Mercury Cycling in the Florida Everglades Project

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.

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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 (fig. 3), 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.

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Initially, this project is focusing on field sites in the northern Everglades (fig. 4), 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 (as indicated in the satellite image map, fig. 4) to the middle of WCA2 and WCA3 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.

The following are brief descriptions of each of the subprojects that make up the "Mercury Cycling in the Everglades" project.


Geochemical Process Studies of Mercury

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To understand how mercury is transported and transformed in the environment, a basic understanding of the spatial occurrence and predominance of the forms of mercury is necessary. Mercury concentrations in water are so low, however, that samples collected for this part of the project require the use of ultra-clean techniques (fig. 5). Specific study objectives for this aspect of the project include:

1. determination of spatial variability and predominance of the specific forms of mercury in water and suspended particles;
2. investigation of the factors controlling the short-term variations in mercury transformation processes;
3. examination of factors controlling the spatial and temporal variability of mercury methylation; and
4. determination of the important locations and mechanisms of food-chain uptake of mercury.

Methylmercury Degradation Studies
Because methylmercury is the most bioaccumulative form of mercury and thus the most toxic, it is important to understand what controls the detoxification process of demethylation. Scientists currently hold that demethylation can proceed along two pathways: methyl-cleavage and oxidative demethylation. The objective of this study is to understand the environmental factors regulating these two processes.


Dissolved Organic Carbon-Hg Interactions
By effectively binding mercury, DOC provides a mechanism to mobilize mercury and many other trace metals in the environment which would otherwise be virtually immobile. This projects seeks to:

1. identify the origin(s) of the DOC in the Everglades system;
2. define how the quantity and quality (molecular makeup) of DOC changes throughout the system;
3. determine what controls the reactivity (binding) of mercury with DOC, and how it varies across the ecosystem; and
4. understand how land use and hydrologic changes affect DOC.

Sulfur Cycling Studies
Mercury transport, accumulation, and cycling are controlled by several microbially mediated processes, many of which are related to sulfur cycling. Although it is now known that sulfate reducing bacteria are the principal organisms responsible for mercury methylation in the Everglades, the relation between sulfur cycling and mercury methylation is not well understood in general. The objective of this study is to relate sulfur reactions and isotopic composition and their relation to changes in nutrient concentrations, season, rates of sedimentary deposition, and ultimately to mercury cycling.


Mercury Accumulation and Diagenetic Processes in Peat
Like most wetlands, the Everglades has an accumulation of surficial peat. Because mercury has a strong affinity for organic matter, mercury that has accumulated in the peat represents the vast majority of what is found in the entire ecosystem. Peat deposits, however, are also known to be areas of significant physical and chemical change (diagenesis). Many biogeochemical processes that control the mobility of most nutrients and trace metals, including mercury, operate in peat. Therefore, it is important to understanding the processes that result in mercury accumulation and potential remobilization in peat. The objectives of this study are to determine the size of the mercury reservoir within the peat, ascertain how diagenetic processes may be affecting the stability of this reservoir, and document historical changes in mercury accumulation rates in the peat.


Biological Uptake of Mercury and Lower Food Chain Transfer Pathways
The assemblage of microalgae that live on shallow submerged substrates are referred to collectively as periphyton. This periphyton covers most submerged plants and forms a thick mat on the sediment surface in many locations in the Everglades. In this ecosystem, periphyton growth is responsible for the majority of primary production, and thus is an important food source. The linkages between the primary producers (periphyton), primary consumers (invertebrate organisms consuming and living in and around the periphyton) and secondary consumers (predaceous fish) are important to document to fully understand how the bioaccumulation process operates in the Everglades. The objectives of this study are to:

1. determine whether mercury methylation is actively occurring in the periphyton, and if so, is this mercury being transferred to the food chain, and
2. document the important food chain linkages of mercury transfer from the primary producers to predaceous fish.

Ground-Water/Surface-Water Exchange
One potentially important source of mercury to the Everglades, yet currently not quantified, is ground-water discharge. Ground water also contains other important ingredients for the methylation process, such as sulfate and DOC. To date, however, very few studies have examined the nature of ground-water/surface-water exchange in the Everglades. The specific objectives of this study are to:

1. determine water fluxes and hydraulic properties of sites in the ENR and WCA2;
2. relate those fluxes to hydrogeologic properties, climatic variability, and water-level management strategies; and
3. estimate ground-water fluxes of mercury and nutrients to surface water.

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