ABSTRACT

At present it is known that internal (in-lake) production of MeHg and external catchment production and atmospheric deposition can be important MeHg sources to freshwater ecosystems. Terrestrial catchments especially those containing wetlands have been identified as important sources of MeHg to downstream aquatic systems.

We simulated the relative importance of MeHg sources to aquatic ecosystems with the help of simple mathematical model.

For the drainage lake with a 10 km² watershed, when only 10% of the watershed area was wetland, about 67% of the MeHg input to the lake originated from the watershed. In contrast, for a seepage lake only 5% of total MeHg input originated from the watershed even if 100% of the watershed was composed of wetland.

The simulations suggest that if the internal mehtylation inputs of MeHg to the lake surface could be important for seepage lakes (up to 67% of total input) and significant for drainage lakes (35%).

INTRODUCTION

It is known that internal (in-lake) production of MeHg, external catchment production and atmospheric deposition can be important MeHg sources to freshwater ecosystems. The long-range transport of anthropogenic mercury emissions to the atmosphere has led to deposition levels well in excess of natural levels in Europe and in many other countries. Much of the atmospheric Hg deposition in Fenno-Scandia has been retained in the soil. The proportion of MeHg in surface waters that comes from soils via runoff varies substantially. It is suggested that the catchment contribution to Hg/MeHg in lakes is of importance for bioaccumulation of Hg in fish.

METHYLMERCURY SOURCES

Atmospheric deposition

Deposition of MeHg has been measured at several sites in the northern hemisphere and has been found to vary considerably from one region to another. The origin of MeHg in deposition is not known but may be related to industrial activity but also MeHg inputs via litterfall may also be important. In the Gårdsjön area, Sweden MeHg deposition via litterfall is equal to wet/bulk deposition and litterfall deposition is 2.5 fold compared to wet/bulk deposition. In southern Finland the litterfall input is much more higher giving 6 fold higher input fluxes than wet/bulk deposition. The source of MeHg in litterfall is not known.

Table 1. MeHg in wet/bulk deposition in different regions of the northern hemisphere.

Region	Wet/Bulk deposition g km^-2 yr^-1	Reference
ELA, NW Ontario, Canada	0.039	St. Louis et al. (1995)
N. Sweden	0.07	Lee et al. (1998)
Wisconsin, USA	0.088	Fitzgerald et al. (1991)
S. Finland	0.1	Verta et al. 1994
C. Sweden SW Sweden	0.1 0.24-0.46 0.21-0.43 0.21	Hultberg et al. (1994) Lee et al. (1998) Munthe et al. (1995) Hultberg et al. (1995)

Terrestrial catchments

Terrestrial catchments especially those containing wetlands have been identified as important sources of MeHg to downstream aquatic systems. The upland catchment in ELA exported ten fold less MeHg than wetland catchment. Studies in ELA and in southern Sweden have demontrated that pure uplands are sinks for MeHg in precipitation. The fate of MeHg retained in uplands is unknown. Some is stored in soils. Feather mosses may also be important upland storage sites It is also possible that some of the MeHg retained by uplands is demethylated.

Table 2. MeHg export from catchments of different regions of the northern hemisphere.

Region	Catchment type	Wet/wet+dry precipitation	Export	Production	% Retention or demethylation	Reference
ELA	Upland Upland/riparian wetland Upland/headwater wetland	0.039/0.08 0.039 0.039	0.007 0.078 0.19	-0.032 0.039 0.148	91	St. Louis et al. (1994)
Gårdsjön, S. Sweden Svartberget, N. Sweden Dybäcken, C. Sweden	Upland/small wetland Upland/wetland Swamp forest	0.21/0.42 0.40/0.80 0.07 0.10	0.03 0.20 0.11 0.16	-0.18 -0.20 0.04 0.06	93 75	Lee et al. (1998) Hultberg et al. (1994) Lee et al. (1998) Lee et al. (1998)
Paroninkorpi, S. Finland	Podzolic soil	0.10/0.20	0.09	-0.01	55	Lee et al. (1998)
Hakojärvi, S. Finland	Podzolic soil/ wetland/upland	0.10	0.21-0.24	0.11-0.14		Verta et al. 1994
Lakkasuo, S. Finland	wetland	?	0.33	(0.23)		Verta & Porvari (1996)

Even though a large percentage of MeHg is retained and/or demethylated in upland terrain, runoff of the residual MeHg from uplands may be an important source to lakes in regions where atmospheric inputs are very high. Possible biogeochemical mechanisms for controlling MeHg output include transferring MeHg from soil to soil water, via degradation of the soil organic matter to DOC, and methylation of inorganic Hg in the soil pool to MeHg. Both processes make MeHg available for hydrological transport out of the soil.

