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The subalpine wetlands found just below tree line in Colorado’s Rocky Mountains are enchanting. Covered with mosses and sedges, these open expanses flanked by evergreens are a breathtaking sight for hikers. Moose graze there, and elk gather during their mating season.
These wetlands also play a crucial role in regulating the supply of clean water from highlands to metropolitan regions downslope, including Denver. However, new research reveals that these wetlands pose a health risk due to methylmercury production just below the surface of their soils.
Methylmercury is a potent neurotoxin that biomagnifies and bioaccumulates, becoming more concentrated as it moves up the food chain. Predatory birds and fish high on this chain are most susceptible to its devastating effects on the nervous and reproductive systems, as are human populations that consume them.
In 1950s Japan, hundreds of people died from methylmercury poisoning linked to consuming water, fish, and shellfish near chemical plants discharging mercury into nearby waters. Mercury methylation is a delicate process involving bacteria that require inorganic mercury, energy, and oxygen-free conditions, with sulfate concentrations being particularly critical.
Too little or too much sulfate is unsatisfactory for the methylating microbes. Too little sulfate does not stimulate mercury methylation, while too much sulfate sequesters mercury in mineral form, minimizing its risk to living organisms. The highest concentrations of methylmercury are produced when sulfate levels fall within an optimal range – just right.
Prior studies found wetlands with methylmercury pollution mostly in lowland areas like Florida’s Everglades. Our study demonstrates that these conditions exist even in seemingly remote mountain locations.
Several factors contribute to the perfect conditions for methylmercury production in Colorado’s subalpine wetlands:
1. **Abundant Organic Matter:** The soil has ample organic matter, providing a deep store of energy in the form of carbon to fuel methylation.
2. **Mercury Pollution:** Industrial mercury pollution from distant sources like China and India reaches these high-elevation regions via rain or dust.
3. **Excess Sulfate:** Rising air temperatures drive ice thaw and quicker mineral weathering, increasing sulfate flow into streams.
As a result, the ingredients mix in flooded, often oxygen-free wetland soils, allowing bacteria to produce methylmercury. Our study showed that concentrations of methylmercury were higher at the outlet than the inlet of subalpine wetlands we studied, confirming their role as sources of the contaminant.
This contamination poses local risks to wildlife and broader concerns for water supplies. Over 3 million people in the Boulder-Denver metropolitan area rely on clean mountain water. Contamination by methylmercury could lead to costly treatment measures for the entire Colorado Front Range’s drinking water supply.
High-elevation ecosystems worldwide are experiencing effects that can feed methylmercury production. In every U.S. state, at least one mercury toxicity warning exists for surface waters. Greater methylmercury production and its threat to food and water sources is now a part of our changing world.
To mitigate the risk:
1. **Curbing Industrial Emissions:** Over 140 nations, including the U.S., signed the Minamata Convention on Mercury in 2013, committing to regulate and monitor industrial mercury sources.
2. **Reducing Sulfate Flow:** Addressing climate change is necessary to reduce the flow of sulfate from ice and rock weathering.
People, governments, and industries can take many steps to slow rising air temperatures that are increasing ice thaw, such as reducing reliance on gas-powered vehicles and regulating carbon dioxide emissions from power plants and factories. Our new research on methylmercury shows another reason why taking steps to slow climate change is worth the effort.
