Mercury Accumulates in Forests, and Prof. Obrist Wants to Know How

03/27/2019
By Edwin L. Aguirre

The National Science Foundation (NSF) has awarded a three-year, $873,000 grant to a research project led by Prof. Daniel Obrist, chairman of the Department of Earth, Environmental and Atmospheric Sciences, to study mercury pollution in forests.

Mercury gets deposited in forests through rainfall and vegetation uptake – that is, plants absorb the gaseous form of elemental mercury from the atmosphere and subsequently transfer it to the soil when the plants die or shed leaves. The goal of Obrist and his collaborator, Asst. Prof. Roisin Commane of Columbia University, is to better understand this deposition process, which Obrist describes as “vegetation pump,” and to take direct measurements of the mercury uptake at two sites – Harvard University’s forest near Petersham, Mass., and the Texas A&M Soltis Center forest near San Isidro de Peñas Blancas in central Costa Rica.

“Scientists now understand that this vegetation pump dominates as the source of mercury on land, accounting for 54 percent to 94 percent of mercury deposits in soils,” says Obrist. “But no direct measurements of this process are available from forests, which constitute the most abundant ecosystems globally.”

According to Obrist, forest measurements are currently limited to polluted sites or short periods of time. “Our goal is to directly measure the uptake of atmospheric mercury at the two sites for one full year each, providing the first such records in forests,” he says.

Understanding the distribution of mercury is important because the heavy metal is a highly neurotoxic environmental pollutant that threatens fish, birds, mammals and humans worldwide. Hundreds of tons of it are released into the atmosphere each year by power plants through the burning of coal, as well as through mining activities and other industrial processes, and the pollutant is distributed across the globe. Airborne mercury eventually falls back to the ground, accumulating in soils, rivers, lakes and oceans.

“Human exposure to mercury is primarily driven by the consumption of fish containing high levels of methylmercury,” notes Obrist.

Methylmercury, which is very poisonous, forms when anaerobic bacteria add a carbon atom to mercury in the water, sediments and soils. It tends to bio-accumulate in the food web, ending up in fish and eventually contaminating wildlife and humans.

Exposure to high levels of methylmercury over long periods can have adverse effects on the body’s reproductive, immune, cardiovascular and neurological systems. Infants and young children can suffer from developmental disorders, reduced memory performance and increased risk of attention problems.

“The dataset we will obtain from this project will be used to better understand how mercury moves through trees, soil and the atmosphere,” says Obrist. “The knowledge gained will be useful for assessing the global distribution of mercury so that internationally recognized goals for reducing mercury pollution can be achieved.”

Mercury Hot Spots

Obrist says the densely forested regions of the northeastern United States are known hot spots of mercury deposition. “The tropics also show very high mercury deposition, probably because the regions’ vegetation pump remains active all year long. But nobody is measuring them directly – that is, until now,” he says.

The NSF project will study two contrasting ecosystems. The Harvard forest is an example of a temperate deciduous forest, where broadleaf trees like read oak and red maple shed their leaves every year, while the Soltis Center forest represents an evergreen subtropical rainforest, where many evergreen species like mulberry and mallows retain their leaves throughout the year.

Obrist, Commane and their students will measure the forests’ uptake of atmospheric mercury by installing atomic fluorescence analyzers and other automated sensors on existing large towers at various heights, from above the tree canopy down to near the forest floor. The researchers will download the data daily for analysis via the web.

“We will measure how the forests’ mercury deposition changes over seasons and across different forest types,” says Obrist. “Additional measurements of trace gases, including carbon dioxide, ozone, water vapor and carbonyl sulfide, will allow us to assess the mechanisms of mercury uptake by plants and underlying soils.”

Obrist is an expert in atmospheric cycling and biogeochemistry of mercury, and he has been studying this environmental hazard for over a decade.

For two years, beginning in fall 2014, Obrist led an international group to conduct the most comprehensive study to find the source of mercury pollution in the Arctic. The team’s findings, which were published in the journal Nature, showed that the absorption of mercury from the atmosphere by the tundra is shown to drive high loads of mercury in Arctic tundra soils. Mercury runoff from tundra soil then supplies 65 to 85 tons of the toxic heavy metal to Arctic lakes, rivers and the Arctic Ocean each year.

“This mercury from the tundra soil explains half to two-thirds of the total mercury input to the Arctic Ocean,” says Obrist. “Now, we’re going to find out how forests contribute to mercury inputs in watersheds in our latitudes.”