How Hydroperiod and Tree Cover Influence Mercury Cycling in Carolina Bay Wetlands
By Tyjaha Steele
Pictured above is a photograph of New Ellenton Bay (Photo courtesy of Amanda Hurst).
In wetlands, small changes in water levels and vegetation can have a significant impact on how elements move through the environment. Carolina bays are shallow, isolated, oval-shaped wetlands found throughout the southeastern United States that naturally fill and dry over time. Mercury, a naturally occurring element introduced through both natural processes and human activities, may remain stored in wetland sediments or be transformed into methylmercury, a form that more readily accumulates in plants and animals and moves through food webs. These pathways are shaped by local conditions, which vary widely among isolated wetlands in their water levels and vegetation structure.
To better understand how these factors influence mercury cycling, researchers from the University of Georgia’s Savannah River Ecology Laboratory (SREL) examined mercury dynamics in Carolina Bay wetlands at the Savannah River Site (SRS) in South Carolina. Led by Xiaoyu Xu, an associate research scientist at SREL, the study focused on ten Carolina Bay wetlands that differ in hydroperiod, the length of time water remains in a wetland each year. Some bays dry out seasonally (short-hydroperiod bays), while others remain flooded for extended periods (long-hydroperiod bays).
“In Carolina bay wetlands, mercury behavior is largely dictated by how long a wetland stays underwater, a cycle known as its hydroperiod. Short-hydroperiod wetlands, which are only seasonally flooded, typically act as mercury ‘sponges’ because their dense tree canopies capture mercury from the atmosphere and drop it into the soil via falling leaves,” explains Xu. “However, long-hydroperiod wetlands, those that stay flooded for most of the year, create the perfect environment for bacteria to transform inorganic mercury into methylmercury.”
Researchers collected surface sediment samples from each wetland during both wet and dry seasons and measured concentrations of total mercury and methylmercury. Sampling across seasons and wetland types allowed the research team to compare the amount of mercury accumulated in sediments and its rate of conversion into its more toxic form under changing environmental conditions.
Cara Love, one of the co-authors listed on the paper, is seen collecting a sample at a wetland edge (Photo courtesy of David E. Scott).
The study suggests that wetlands with shorter hydroperiods, those that dry out part of the year, accumulated higher levels of total mercury, particularly when flooded. This pattern was associated with greater tree canopy cover, which can influence how mercury enters wetland sediments through natural processes such as leaf uptake and litterfall.
“The density of the tree canopy controls how much mercury enters the wetland in the first place and the leaves absorb mercury from the atmosphere and then deliver it directly to the ground,” says Xu. “The shade from a dense canopy protects the soil from sunlight, which would otherwise help ‘burn’ the mercury back into the atmosphere when the wetland dries out.”
Short-hydroperiod bays primarily functioned as storage sites for total mercury, whereas long-hydroperiod bays showed a greater capacity to convert mercury into methylmercury. The highest methylmercury concentrations were observed in the central areas of long-hydroperiod bays, where prolonged flooding supports conditions favorable to mercury methylation.
“Methylmercury is most concentrated in the deeper center areas of long-hydroperiod wetlands, which stay underwater for most of the year. These locations maintain waterlogged, oxygen-poor conditions where bacteria can convert inorganic mercury into methylmercury,” says Xu. “Identifying these hotspots allows us to better predict which species are most at risk and how methylmercury might move and magnify through the food web.”
The study suggests that mercury methylation potential decreased with increasing total mercury concentrations and tree canopy cover, with short-hydroperiod bays functioning primarily as sinks for atmospheric mercury and long-hydroperiod bays favoring methylmercury production.
The full study, Influence of hydroperiod and canopy cover on mercury accumulation and methylation in Carolina bay wetland sediments in the Southeastern United States, was published in Environmental Research. Authors include Chongyang Qin, David E. Scott, Stacey L. Lance, Demetrius Calloway, Cara N. Love, and Xiaoyu Xu.