Total Mercury and Methylmercury Response in Water, Sediment, and

Apr 12, 2017 - The construction of a railroad causeway in 1959 restricted water flow between the north arm (Gunnision Bay) and the south arm (Gilbert ...
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TOTAL MERCURY AND METHYLMERCURY RESPONSE IN WATER, SEDIMENT, AND BIOTA TO DESTRATIFICATION OF THE GREAT SALT LAKE, UTAH, USA Carla Valdes, Frank J Black, Blair Stringham, Jeffrey N. Collins, James R Goodman, Heidi J. Saxton, Christopher R. Mansfield, Joshua N. Schmidt, Shu Yang, and William P. Johnson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05790 • Publication Date (Web): 12 Apr 2017 Downloaded from http://pubs.acs.org on April 13, 2017

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Photo credit: NASA ISS040-E-043230 (6 July 2014) 82x49mm (300 x 300 DPI)

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Draft 2016-03-25

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TOTAL MERCURY AND METHYLMERCURY RESPONSE IN WATER, SEDIMENT, AND

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BIOTA TO DESTRATIFICATION OF THE GREAT SALT LAKE, UTAH, USA

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Carla Valdes‡, Frank J Black §, Blair Stringham †, Jeffrey N Collins §, James R Goodman §,

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Heidi J Saxton §, Christopher R Mansfield §, Joshua N Schmidt §,

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Shu Yang‡, and William P Johnson *,‡

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§

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Department of Geology & Geophysics, University of Utah, Salt Lake City, UT 84112



Department of Chemistry, Westminster College, Salt Lake City UT 84105

Division of Wildlife Resources, Utah Department of Natural Resources, Salt Lake City, UT 84114

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*

Corresponding author, [email protected] (801)-664-8289

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Abstract:

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Measurements of chemical and physical parameters made before and after sealing of culverts in

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the railroad causeway spanning Great Salt Lake in late 2013 documented dramatic alterations in

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the system in response to the elimination of flow between the Great Salt Lake’s north and south

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arms. The flow of denser, more saline water through the culverts from the north arm (Gunnison

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Bay) to the south arm (Gilbert Bay) previously drove the perennial stratification of the south arm

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and the existence of oxic shallow brine and anoxic deep brine layers. Closure of the causeway

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culverts occurred concurrently with a multiyear drought that resulted in a decrease in the lake

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elevation and a concomitant increase in top-down erosion of the upper surface of the deep brine

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layer by wind-forced mixing. The combination of these events resulted in replacement of the

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formerly stratified water column in the south arm with one that is vertically homogeneous and

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oxic. Total mercury concentrations in the deep waters of the south arm decreased by

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approximately 81%, and methylmercury concentrations in deep waters decreased by roughly

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86%, due to destratification. Methylmercury concentrations decreased by 77% in underlying

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surficial sediment whereas there was no change was observed in total mercury. The dramatic

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mercury loss from deep waters and methylmercury loss from underlying sediment in response to

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causeway sealing provides new understanding of the potential role of the deep brine layer in the

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accumulation and persistence of methylmercury in the Great Salt Lake. Additional mercury

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measurements in biota appear to contradict the previously implied connection between elevated

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methylmercury concentrations in the deep brine layer and elevated mercury in avian species

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reported prior to causeway sealing.

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Introduction: Mercury is a global pollutant that migrates through natural systems by complex

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transformation and transport processes that are in large part governed by redox conditions in the

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environment.1 Mercury exists in nature as elemental mercury Hg(0), divalent mercury Hg(II),

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and organomercury compounds such as monomethylmercury (MeHg), the bioaccumulative form

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of mercury. Biomagnification of MeHg can result in potentially toxic levels of MeHg exposure

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to humans and wildlife through consumption of prey, causing detrimental neurological,

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behavioral, and reproductive effects.2

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The methylation of inorganic mercury in the environment is predominantly biologically

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mediated, occurs in anoxic environments, and is carried out largely by sulfate-reducing

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bacteria.3-5 Methylation by iron-reducing bacteria and methanogens can also be important, and

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many other microbes have been shown to contain the hgcAB gene cluster responsible for

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mercury methylation.6-9 Abiotic parameters, such as dissolved organic matter (DOM), have also

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been shown to increase mercury methylation under sulfidic conditions in lab experiments,5,10 and

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MeHg concentrations in aquatic sediments are often positively correlated with concentrations of

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dissolved organic carbon (DOC), although this is not always the case due to the complex effects

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of DOM on mercury’s biogeochemical cycling via its role as a substrate for microbial respiration

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and its role in inorganic mercury complexation and chemical speciation.11-13

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The Great Salt Lake (GSL) is the largest terminal lake in the Western Hemisphere, and

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the Western Hemisphere Shorebird Reserve Network recognizes it as a habitat of hemispheric

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importance for millions of migratory birds (Figure 1). Over 1.4 million shorebirds use the GSL

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for breeding and staging areas, and over 7 million waterfowl utilize the GSL and its adjacent

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1,900 km2 of freshwater and brackish wetlands during some portion of their biannual migration.

