Coping with Heavy Metals and Ecosystem Disservice

The Everglades is a vast freshwater wetland that originally covered an area of more than 10,000 km2 in south Florida, United States. It is part of a 100-km-long basin in which water flows along a gradual gradient of 3 cm/km from shallow Lake Okechobee to the mangroves lining Florida Bay. Exploitation of rich organic soils for agriculture, drainage for urban development, the construction of canals, and the impoundment of surface water for flood control and water storage have led to dramatic changes in flooding and fire regimes and nutrient inputs to the wetland. Because of draining and modifications in hydrologic regime, the area of the Everglades is now, in 2004, 35 percent of its original size.

The Everglades provides numerous ecosystem services for human well-being. Even in its much altered state, the Everglades filter polluted runoff from agricultural fields, yielding fresh, clean water for a variety of uses, including support of the estuarine ecosystems at its terminus. It harbors and produces a great quantity and diversity of wildlife, most notably alligators, crocodiles, the Florida panther, manatees, and a rich variety of aquatic birds. The fresh water it supplies to Florida Bay comes in a quality, quantity, and pattern of delivery that enables coastal ecosystems to provide their own suite of services. Finally, the Everglades provide aesthetic values, including recreation, to an audience that extends well beyond the boundaries of the United States.

Among the many changes to the Everglades that alerted scientists and resource managers to potential "ecosystem disservices," one that was particularly difficult to diagnose was the increase in concentrations of mercury in several species of vertebrates. Mercury contamination has been particularly pronounced for Everglades sport fishes; high levels have been detected in other vertebrates, including alligators, wading birds, and the Florida panther (Fink et al. 1999). Fear arose that agricultural pollutants introduced primarily into the north and eastern ends of the wetland were finding their way into and up the food chain, instigating a closer look at the mercury cycle in the Everglades. In fact, the emerging pattern of cause-and-effect is complex and, in some ways, very difficult to counteract.

Most of the mercury introduced to the Everglades comes from atmospheric sources, not from agriculture (Fink et al. 1999). Although some is from natural sources, such as volcanoes and outgassing from oceans, approximately 95 percent of the atmospheric mercury is released with coal combustion, waste incineration, and industrial processing (Krabbenhoft et al. 2003). Mercury in the atmosphere is primarily elemental mercury, which is relatively inert. Once deposited, it is subject to conversion to the more toxic methylmercury, a process performed primarily in an anoxic environment by sulfur-reducing bacteria, which are responsible for much of the organic carbon decomposition in the Everglades' sediments. An unusual feature in the Everglades' food chains is the dominance of periphyton over phytoplankton as the base of food chains (Browder et al. 1994). Periphyton is an assemblage of algae, bacteria, and associated microfauna that form a mat that overlies the surface sediments and often includes filamentous blue-green algae. Both mercury and methylmercury accumulate in periphyton, but it is still unclear how mercury becomes so concentrated in fishes near the top of the food chain. Complex interactions that change seasonally with fish diets (which include benthic invertebrates) and are affected by wetting cycles, fire, sunlight, total mercury concentrations, sulfate concentration, and levels of anoxia remain to be clarified (Gilmour et al. 1998;

Krabbenhoft et al. 2003). It is clear that in areas of nutrient enrichment, accumulation of biomass (often attributed to increased abundance and rates of growth of Typha lati-filiaand T. domingensis) increases, rates of microbial activity and decomposition increase, and there is an increased tendency for mercury methylation (Gilmour et al. 1998).

Through our use of the atmosphere to perform the service of waste mercury disposal, humans are compromising animal and human food chains in the Everglades. Atmospheric deposition of mercury to the Everglades is approximately double the rate in rural Wisconsin for example, but it is difficult to determine the source of this input. Although it may be possible to manipulate Everglades water levels and mercury release patterns to minimize formation of methylmercury, the parts of this wetland that are most affected are the parts with the most natural fire and water regimes. Maintaining an environment that can continue to produce sustainable populations of sport fish and wildlife may not be compatible with atmospheric release of waste mercury. This example of the Everglades' ecosystem demonstrates the extent to which freshwater systems are often compromised by the use of ecosystem services in other realms.

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