Ecologic and precursor success criteria for south Florida ecosystem restoration

A Science Sub-Group Report to the Working Group of the South Florida Ecosystem Restoration Task Force

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Principal author: Dan Scheidt, USEPA Region 4. Reviewers / participants/ information provided by: Dr. Jerry Stober; USEPA Region 4 Mercury Project Leader; Dr. Tom Atkeson, FDEP Mercury Coordinator; Larry Fink, SFWMD Mercury Coordinator; Ted Lange, FGFWFC; Dr. Peter Frederick, University of Florida; Dr. Marilyn Spalding, University of Florida; Tom Logan, FGFWFC; Michael Troyer, USEPA ORD; Duncan Powell, USEPA REGION 4.


Mercury is a naturally occurring element with no known biological function. Some forms of mercury, such as methylmercury, are highly toxic. Mercury can be bioconcentrated in organisms and highly biomagnified through aquatic food webs. Since the initial detection of elevated levels of mercury in freshwater fish from the Everglades in 1989 (Ware et al. 1990), it has become increasingly apparent that the south Florida ecosystem has an extensive mercury problem. In response to data which increasingly demonstrate significant mercury concentrations in gamefish, since 1989 the State of Florida has issued human health advisories that ban or restrict consumption of largemouth bass and other gamefish from over two million acres encompassing the Everglades and the Big Cypress Swamp. In 1995 a limited consumption advisory was initiated for jack crevalle, spotted seatrout and gafftopsail catfish taken from Florida Bay. Although mercury has been detected at levels of concern in largemouth bass throughout Florida (Lange et al. 1993), the maximum concentrations found in Everglades WCA3A largemouth bass (4.4 mg/kg wet weight, fillet) and bowfin (over 7 mg/kg) are the highest gamefish mercury levels reported thus far from Florida waters. Mercury accumulation through the food web may reduce the breeding success of Everglades wading birds (Frederick and Spalding, 1994), and the viability of the endangered Florida panther (Roelke et al. 1991). The maximum mercury concentrations reported to date for Everglades water, soil, sediment and biological tissues are reported in Table 1. Biomagnification with higher trophic levels is apparent.

The issue of mercury contamination of Everglades biota is extremely complex. Preliminary studies of mercury in wet deposition suggest increased concentrations in south Florida over those found in other rural parts of North America (Landing, personal communication, 1995). Present information indicates that the region of highest mercury concentration in gamefish, mosquitofish and certain wading birds is remote and, in general, extends from Loxahatchee National Wildlife Refuge (WCA-1) south and west through Water Conservation Areas 2 and 3 and into the Shark Slough region of Everglades National Park. The zone within WCA3A near the L-67A canal and the Miami Canal has the highest reported concentrations for several species including great white herons, mosquitofish, largemouth bass and bowfin (Table 1). No evidence has been found of elevated mercury concentrations in human hair or blood, although two independent studies have focused on this issue in human populations potentially at increased risk, such as sport fisherman, Everglades residents such as the Miccosukee Tribe of Indians of Florida, and subsistence fisherman (Centers for Disease Control 1993; Fleming et al., 1995). Although it has been suggested that the health or viability of certain Everglades species at upper levels of the aquatic-based food web, such as the endangered Florida panther and the great white heron, may be adversely affected by mercury contamination, definitive information is presently not available. Further, the data required for a mercury mass balance model for the Everglades region presently do not exist. The relative contribution of anthropogenic sources of mercury to the south Florida region has not been quantified relative to the cycling of the historic natural mercury pool internal to the system. Finally, the specific processes or aquatic environments in south Florida that are creating conditions conducive for mercury bioaccumulation or bioconversion to methylmercury have not been identified (Stober, et al., 1995).

It is imperative that collective scientific efforts directed at understanding mercury in the south Florida ecosystem be focused on answering critical policy questions. In order to guide the development of this integrated scientific approach for understanding mercury contamination in the south Florida ecosystem, USEPA Region 4 developed seven policy-relevant questions to be addressed by an interagency research effort (USEPA, 1993):

  1. Magnitude. What is the magnitude of the problem? What are the current levels of mercury contamination in various species? What ecological resources of interest are being adversely affected by mercury?

