Mountain wetlands are turning into mercury hotspots
When glaciers melt, they release water that carries sulfate-rich minerals previously trapped in the ice. This sulfate flows into streams and wetlands, increasing its concentration in watersheds.
As sulfate accumulates, it creates conditions that can trigger the production of toxic methylmercury, posing risks to ecosystems and wildlife.
Methylmercury, a potent neurotoxin, forms in waterlogged, sulfate-rich wetlands and enters water systems. It accumulates in fish, moves up the food chain to birds, and eventually reaches people who consume contaminated wildlife.
“Very little research has looked at methylmercury production in high-elevation wetlands,” said Hannah Miller, a PhD student at CU Boulder and lead author of a recent study.
“This leaves important knowledge gaps regarding how much is being produced in mountain wetlands and how this may change with ongoing climate change.”
How mercury turns toxic in wetlands
Methylmercury production from glacier melt sulfate only occurs within a specific range in wetland soils. If glacier-derived sulfate levels are too low, the conversion process doesn’t start; too high, and it stalls.
The ideal range – 0.60 to 1.4 milligrams per gram of dry soil – creates optimal conditions for mercury to transform into its toxic methylated form.
Miller’s team pinpointed this “Goldilocks zone” after testing soil from peatlands, alpine meadows, and riparian zones. Above the treeline, methylmercury levels barely budged. Below the treeline, peatlands pulsed with microbial activity, converting mercury to its toxic form.
“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.
Peatlands as mercury hotspots
In the lab, Miller pumped different sulfate concentrations into soil samples. The response? Peatlands with moderate sulfate additions churned out the most methylmercury.
But there’s a catch. Wetlands above the treeline don’t have much vegetation. Fewer plants mean less carbon-rich material to feed the microbes that convert sulfate to methylmercury. But in the subalpine peatlands, dense vegetation and waterlogged soils create the perfect storm.
Sulfate-reducing bacteria thrive in these oxygen-starved environments. They grab onto sulfate, munch through carbon, and churn out methylmercury as a byproduct. And as glaciers keep melting, more sulfate keeps coming.
Melting glaciers spread sulfate
Sulfate doesn’t stay put. It migrates downstream, riding currents and seeping into soils. In the past 30 years, sulfate levels in Boulder’s watershed have shot up by 200%. That spike mirrors a global pattern.
Glacier-fed streams in the Andes, Alps, and Himalayas show similar trends – up to a 2000% increase in sulfate concentrations. As these minerals flow, so does the risk of methylmercury contamination.
“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.”
Mercury moves through food chains
Methylmercury doesn’t just stay in the soil. It slips into water, where fish absorb it through their gills. Small fish pass it to bigger fish. Birds swoop down, eat the fish, and the toxin moves up the food chain.
“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,” noted Miller.
“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.”
Sulfate isn’t the only player
Sulfate takes the spotlight, but it’s not alone. Iron, carbon, and water levels also play crucial roles in the methylmercury cycle.
In Boulder’s watershed, dry conditions could limit the spread of peatlands, reducing the areas where methylmercury thrives. But increased precipitation could do the opposite – more wetlands, more methylmercury.
Miller’s study also hints at the role of iron. Sulfate additions alone triggered methylmercury production, but iron concentrations could magnify or mitigate that effect.
Melting glaciers, rising mercury levels
Glaciers worldwide are melting, releasing sulfate-rich minerals into nearby streams and wetlands. This surge in sulfate is not limited to Boulder but is occurring in mountain watersheds globally.
As sulfate levels rise, so does the risk of toxic methylmercury formation, impacting ecosystems far beyond Colorado.
Glacial melt in Peru’s Andes has turned once-pristine wetlands into mercury-contaminated hotspots. The European Alps and the Himalayas face similar risks as warming temperatures increase sulfate runoff.
As glaciers recede, more sulfate spills into wetlands, driving methylmercury production and threatening ecosystems globally.
Controlling mercury in wetlands
Monitoring sulfate is one thing. Mitigating its impact is another. Restoring wetlands, managing runoff, and regulating upstream mining could reduce sulfate flow. But Miller and her team aren’t stopping there.
“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,” said Eve-Lyn Hinckley, co-author and CIRES Fellow.
Glaciers keep melting, releasing more sulfate into streams and wetlands. This ongoing process fuels methylmercury production, posing increasing risks to ecosystems.
The outcome hinges on monitoring efforts and proactive intervention to manage these toxic threats.
The study is published in the journal Environmental Research Letters.
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