Factors affecting methyl mercury partitioning to DOC and UVB photodegradation in fresh waters

The partitioning of methyl mercury (MeHg) to dissolved organic carbon (DOC) and the photodegradation of mercury by UVB radiation (280-320nm) are two important processes that influence the availability of MeHg to the base of the aquatic food chain.; Water samples from 20 sites across Eastern Ontario and Western Quebec were filtered sequentially using tangential flow ultrafiltration to determine the size distribution of MeHg and DOC and to test whether the concentrations and distribution of these two variables varied in wetlands, lakes and rivers. These filtrates were also analyzed for DOC fluorescence and absorbance. The highest proportions of mean MeHg (47.3 +/- 25.4%), DOC (56.8 +/- 14.5%) and DOC FL (74.5 +/- 11.4%) were found in the low molecular weight fractions (<5 kDa). Significant differences in the distribution and concentration of MeHg amongst filtered samples were found between wetlands, lakes and rivers. MeHg was related to DOC at all size fractions. The low molecular weight organic compounds may be an important contributor to MeHg biomagnification through uptake by bacteria and/or algae.; St. Lawrence river water was collected to test factors that affect the rate of photodegradation of MeHg. Samples exposed to UVB irradiance from a fluorescent lamp and spiked with MeHg(5 ng/L) illustrated significant decreases in concentration with a 31% average loss after 6 hours. No significant difference in photodegradation was found between samples with and without added Fe(II). MeHg concentrations decreased 35.4% and 41.7% after 6 hours of exposure at pH 3 and 5, respectively. It appears that photo-demethylation is a function of UVB exposure, is more rapid in acidic conditions and likely occurs slower under most natural freshwater conditions due to the attenuation of UVB.