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Aquatic Cycling of Mercury in the Everglades

photo of the everglades
Project Investigators: David P. Krabbenhoft, William H. Orem, George Aiken, Carol Kendall


Summary

The goal of this project is to describe the mercury contamination problem in South Florida.

New Key Findings!

For most aquatic ecosystems, atmospheric deposition is the primary source of mercury, although there are numerous instances of geologic and anthropogenic point-source contamination. There are many sources of mercury to the atmosphere, both natural and human related. Natural sources include outgassing from the oceans, volcanoes, and natural mercury deposits. Coal combustion, waste incineration, chloralkai production, and metal processing are the dominant human-related sources to the atmosphere. In ecosystems for which atmospheric deposition is the dominant source, resulting concentrations of total mercury in water are very low, generally less than 10 nanograms per liter (ng/L). The challenge to scientists is to explain the series of processes that lead to toxic or near-toxic levels of mercury in organisms near the top of the food chain (bioaccumulation), when aqueous concentrations and source-delivery rates are so low. To understand this phenomenon adequately, scientists must apply an interdisciplinary approach wherein various components of an ecosystem (atmosphere, biota, surface water, ground water, and sediments) are studied contemporaneously. The purpose of this fact sheet is to describe the mercury contamination problem in south Florida, and the interdisciplinary project that was assembled under the auspices of the U.S. Geological Survey South Florida Ecosystem Program to investigate the underlying processes that cause mercury bioaccumulation.

In response to this request from resource managers for more scientific information on mercury cycling in the Everglades, the USGS South Florida Ecosystem Program, South Florida Water Management District (SFWMD), and United States Environmental Protection Agency (USEPA) are co-funding a group of scientists to study mercury bioaccumulation in the Everglades. Participating scientists are from several agencies, including: USGS, SFWMD, Florida Department of Environmental Protection (FDEP), USEPA, Wisconsin Department of Natural Resources, and University of Wisconsin-Madison. The overall objective of this project is to provide resource managers scientific information on the hydrologic, biologic, and geochemical processes controlling mercury cycling in the Everglades. It is anticipated, however, that information from this project will be transferrable to other ecosystems where mercury problems arise. Specific areas of research among the group includes: geochemical studies of mercury, mercury methylation and demethylation studies, DOC-Hg interactions, mercury accumulation in sediments, diagenetic processes in peat, sulfur cycling studies, biological uptake of mercury and lower food chain transfer pathways, and groundwater/surface-water exchange.

Specific key findings of Phase I of the ACME project are the following:

  1. MeHg bioaccumulation is driven by internal MeHg production, mainly in surface sediments;
  2. MeHg concentrations in all matrices (sediment, surface water, pore water, and biota) are maximal in the central Everglades (southern WCA2A, WCA2B and north-central WCA3);
  3. MeHg is somewhat lower in more pristine areas like Everglades National Park and WCA1, and much lower in the most eutrophic areas of WCA2A and the constructed nutrient-retention wetlands;
  4. The spatial MeHg pattern is not driven primarily by inorganic Hg concentration, although there is weak but significant relationship between Hg and MeHg concentrations in surface sediments;
  5. Photochemical reduction and photo-demethylation are important mechanisms for removal of mercury and destruction of MeHg, respectively, over much of the Everglades;
  6. Sulfur inputs from areas north of the Everglades have a large impact on MeHg production, but the magnitude and even direction of the impact varies with the sulfate and sulfide concentration;
  7. Phosphate and nitrate generally have no direct effect on MeHg production rates in sediment cores;
  8. Anaerobic microbial processes, including sulfate reduction, are key components of microbial organic carbon decomposition in Everglades sediment;
  9. Microbial dissimilatory sulfate reduction (rather than assimilation by plants) appears to be the most important mechanism for reduced sulfur storage in Everglades peat;
  10. Natural fires and extended periods of peat exposure can greatly exacerbate MeHg production (for example 10x increases in sediment MeHg levels), and this phenomenon appears to be driven by sediment oxidation and release of sulfate after re-inundation;
  11. Bioaccumulation of MeHg in Gambusia appears to be facilitated by the movement of benthic invertebrates (insects and zooplankton) into the water column, and less importantly by direct grazing on surface sediments and periphyton;
  12. Methylation occurs only in periphyton “mats” where microbial sulfur cycling occurs, and is most common in the less-calcareous periphyton found in eutrophic areas.

graphic showing the theoretical mercury cycle within the Everglades
Theoretical mercury cycle within the Everglades. [larger image]

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Script last updated: 29 May 2014 @ 12:08 PM by THF. Record creator: BJM. Record last updated by: BJM.