Mercury (Hg) in the Great Salt Lake
The Great Salt Lake (GSL) hosts some of the highest environmental mercury (Hg) concentrations in the US. The lake’s water level and salinity fluctuate significantly with annual precipitation impacting the amount of Hg available to the animals who live in and around the lake. The GSL has a relatively simple food web dominated by aquatic arthropods—brine shrimp (Artemia franciscana) and brine flies (Ephydra spp, with aquatic larval stages). However, it is unclear how: 1) Hg in GSL biota varies year to year, and 2) how much Hg emergent brine flies transfer from the GSL to the adjacent terrestrial food web. This is of significant ecological importance as brine flies make up ~75% of the diet of the millions of breeding and over-wintering waterfowl and shorebirds in this region. This includes the Wilson’s Phalarope (Phalaropus tricolor), a species which has just been petitioned to be listed as Threatened under the Endangered Species Act. Approximately 60% of the global population of Wilson’s Phalaropes returns to the GSL each summer to feed on brine flies and shrimp. The western spotted orb weaver spider (Neoscona oaxacensis), another known brine fly consumer, is an ephemerally abundant terrestrial predator that breeds along the shorelines of the GSL. With a diet similar to many shorebirds, it can serve as a suitable biomonitor of the trophic transfer of Hg from the GSL to the adjacent terrestrial food web. Together, these two invertebrates are being used to document intra- and inter-annual variation of Hg in the GSL food web.
Where? Antelope Island State Park, Great Salt Lake, UT
The GSL is an interesting ecosystem in which to study Hg as it is a terminal lake (4th largest in the world) and it's super salty! The salinity of the lake ranges from 5-27% whereas the ocean averages 3.5%. Elevated Hg has previously been documented in the lake, the likely result of runoff from rivers and atmospheric deposition from local emissions. Antelope Island is the largest of the ten islands in the GSL. The island provides easy access to shorelines full of brine flies, and their predators, during the summer months.
Who? Brine flies
Brine flies are emergent aquatic insects whose larval stages develop in the GSL. As larvae, they feed on organic matter and algae where they readily accumulate Hg. And they sure are abundant--up to 370 million per mile of shoreline during the summer months! A single brine fly can have as much as eight times the calories of a brine shrimp, making them a GSL super food. Therefore, brine flies are the main prey of the millions of birds who live in and visit the GSL each year.
Who else? Spiders...big ones
The western spotted orbweaver is the most common spider on Antelope Island. They build webs between sage brush bushes along the shorelines of the GSL, hoping to catch brine flies. As orbweavers consume brine flies similarly to birds, they can be used to understand how Hg is accumulated by brine fly predators. They number in the millions during their brief breeding season in the late summer. In fact, their abundance inspired the Antelope Island Spider Festival held each year!
When? Sample collection
We collect brine flies and spiders when they are most abundant--during the summer months. To answer questions about temporal variation in Hg coming out of the lake, we collect brine flies in June, July, and August. Spiders are collected in July as they are only abundant for a short period. We began collecting in 2019 and havent stopped!
How? Hg analysis in the lab
Flies and spiders are analyzed for total Hg in our lab using a Nippon Direct Mercury Analyzer MA-3000. Students are primarily responsible for running samples in the analyzer in support of their individual and lab projects. Flies and spiders are loaded into the analyzer and after combustion, amalgamation, and absorption spectrophotometry we get the concentration Hg.
Why? Effects of Hg on wildlife
Hg is a potent neurotoxin at high, chronic levels of exposure. However, even small concentrations of Hg can cause reproductive impairment, immune dysfunction, and endocrine disruption. Animals accumulate Hg in their tissues by consuming prey containing Hg, such as brine flies. When animals accumulate Hg at a greater rate than they can excrete it, tissue concentrations can reach dangerous levels and cause harm. The purpose of our research is to understand how much Hg is accumulated by brine fly consumers and if it changes over time.
What's next?
The water level of the GSL has declined significantly over the past several years, reaching a record low in 2022. In 2023, owing to an amazing year of snow, the lake began to return to near normal levels. Preliminary data analysis shows Hg concentrations in flies and spiders declined from 2019-2022 as the lake dried. In 2023, Hg concentrations began to rise again as the lake returned. We are currently trying to understand the factors that led to the changes in Hg in the flies and spiders. Was there a change in the production of organic Hg in the lake as water levels dropped and salinity increased? Or, did predators shift their diet as flies weren't abundant along the shorelines? Meanwhile, annual sampling continues each summer building an longer timeline of Hg availability in the GSL food web.