Methylmercury (MeHg), the known toxic form of mercury is generally only 1-3% of total mercury (THg) in precipitation. As such it was thought to be an insignificant source to aquatic ecosystems; however, since other forms of mercury can be re-volatilized from the snowpack back to the atmosphere shortly after deposition the proportion and biological significance of MeHg in the snowmelt runoff is indeed an important source to downstream ecosystems. To assess the seasonal contribution of MeHg in runoff to the overall annual load of MeHg, the tributaries of two lakes, Dickie (DE) and Harp (HP) in the Haliburton-Muskoka district of Ontario were sampled on a weekly basis for MeHg, dissolved organic carbon (DOC) and conductivity over two years. Stream discharge was monitored on a continuous basis. In order to assess the amount of MeHg in the snowpack, snow cores from the watersheds were sampled during early spring. Additionally, bulk precipitation samples were collected using a wet-dry automated precipitation collector in the mercury deposition network (MDN) operated station in St. Anicet, Quebec every 4-6 weeks, for 26 months. The samples were analyzed for MeHg, and an annual deposition rate was calculated. THg and MeHg data obtained from the MDN was used to calculate a deposition rate for THg, and the proportion of MeHg. THg in wet deposition was 5.29-6.88 mug m-2 year-1, and MeHg was 0.15-0.19 mug m -2 year-1 and equivalent to 2.7-2.9 % of THg. Although low MeHg concentrations were measured in runoff during spring freshet, the high discharge rates resulted in quantities which were up to 33.9% of the annual amount of MeHg. The seasonal contribution of MeHg in runoff was found to be: spring>summer>fall> winter for the wetland rich DE and fall> spring> winter> summer for the wetland dull HP. Multiple regression analysis illustrated that DOC was the most significant predictor of MeHg (r2=0.32, p<0.01) across all data with a strong seasonal dependence of the DOC-MeHg correlation. The strongest correlation was found during spring freshet (r 2=0.65, p=0.001) followed by summer, winter and fall (r2= 0.37, 0.23 and 0.22 respectively, p<0.05 for all). The constant flow of water during the winter and strong correlation with DOC during snowmelt suggests a terrestrial source of MeHg even during the winter months, however, direct precipitation during spring and snowmelt were also found to contribute to the MeHg load during snowmelt runoff.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/28897 |
| Date | January 2009 |
| Creators | Bodek, Tamar |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 112 p. |
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