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The GSL avian community feeds on aquatic organisms adapted to highly saline conditions,

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which includes brine shrimp (Artemia fransicana), brine fly (Ephydra spp.), water boatman

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(corixid) and numerous algal species.14 In 2007, human consumption advisories were placed on

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three duck species (Cinnamon Teal, Northern Shoveler, and Common Goldeneye) at the GSL

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based on elevated mercury levels in breast muscle tissue exceeding the EPA screening value (0.3

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µg/kg, ww).15-17

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The construction of a railroad causeway in 1959 restricted water flow between the north

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arm (Gunnision Bay), and the south arm (Gilbert Bay).15 Because the south arm receives nearly

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all surface water inflows of freshwater to the GSL, the north arm is evaporatively concentrated to

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a greater extent than the south arm, yielding brine in the north arm (250-280 g/L) that is 1.4 to

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1.6 times more saline than the south arm (110-180 g/L), the latter being 3–5 times more saline

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than seawater (33-36 g/L).18-19 Culverts in the causeway allowed bidirectional flow, with

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shallow water flowing from the south arm to the north arm, and deeper denser water flowing

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from the north to south arm. This flow of dense brine from the north arm, along with limited

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wind-driven mixing, resulted in saline based density-driven stratification and the formation of a

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monimolimnion, herein referred to as the deep brine layer (DBL), in the south arm that persisted

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6-7 m below the surface of the lake, and that was not subject to annual turnover.15,18,19,20 The

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DBL was anoxic with elevated DOC (59 – 88 mg/L) and sulfide (7 –29 mg/L)18,22,23, high

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activities of sulfate-reducing bacteria, 24 and elevated MeHg (20 to 32 ng/L) and total mercury

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(HgT) (38-80 ng/L) with a high percentage of HgT existing as MeHg.22 Several studies20,25-27

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have implicated the elevated MeHg in the DBL as a potential source of elevated mercury in

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biota, possibly via mixing into the oxic upper brine layer where uptake into the food chain could

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subsequently occur. However, a previous laboratory study23 reported no difference in the HgT

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concentration of brine shrimp grown for 14 days in a water column where a DBL was absent

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compared to a stratified water column where an anoxic DBL was present. And they reported that

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brine shrimp grown in water created by mixing different ratios of the upper brine layer and DBL

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accumulated Hg in proportion to the Hg:POC (particulate organic carbon) ratio, such that at least

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the short term effect of mixing in water from the DBL was a decrease in the HgT concentration

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in the brine shrimp.

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Mixing between the DBL and upper brine layer in the south arm of the GSL is believed

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to be relatively limited, with previous research28 indicating that the pools of dissolved inorganic

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nitrogen (DIN) and total dissolved phosphorous (TDP) in the shallow brine layer are only weakly

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coupled to those in the deep brine layer. But despite persistent stratification in the GSL, Beisner

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et al.29 reported evidence that limited mixing between the DBL and shallow brine layer occurred

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during conveyance of the DBL from the northern to southern areas of the south arm, and Jones

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and Wurtsbaugh23 estimated that 40% of the DBL was entrained into the shallow brine layer

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annually. Such mixing would allow for the transfer of MeHg from the DBL to the overlying

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shallow brine layer, and by extension, into the base of the food chain, and eventually to brine fly

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larvae and brine shrimp that serve as diet for many avian species.23,27,28,30,31 Such an indirect

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connection was inferred by Johnson et al.22 based on elevated Hg concentrations previously

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reported in birds (eared grebes)32 and brine flies26 on the southwestern side of Gilbert Bay

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(where the DBL existed at depth in the most proximal water body), as well as due to the

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temporal correspondence of reduced Hg(II)-methylation rates in the DBL and reduced Hg

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burdens among aquatic invertebrates (brine shrimp30,33 and brine flies26).

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In November 2012 and December 2013, two railway causeway culverts that allowed limited water flow between the north and south arms of the GSL were closed due to deteriorating

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structural integrity. These closures provided the opportunity to determine the geochemical

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response of the system to elimination of flow between the north and south arms and

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destratification and disappearance of the DBL in the south arm (described below). This event

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also allowed us to quantify how HgT and MeHg concentrations in the water column, surficial

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sediment, and biota responded to the disappearance of the DBL, thus providing insight into the

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role of the DBL in the biogeochemical cycling of mercury in the GSL.

