Â鶹ÒùÔº

Climate change is melting glaciers and permafrost in the mountains outside of Boulder, Colorado, exposing rocks and freeing up minerals containing sulfate, a form of sulfur, to flow downstream into local watersheds.

CIRES researchers studied the impacts of sulfate in mountain wetlands and confirmed that elevated levels can increase methylmercury, a potent neurotoxin that accumulates up the food chain and can lead to a wide range of health concerns.

"Very little research has looked at methylmercury production in high-elevation wetlands," said Hannah Miller, CU Boulder and CIRES Ph.D. student and lead author of the study. "This leaves important knowledge gaps regarding how much is being produced in mountain wetlands and how this may change with ongoing climate change."

The work, in Environmental Research Letters, is the first to catalog baseline measurements of the toxin in Boulder's watershed. The results could help land managers monitor future impacts on soils, water flowing in creeks downstream, and local wildlife. CIRES Fellow Eve-Lyn Hinckley, who leads CU Boulder's Environmental Biogeochemistry Group, co-authored the study.

"This research is timely, given the confluence of global changes, including climate warming, changes in the supply of reactive elements, and the occurrence of wildfires that are threatening sensitive high-elevation ecosystems," Hinckley said.

In a warming world, runoff can alter ecosystems

Worldwide, mountain ecosystems experience disproportionate impacts of climate change, with glaciers and permafrost melting faster than ever before. Recent studies have found that water in glacier-fed mountain creeks and streams is increasingly loaded with sulfate. Over the past 30 years, the North Boulder watershed has seen about a 200% increase in the sulfate concentrations downstream, and similar trends have been documented in more than 150 lakes and streams globally.

"It's a really interesting climate-driven signal that is starting to get more attention," Miller said. "And while increasing sulfate concentrations have direct implications for ecosystems, they can also have indirect impacts, including changing how mercury is transformed within wetlands."

The group's work dug further to see how sulfate runoff impacts mercury cycling, specifically how much mercury is converted to its most toxic form: methylmercury.

Marshy wetlands and large bodies of water lack oxygen in their soils, so microbes have evolved to use other elements, like sulfate, to produce energy and grow. When sulfate travels downstream and interacts with mercury, in this case in waterlogged peatlands just below treeline, complex chemistry drives sulfate-reducing bacteria to convert mercury into toxic methylmercury.

Digging for soil above and below treeline

Miller sampled soils in the mountains 25 miles northwest of Boulder to confirm whether increasing sulfate runoff may lead to increased methylmercury production. The study had two parts: documenting methylmercury levels in wetlands above and below treeline, and second, identifying the level at which sulfate can trigger methylmercury production.

With a backpack full of soil samples, Miller loaded her car and drove to the U.S. Geological Survey Mercury Research Lab in Madison, Wisconsin. There, she analyzed the soils to see which sites produced the highest levels of toxic mercury. She ran additional experiments on the subalpine peatland soils, adding increasing amounts of sulfate to mimic what researchers observed in the field. She tracked methylmercury levels with each additional injection of sulfate.

In the wetlands above treeline, researchers found very low amounts of methylmercury. But in the peatlands below treeline, levels were high. The results point to the difference in vegetation: more trees, shrubs, and plants create a welcoming carbon-rich environment for microbes, which can help stimulate methylmercury production.

"We also found in our experiment that moderate sulfate additions to the subalpine peatlands resulted in the greatest production of methylmercury within the soils," Miller said.

Why would moderate sulfate concentrations result in the highest levels of methylmercury? A 1990s study that coined the "Goldilocks effect" concluded that "just right"—or moderate amounts—of sulfate mixed with mercury leads to a spike in methylmercury production. In contrast, methylmercury is lower or not present in soils when sulfate levels are high or low.

The results present the first-ever sulfate thresholds identifying at what point methylmercury levels may be highest in the North Boulder Watershed. The study provides land managers with information on how continued sulfate export may impact methylmercury levels in these ecosystems in the future.

"In semi-arid mountain ecosystems, poorly developed soils and limited water bodies make it easy to think there is a lower risk for mercury contamination," Miller said. "But our findings show there is a risk, particularly in subalpine peatlands, and raise important questions about how this toxin may impact the communities and wildlife that rely on that water source now and in the future."