  2. Extent. What is the extent of the mercury problem? What is the geographic distribution of the problem? Is it habitat specific?

  3. Temporal trend. Is the problem getting better, worse or staying the same? Causative factors. What factors are associated with, or contributing to, methylmercury accumulation in sensitive resources?

  4. Sources. What are the relative contributions and importance of mercury from different sources (i.e., fossil fuel power plants, waste incinerators, cement plants, agricultural management practices, geological pools, natural peat deposits, global atmospheric background, etc.)?

  5. Risks. What are the relative risks to different ecological systems and species from mercury contamination?

  6. Solutions. What management alternatives are available to ameliorate or eliminate the mercury contamination problem?


The stated precursor success index for mercury is "reduction in body burdens of mercury in top carnivores". This index is clearly directed at the third policy-relevant question above, temporal trend, but it is also relevant at a minimum to magnitude, extent, and risks. Several top carnivores are proposed - largemouth bass for human health and the Florida panther, wading birds and the American alligator for ecological interest.

1. Human Health Indicator Species - Largemouth Bass.

A. Statement of the success criteria

Age-standardized (3-year old) fillet mercury concentration (wet weight) in largemouth bass collected from the vicinity of L-67A and the Miami Canal. At least 20 fish must be sampled each year, and fish must be aged, sexed and weighed. The obvious Everglades top carnivores that are a human food source are gamefish. In recent years the FGFWFC has been sampling several fish species throughout Florida including the Everglades region and analyzing fillets for mercury content. The most data are available for largemouth bass. Bass is a popular gamefish species that is widespread and is recognizable by the public. As previously noted, a consumption advisory has been posted for the Everglades and Big Cypress region, and bass fishing in the Everglades is an activity of economic interest. Since fish age and size are known to be related to fish contaminant body burden, mercury concentration data for individual fish must be standardized to reflect a standard age fish before meaningful statements can be made with regard to whether the mercury concentration in a given fish species is increasing, stable or decreasing over time. It is critical that an adequate sample size of largemouth bass be consistently collected at the same location over time.

B. Reference Baseline.

The FGFWFC has been sampling largemouth bass at scores of canal locations in the Everglades region since 1989. They have reported adjusted mean annual bass mercury concentrations for one location, the L67A canal (within the zone of highest mercury concentrations) from 1989 through 1995 (Table 2). The highest adjusted mean occurred in 1992 (2.24 mg/kg wet weight) and the lowest was in 1995 (0.83 mg/kg). The adjusted mean for 1995 was significantly different (p < 0.05) than all other dates except 1989, and 1992 was significantly different (higher) than 1993 and 1994. It is difficult to determine if this represents a downward trend or is simply a low point in the natural variation of mercury concentration in largemouth bass (Ted Lange, personal communication, 1996). It has been suggested that bass mercury concentrations may be influenced by hydrologic conditions, with the low concentrations observed in 1995 due to or coincident with high water conditions. This hypothesis has yet to be substantiated. The proposed baseline for the L67A sampling location is 1989.

C. Historic condition.

Unknown. Prior to 1989 when the high mercury concentrations in Everglades largemouth bass were inadvertently discovered by Florida agencies, there are no data on Everglades gamefish fillet mercury content.

D. Expectation from restoration.

Reduction in the mercury content of Everglades gamefish fillets below 0.5 mg/kg, the concentration that when exceeded results in Florida posting a consumption advisory for protection of human health.

E. Scores and scoring methodology.

Age-standardized (3-year old) fillet mercury concentration (wet weight) in largemouth bass will be collected from individual sampling locations on an annual basis. At least 20 fish must be sampled per year at each sampling location in order to have a sample that is representative of the species population. A comparable number of fish of comparable age must be sampled from each location each year in order to make accurate statements about the status of population mercury content over time. Fish must be aged, sexed and weighed. An age-standardized mercury concentration will be calculated for each sampling location each year. Comparisons can then be made each year to see if the mercury concentration is decreasing, stable or increasing. Several years of data are required before definitive statements can be made about trend over time. If a trend is observed, it is not possible from these data alone to make statements as to the cause for the observed trend (ie., is it the result of varying hydrological conditions, influence of local mercury atmospheric emissions, influence of changes in global mercury emissions, changes in local Everglades marsh conditions that influence internal mercury cycling and bioaccumulation processes, etc.). The highest concentrations observed in the Everglades are in the region of the L-67A canal and the Miami Canal. This zone is the most critical area for this sampling to be conducted.

