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Mercury Studies in the Florida Everglades

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Identification_Information:
Citation:
Citation_Information:
Originator: David P. Krabbenhoft
Publication_Date: Unpublished material
Title: Mercury Studies in the Florida Everglades
Publication_Information:
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Description:
Abstract:
Public concern for wildlife and human health problems due to mercury (Hg) toxicity has increased substantially since the mid-1980's. These concerns are manifested primarily by the issuance of fish consumption advisories in the majority of U.S. states, Canada, and several European countries because of high levels of mercury in game fish. Although the precise causes for this contamination problem are not completely understood, it appears that there are both source and ecosystem-specific factors that can result in elevated levels of mercury in game fish. Because mercury is known to adversely affect the human brain and nervous system, health concerns arise when elevated concentrations of mercury are detected in game fish from ecosystems where there is subsistence level consumption of fish. In extreme cases such as the Everglades, where mercury concentrations in fish consistently exceed the Florida advisory level of 1.5 parts per million, even occasional fish consumption is not recommended.

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.

Purpose:
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, 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.

Supplemental_Information:
Time_Period_of_Content:
Time_Period_Information:
Range_of_Dates/Times:
Beginning_Date: 19950301
Ending_Date: 19990930
Currentness_Reference: Publication date
Status:
Progress: in work
Maintenance_and_Update_Frequency: as needed
Spatial_Domain:
Bounding_Coordinates:
West_Bounding_Coordinate: -81.25
East_Bounding_Coordinate: -80.30
North_Bounding_Coordinate: 25.80
South_Bounding_Coordinate: 24.75
Keywords:
Theme:
Theme_Keyword_Thesaurus: none
Theme_Keyword: Mercury toxicity
Theme_Keyword: Game fish
Theme_Keyword: Mercury contamination
Theme_Keyword: Bioaccumulation
Theme_Keyword: Atmospheric deposition
Theme_Keyword: Groundwater discharge
Theme_Keyword: Methylation
Theme_Keyword: Dissolved organic carbon
Theme_Keyword: Biomagnification
Theme_Keyword: Phosphorus loading
Place:
Place_Keyword_Thesaurus: none
Place_Keyword: South Florida
Place_Keyword: Everglades
Place_Keyword: L-39 canal
Place_Keyword: L-67 Canal
Place_Keyword: Central Everglades
Place_Keyword: Greater Lake Okeechobee
Access_Constraints: none
Use_Constraints: none
Point_of_Comments and suggestions? Contact:
Contact_Information:
Contact_Person_Primary:
Contact_Person: David Krabbenhoft
Contact_Organization: U.S. Geological Survey
Contact_Position: Project chief
Contact_Address:
Address_Type: mailing address
Address: 8505 Research Way
City: Middleton
State_or_Province: WI
Postal_Code: 53562
Contact_Voice_Telephone: 608 821 3843
Contact_Facsimile_Telephone: 608 821 3817
Contact_Electronic_Mail_Address: dpkrabbe@usgs.gov
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Mercury in the Geochemical Cycle and Food Chain of the Everglades: The fate and toxicity of mercury in the Everglades ecosystem is controlled by the interactions between the geochemical cycle and food-chain transfer steps. Although most mercury is likely derived from atmospheric deposition, other potential mercury sources exist, such as ground-water discharge and water from drainage canals. The dominant form of mercury in atmospheric deposition is ionic mercury [Hg (II)], but once in surface water of an aquatic ecosystem, rapid geochemical transformations can occur. The transformation of Hg (II) to methylmercury [CH3Hg+] is referred to as methylation. From a toxicity perspective, methylation is an important step, because CH3Hg+ is the most bioaccumulative form of mercury and comprises almost all the mercury in consumable fish. Although several biological and non-biological processes can methylate mercury, scientists generally agree that methylation by sulfate-reducing bacteria is most important. This process is localized where the bacteria concentrate, such as at the sediment/water interface or in algal mats. Demethylation also occurs, which is the process of transforming CH3Hg+ to Hg(II) or to elemental mercury [Hg0]. This is an important process because the mercury byproducts of this process are less bioaccumulative and Hg0 is removed from the water surface by transfer to the air (evasion). Dissolved organic carbon (DOC) in Everglades water is not only responsible for its characteristic brown color, but also is an important transport vehicle for mercury and can facilitate many of the important biogeochemical reactions. Mercury associates with DOC in water and generally increases the concentration of mercury that can be maintained in water. Depending on local conditions, DOC-Hg binding can either increase or reduce mercury uptake by organisms. If DOC-Hg bound mercury is transported to a site where methylation is occurring, enhanced toxicity results; however, if DOC-Hg binding is strong enough, DOC can limit the availability of mercury for methylation.

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.

Process_Date: not complete
Process_Comments and suggestions? Contact:
Contact_Information:
Contact_Person_Primary:
Contact_Person: David Krabbenhoft
Contact_Organization: U.S. Geological Survey
Contact_Position: Project chief
Contact_Address:
Address_Type: mailing address
Address: 8505 Research Way
City: Middleton
State_or_Province: WI
Postal_Code: 53562
Contact_Voice_Telephone: 608 821 3843
Contact_Facsimile_Telephone: 608 821 3817
Contact_Electronic_Mail_Address: dpkrabbe@usgs.gov

Distribution_Information:
Distributor:
Contact_Information:
Contact_Person_Primary:
Contact_Person: David Krabbenhoft
Contact_Organization: U.S. Geological Survey
Contact_Position: Project chief
Contact_Address:
Address_Type: mailing address
Address: 8505 Research Way
City: Middleton
State_or_Province: WI
Postal_Code: 53562
Contact_Voice_Telephone: 608 821 3843
Contact_Facsimile_Telephone: 608 821 3817
Contact_Electronic_Mail_Address: dpkrabbe@usgs.gov
Resource_Description:
Distribution_Liability: The data have no explicit or implied guarantees.

Metadata_Reference_Information:
Metadata_Date: 19981116
Metadata_Comments and suggestions? Contact:
Contact_Information:
Contact_Person_Primary:
Contact_Person: Jo Anne Stapleton
Contact_Organization: U.S. Geological Survey
Contact_Address:
Address_Type: mailing address
Address: 521 National Center
City: Reston
State_or_Province: VA
Postal_Code: 20192
Contact_Voice_Telephone: 703 648 4592
Contact_Facsimile_Telephone: 703 648 4614
Contact_Electronic_Mail_Address: jastapleton@usgs.gov
Metadata_Standard_Name: Content Standard for Digital Geospatial Metadata
Metadata_Standard_Version: FGDC-STD-001-1998

Generated by mp version 2.2.6 on Mon Nov 16 10:43:13 1998