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Methods

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Water and Sediment Sampling and Analysis

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Periodically from October 2015 to December 2016, water column chemical and physical

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conditions were characterized at seven locations across the south arm of GSL (Figure 1). Water

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samples were collected at five stations (2267, 2565, 2767, N1018, 2820, 3510, and 4069) at 0.2

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m below lake surface and 0.5 m above lake bottom, referred to as shallow and deep samples,

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respectively. The data from these samples collected after the disappearance of the DBL were

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compared to previous data collected from the shallow and deep brine layers from May 2007 to

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December 2012, prior to causeway closure when the DBL was present22,34 (Figure 1). Water

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column temperature, specific conductance, pH, and dissolved oxygen (DO) were measured in the

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field using a YSI Professional Series Quatro probe that was calibrated within 12 hours prior to

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sampling.22 The YSI probe corrects for temperature but not salinity. Thus, the DO values

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presented have a positive bias and are qualitative, but are still useful for accurately denoting oxic

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versus anoxic conditions. Sulfide was measured in filtered water samples in the field

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immediately after collection using a photometric method (V-2000 Multi-analyte LED

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Photometer and Vacu-vials®, CHEMetrics).22

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Clean hands - dirty hands protocol was followed during sample collection and analysis.35

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Unfiltered Hg water samples were collected by peristaltic pump using acid-washed PTFE tubing

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into pre-cleaned FLPE bottles. Bottles used to collect anoxic water samples were filled to

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overflowing in order to minimize headspace. Filtered water samples were passed through a 0.45-

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micron pore size, pre-acid rinsed capsule filters in the field (Geotech Environmental). After

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collection, water samples for HgT and MeHg analyses (unfiltered) were stored on ice in the field

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and acidified to 0.5% using trace-metal grade sulfuric acid for preservation the same day as

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sampling. All samples were then refrigerated and analyzed within two weeks of collection.

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HgT samples were oxidized by amendment to 5% BrCl at least 24 hours prior to analysis.

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This higher BrCl concentration relative to established protocols that call for 1% BrCl was

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necessary36 to fully oxidize the high levels of dissolved organic matter (75.4 ± 11.1 mg/L) in

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these water samples (as described below). HgT concentrations in water were determined via

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reduction with SnCl2, purge and trap onto gold traps, thermal desorption, with quantification by

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cold vapor atomic fluorescence spectroscopy (CVAFS) using a MERX-T automated system

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(Brooks Rand) using established techniques.36 Mean laboratory blank concentrations were 0.07 ±

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0.008 ng/L HgT. HgT matrix spike recovery averaged 98.3 ± 3.2% (n=3). MeHg concentration

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in water was measured after distillation with ammonium pyrrolidine dithiocarbamate (APDC),

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followed by aqueous phase ethylation, purge and trap onto tenax traps, thermal desorption,

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pyrolitic decomposition, and CVAFS detection using a MERX-M automated system (Brooks

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Rand) using established techniques.37 MeHg blanks averaged 0.15 ± 0.10 ng/L. Because no

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certified reference material exists for MeHg in water, MeHg matrix spikes were included in each

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distillation; MeHg spike recoveries (n=2) averaged 93 ± 29%. Water samples for dissolved

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organic carbon (DOC) analysis were placed on ice in the field then placed in a refrigerator upon

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return tio the laboratory. DOC was measured in water samples (TOC-5000a, Shimadzu) within

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one week of sample collection using EPA Method 1684.38

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Surficial sediment was sampled at eleven sites (A1, A2, B1, B2, C1, C2, CB, 2565, 3510,

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4069, and N1018) in the GSL (Figure 1), and were collected by peristaltic pump using acid-

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washed PTFE tubing into pre-cleaned 500 mL FLPE bottles filled to overflowing to minimize

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headspace, and stored on ice in the field. The preferred method for examining vertical variations

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involves collecting a sediment core that is subsequently sectioned. However, because our focus

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was geographic variations in surficial sediment, we employed a faster method to collect

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sediment. The method composites surficial sediment across depths of a few cm. Naftz et al.39

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reported 40% variation of HgT concentration within the top 2 cm of GSL sediment. Previous

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studies22 using this method showed relatively low variability in HgT and MeHg in surficial

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sediment, indicating that collected sediment was representative.

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Subsamples were oven dried at 105°C for 12 hours and re-weighed to determine water

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content and allow conversion between wet and dry weight concentrations (without salt

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correction). HgT was extracted from sediment by digestion in a 7:3 mixture of HNO3:H2SO4

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(trace metal grade) at 80°C for 6 hours, followed by amendment to 5% BrCl.36 MeHg was

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leached from sediment with a mixture of potassium bromide, sulfuric acid, and copper sulfate,

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extracted into methylene chloride, back extracted into water, followed by aqueous phase

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ethylation, purge and trap, thermal desorption, pyrolitic decomposition, and CVAFS detection

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according to established techniques.36,37,41 Certified reference materials (CRMs) for HgT

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(MESS-3) and MeHg (CC-580) in sediment were also analyzed, with recoveries averaging 99%

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and 119%, respectively.

