What lies beneath: methylmercury production and cycling in the Great Salt Lake

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In the northern part of Utah lies the Great Salt Lake, a unique ecosystem providing an important stopover for millions of migratory birds. At the bottom of the lake lies the deep brine layer, a layer of dense, salty water, exceptionally high in toxic methylmercury (MeHg). Recent abrupt changes in physical conditions have seen the apparent disappearance of almost 90% of this mercury. Professor Frank Black, his team of students at Westminster College of Salt Lake City, and collaborators at the University of Utah have been investigating the impact of the deep brine layer on the production and cycling of mercury in the Great Salt Lake and the subsequent effects on local and migratory wildlife.
great salt lakeAs the largest terminal lake in the western hemisphere, the Great Salt Lake (GSL) in northern Utah is widely recognised as providing an important habitat for millions of birds. Over 8 million shorebirds and waterfowl use the GSL and the adjacent wetlands for feeding, breeding and resting during migration. The GSL is also of interest due to its unique physical and chemical conditions. Construction of a railroad in the 1950s restricted the flow of water between the north and south arms of the lake. Nearly all of the freshwater inflow to the GSL flows into the south arm, resulting in highly saline conditions in the north arm. Culverts in the causeway allowed for limited flow of denser, higher salinity water from the north arm into the south arm, where it sinks and forms what is known as the Deep Brine Layer (DBL), an anoxic layer of highly saline water that fills the depths of the south arm.
Murky Waters
Previous investigations into the depths of the lake have revealed some of the highest levels of methylmercury (MeHg) ever detected in a natural water body. Mercury is a global pollutant and toxic heavy metal and MeHg is of particular concern as it can be biomagnified up food chains to toxic levels, posing risks for both biota and humans. The discovery of exceptionally high levels of mercury in some duck species has led to the issue of duck consumption advisories for multiple species at the GSL. This is the first time globally that such precautionary measures have been issued. Elevated concentrations across multiple species seemingly pointed towards a strong connection between mercury in the DBL and in birds feeding in the area.

MeHg is of particular concern as it can be biomagnified up food chains to toxic levels, posing risks for both biota and humansQuote_brain

Beneath the Surface
Whilst the elevated levels of MeHg in the water, sediment and wildlife surrounding the lake have been well documented, research by Professor Frank Black and collaborators focuses on where this MeHg is coming from and why the levels in the depths of the GSL are so exceptionally high. MeHg is produced by bacteria in anoxic environments, such as the DBL, leading to speculation that the DBL is the primary source. However, the exact source has been challenging to pinpoint: the MeHg may originate in the DBL, the sediments beneath the lake, or in the surrounding wetlands.
Seizing the Day
great salt lakeIn 2013, a unique opportunity arose for Professor Black and his team to investigate the cycling of mercury and the production of MeHg in the GSL. Closure of the culverts for maintenance in 2013 stopped the flow of salty water into the south arm of the lake, and resulted in the total disappearance of the DBL in 2014. Sampling before, during and after the culvert closures, Professor Black and collaborator Bill Johnson at the University of Utah, assisted by a number of graduate and undergraduate students, collected data on the water, sediment and biota of the GSL. What they found seemed to pose more questions than answers.
A Mercury Mystery
The data collected appears to show a significant overall removal of mercury from the GSL. Following the disappearance of the DBL, the concentration of MeHg in deep waters of the GSL decreased by 86%. This loss cannot be attributed merely to mixing: surface water concentrations also showed a slight reduction, in stark contrast to the 75% increase expected had the MeHg been redistributed throughout the water column. Similarly, a concurrent decrease (77%) of MeHg in sediments previously underlying the DBL indicates that the MeHg previously present in deep waters wasn’t simply lost to sediments. These findings have led Professor Black and his team to conclude that something more complicated is occurring within the lake. Whilst the findings clearly indicate that the changes in MeHg concentrations were connected to the loss of the DBL, Professor Black suspects that the DBL may not be where the MeHg was produced. These findings suggest that the DBL acts as a cap and promotes the accumulation of MeHg in surface sediments and the DBL itself. Whilst the exact processes are still unknown, Professor Black speculates that the absence of the DBL made it easier for MeHg to travel from the sediments to overlying surface waters, leading to a loss of MeHg from the system e.g., through reactions at the surface.

great salt lake
Westminster College undergraduate researchers collecting brine flies and brine shrimp at the Great Salt Lake.
Deeper Impacts
In addition to the geochemical factors, Professor Black is also interested in the relationships between the concentrations of MeHg found in lake waters and sediments, and those found in the biota of the lake. MeHg is readily bioaccumulated up through the food chain, from brine flies, to orb weaver spiders to numerous birds feeding on and around the lake. Therefore, it is sensible to expect that changes in MeHg concentrations in and around the lake would be reflected in the biota. However, whilst lake sediment and water concentrations of MeHg showed dramatic decreases following the culvert closures, no such decrease was observed in the four avian species sampled. Likewise, no decrease was detected in brine flies, a keystone species at the GSL and a primary food source for many migratory birds. This surprising result questions previously held assumptions that the DBL is the primary source of mercury found in organisms around the GSL. An alternative explanation could be that the link between water concentrations in the GSL and the species that feed in the area is temporally slow and we are yet to see the impacts.

