DIURNAL TRENDS IN METHYL MERCURY CONCENTRATION IN A WETLAND ADJACENT TO GREAT SALT LAKE, UTAH, USA

The open water and adjacent wetlands of the Great Salt Lake (GSL) ecosystem support millions of migratory waterfowl from throughout the Western Hemisphere and a multi-million dollar brine shrimp industry. Recent biogeochemical assessments of the GSL ecosystem have found elevated levels of methyl mercury (MeHg) (> 30 ng/L) in whole-water samples and Hg in three duck species that consistently exceeded the USEPA screening level for human consumption. The 192,000 hectares of wetlands surrounding GSL play a key role in the transformation of inorganic Hg to the significantly more toxic MeHg species. Only a few studies have examined short-term cycling of Hg in wetland systems. The objective of this work was to measure changes in MeHg concentration and loads in water discharging from the Howard Slough wetland adjacent to GSL over a 24-hour interval during July 2008. During the start of the diel experiment, the concentration of filtered (< 0.45 mm) MeHg consistently decreased during daylight hours to a low of 0.34 ng/L at sunset. After sunset, MeHg increased by > 100 % to a maximum concentration of 0.70 ng/L between 0100 and 0200 hours. After sunrise on July 24^th, MeHg steadily decreased, returning to a low value of 0.30 ng/L at 1100 hours. Dissolved oxygen concentrations decreased significantly after sunset and continued to decrease to near 0 mg/L prior to sunrise on July 24^th. The percentage of total Hg in the MeHg form averaged about 25 % during daylight hours and peaked to almost 40 % during non-daylight hours. Variations in the concentration of dissolved organic carbon did not correlate with MeHg concentrations. The likely mechanism for the nighttime increase in MeHg was convective turnover of the wetland water column driven by dropping nighttime air temperature which cooled the surface water. During this convective overturn of the water column, MeHg-rich water near the sediment-water interface was likely mixed into the wetland water column resulting in the observed increase in MeHg concentration at the outflow structure. Vertical water temperature profiles at the outlet structure indicated thermally stratified conditions during daylight hours and a convergence to homogenous water temperature during nighttime cooling. Simulations of convective overturn during nighttime cooling using hydrodynamic modeling are currently ongoing.