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Biota Collection and Analyses Adult brine flies (Ephydra spp.) were collected from Lady Finger Point on Antelope

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Island of the GSL (Figure 1) from spring to fall of 2012 to 2015. Flies were collected with nets,

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transferred into polypropelene tubes, placed on ice in the field, and frozen back in the lab the

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same day. Seventy-six waterfowl were harvested in mid-September of 2014 and ninety-nine

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were harvested in late August and early September of 2015 from the GSL and surrounding

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wetlands by the Utah Division of Wildlife Resources. The waterfowl were harvested from the

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Farmington Bay, Ogden Bay, Howard Slough, and Harold Crane Waterfowl Management Areas

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(Figure 1). Ducks were harvested with shotguns using non-lead shot then frozen. The age of each

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bird was determined by physical characteristics; examining rectrices, wing and body plumage,

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and cloacal characters. The species sampled were Northern Shovelers (Anas clypeata) (DBL

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present: n = 16; DBL absent: n= 16), Mallard (Anas platyrhynchos) (DBL present: n= 16; DBL

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absent: n= 21), Gadwall (Anas strepera) (DBL present: n = 15; DBL absent: n =22), and

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Cinnamon Teal (Anas cyanoptera) (DBL present: n = 23; DBL absent: n = 31). Tissue samples

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were thawed, then a scalpel was used to remove the skin, and breast muscle tissue was harvested

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and stored in a Whirl-Pak (NASCO) polyethylene bag and frozen. Breast muscle was analyzed

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because it is the most likely tissue to be consumed by duck hunters.

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Both brine fly and waterfowl samples were freeze-dried and homogenized prior to

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analysis, and thus all HgT concentrations in biota are reported on a dry weight basis. Brine flies

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were digested in Teflon vials with 10 mL of a 2:1 mixture of trace metal grade nitric and sulfuric

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acids. Samples were allowed to predigest at room temperature for 1 hour, then heated to 100 °C

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for 4 hours. Following the digestion, samples were amended to 1% BrCl. All digestions included

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at least 2 digestion blanks and 2 certified reference materials (TORT-2 and DORM-3, National

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Research Council Canada), each digested in duplicate. Aliquots of the digested samples were

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measured by oxidation with BrCl, reduction with SnCl2, purge and trap using dual-stage gold

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trap amalgamation and quantification by CVAFS.36 The average daily HgT detection limit,

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based on 3 times the standard deviation of digestion blanks, was 2.3 ng g-1, assuming a 100-mg

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sample. Recoveries for HgT in the biota certified reference materials averaged 101.4% ± 7.8%, n

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= 88. The duck muscle tissue was analyzed using thermal decomposition, amalgamation, and

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atomic absorption spectrophotometry using a DMA-80 (Milestone) following established

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protocols.42 Muscle tissue samples were only analyzed for HgT because previous studies have

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shown that >95% of Hg in most bird muscle is MeHg43. Each analysis run included each of the

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two certified reference materials (TORT-2 and DORM-3) analyzed in duplicate, with recoveries

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of HgT ranging from 88% to 123%. Analytical precision of the HgT measurements was

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assessed by analyzing eight samples in triplicate, and the average percent relative standard

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deviation was 5.2%.

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The HgT concentrations in the waterfowl were log transformed prior to statistical

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analysis in order to meet the assumptions of parametric statistics. The data were analyzed using

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multifactor ANOVA using a crossed design. While we did not anticipate a priori that the sex of

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the waterfowl would have any effect on their HgT concentrations, we initially included sex in the

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statistical model, which included the following five variables as fixed factors: duck species, age,

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year, site, and sex. The fully crossed model could not be run due to missing cells because not all

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sites were sampled both years. We therefore used a reduced model made by removing the five-

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way and four-way interactions.44 In this model none of the interactions involving sex, nor sex as

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a main factor, were significant (p>0.49 in all cases). Given this result along with our a priori

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assumption regarding the importance of sex, this factor was removed from the model. The

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multifactor ANOVA using a crossed design with the fixed factors duck species, age, year, and

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site was then reduced by removing the four-way interaction and the three-way interaction Age ×

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Site × Year due to the missing cells, as described above.

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Results

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Water Quality Parameters

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Before 2013, prior to closure of the causeway culverts, shallow waters in the south arm

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were oxic (DO = 9.48 ± 0.70 mg/L), whereas deep waters in areas where the DBL exists were

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anoxic (DO = 0.05 ± 0.02 mg/L) (Figure 2). After causeway closure, mean DO was 9.1 ± 3.4

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mg/L in shallow waters and 7.2 ± 5.2 mg/L in all deep waters. Prior to culvert closure DO

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concentrations were significantly higher in surface waters than in the DBL (t-test, p