Mercury, chemical symbol Hg, atomic number 80, is one of the more dangerous contaminants found in a large percentage of Florida’s coastal, estuarine, and fresh waters, including those of the Everglades. Mercury is a heavy-metal neurotoxin that occurs naturally in the environment. However, according to NOAA, solid waste incineration and fossil fuel combustion facilities account for approximately 87 percent of airborne emissions of mercury in the U.S.

Most of the mercury from industrial sources is released to the atmosphere as elemental (also called inorganic) mercury vapor. This inorganic mercury can circulate in the atmosphere for up to a year and, therefore, can be widely dispersed thousands of miles from likely sources of emission [USEPA]. Numerous sources, including the 2009 South Florida Environmental Report, confirm that mercury deposition from the atmosphere is the main source of mercury in the Everglades. In aquatic systems, inorganic mercury is converted (methylated) to an organic form called methylmercury (MeHg) by sulfate-reducing bacteria in the soils.

The 2009 South Florida Environmental Report, created by the South Florida Water Management District (SFWMD), stresses that methylmercury is the problem:

A key to understanding the Everglades mercury problem is recognizing that it is primarily a methylmercury (MeHg) problem, not an inorganic or elemental mercury problem. MeHg is more toxic and bioaccumulative than the inorganic or elemental form.

That being said, it should be noted that inorganic mercury depositing from the atmosphere to aquatic systems is still the underlying cause, because without the mercury present in the system, there could be no methlymercury. For reasons unknown to scientists, South Florida continues to receive elevated mercury deposition relative to the rest of the continental U.S. Experts believe that the abundance of rain is one reason for greater mercury deposition, but studies attempting to explain the phenomenon continue. Another problem with mercury is that it remains for long periods in the environment and cycles from soil, air, and water. Soil disturbances can launch mercury into the water or atmosphere again, and again. Although mercury emission sources are now more tightly controlled, the mercury released since the beginning of the Industrial Revolution will be with us for some time.

The Mercury Methylation Process

Another reason for elevated methylmercury problems in South Florida is that the methylation process appears to be extremely efficient due to the availability of sulfate and the large pool of dissolved organic matter, in addition to the elevated atmospheric deposition of mercury [2009 SFER, App 3B-1]. In fact, the SFWMD is looking at reducing sulfate concentrations as a method of reducing methylmercury formation.

Bacteria in the soils of aquatic systems take up sulfate and inorganic mercury and, through metabolic processes, convert the mercury into organic methylmercury. This conversion to methylmercury is important because methylmercury is much more toxic than inorganic mercury and organisms require considerably longer to eliminate methylmercury from their bodies 2009 SFER, App 3B-1].

Sulfate-reducing bacteria are the predominant producers of methylmercury in aquatic ecosystems, and the methylation process is dependent on the availability of sulfate. The USEPA has not issued water quality guidance criteria for sulfate in surface waters. However, the Comprehensive Everglades Restoration Plan (CERP) has set a goal of reducing sulfate concentrations to 1 milligram per liter (mg/L). Studies indicate that approximately 60 percent of Everglades marsh area has sulfate concentrations exceeding the 1 mg/L goal. Studies also indicate that, at sulfate concentrations less than 1 mg/L, the methylation process would be slowed because sulfate becomes a limiting factor. The SFWMD is viewing sulfate reduction as a potential option for reducing methylmercury bioaccumulation. Major sources of sulfate include the Everglades Agricultural Area canals and other non-point sources from fertilizer use [2009 SFER, Ch-3B].

Effects of Mercury in Animals

Mercury and methylmercury concentrate in the muscle tissue. Unlike some inorganic contaminants, such as PCBs and dioxins, which concentrate in the skin and fat, mercury cannot be filleted or cooked out of fish or other animals. This has far-reaching implications regarding the health of both humans and wildlife.

A look at human health advisories is helpful in providing a sense of scale to the problem of mercury contamination. Health advisories issued by each state alert residents to potential health risks of eating contaminated fish caught locally in lakes, rivers, and coastal waters. The USEPA maintains a database from which a National Listing of Fish Advisories (NLFA) is published. The latest NLFA was for 2004 and was released in 2005 [NLFA].

The 2004 advisories, the vast majority for mercury contamination, covered the following areas within the 48 contiguous states:

  • 35% of the nation’s total lake area (over 14 million acres), excluding the Great Lakes,
  • 100% of the Great Lakes and their connecting waters,
  • 24% of the nation’s total river miles (over 800,000 miles),
  • 92% of the nation’s Atlantic coastal waters, and
  • 100% of Gulf coastal waters.

