Christine Rumsey; Andrew Piskadlo; Adele Reynolds; Ryan Rowland; Shu Yang; Bill Johnson; Anna Robert; Gaurav Pandey, Westminster
The Great Salt Lake (GSL) in Utah is one of the largest hypersaline lakes in the world, and is used by millions of migrating birds each year. The GSL also has some of the highest concentrations of mercury (Hg), which is a neurotoxin, of any natural water body globally. The south arm (SA) of the GSL perennially had a stratified water column, consisting of a lower salinity upper brine layer (UBL) and a higher salinity, anoxic deep brine layer (DBL). The highest concentrations of Hg in the GSL are found in the DBL, which is formed by the flow of denser, higher salinity water from the north arm (NA) into the SA, where it sinks and does not mix with the UBL. The closure of culverts in 2013 prevented this flow of water, which eventually resulted in the DBL vanishing. A new bridge built in 2016 has recently allowed for water to again flow from the NA to the SA, creating conditions needed for the reestablishment of the DBL. To evaluate the effect of the disappearance and reformation of the DBL on Hg cycling in the GSL, we measured total mercury (HgT) in filtered and unfiltered surface and deep waters at 6 sites monthly in 2017. We observed stratification of the SA in spring due to freshwater inputs associated with spring runoff, followed by wind driven mixing. In mid-summer this was followed by re-stratification when the DBL began to reform due to inflows of denser NA water. Concentrations of HgT in unfiltered surface waters decreased during the spring and summer, while HgT in filtered surface waters did not change in the spring and increased during the summer. HgT concentrations in deep waters were notably lower than before 2014 prior to the disappearance of the DBL. Elevated HgT in deep waters were measured at multiple sites during dates when depth profiles of dissolved oxygen and salinity provided evidence of water column stratification and reformation of the DBL. HgT concentrations in deep waters increased during the late summer at more northern sites, coincident with increased salinity and decreased oxygen at depth, indicative of the reformation of the DBL. Such conditions were not measured in the southern sites, where concentrations of HgT in deep waters remained low. Our results document the beginning stages of the reformation of the DBL and the influence this has on HgT levels in the lake.