Cooperator: Critical Ecosystems Program, U.S. Geological Survey, Reston, Virginia
Project Chief: David P. Krabbenhoft
Location: Florida Everglades
Project Number: OH712
Period of Project: January 1995–Continuing
Problem
Mercury contamination is one of the largest potential health risks to aquatic organisms, predatory animals, and humans. This great concern is the result of two observations: (1) mercury biomagnifies in the food chain to toxic concentrations even though it is found at very low aqueous concentrations, and (2) the principal source to most areas is atmospheric deposition. Thus, almost any aquatic ecosystem with a food chain is potentially susceptible to mercury contamination.
Objective
The overall objective of this project is to provide a better understanding of the mercury contamination problem in the Florida Everglades and other aquatic ecosystems. Specific processes and applied, in-field experiments are used to determine the most important processes regulating the transport, fate, and toxicity of mercury in the Everglades, including methylation, volatilization, biological uptake, and interactions with dissolved organic carbon.
Approach
Mercury contamination of the Florida Everglades has been recognized as a serious problem for over a decade, yet solutions to the problem have remained elusive. Research conducted by the Wisconsin District Mercury Research Laboratory from 1995 to 2000 on the factors leading to high levels of mercury bioaccumulation revealed that in addition to mercury inputs, sulfate, phosphate, and organic carbon loading from upstream point sources, as well as water-level management, all likely contribute to the problem. To aid in the decision-making process for the Consolidated Everglades Restoration Program (CERP), which is an interagency program charged with developing the restoration plan for the Everglades, we have taken a new direction in our research to directly evaluate the relative importance of each of the co-factors. We have installed 75 wetland enclosures at five sites in the Everglades, and within them we are performing chemical addition studies using stable isotopes of mercury, sulfate, phosphate, and organic carbon to quantify the dose response of each to the formation of methylmercury and bioaccumulation in the food web. Following each amendment, sediment, surface water, pore water, plant, and fish samples are collected at several time intervals for periods up to six months. |
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With this type of information, we will be able to provide a much clearer quantitative context of the relative importance of each of the contributing factors to the mercury problem, and agencies responsible for direction.
Progress (July 2003–June 2004)
During this time period, we executed our last planned mesocosm dosing experiment. Experimental variables included dosing at different rates with mercury (202Hg and 200Hg), sulfate and dissolved organic carbon (hydrophobic acid extracted from water taken from the northern Everglades). In addition, in November 2003, we initiated a sulfur toxicity experiment to determine whether sulfur loading derived from agricultural fields may be at least partially responsible for the die-off of sawgrass and replacement by cattails (sulfur tolerant) and net habitat degradation. |
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Results to date show substantial methylmercury production response to mercury, sulfate, and dissolved organic carbon additions, the last being somewhat unexpected. Given that there are external sources of all three of these controlling factors on methylmercury production, and all of which are potentially influenced by existing Everglades Restoration plans, a robust plan is necessary. Presently, sulfur abatement is not part of Everglades Restoration planning, and if our toxicity experiments show a definitive and substantial response, new considerations may be necessary for Restoration Planners.
Plans (July 2004–June 2005)
Plans for this time period call for planning and beginning a new phase of mercury cycling research in south Florida that would include evaluations of methylmercury production and biological uptake in near coastal zones and embayments (for example, Florida Bay). Presently, most human exposure to methylmercury is through the consumption of marine fish, yet very little is known about how marine fish derive their methylmercury. This effort would begin to evaluate where this important exposure pathway originates.
Reports
The Aquatic Cycling of Mercury in the Everglades (ACME) project: A Synthesis of Scientific Findings (complete first draft by September 30, 2004). |
