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Does Everglades Restoration Mean More Florida Bay Mercury?
Scientists are trying to determine if the largest ecological restoration project ever attempted - now underway in Florida's Everglades, the largest remaining subtropical wilderness area in the U.S. - will lead to increased human exposures to toxic mercury concentrations from seafood in Florida Bay.
Researchers from the National Oceanic and Atmospheric Administration (NOAA), in Beaufort, N.C., and from the South Florida Water Management District (SFWMD) have teamed up to determine whether redirected freshwater flows aimed at restoring damaged habitats and supplying water to adjoining coastal areas will increase human and wildlife exposure to methylmercury. Although many of the changes in water deliveries to Florida Bay planned as a result of the Comprehensive Everglades Restoration Plan (CERP) have already taken place, there are scenarios under consideration which would provide substantial additional deliveries in order to restore the southern Everglades and Florida Bay to a more natural condition existing before flood control and diversions were implemented. These additional flows would change the spatial patterns of inundation in the Everglades wetlands with the possibility that more methylmercury could be produced there and delivered to Florida Bay.
Florida Bay has been identified as one of two regional "hotspots" for mercury accumulation in fish from the Gulf of Mexico in the Environmental Protection Agency's Gulf of Mexico Program report, A Survey of Mercury in the Fishery Resources of the Gulf of Mexico. Unlike the other hotspot at a Superfund site in Texas, there are no known industrial point sources of mercury in Florida Bay.
The researchers' preliminary findings suggest that high mercury concentrations in many Florida Bay fish result from multiple runoff sources and atmospheric deposition combined with special environmental conditions in Florida Bay and its watershed. These conditions facilitate the transformation of inorganic mercury from external sources into methylmercury which is readily bioaccumulated by fish and other organisms in their food web.
Concern over methylmercury concentrations in south Florida is not a new phenomenon, and Florida Bay since 1995 has been under a health advisory concerning limited fish consumption because of elevated mercury levels. Neither are mercury concerns limited to south Florida. As of December 2000, mercury contamination had led to issuance of more than 2,200 fish consumption advisories in 41 different states, increasing 149 percent from the 899 mercury-related advisories issued in 1993. (http://map1.epa.gov/)
Methylmercury Sources and Risks
What makes the issue of increasing public health concern is the possibility that increased methylmercury flows could be directed toward Florida Bay and its numerous recreational fishers, potentially increasing total human exposures.
Methylmercury, an organic form of mercury derived from inorganic mercury released into the environment, accumulates up the food chain, reaching high concentrations in predatory fish and leading to increased risks to humans when they consume those fish.
"Methylmercury biomagnifies up the food chain as it is passed from a lower food chain level to a subsequently higher food chain level through consumption of prey organisms by predators," the U.S. Environmental Protection Agency has reported. "Fish at the top of the aquatic food chain…bioaccumulate methylmercury approximately 1 to 10 million times greater than dissolved methylmercury concentrations found in surrounding waters."
"Exposure to methylmercury can result in adverse effects in several organ systems throughout the life span of humans and animals," The National Academy of Sciences' Committee on Life Sciences has reported. The Academy group points to adverse effects on developing and adult cardiovascular systems (blood-pressure regulation, heart-rate variability, and heart disease) and to damage to cognitive, motor, and sensory functions, along with central nervous system effects and neurological impairment.
Mercury in the environment comes both from natural sources and human activities. Solid waste incineration and fossil fuel combustion facilities account for approximately 87% of airborne emissions of mercury in the United States. Mercury is released into surface waters by naturally occurring processes, and industrial activities such as pulp and paper mills, leather tanning, electroplating, and chemical manufacturing.
In addition, mercury in soils results from direct application of fertilizers and fungicides and from disposal of solid wastes, including such common household items as batteries and thermometers.
Once released into the aquatic environment, inorganic mercury can be converted to an organic form of mercury, methylmercury, which is the primary form that accumulates in fish and shellfish.
