.

Acknowledgments We appreciate the support of the Experimental Lakes Area. We thank A. Bordeleau, J. Heibert, and M. Lyng for helping with the MeHg and THg field sampling and B. Gooding and D. Mackay for analytical support. M. Lyng helped collect and prepare the hydrology data. G. McCullough provided tracings of aerial photographs for calculations of percent wetland area within each catchment and helped calculate water yields from the uplands of the basin wetland catchment. We thank R. A. Bodaly, M. Paterson, and three anonymous reviewers for their comments on earlier versions of this manuscript. We appreciate the financial support of Ontario Hydro, Hydro Quebec, Manitoba Hydro, the Department of Fisheries and Oceans, and an NSERC postdoctoral fellowship to V.L.St.L.

Literature Cited (1) Rudd, J. W. M. Water Air Soil Pollut. 1995, 80, 697-713. (2) St. Louis, V. L.; Rudd, J. W. M.; Kelly, C. A.; Barrie, L. A. Water Air Soil Pollut. 1995, 80, 405-414. (3) St. Louis, V. L.; Rudd, J. W. M.; Kelly, C. A.; Beaty, K. G.; Bloom, N. S.; Flett, R. J. Can. J. Fish. Aquat. Sci. 1994, 51, 1065-1076. (4) Mierle, G. Environ. Toxicol. Chem. 1990, 9, 843-851. (5) Aastrup, M.; Johnson, J.; Bringmark, E.; Bringmark, L.; Iverfeldt, A. Water Air Soil Pollut. 1991, 56, 155-167. (6) Johansson, K.; Aastrup, M.; Andersson, A.; Bringmark, L.; Iverfeldt, A. Water Air Soil Pollut. 1991, 56, 267-281. (7) Meili, M. Water Air Soil Pollut. 1991, 56, 719-727. (8) Mierle, G.; Ingram, R. Water Air Soil Pollut. 1991, 56, 349-357. (9) Lindqvist, O.; Johansson, K.; Aastrup, M.; Andersson, A.; Bringmark, L.; Hovsenius, G.; Hakanso, L.; Iverfeldt, A.; Meili, M.; Timm, B. Water Air Soil Pollut. 1991, 55, 1-261. (10) Lee, Y. H.; Borg, G. C.; Iverfeldt, A.; Hultberg, H. Fluxes and Turnover of Methylmercury: Mercury Pools in Forest Soils. In Mercury Pollution: Integration and Synthesis; Watras, C. J., Huckabee, J. W., Eds.; Lewis Publishers: Chelsea, MI, 1994; pp 329-341. (11) Lindberg, S. E.; Kim, K.-H.; Munthe, J. Water Air Soil Pollut. 1995, 80, 383-392. (12) Winfrey, M.; Rudd, J. W. M. Environ. Toxicol. Chem. 1990, 9, 853-869. (13) Brunskill, G. J.; Schindler, D. W. J. Fish. Res. Board Can. 1971, 28, 139-155. (14) Beaty, K. G.; Lyng, M. E. Can. Data Rep. Fish. Aquat. Sci. 1989, 759, v + 280 pp. (15) Bayley, S. E.; Schindler, D. W.; Beaty, K. G.; Parker, B. R.; Stainton, M. P. Can. J. Fish. Aquat. Sci. 1992, 49, 584-596. (16) Beaty, K. G. Can. J. Fish. Aquat. Sci. 1994, 51, 2723-2733. (17) Dyck, B.; Shay, J. University of Manitoba, Manitoba, unpublished results, 1996. (18) Bloom, N. S.; Crecelius, E. A. Mar. Chem. 1987, 14, 49-59. (19) Bloom, N. S. Can. J. Fish. Aquat. Sci. 1989, 46, 1131-1140.