We simulated the relative importance of MeHg sources to aquatic ecosystems with the help of simple mathematical model. The original idea of simulatation was adapted from Rudd (1995). Inputs to the model included lake area, watershed area, and percent wetland in the watershed. Other inputs were fluxes of MeHg from direct precipitation to the lake surface, runoff from uplands, runoff from wetlands, and in lake production.

Figure 1. The relative importance of the watershed as a source of MeHg as a function of the watershed:lake area ratio, the % wetland in the watershed, and the methylation rate.

For the drainage lake with a 10 km² watershed, when only 10% of the watershed area was wetland, about 67% of the MeHg input to the lake originated from the watershed (Fig. 1). In contrast, for a seepage lake only 5% of total MeHg input originated from the watershed even if 100% of the watershed was composed of wetland (Fig. 1).

For a seepage lake with high in-lake methylation rate (3 g km^-2 yr^-1) the relative importance of the wateshed as a source of MeHg was extremely low even if 100% of the watershed was wetland. For a 10:1 drainage lake, if the watershed was 15% wetland, about 30% of total MeHg inputs was from the wateshed and if 100% of the watershed was wetland, 52% of total input was from the watershed. For this drainage lake situation, where in-lake methylation rate was high and atmospheric input was

low, this simulation suggests that both the watershed and internal production were important sources of MeHg.

Simulations of the possible importance of precipitation as a source of MeHg to lakes were made by manipulating the following inputs to the model: 1) watershed:lake area ratio, 2) rate of direct precipitation onto the lake, 3) rates of internal methylation. For this exercise, fixed inputs to the model were the percentage wetland in the watershed and the flux of MeHg from wetland areas.

Figure 2. The relative importance of the precipition as a source of MeHg as a function of the watershed:lake area ratio, the MeHg precipitation rate, and the in-lake methylation rate

The simulations suggest that if the internal methylation inputs of MeHg to the lake surface could be important for seepage lakes (up to 67% of total input) and significant for drainage lakes (35%) (Fig 2).

References

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Hultberg, H., Iverfeldt, Å. & Lee, Y.-H. 1994. Methylmercury input/output and accumulation in forested catchments and critical loads for lakes in south-western Sweden. In: Watras, C.J. & Huckabee, J.W. (eds.) Mercury Pollution - Integration and Synthesis. Lewis Publishers, Boca Raton Fl., USA, p. 313-322.

Lee, Y.H., Bishop, K., Munthe, J., Iverfeldt, Å., Verta, M., Parkman, H. & Hultberg, H. 1998. An examination of current Hg deposition and export in Fenno-Scandian catchmets. Biogeochemistry 40: 125-135.

Rudd, J. 1995. Sources of methyl mercury to freshwater ecosystems: a review. Water, Air, and Soil Pollution 80:697-713.

St. Louis, V., Rudd, J., Kelly, C. & Barrie L. 1995. Wet deposition of methyl mercury in northwestern Ontario compared to other geographic locations. Water, Air and Soil Pollution 80: 405-414.

St. Louis, V., Rudd, J., Kelly, C., Beaty, K., Bloom, N. and Flett, R. 1994. Importance of wetlands as sources of methyl mercury to boreal forest ecosystems. Canadian Journal of Fisheries and Aquatic Science 51:1065-1076.

Verta, M., Matilainen, T., Porvari, P., Niemi, M., Uusi-Rauva, A. & Bloom, N. 1994. Methylmercury sources in boreal lake ecosystems. In.: Watras, J. & Huckabee, J. (eds.), Mercury as a global pollutant: Intergation and synthesis, p. 119-136.

Verta, M. & Porvari, P. 1996. Total and methylmercury concentrations and fluxes from small catchments in South-Finland. In.: Ebingehaus, R. et al. (eds.) Fourth International Conference on Mercury as a global pollutant, August 4-8, 1996, Hamburg, Germany. Book of Abstracts p. 22.