In 2013, a unique opportunity arose for Professor Black
and his team to investigate the cycling of mercury
and the production of MeHg in the GSLQuote_brain

The opportunistic investigation of this unique system by Professor Black and his team has provided a unique insight into the role of the DBL in MeHg production and cycling in the GSL. The findings have implications for the ongoing management of the lake and the reduction of mercury levels in wildlife at the GSL and within the lake itself.

Your research has shown that the apparent disappearance of mercury at the Great Salt Lake (GSL) is strongly linked to the deep brine layer. Do you have any plans for further research to investigate exactly where the mercury went?
Yes, but at this point our research is focused on understanding how mercury cycling and methylmercury production in the GSL will respond to the anticipated reestablishment of the deep brine layer due to the completion of a new bridge in the railroad causeway in December 2016 that has allowed for water exchange between the north and south arms of the GSL for the first time in over three years.
How can the results of your recent research be used to improve management methods at the GSL?
I think a relatively widely held belief before our study was that the mercury problem at the GSL was directly tied to the deep brine layer with its very high concentrations of methylmercury. As a result, one management option I heard suggested was to try to eliminate the deep brine layer, even though many options that might achieve this, such as mechanically aerating bottom waters of the GSL were not likely to be financially feasible. Our research suggests that the deep brine layer may play more of a role in allowing methylmercury to build up in deep waters of the lake than it does as an important location for the production of methylmercury that eventually makes its way into the food chain. So getting rid of the deep brine layer may not be as helpful as some previously thought.

The predominant source of MeHg in
the Great Salt Lake has been challenging to pinpointQuote_brain

Your team includes many undergraduate and graduate students. What are the benefits of involving students in this type of research?
Westminster College is a small liberal arts college where our focus is primarily on teaching and training undergraduate students. So, at the core of much of our research is using it as a tool to engage and train the next generation of scientists and give our students the skills to be successful regardless of where their career path leads in the future. The most important of these are not technical skills, but rather problem solving, critical thinking, and communication skills.
In comparison to many institutions, Westminster College of Salt Lake City is relatively small in size. What, in your view, are the key factors to producing cutting-edge research in smaller research environments?
Because we’re small and our undergraduate students are the ones conducting the research (we don’t have graduate students or post-docs), we focus our research on the things we do best and don’t try to extend ourselves beyond our capabilities. We also benefit from working with fantastic collaborators at larger research institutions in Utah that do have graduate students, full time research staff, and equipment that we wouldn’t have access to otherwise.
Given that methylmercury is bioaccumulated through the food chain, do high concentrations at the GSL pose a risk to humans?
In most aquatic ecosystems, the primary concerns for human health are due to the consumption of fish, especially the large fish near the top of the food chain that have high mercury concentrations. Most of the GSL is too salty for fish to live in it, so we don’t have those concerns. However, duck hunting is quite popular in the large tracts of wetlands along the margins of the GSL, and some duck species here have extremely high levels of mercury, which has prompted the state of Utah to issue duck consumption advisories due to mercury, a world’s first.

Research Objectives
Professor Black, his research team, and collaborators have been investigating the impact of the deep brine layer on the production and cycling of mercury in the Great Salt Lake and the subsequent effects on local and migratory wildlife.
Funding
Utah Division of Forestry, Fire, and State Lands of the Utah Department of Natural Resources, iUtah, and the National Science Foundation (NSF).
Collaborators

  • Bill Johnson’s research group and graduate students Carla Valdes and Shu Yang at the University of Utah.
  • Westminster undergraduate students, including Jeff Collins, Jim Goodman, Heidi Saxton, Chris Mansfield, Josh Schmidt, Abby Scott, Alex Martin, Maddy Trentman, Adele Reynolds, Anna Robert, Gaurav Pandey, and Andrew Piskadlo.
  • Ryan Rowland and Christine Rumsey at the US Geological Society.

Bio
Professor Frank BlackProfessor Frank Black is a trace metal biogeochemist and associate professor of chemistry at Westminster College in Salt Lake City, Utah. His research, carried out with college undergraduate students, focuses on the biogeochemical cycling of heavy metals and trace elements in the environment.
Contact
Professor Frank Black
Westminster College of Salt Lake City
1840 South 13th East
Salt Lake City, Utah
USA
E: fblack@westminstercollege.edu
T: +1 801 832 2351
W: www.researchgate.net/profile/Frank_Black4 and
www.westminstercollege.edu/undergraduate/programs/chemistry

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1 thought on “What lies beneath: methylmercury production and cycling in the Great Salt Lake”

  1. I found this article upon researching for an upcoming trip to Great Salt Lake, it has truly fascinated me for years! I look forward to seeing it in person!

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