Mercury advisories in South Florida were estimated by the U.S. Geological Survey (USGS) to cover 2 million acres in 1996 [USGS – SFIS 1996]. In Florida, the Department of Health (FDOH) issues fish consumption advisories. The latest version of Your Guide to Eating Fish Caught in Florida (May 2008, including the July 2008 update) suggests fish consumption limits for 351 freshwater bodies in Florida. If the lake or stream is not listed, the Guide suggests that meals (= 6 oz, cooked) be limited to 1 per week for largemouth bass, bowfin, and gar for women of childbearing age and small children.

Additionally, all Florida coastal waters are under an advisory for mercury contamination and the Guide lists 63 marine fish with consumption limits. Seven of these fish have more restrictive advisories for fish caught in Florida Bay, Biscayne Bay, and the Florida Keys. According to the 2009 South Florida Environmental Report, Floridians are predominantly exposed to methylmercury from eating marine fish.

Methylmercury exposure can result in adverse effects in the brain and nervous systems of humans and animals. According to a study by a San Francisco doctor, even low-level mercury poisoning has been found to cause memory loss, hair loss, fatigue, depression, difficulty concentrating, tremors, and headaches [].

Methylmercury and Wildlife

According to a NOAA report [NOAA NCCOS 2003], mercury contamination in Florida gained public attention with “the discovery beginning in 1989 of three dead Florida panthers with high mercury body burdens.”

In 1999, a workshop organized by Florida’s Department of Environmental Protection (FDEP), SFWMD, and the University of Florida was held to discuss fish-eating birds and mercury contamination [a PDF version of the report is available on this ftp site; requires ftp-capable browser and PDF software]. One consensus reached during the workshop was that mercury levels were clearly associated with toxic effects on individual piscivorous birds, but it was not clear if mercury levels were having a toxic effect on entire populations. It was generally agreed that limits established as human health criteria were not adequate to protect non-human animals.

It was expected that fish-eating wading birds, because they were relatively long-lived, would show signs of toxicity relatively early. Concerns were also expressed about wading birds because any loss of motor skill due to mercury toxicity could diminish food intake, especially important at times when other stressors, such as low densities of fish, were also occurring.

The primary evidence of mercury impacts in South Florida was a relationship between great white heron disease and elevated mercury in the liver. Other plausible effects of mercury exposure include altered adult behavior, decreased immune function, embryo mortality, juvenile survival/behavior, and delayed recruitment into the breeding population.

Further concern was expressed about chronic exposure to methylmercury at levels too low to cause overt signs of toxicity. These low-level exposures could render an animal susceptible to infection that might otherwise be resisted. One of the participants noted that these sublethal effects could be more serious than outright mortality events because of the difficulty in linking the effect to the cause.

One study mentioned in the workshop found that larger birds have higher levels of mercury concentration: great blue herons and great egrets were found to have higher levels than those found in little blue heron, snowy egret, tricolored heron, roseate spoonbill, and white ibis in South Florida. This was attributed to the fact that these larger birds could consume.larger fish, which have a correspondingly higher methylmercury concentration in their tissues. This study, it was noted, was consistent with numerous other studies conducted in mercury-impacted aquatic systems throughout North America.

Other than fish-eating birds, methylmercury can accumulate to high levels in Florida panthers, raccoons, otters, alligators, turtles, Everglades mink, and pig frogs. And finally, those fish we have generally viewed dispassionately as mere methylmercury vectors also suffer from the toxin. Methylmercury in fish has been shown to reduce growth, prevent gonad development, and to cause testicular atrophy.

Although mercury contamination is believed to be less than it once was, the 2009 South Florida Environmental Report concludes that piscivorous avian and mammalian wildlife may be exposed to unhealthy methylmercury concentrations in broad areas within Everglades National Park and the three Water Conservation Areas.

Mercury will continue to be an issue for Everglades conservation and an important step is expected to occur in 2012. FDEP is required to develop Total Maximum Daily Load (TMDL) limits for mercury-impaired waters in the state and submit them to the USEPA for approval.

Useful web sites

Does Everglades Restoration Mean More Florida Bay Mercury? Accessed 14 May 2009.

Mercury Study Report to Congress: Overview. Accessed 18 May 2009.

2009 South Florida Environmental Report (SFER) (links page),14424181,2714_14424212&_dad=portal&_schema=PORTAL

EPA National Fish Advisories:
The National Listing of Fish Advisories (NLFA). Accessed 21 July 2009.

USGS site for
Mercury Studies in the Florida Everglades. 1996? Accessed 14 May 2009.

Florida DOH fish consumption advisory
Your Guide to Eating Fish Caught in Florida. May 2008.
Fish In Your Diet – Not Health Food Anymore. Accessed 8 July 2009.

Mercury assessment
Effects Assessment for Fish-eating Birds: Setting an Avian Wildlife Criterion. Peer Reviewers’ Reports of Workshop Held October 6-7, 1999, at River Ranch, Florida. Accessed May 2009.