Everglades Changes Over the Decades
A century ago, the Everglades covered much of south Florida stretching from Lake Okeechobee south and east toward Miami. Partly drained over the decades for agriculture, dammed and diverted for flood control, and filled for urban development, the Everglades has had much of its water flow diverted over the years to help support population growth along Florida's Gulf and Atlantic coasts.
The current Everglades system now comprises less than half of the total land area it did in 1900. But the system still maintains its essential core functions, with a complex of habitats including estuaries, fringing mangrove forests, coastal prairies, sloughs, cypress swamps, tree hammocks, and pinelands. Plans call for spending more than $8 billion over the next two decades to restore historic freshwater flows toward the southwest and to Florida Bay, bordered on the southeast by the strand of Keys islands.
With that restored water flow comes the challenge the NOAA and SFWMD scientists are confronting in their research: Will tradeoffs in meeting the ecosystems's water needs result in increased mercury contamination in some prize species of Florida Bay fish?
Lead researcher David W. Evans, of NOAA's Beaufort, North Carolina, Center for Coastal Fisheries and Habitat Research, says initial findings may raise just such a concern.
"High levels of mercury already exist in fish from northeastern Florida Bay. We have observed seasonal increases in methylmercury in water, sediments, and forage fish associated with seasonal Everglades runoff, he says.
The Everglades National Park has been recognized as an International Biosphere Reserve, a World Heritage Site, and a Wetland of International Importance. The Everglades is home to the only population of American crocodiles in the U. S., along the mangrove fringe separating the Everglades mainland from Florida Bay. It is home also to a number of endangered species, four species of marine turtles, the West Indian Manatee, the Cape Sable Seaside Sparrow, Snail Kites, and Wood Storks.
Dead Panthers Stir Public Support
Sparking public interest and public support for the wide-ranging effort to study and control mercury pollution in the Everglades was the discovery beginning in 1989 of three dead Florida panthers with high mercury body burdens. Subsequent research has found extraordinarily high levels of mercury in fish from the central Everglades. In addition, researchers have found wetlands methylmercury concentrations that could bioaccumulate to concern levels in gamefish and carnivores such as the Florida panther.
In recent years, fish consumption advisories have been posted throughout much of the Everglades region, cautioning people on risks of eating too much local fish. The warnings extend into Florida Bay, the shallow estuarine region that receives freshwater runoff from the Everglades.
The highest mercury concentrations in both forage and game fish have been found in the northeastern section of Florida Bay, adjacent to the major freshwater inflows from the Everglades. Researchers say that more than 30 percent of spotted sea trout in this area exceed the state's no-consumption threshold.
"Will redirecting freshwater flows in the Everglades improve conditions for plants and animals there at the expense of increasing mercury concentrations for Florida Bay fish and for the recreational fishing population and other species dependent on them?" asks Evans.
For humans, fish and other seafood are the main source of methylmercury in the diet. EPA says methylmercury concentrations in fish and shellfish can be thousands of times higher than in other foods. The agency's 1997 Mercury Study to Congress concludes that the typical consumer is in no danger of consuming harmful levels of methylmercury from fish. But it says those eating more fish than is typical - or eating certain species of fish from particular localities - may face risks for developing fetuses.
Among persons potentially at higher risk are recreational and subsistence fishers regularly consuming large amounts of locally caught fish and subsistence hunters routinely consuming meat or organ tissues of marine mammals. Concern over potential risks resulting from consumption of marine mammals is not a major concern in the U.S., where marine mammals are protected species not to be injured or harvested without special permit. Only a few native Americans such as the Inuit and Makah consume marine mammal tissues. North of the United Kingdom, studies among Faroe islanders who harvest whales for food are among those raising concerns over potential health risks to subsistence whalers.
