Response of density stratification, aquatic chemistry, and methylmercury to engineered and hydrologic forcings in an endorheic lake (Great Salt Lake, U.S.A.)

Abstract

AbstractElimination of limited flow through the earthen railroad causeway separating the north and south arms of Great Salt Lake, Utah, U.S.A., in late 2013 destratified this meromictic endorheic lake and effectively eliminated previous elevated methylmercury (40–90 ng L−1) in deep waters. Reestablishment of north–south flow in late 2016 offered the opportunity to understand the drivers of density stratification, which in turn drive geochemical conditions supporting mercury methylation at depth. Both engineered (causeway flow reopening) and hydrologic (snowmelt) forcings were observed to initiate meromixis (stratification). Return to meromixis drove anoxia at depth with increased sulfide and methylmercury concentrations. However, shallow brine oxygen concentrations were lower under destratified relative to meromictic conditions, and brief periods of anoxia and elevated sulfide and methylmercury occurred at depth even under destratified conditions, demonstrating oxygen demand by underlying sediment organic matter. Mercury demethylation in deep waters was observed only under destratified conditions when oxygen reached bottom waters. These findings indicate that the deep brine layer acts as a cap that prevents oxygen from the overlying mixolimnion from coming into contact with sediment organic matter and drives accumulation of methylmercury in deep waters under meromictic (stratified) conditions.