2.The Environment. Indicator Species: Florida Panther.

A. Statement of the success criteria.

There are numerous Everglades top carnivores that could be used for a mercury body burden index (see Table 1). Attached are two aquatic food web diagrams from Gunderson and Loftus (1993) depicting known trophic relationships among characteristic Everglades animals with a macrophyte base (top) and detritus/periphyton base (bottom). What is proposed below is the bare minimum number of top carnivore indicator species. Mercury body burden information for several additional species is required for development of a critical path analysis and elucidation of how mercury moves through south Florida food webs and aquatic systems.

The first of the proposed indicators for the environment is the Florida panther. Blood and hair mercury concentration in samples taken every two years from radio-collared Florida panthers, and taken from livers of all panthers post-mortem. The Florida panther is an endangered species of high public visibility. The Everglades and Big Cypress region is its last long standing refuge within the eastern part of the United States, although efforts of limited effectiveness are underway to reestablish it in other parts of Florida and Georgia. It has also been designated in Florida as the state animal.

B. Reference Baseline.

The proposed baseline is the existing data for panther liver mercury concentrations. In 1989 a 3- to 4-year old radio collared female Florida panther was found dead within the Northeast Shark Slough portion of Everglades National Park. The carcass was retrieved within 24-36 hours postmortem. The relatively good condition of the carcass and lack of significant pathological findings coupled with concern over the loss of a prime breeding age female prompted the analysis of selected tissues for metals and organic compounds. The liver contained 110 mg/kg of mercury (wet weight) along with several other contaminants. Death due to mercury toxicosis was implicated as a possibility, and between 1989 and 1991 mercury toxicosis was implicated in the deaths of two other panthers (Roelke, et al., 1991). Unfortunately, the critical liver mercury concentration above which undesirable effects occur in panthers remains unknown. Mercury toxicosis has not been implicated as a cause of death in panthers since 1991. This may be in part due to the success of the program to assure that there are adequate deer as a panther food source, as opposed to raccoons.

Panthers with an aquatic-based food source such as raccoons are of particular interest as they are potentially exposed to higher mercury intake via food. Panthers with a terrestrial-based food source such as white-tailed deer and wild hogs are of presumably at less risk. FGFWFC is presently taking blood samples from panthers as they are routinely captured every two years. No data summaries have been performed since 1991, and it is presently not possible to make statements as to whether mercury concentrations in Florida panther are reducing, increasing or stable (Tom Logan, personal communication, 1996).

C. Historic condition.

Unknown. Prior to 1989 there are no data on mercury body burdens (liver, blood or hair) in the Florida panther. It is not possible to determine mercury content of museum specimens because of the common historic use of mercury containing preservatives, and the uncertainty that would therefore be associated with any mercury data obtained from museum specimens.

D. Expectation from restoration.

The critical concentration of mercury in panther livers that represents the breakpoint between a safe and unsafe concentration is unknown. The mercury dose liver concentration response relationship for the panther is unknown. However, the 110 mg/kg concentration observed in the 1989 incident is unquestionably high. Restoration should result in lower concentrations, but a safe level has yet to be defined.

E. Scores and scoring methodology.

Simply compare more recent body burdens as determined by post-mortem liver mercury content and blood and hair mercury content of live animals and determine if concentrations are increasing, stable, or decreasing.