For the Florida Bay, the state's Department of Health and Rehabilitation Services has found mercury levels in spotted seatrout, crevalle jack, ladyfish, and bluefish exceeding state limits for human consumption. Postings caution adults against eating those fish species more than once per week - or more than once per month for women of childbearing age and children. In upstream parts of the Everglades, state postings caution against eating any largemouth bass, bowfin, or gar, and advise only limited consumption of certain other species.
The research team set out to understand the extent to which high levels of mercury found in Florida Bay fish may originate from Everglades freshwater runoff. Along two major freshwater flow pathways, they sampled total mercury and methylmercury in water, sediments, small forage fish and larger gamefish. They found a pattern of elevated methylmercury concentrations in water and sediments in the mangrove transition zone where Everglades runoff mixes with saline water from the Florida Bay.
The researchers also observed variations over time in mercury concentrations. "We have sampled often enough now to begin to see seasonal patterns in mercury concentrations," says Evans. The lowest methylmercury concentrations occur in the dry season of mid-winter. The methylmercury concentrations in water rise when freshwater flow increases in response to summer rains at a time when temperatures rise and biological activity is greatest. The increases in mercury concentrations with seasonal runoff are thought to serve as an analog for changes in mercury concentrations that might accompany altered freshwater flows during Everglades restoration.
Total mercury concentrations in silversides, a fast growing forage fish, also increase at this time, suggesting a higher exposure of parts of the food web to methylmercury in water. This linkage between methylmercury concentrations in water and total mercury concentrations in fish is strongest at sites in the brackish water of the mangrove transition zone of Joe Bay and Little Madeira Bay, where the study team observed the highest methylmercury concentrations in water and sediments. They say this conclusion suggests particle deposition and enhanced methylmercury formation within the transition area. The highest methylmercury concentrations in most fish were found downstream of this transition zone.
Eastern Florida Bay is unique in many ways that may contribute to methylmercury accumulation in fish. Because it is cut off from the ocean by a series of islands, the Keys, and mud banks, it has restricted circulation, limiting the dilution which otherwise might reduce the mercury concentrations.
The researchers say Eastern Florida Bay's shallow well-mixed carbonate sediments limit the binding of mercury, making it more available for methylation, through which inorganic mercury is transformed to the more dangerous organic mercury. The organic or methylmercury can then be bio-accumulated in microalgal or detrital foods; it then can make its way up the food web to fish and, through fish, to humans as consumers. In collaboration with the U. S. Geological Survey, the researchers also found significant methylation of inorganic mercury in the sediments of the Everglades wetlands, the mangrove transition zone, and in Florida Bay itself.
On the other hand, and somewhat to their surprise, the study team observed little dependence of mercury concentrations in fish on salinity, or on stable isotope measurements of carbon or nitrogen which served as co-tracers of freshwater runoff. It now seems likely that under existing conditions Everglades runoff is not the dominant source of mercury accumulating in Florida Bay fish, only one of several sources.
"Methylmercury - either imported from the Everglades, the mangrove transition zone, or produced within the Bay - is relatively well mixed in the eastern bay, resulting in fairly uniform exposures," says NOAA researcher Evans. "Sorting out the contribution of Everglades freshwater runoff is made more difficult as a result, as is predicting changes that will result from Everglades restoration."
Continued sampling of mercury levels in water, sediments, and fish are needed to better understand effects of seasonal runoff fluctuations and to quantify the extent to which mercury found in the Bay results from runoff upstream in the Everglades. The researchers expect to use water quality and flow models to predict potential impacts from increased or redirected flows during the restoration. And throughout the restoration effort, they hope to continue monitoring to assess possible human health risks and to evaluate the need for new or different fish consumption advisories in both Florida Bay and other areas of south Florida expected to receive altered freshwater flows. A better understanding of food web linkages, fish movements between ecotones, and the biogeochemistry and bioavailability of mercury and methylmercury will be needed before the impact of altered freshwater flows during Everglades restoration can be predicted.
