Mercury in the Geochemical Cycle and Food Chain of the Everglades

figure 1 [larger image]

The fate of mercury in the Everglades ecosystem is controlled by the geochemical cycle and food-chain transfer steps (fig. 1). 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. 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.

figure 2 [larger image]

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 (fig. 2). 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); and
3. hydrologic changes resulting from the Central and South Florida Flood Control Project.

Next: The USGS South Florida Ecosystem Program

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