(20) Horvat, M.; Bloom, N. S.; Liang, L. Anal. Chim. Acta 1993, 281, 135-152. (21) Iverfeldt, A. Water Air Soil Pollut. 1991, 56, 553-564. (22) Fitzgerald, W. F.; Mason, R. P.; Vandal, G. M.; Dulac, F. AirWater Cycling of Mercury in Lakes. In Mercury Pollution: Integration and Synthesis; Watras, C. J., Huckabee, J. W., Eds.; Lewis Publishers: Chelsea, MI, 1994; pp 203-220. (23) Watras, C. J.; Bloom, N. S.; Hudson, R. J. M.; Gherini, S.; Munson, R.; Claas, S. A.; Morrison, K. A.; Hurley, J.; Wiener, J. G.; Fitzgerald, W. F.; Mason, R.; Vandal, G.; Powell, D.; Rada, R.; Rislov, L.; Winfrey, M.; Elder, J.; Krabbenholf, D.; Andren, A. W.; Babiarz, C.; Porcella, D. B.; Huckabee, J. W. Sources and Fates of Mercury and Methylmercury in Wisconsin Lakes. In Mercury Pollution: Integration and Synthesis; Watras, C. J., Huckabee, J. W., Eds.; Lewis Publishers: Chelsea, MI, 1994; pp 153-177. (24) Lamborg, C. H.; Fitzgerald, W. F.; Vandal, G. M.; Rolfhus, K. R. Water Air Soil Pollut. 1995, 80, 189-198. (25) Munthe, J.; Hultberg, H.; Iverfeldt, A. Water Air Soil Pollut. 1995, 80, 363-371. (26) Newbury, R. W.; Beaty, K. G. Water budgets in small Precambrian lake basins in northwestern Ontario, Canada. In Second Conference on Hydrometeorology; American Meteorological Society: Boston, MA, 1977; pp 132-139. (27) Roulet, N. T. McGill University, Montreal, Quebec, unpublished results. (28) Goodison, B. E. J. Appl. Meteorol. 1978, 17, 1542-1548. (29) Winter, T. C. Water Res. Bull. 1981, 17, 82-115. (30) Kelly, C. A.; Rudd, J. W. M.; St. Louis, V. L.; Heyes, A. Water Air Soil Pollut. 1995, 80, 715-724. (31) Bloom, N. S.; Horvat, M.; Watras, C. J. Water Air Soil Pollut. 1995, 80, 1257-1268. (32) Bodaly, R. A.; Rudd, J. W. M.; Fudge, R. P. J.; Kelly, C. A. Can. J. Fish. Aquat. Sci. 1993, 50, 980-987. (33) Lee, Y. H.; Bishop, K.; Hultberg, H.; Pettersson, C.; Iverfeldt, A.; Allard, B. Water Air Soil Pollut. 1995, 80, 477-481. (34) Stainton, M. P. Department of Fisheries and Oceans, Winnipeg, Manitoba, unpublished results, 1996. (35) Brunskill, G. J.; Povoledo, D.; Graham, B. W.; Stainton, M. P. J. Fish. Res. Board Can. 1971, 28, 277-294. (36) Jonasson, I. R.; Boyle, R. W. Mercury Man’s Environ., Proc. Symp. 1971, 5-21. (37) Hurley, J. P.; Benoit, J. M.; Babiarz, C. L.; Shafer, M. M.; Andren, A. W.; Sullivan, J. R.; Hammond, R.; Webb, D. E. Environ. Sci. Technol. 1995, 29, 1867-1875. (38) Mierle, G. Ontario Ministry of the Environment, Dorset, Ontario, personal communication, 1996. (39) Branfireun, B. A.; Heyes, A.; Roulet, N. T. Water Resour. Res. 1996, 32, 1785-1794. (40) Bubier, J. L. J. Ecol. 1995, 83, 403-420. (41) Heyes, A. McGill University, Quebec, unpublished results, 1996. (42) Mills, K. D. Department of Fisheries and Oceans, Winnipeg, Manitoba, unpublished results, 1996. (43) Klaverkamp, J. F. Department of Fisheries and Oceans, Winnipeg, Manitoba, unpublished results, 1996. (44) Fudge, R. J. P.; Bodaly, R. A.; Strange, N. E. Can. Data Rep. Fish. Aquat. Sci. 1994, 921, v + 96 pp. (45) Bloom, N. S. Can. J. Fish. Aquat. Sci. 1992, 49, 1010-1017. (46) Sellers, P.; Kelly, C. A.; Rudd, J. W. M.; MacHutchon, A. R. Nature 1996, 380, 695-697. (47) Harris, R. C. M.Eng. Thesis, McMaster University, 1991. (48) Hudson, R. J. M.; Gherini, S. A.; Watras, C. J.; Porcella, D. B. Modelling the biogeochemical cycle of mercury in lakes: The mercury cycling model (MCM) and its application to the MTL study lakes. In Mercury Pollution: Integration and Synthesis; Watras, C. J., Huckabee, J. W., Eds.; Lewis Publishers: Chelsea, MI, 1994; pp 473-523. (49) Henry, E. A.; Dodge-Murphy, L. J.; Bigham, G. N.; Klein, S. M. Water Air Soil Pollut. 1995, 80, 489-498.

Received for review November 15, 1995. Revised manuscript received May 1, 1996. Accepted May 3, 1996.X ES950856H

X

Abstract published in Advance ACS Abstracts, July 1, 1996.


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Production and Loss of Methylmercury and Loss of Total Mercury from

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