MAJOR AND TRACE ELEMENT INVESTIGATIONS AND MERCURY CONCERNS

In addition to total Hg, this study focuses on the biogeochemistry of selected major nutrient (essential) elements (Ca, Fe, K, Mg, and P) as well as environmentally important trace elements (Cr, Cu, Ni, Pb, and Zn). Data for other elements of possible environmental importance are given but not discussed. We present chemical analysis data for organic-rich sediments from selected cores, selected plant materials, and surface waters.

Everglades Peat Lands

The Everglades peat lands have been described as ombrotrophic and are thus nutrient poor (Koch and Reddy, 1992). Historically, the peat lands developed from organic matter accumulation in standing water derived predominantly from atmospheric precipitation. The waters are near neutral in pH because of extensive calcareous bedrock (Gleason and Stone, 1994; Orem and others, 1997). The peat that forms in this system is ideal for our type of study (the monitoring of historical changes in metal cycling) because it serves as a receptor surface that is discrete from ground water influences (Jones and Hao, 1993). Recent new research by the ACME project does show, however, that ground water influence on nutrient and perhaps metal cycling can occur and should not be discounted (Hurley and others, 1998; Orem and others, 1997).

Figure 2. Sampling sites for organic-rich sediment cores, surface waters, and sawgrass vegetation in the Taylor Slough study area of Everglades National Park. Boundary markers of the Slough and depiction of the canal system east of the Slough are diagrammatic only (see discussion). Click on image to open larger version.

Taylor Slough Taylor Slough is a critical part of the overall south Florida Everglades system because (1) it is a terminus for surface water sheet flow from the WCA and ultimately EAA, and is thus the "end of the pipe" for solutes being transported into Everglades National Park (ENP), (2) it discharges over a broad "delta" region into eastern Florida Bay thus directly affecting water quality of the Bay, and (3) it is a major breeding area for both terrestrial and aquatic biota within ENP. In addition, it may be a sink for elements being transported by the sheet flow. Figure 2 shows Taylor Slough and the location of the 1996 study sites. Figure 3 gives the monthly precipitation for three areas that surround Taylor Slough and thus have similar water regimes. The data are grouped by month over a seven-year period from 1990-97. Our sampling occurred in May of 1996 during and right after an unusually wet spring. Loxahatchee National Wildlife Refuge is adjacent to EAA in the north, Royal Palm Ranger Station is in the Slough on its northwestern edge, and Flamingo Ranger Station is immediately west of the southern Slough terminus (fig. 1). Modern Everglades rehabilitation efforts seek to reestablish more of the surface water sheet flow that was characteristic of the area in the past. Concern exists, however, that such an effort will exacerbate current undesirable north/south trends (i.e., nutrient flow, cattail expansion, and mercury bioaccumulation). Our work in Taylor Slough extends the examination of these north/south trends south of WCA (fig. 1).

Mercury

A brief discussion of the Hg concerns in the Everglades is presented. In general, Hg in the environment comes from a number of geogenic and anthropogenic sources. In south Florida these include peat deposits (organic matter oxidation), atmospheric deposition (fossil fuel-fired electrical generating facilities, garbage incinerators, medical laboratories, paint, pulp and paper production), and agricultural operations (herbicide and pesticide application) (Stober and others, 1995). Very recent studies suggest that the atmospheric deposition of Hg in south Florida is generally driven by large-scale regional or hemispheric processes as opposed to local emission/deposition processes (Pollman and others, 1995). These authors also show that deposition is seasonally variable with fluxes 4- to 6-fold higher during April-September (wet season) compared with October-March (dry season). Mason and others (1994) found that deposition on land was the dominant sink for atmospheric Hg and that the atmospheric and ocean surface components are in rapid equilibrium; i.e., the evasion of Hg^o from oceans is balanced by the total oceanic deposition of Hg(II) from the atmosphere. Rood and others (1995), however, note from the literature that gaseous elemental Hg can have an atmospheric residence time of 0.7 to 2.0 years.

As is characteristic of the biogeochemical cycling of Hg in warm-water wetlands, the Hg(II) form is typically methylated in eutrophic, sulfate-reducing sediments and is bioaccumulated in secondary consumers, particularly fish (Zillioux and others, 1993; Zhang and Planas, 1994). Recent work has shown a gradient in methylmercury (MeHg) production in sediment and bioaccumulation in biota in the Water Conservation Areas north of Taylor Slough (fig. 1; Gilmour and others, 1998; Cleckner and others, 1998; Hurley and others, 1998). In general, lower MeHg concentrations occurred in the more eutrophic north and higher concentrations in the more pristine south (south of WCA). This is not to imply, however, that eutrophic conditions "retard" Hg methylation. Typically, Hg appears to be associated with the organic fraction of both soils and sediments (suspended and bottom) (Gill and Bruland, 1990). Humic substances in organic sediments can serve as strong reducing and complexing agents and can influence the processes that control mobilization of many toxic metals including Hg. 

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