3. The Environment. Indicator Species. American Alligator.

A. Statement of the success criteria.

The American Alligator is a logical top predator for monitoring mercury body burden over the long-term. The alligator's primary food items are aquatic organisms, they are no longer federally protected, and they are much more common throughout the ecosystem than the Florida panther. As compared to the panther, mercury levels in the alligator can be measured much more frequently with less effort and expense. Tail meat from alligators between three and five years old should be sampled within WCA3A. This age range should integrate mercury in the aquatic system. Tail meat is of interest not only from an ecological consideration as a top predator, but also from a potential human health consideration since alligator meat is sold and served in restaurants for human consumption.

B. Reference baseline.

The FGFWFC has some data for tail meat mercury in Everglades alligators from the Miami Canal dating from 1989. These data are the suggested reference baseline. The concentrations as of 1989 were in the 0.7 mg/kg to 3.6 mg/kg range.

C. Historic condition.

Unknown. Prior to 1989 there are only sporadic data on mercury body burdens of Everglades alligators. It is not possible to determine mercury content of museum specimens because of the common historic use of mercury containing preservatives, and the uncertainty that would therefore be associated with any mercury data obtained from museum specimens.

D. Expectation from restoration.

Restoration should result in lower body burdens, but a safe level has yet to be defined.

E. Scores and scoring methodology.

Compare more recent body burdens as determined by tail meat and liver mercury content and determine if concentrations are increasing, stable, or decreasing as compared to the baseline.

4. The Environment. Indicator Species - Wading Birds.

A. Statement of the success criteria

The proposed indicator of mercury body burdens for wading birds is feather mercury concentrations in great blue herons, great egrets and wood storks from the Everglades. Spalding, et al. (1994) investigated mercury as a potential cause of death in Everglades great white herons from 1987-1989. They radio-tagged juvenile birds, recovered them soon after death, and determined liver mercury content and cause of death. They found that birds that died of acute causes had less liver mercury than birds that died from chronic, often multiple diseases, and birds that migrated to mainland Florida accumulated more mercury than birds remaining in Florida Bay. They concluded "it is likely that detrimental effects of mercury toxicosis on reproduction are contributing to the decline of breeding wading bird populations in southern Florida". A companion study by Sundlof, et al. (1994) looked at liver mercury in other Everglades wading birds from 1987-1991. They found that birds collected from the central Everglades to eastern Florida Bay had significantly higher mercury levels than birds from other areas, bird species whose prey base consisted of larger fish (great blue heron, great egret) had about four times the liver mercury concentration than species which consume smaller fish or crustaceans (little blue heron, snowy egret, white ibis, roseate spoonbill, tricolored heron), between 30% and 80% of potential breeding age birds collected from the central Everglades area contained liver mercury concentrations associated with reproductive impairment in pheasants and ducks, and they concluded that "these data suggest that declining numbers of nesting ciconiiform birds in Florida may be due, in part, to mercury contamination of their food supply". They noted several caveats, including that the mercury dose - wading bird response relationship is unknown for the species of wading birds in southern Florida.

In subsequent work Sepulveda, Spalding, Frederick and others have been attempting to better understand the mercury dose - wading bird response relationship in wading birds. While their collective results through 1995 are informative, what a specific mercury level in a wading bird means has yet to be understood or defined. They have determined that feather mercury content can be used as an accurate predictor of liver mercury content, and blood mercury is less informative (Peter Frederick and Marilyn Spalding, personal communication, 1996). Collection of feathers from nestlings is a feasible monitoring approach.

The Wood Stork is another bird species of special interest. It is a top carnivore, it has high visibility and it is an endangered species. Although it is a large bird, similar in size to the great blue heron and great egret, the wood stork may consume smaller size fish more equivalent to the food sizes consumed by smaller wading birds such as the tricolored heron, little blue heron and snowy egret. Since the wood stork is an endangered species individuals cannot be sacrificed to determine liver mercury content, although samples could be taken from the blood or feathers of captured birds. Individuals that are found dead should be preserved and tissues such as the liver should be analyzed for mercury content. To date this type of work has not been done for wood storks. The obvious agencies to perform or be responsible for funding this work would be the USFWS and FGFWFC. An adequate number of birds should be sampled in order to make meaningful statements about the population over time. USFWS should collect wood stork feathers to compare mercury concentrations to those observed in other small and large wading birds.

