The Everglades Handbook

Section 4: Everglades Water Quality Issues

Everglades Water Quality Issues

Water quality includes many aspects such as nutrients used by plants for growth, dissolved substances that affect pH (acid/base scale), hardness, salinity, and toxins that may inhibit or poison living processes. Degraded water quality has created widespread changes in the Everglades. Water quality concerns in the freshwater Everglades have focused on tow main issues: phosphorus enrichment and elevated mercury in wildlife.


Phosphorus is a limiting nutrient in Everglades water quality. Natural levels between 4 and 10 ppb characterized the original system, although it is thought that incoming waters from Lake Okeechobee, prior to agricultural development of its watershed, were naturally higher—at least 20 ppb. The northern Everglades acted as a natural nutrient-removal system as it grew and laid down peat soils. Considerable research has attempted to quantify the level of phosphorus that the natural Everglades could assimilate without causing an imbalance in natural populations of flora or fauna—the legal definition of allowable change. Consensus among most scientists is that the maximum level for sustainability in the Everglades is 10 ppb, which is now the center of a complex standard for waters in the Everglades Protection Area, basically the water conservation areas and Everglades National Park.

With its conversion into the EAA, the northern Everglades became a nutrient source instead of its historic role as a “sink.” Without treatment, EAA stormwater contained high phosphorus levels from the fertilizer applications, often in excess of 500 ppb. As the entire EAA became fully developed in the 1970s, the problem of where to put excess stormwater was increasingly solved by backpumping to Lake Okeechobee. It was recognized, however, that Lake Okeechobee was suffering from eutrophication from intensely used agricultural lands in its watershed. A water management decision in 1979 protected the lake from EAA nutrients, redirecting stormwater to the water conservation areas, including WCA-1, the Loxahatchee National Wildlife Refuge.

Sulfur and Mercury

Everglades surface-water background levels of sulfate are 0.5 ppm or less (about six thousand times less than sea water). It is now clear that enrichment in the element sulfur, caused by agricultural releases of sulfate into the freshwater Everglades, is a major concern because of deleterious effects on the ecosystem.

Two main problems result from sulfate enrichment:

  1. Biomagnification of toxic mercury in Everglades wildlife and consequent increased human exposure;
  2. Formation of hydrogen sulfide in Everglades soils at levels potentially toxic to wetland vegetation and aquatic animals, particularly burrowing species like crayfish.

Sulfate and Sulfide

For decades, and continuing today, elevated sulfate has been passing into the Everglades from the EAA so that most areas are far above the background of 0.5 ppm or less. In descending order, the major sources have been: (1) release of legacy sulfate from the EAA due to soil oxidation/subsidence, (2) Lake Okeechobee waters passing through the EAA via canals, and (3) ongoing agricultural applications in the EAA. Sulfate from these sources has passed from the EAA into the Everglades, and since the late 1990s has moved through the EAA stormwater treatment areas (STAs, see “The Everglades Forever Act and Everglades Construction Project,” this chapter). Sulfate levels in waters released from one STA have been as high as 80 ppm. STAs were designed to treat phosphorus, not sulfate. As a result, they have little effect on sulfate.

Mercury and Methylmercury

Mercury (chemical symbol Hg) is a naturally occurring element, most familiar as a silvery metallic liquid. It has no known biological function but forms chemical compounds of various toxicities. The most dangerous is an organically bound form, methylmercury (CH3Hg+), which is a powerful teratogen, endocrine disruptor, and neurotoxin. While other forms of mercury are also toxic, they do not readily enter living organisms. Methylmercury, however, is readily absorbed and then biomagnified through the food web. It can cause mutations, abnormal growth, and neurologic/behavioral disorders leading to death at higher concentrations. In humans, embryonic development through infancy is most susceptible to mercury toxicity because of interference with tissue differentiation, even at mercury levels that are not dangerous to adults.

Consumption warning for bass, posted in Everglades National Park at the path to Ficus Pond along the main park road between Pa-hay-okee Overlook and Mahogany Hammock. Photo by Tom Lodge, May 1990

Alarmingly high mercury in Everglades wildlife was first reported in 1974 in largemouth bass and other predatory fish. Especially surprising were levels dangerous for human consumption in remote locations inside Everglades National Park’s Shark River Slough.

Current concerns about mercury in the Everglades are:

  • Largemouth bass of edible size in the Everglades continue to have high levels of mercury. The Florida Department of Health has continuing “Do Not Eat” advisories, directed at women of childbearing age and young children, for bass of 14 inches in length or more in most areas of the Everglades, and for all bass in Everglades National Park
  • Wading birds: Mercury is deposited in the growing feathers of birds. Feathers have been used extensively to monitor mercury levels in Everglades wading birds. Research has shown egret nestlings with high levels of mercury began to exhibit severely disoriented behavior including substantially decreased breeding success, and males exhibiting male-male pairing.
  • Pythons have been found to harbor excessive mercury in muscle tissue, to the extent that researchers are puzzled about their survival. Python mercury levels have been found three to four times higher than in alligator meat, previously thought to be the highest in the ecosystem.

Lodge, Thomas E.. The Everglades Handbook. CRC Press. Kindle Edition.