The USGS South Florida Ecosystem Program is part of an intergovernmental effort to restore and maintain the ecosystem of south Florida. One element of the restoration effort is the development of a scientific basis for resource management decisions. Mercury contamination in the Everglades has been identified by local, state, and national agencies as a topic of great concern, and in need of research to provide the information to base restoration plans. The South Florida Water Management District (SFWMD), Florida Department of Environmental Protection (FDEP), U.S. Environmental Protection Agency (USEPA), and the National Marine Fisheries Service need information on mercury cycling to predict the effects of proposed restoration plans on mercury exposure. The Everglades Forever Act of 1994 has mandated management decisions regarding what can be done to mitigate the toxic effects of mercury in the Everglades.
In response to this request from resource managers for more scientific information on mercury cycling in the Everglades, the USGS South Florida Ecosystem Program, SFWMD, and USEPA are co-funding a group of scientists to study mercury bioaccumulation in the Everglades. Participating scientists are from several agencies, including: USGS, SFWMD, 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.
The precise mechanism for transfer of CH3Hg+ to the food chain is unknown, but likely involves the consumption of methyl-mercury containing bacteria by the next higher level in the food chain (likely plankton) or direct adsorption of CH3Hg+ dissolved in water. The initial food chain transfer step is vitally important, because concentrations of mercury in plankton increase about ten thousand fold over water concentrations. This process is called biomagnification. Because organisms cannot eliminate mercury as fast as it can be ingested, mercury tends to accumulate as one proceeds up each remaining food-chain level. However, the bio-magnification factor between each of these levels is about ten fold or less. Although the transfer routes and controlling processes of mercury in the food chain are generally known, many complicating factors make food-chain studies difficult, including: precise knowledge of what certain organisms consume, seasonal presence/absence of prey, and the fact that mercury concentrations generally correlate with the age of an organism.
Is mercury contamination in the Everglades different than elsewhere, and if so, why? Mercury concentrations in game fish from in the Everglades region are some of the highest observed anywhere in the world. A statewide sampling of Largemouth Bass in the late 1980's revealed that the fish in one-half to two-thirds of Florida's lakes contained elevated levels of mercury. Many of the lakes and streams across northern and central Florida were found to have Largemouth Bass with average mercury concentrations between 0.5 and 1.5 parts per million (ppm), which is cause for issuing a limited consumption advisory for the general population; with even more stringent recommendations for women of child-bearing age, and children. A much more severe problem was revealed in the Everglades, however, where nearly a million acres of this ecosystem was found to have average mercury concentrations in Largemouth Bass exceeding 1.5 ppm, resulting in a "do not consume advisory" for this region.
The severe mercury problem in the Everglades is likely the result of naturally occurring conditions that make the ecosystem prone to mercury methylation and bioaccumulation, and the exacerbating effects of many disturbances caused by a large, nearby human population. Most wetland systems, like the Everglades, have the necessary ingredients that tend to promote elevated levels of CH3Hg+ in organisms, such as ample DOC, organic substrate (peat), and low to neutral pH. In addition, relatively high sulfate levels and a subtropical climate in the Everglades region provide optimal conditions for sulfate-reducing bacteria to methylate mercury. The human effect on the mercury problem in the Everglades centers on three issues: (1) Hg-containing emissions from incinerators and power generating utilities; (2) increased soil-mercury mobilization promoted by drainage and soil disturbance in the Everglades Agricultural Area (EAA);(3) hydrologic changes resulting from the Central and South Florida Flood Control Project and (4) other chemical additions to the Everglades, such as sulfate from acid rain or runoff from agricultrual areas, that may stimulate mercury methylation.
Mercury cycling in the Florida Everglades project: From a resource management perspective, one of the primary concerns of this project is the long- and short-term effects of the Everglades Nutrient Removal (ENR) project, which is a crucial component of the SFWMD's restoration plans. The ENR project calls for the construction of stormwater treatment areas (STAs), which are reclaimed agricultural lands that will be permanently flooded with water draining from the EAA and thus reduce phosphorus loads to the Water Conservation Areas (WCAs) by sequestering phosphorus through biological uptake. Questions have arisen concerning whether enhanced mercury methylation might result within the STAs and present a toxicological hazard for wildlife residing there, or whether potentially high levels of CH3Hg+ in outflows from the STAs might present an environmental hazard to wildlife in the WCAs.
Initially, the project focused on field sites in the northern Everglades, where phosphorus loading from the EAA and its impact on mercury cycling is of concern. Sampling stations include several sites within the ENR, canals, and marshes. Sites along the L-39 canal were chosen to examine how mercury levels change with distance from the EAA and as water leaves the canals through levee spillways and encounters more quiescent conditions of WCA2. Sites within WCA2 are along a transect that spans the region of greatest phosphorus impact where more natural phosphorus conditions prevail. The two sites along the L-67 canal were chosen because this area showed the greatest Hg concentrations in Largemouth Bass. More recently, the project has added sampling sites in the central and southern parts of the system (WCA3A and Everglades National Park). Fish sampling surveys have shown that game fish tend to have the highest concentrations of mercury in this part of the Everglades, and our research is focused at determining what factors promote mercury methylation and bioconcentration in this area.