B. Reference baseline.

The suggested reference baseline is the 1987-1989 liver and feather data discussed above for the great blue heron and the great egret. There are no reference data for the wood stork.

C. Historic condition.

Unknown. Prior to 1989 there are only sporadic data on mercury body burdens (liver, blood or feather) of Everglades wading birds. It is not possible to determine mercury content of museum specimens because of the common historic use of mercury containing preservatives, and the uncertainty that would therefore be associated with any mercury data obtained from museum specimens.

D. Expectation from restoration.

The critical concentration of mercury in wading bird tissues that represent the breakpoint between safe and unsafe concentrations are unknown. There is mercury dose- wading bird response work being conducted by the University of Florida that is directed at this issue. As noted above, previous published work by Spalding, Sundlof and others suggests that south Florida breeding wading birds may be adversely affected by mercury toxicosis. Restoration should result in lower body burdens, but a safe level has yet to be defined.

E. Scores and scoring methodology.

Compare more recent body burdens as determined by post mortem liver mercury content and feather content of live animals and determine if concentrations are increasing, stable, or decreasing.


Among the supporting information necessary for implementing and interpreting the proposed mercury success indices are the following:

A. Largemouth bass summary report.

The FGFWFC should produce a report summarizing existing age-standardized Everglades largemouth bass data. This report should include all raw data and all standardizing method calculations. The report should include a determination of whether statistically sound statements can be made as to whether mercury concentrations in the population are in fact increasing, decreasing or stable at selected Everglades sampling locations including but not limited to the L-67A location. Perform appropriate statistical tests to determine whether there is a relationship between hydrologic conditions (such marsh stage, canal stage, basin precipitation) and fillet mercury content at these various locations.

B. Florida panther mercury summary report.

FGFWFC should prepare a report summarizing all mercury data collected from Florida panthers since 1989. The most recent report available dates from 1991. None of the panther mercury data collected since that time have been summarized. The report should state whether any statements can be made with regard to changes in population mercury body burden over time.

C. Panther mercury dose-response research.

The mercury concentrations in panther tissues such as liver, blood or hair above which there is some adverse effect on health, viability or reproduction are not known. Since the panther is an endangered species and animals cannot be sacrificed, information that relates specific concentrations to health effects can only be inferred. Cause and effect cannot be established. Available panther mercury data for blood, hair and liver should be summarized and compared to existing mercury data for other cat species or similar mammals in order to determine if safe levels can be inferred.

D. Wading bird mercury summary report.

A summary report should be prepared that states what can be said about mercury body burden trends in Everglades wading birds since 1987. No such report exists. Since nearly all of the data have been collected by the University of Florida with FDEP funding, the FDEP and UF are the logical entities to complete this report.

E. Wading bird mercury dose - response relationships.

Mercury tissue concentrations for Everglades wading bird species above which there is some adverse health, viability or reproductive effect are unknown. The commonly cited information for bird liver mercury concentrations causing adverse effects comes from loons. Dose-response information for Everglades wading birds is needed so the observed tissue mercury concentrations can be put in some meaningful perspective. Important work funded by FDEP underway at the University of Florida is directed at this issue.

F. Alligator mercury body burdens report.

FGFWFC should prepare a report summarizing all available data for Everglades alligator mercury body burdens and determine whether any statements can be made about whether concentrations are increasing, stable or decreasing over time.


No other mercury-related success criteria are proposed at this time.


For each of the species identified, a collaborative, multi-agency, long-term monitoring program must be established and maintained to track mercury body burdens in top predators. The primary agencies responsible for this effort are FGFWFC and USFWS. Other agencies that may have a role include NBS, USEPA, SFWMD and FDEP. Present efforts are inadequate. Suggested agencies and their responsibility in the proposed collaborative effort follow:

Largemouth bass: FGFWFC.
Wading birds:
Great Egret: UF.
Great Blue Heron: UF.
Wood stork: USFWS.
Florida Panther: FGFWFC or USFWS.
American alligator: FGFWFC.


