Wetlands of all types play a key role in the biogeochemical cycling of elements. In peatlands, decaying vegetation forms a spongy, acidic habitat for hungry microbes that process substances like mercury and sulfur. A group of microbes known as sulfate-reducing bacteria uses sulfate deposited from the atmosphere to metabolize organic material. These microbes make peatlands an efficient sink for atmospheric sulfate.
Microbes and other organisms also take in mercury from their surroundings. As a side effect of their normal metabolic processes, sulfate-reducing bacteria convert inorganic mercury into the organic compound methylmercury. Methylmercury acts as a neurotoxin in humans and wildlife, and it is biomagnified—meaning it becomes more concentrated as it travels up the food chain, from algae to fish to top predators, including humans.
Drought conditions can effectively transform peat bogs from sulfate sinks to sources by oxidizing the peat deposits and mobilizing the sulfate that they had previously sequestered. The released sulfate can then stimulate microbial populations, producing more methylmercury that can make its way into downstream aquatic environments.
Projected changes in climate, including an increase of droughts and less frequent, more intense rain events, imply that this phenomenon may make peat bogs into larger sources of methylmercury to downstream water resources and ecosystems.
In a new study, Coleman Wasik et al. address the potential of peat bogs to release mercury—particularly its most common organic form, methylmercury. Between 2000 and 2008 the scientists experimentally manipulated the amount of sulfate falling on a small peat bog in northern Minnesota and monitored how variations in water levels and sulfate input affected the release of methylmercury. Two severe droughts that occurred during the study period provided a unique opportunity for the researchers to measure the environmental response as it unfolded.
The study results revealed that drought conditions increased the production of methylmercury in peatlands and its subsequent release into the food chain. When water returned to drought-affected peatlands it prompted the release of previously sequestered sulfate, increasing the overall power of peat bogs as methylmercury sources.
Perhaps unsurprisingly, these effects were amplified in areas where atmospheric sulfate deposition had been experimentally elevated to begin with. As changes in the global climate alter rainfall patterns and increase the frequency and severity of droughts, a resulting rise in peatland sulfate release has severe implications for water resources and downstream ecosystems. (Journal of Geophysical Research: Biogeosciences, doi:10.1002/2015JG002993, 2015)
—Lily Strelich, Freelance Writer
Citation: Strelich, L. (2015), Drought changes how peat bogs cycle mercury and sulfur, Eos, 96, doi:10.1029/2015EO039121. Published on 11 November 2015.