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 21 ) and methylmercury (33 ng L 21 ). 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 21 ), 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.

Brine shrimp grazing and fecal production increase sedimentation to the deep brine layer (monimolimnion) of Great Salt Lake, Utah

“…2) in relation to earlier years (e.g., Fig. 3 in Jones & Wurtsbaugh, 2014). The salinity increased from 15% in the mixed layer to over 20% in the DBL (Fig.…”

“…Conceivably, they chose to feed in the photic mixed layer during the day to capitalize on the ongoing primary production there. We were surprised that the Artemia utilized the top part of the hypoxic layers at night, because in a previous experiment utilizing the same experimental columns, the Artemia rarely entered the hypoxic layers (Jones & Wurtsbaugh, 2014). In that experiment, however, water from the DBL of the lake was used to fill the bottoms of the columns, in contrast to the artificial DBL we utilized.…”

“…The preliminary experiment was started on 4 May 2015 and the main experiment on 22 May 2015: both lasted 5 days. To simulate the stratified water column of the lake, we utilized acrylic plastic columns (19.7 cm diameter and 156 cm high; a detailed description of columns is available in Jones & Wurtsbaugh, 2014). The upper mixed layer of each column was filled with lake water from Great Salt Lake with a salinity of 150 g 9 l -1 , and the deep brine layer (DBL, monimolimnion) was filled with a solution of NaCl and distilled water with a salinity of 189 g 9 l -1 (19.8%) that mimicked that of the actual lake in April 2015.…”

“…We then pumped 15 l of this DBL water at a rate of 285 ml min -1 through the bottom sampling port below the mixed-layer water using a peristaltic pump, giving a total depth of 150 cm in each column. The experiments were performed at a constant room temperature (23°C) with fluorescent lights behind the upper meter of the columns, which provided 310 lmol m -2 s -1 (within the range of the light intensity in the top part of the water column in the lake, e.g., Figure 4 in Belovsky et al, 2011) on a 16:8 L:D cycle, as a standard in studies on zooplankton depth selection, including Artemia (e.g., Jones & Wurtsbaugh, 2014) and other zooplankton taxa (Alekseev, 2004). A covering of black plastic was wrapped around the bottom 50 cm of the columns to simulate the light conditions in the deeper layers of the lake.…”

“…Great Salt Lake (Utah, USA) is a large, shallow, hypersaline, and highly productive lake with a monimolimnion (deep brine layer) that accumulates high concentrations of nutrients, organic matter and mercury (Wurtsbaugh & Berry, 1990;Jones & Wurtsbaugh, 2014). As in other hypersaline lakes, it contains a relatively simple community of halotolerant organisms, with Artemia franciscana Kellogg, 1906 as the only species of macrozooplankton.…”