HOMESTEAD, Fla. -- For the first time, scientists have measured the amount of dissolved organic carbon and mercury moving from a southwest Florida mangrove swamp via tides to coastal waters. The scientists from the U.S. Geological Survey discovered that a large amount of mercury and methylmercury–the form of mercury that is most toxic and the form that accumulates in fish–flows from mangroves into the Gulf of Mexico.
According to the South Florida Water Management District, over 50 species of fish in Florida's coastal waters have elevated concentrations of mercury, and human health advisories regarding consumption have been issued by the Florida Department of Health for several fish species in Florida coastal marine areas. A greater understanding of the sources and environmental pathways resulting in these elevated methylmercury concentrations in fish will help environmental managers, policy makers, and regulators lower human and wildlife mercury exposure.
"Previous USGS research has revealed that these swamps transform mercury to its toxic methylmercury form far from the coal-burning plants that are the original source of the pollution," said USGS Director Marcia McNutt. "Now we are learning that the threat to wildlife and human health does not stop there, but is carried out to sea on the ebb tide."
Scientists pioneered a new high-frequency measurement method to measure dissolved organic carbon, mercury and methylmercury in an extremely complex tidal estuarine environment. The study originated from the need to measure the flux of dissolved organic carbon, but scientists realized that they could simultaneously measure mercury and methylmercury using the same techniques. Previous USGS research in the Everglades has shown that methylmercury and mercury bind to dissolved organic carbon.
Study results revealed that mercury from the mangroves could account for over 90% of the methylmercury and almost half the total mercury supply to the near-shore coastal waters of southwestern Florida. The study's findings represent an important first step in identifying and quantifying a significant source of mercury and methylmercury for coastal fish in southwest Florida-Gulf of Mexico region. The findings are published in the journal Environmental Science & Technology.
Although almost all of the mercury entering the Gulf of Mexico originates from the atmosphere, it is not understood how or where this mercury undergoes the necessary conversion to its more toxic form, methylmercury. Scientists do not understand the relative contribution, but generally agree that the conversion to methylmercury occurs in the following three areas:
- in the landscape, where it could be flushed into coastal areas by runoff and tidal pumping;
- in the estuarine zones, which in southwest Florida often contain mangroves, where it could be flushed into the coastal areas by the tides; or,
- in the deep open waters and/or bottom sediments of the Gulf of Mexico.
Until now, the amount of methylmercury coming from estuarine mangrove zones into the coastal areas was unknown because making accurate measurements of this type is extremely challenging. The tide waters that flow in and out of the estuary twice a day must be measured in addition to the constantly changing dissolved organic carbon, mercury, and methylmercury concentrations that result.
"Once we understand where the mercury is being methylated, and how much methylmercury is coming from various environments, resource managers, regulators, and decision makers will be better able to anticipate how the Gulf of Mexico will respond to reductions in mercury loads. It's a very important start that we at least have one of the rivers, the Shark River, flowing into the Gulf of Mexico, quantified as to how much mercury, methylmercury and dissolved organic carbon, is flowing into the gulf," said USGS researcher David Krabbenhoft.
Researchers believe that a relatively unique combination of circumstances result in the large amounts of methylmercury they measured flowing from mangroves into the coastal ocean. Mangroves forests capture gaseous mercury from the air through their leaves and drop them into the brackish swamp waters, where methylmercury is produced in the presence of seawater sulfate.
"Measuring the amount of mercury that mangrove swamps contribute to coastal waters will help us understand and model the sources of mercury that enter the food web," said lead researcher Brian Bergamaschi with the USGS.
This study is the work of scientists from the USGS, Florida Gulf Coast University, and the State of Florida. The high-frequency dissolved organic carbon measurement methodology was developed for a previous study in San Francisco Bay.
The USGS has studied mercury in the Everglades since 1994. Previous USGS research discovered that mercury in the Everglades was coming from atmospheric releases and deposition, and not from natural land sources such as rocks. They also found that the formation of methylmercury in the Everglades is strongly influenced by land and water uses, including the delivery of sulfate from up-gradient agricultural lands. This study represents a significant contribution to the knowledge about the risks of mercury exposure in south Florida, but additional studies are necessary to better understand how mercury accumulates in coastal fish.
For more information about mercury, please visit the USGS Mercury in the Environment webpage. For more information about carbon cycling and sequestration in terrestrial and marine environments, please visit the USGS LandCarbon website.