All of the proposed top predators have some Everglades baseline data for mercury body burdens. Unfortunately, since no data predate 1989 there are no data on historic conditions. Often the data collected since 1989 have been collected in a sporadic, piecemeal fashion, not as part of an integrated long-term effort to determine whether the body burden of mercury in Everglades top predators is increasing, stable or decreasing. Collecting this information so meaningful statements can be made about the Everglades mercury contamination trends should become a priority of the ongoing collective state-federal-tribal effort concerning mercury science in this ecosystem. In some cases data have been collected since 1989, but there have been no summaries or interpretation. As such, it is not possible to determine if enough data have been collected. In other cases, no data have been collected. Consequently, it presently not possible to state whether mercury contamination of Everglades top predators is getting worse, staying the same, or getting better. The highest priority sampling area is the WCA3A zone of highest south Florida mercury contamination in gamefish such as largemouth bass and wading birds. This is also the general area of northeast Shark Slough where the highest mercury concentrations have been found in Florida panthers.

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Figure 1: South Florida human health freshwater advisories for gamefish consumption (bowfin, largemouth bass, gar) due to mercury contamination
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Thumbnail of Figure 2b
Figure 2: Mercury concentrations in (a) wading birds and (b) largemouth bass fillets
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Figure 3: Aquatic food web diagram showing known trophic relationships among characteristic Everglades animals with (a) macrophyte base and (b) detritus/periphyton base.


Centers for Disease Control. 1993. Mercury Exposure Among Miccosukee Indian Tribe: South Florida, 1993. Memo Report. Centers for Disease Control, Atlanta, GA.

Fleming, L.E., S. Watkins, R. Kaderman, B. Levin, D.R. Ayyar, M. Bizzo, D. Stephens and J.A. Bean. 1995. Mercury Exposure in Humans Through Food Consumption from the Everglades of Florida. Water, Air and Soil Pollution 80:41-48.

Frederick, Peter. 1996. Personal communication.

Frederick, P.C. and M.G. Spalding. 1994. Factors Affecting Reproductive Success of Wading Birds (Ciconiiformes) in the Everglades Ecosystem. pp.659-692. In "Everglades: the Ecosystem and its restoration". S.M. Davis and J.C.Ogden Ed. St. Lucie Press.

Gunderson, L. and W. Loftus. 1993. The Everglades. pp. 199-225 (Chapter 6) in "Biodiversity of the Southeastern United States: Lowland". John Wiley and Sons, Inc.

Landing, William. 1995. Personal communication.

Lange, Ted. 1996. personal communication.

Lange, T.R., H.E. Royals and L.L. Connor 1993. Influence of Water Chemistry on Mercury Concentration in Largemouth Bass from Florida Lakes. Trans. Amer. Fish. Soc 122: 74-84.

Logan, Tom. 1996. Personal communication.

Roelke, M.E., D.P. Schultz, C.F. Facemire, S.F. Sundlof, and H.E. Royals, 1991. Mercury Contamination in Florida Panthers. Prepared by the Technical Subcommittee of the Florida Panther Interagency Committee.

Spalding M.G., R.D. Bjork, G.V.N. Powell, and S.F. Sundloff. 1994. Mercury and Cause of Death in Great White Herons. Wildl. Manage. 58(4) 735-739.

Stober, Q. J., R. D. Jones and D. J. Scheidt. 1995. Ultra trace level mercury in the Everglades Ecosystem, a multi-media canal pilot study. Water, Air and Soil Pollution 80: 991-1001.

Sundlof, S. F., M. G. Spalding, J. D. Wentworth, and C. K. Steible. 1994. Mercury in livers of wading birds (Ciconiiformes) in southern Florida. Archives of Environmental Contamination and Toxicology 27:299-305.

USEPA. 1993. Region IV: Ecological Risk Assessment of Mercury Contamination in the Everglades Ecosystem, pages 25-36 in "R-EMAP: Regional Environmental Monitoring and Assessment Program". USEPA Office of Research and Development EPA/625/R-93/012.

Ware, F.J., H. Royals and T. Lange. 1990. Mercury Contamination in Florida Largemouth Bass. Proc. Ann. Conf. SEAFWA 44: 5-12.

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