The Great Salt Lake's monimolimnion and its importance for mercury bioaccumulation in brine shrimp (Artemia franciscana)

Abstract

The Great Salt Lake (Utah) is divided by a railroad causeway that causes the lake's south arm to be chemically stratified, when saltier, denser water from the north underflows into the south, creating an anoxic, sulfide‐rich deep brine layer that accumulates high levels of total mercury (Hg; 59 ng L−1) and methylmercury (33 ng L−1). Approximately 40% of this water is advected into the upper mixed layer annually. High mercury levels of brine shrimp (Artemia franciscana) in the mixed layer are passed to waterfowl, creating a human health hazard. We hypothesized that high mercury levels in Artemia are due to exposure when mercury is mixed into the upper layer or when they feed on mercury‐rich organic matter in the chemocline separating the two layers. Surprisingly, in aquaria growth experiments with 0%, 10%, or 25% deep brine water, Artemia exposed to progressively higher concentrations of mercury had significantly less mercury. In column experiments simulating a lake with a deep brine layer, Artemia grazed in the chemocline, but they also had lower mercury concentrations than Artemia in controls without a deep brine layer. This was due to detrital dilution of the mercury because the deep brine layer has very high particulate organic carbon (POC; 11.0 mg C L−1), which reduced the Hg : POC ratio of food 7‐fold compared to that in the overlying mixed layer. Consequently, although Artemia are exposed to the high concentrations of methylmercury generated in the deep layer, the detrimental effect is partially ameliorated by detrital dilution of the mercury.