The use of sulfate in agricultural areas near the Florida Everglades is creating an enormous mercury problem — with seemingly no end in sight.
The Florida Everglades are often thought of as the state’s wildest and most untamed area, well-protected and far from the grind of urban civilization, a lush wetland with a folkloric reputation that separates it from the hustle and bustle of nearby Fort Lauderdale and Miami.
Some of the state’s most ubiquitous creatures call the area home: manatees, alligators, wading birds. But lurking below the surface, amid the highly diverse flora and fauna, is a surprisingly large amount of a notoriously toxic substance: Methylmercury.
According to scientists, the Methylmercury issue rivals other better-known ecological issues in the state, like nutrient overload in Florida waterways. Nutrients like phosphorus and nitrogen have long been thought of as villains in Florida waterways, due to their capacity to breed toxic algal blooms that lead to fish and dolphin kills.
Sulfate is often used as a means to kill those noxious algal blooms. In this process, sulfate (sulfur combined with oxygen) is added directly to water, eventually finding its way into Stormwater Treatment Areas, manmade wetlands that are specifically designed to filter pollution before it enters the Everglades.
The problem, according to scientists, is that the Stormwater Treatment Areas (known as “STAs”) only filter so much. “These STAs are not designed to filter sulfate. They’re designed to filter phosphorus,” says Dr. Melodie Naja, a water quality scientist at the Everglades Foundation, a nonprofit that aims to restore and protect the greater Everglades ecosystem.
Naja says that sulfate in South Florida canals can come from several sources, including groundwater, Lake Okeechobee water, soil oxidation and fertilizer. But the main sources of sulfate, according to a study conducted by Naja and her colleagues, are the latter two: soil oxidation and fertilizers.
In a paper published in 2011, Naja and other scientists found that high levels of Methylmercury (MeHg) are a serious problem in many wetland ecosystems worldwide:
In the Florida Everglades, it has been demonstrated that increasing MeHg occurrence is driven by the sulfate contamination problem. A promising strategy of lowering the MeHg occurrence is to reduce the amount of sulfate entering the ecosystem. High surface water sulfate concentrations in the Everglades are mainly due to discharges from the Everglades Agricultural Area (EAA) canals. [Emphasis added.]
In addition to using sulfate as a fungicide, farmers in the Everglades Agricultural Area use it as a fertilizer counter-ion, and to increase the acidity of the soil, making fertilizer more readily available to plants, which take in sulfate through their roots.
These practices result in sulfate runoff into canals that eventually makes its way to the marshes of the Everglades. Due to the anaerobic conditions of the sediment in the marshes, sulfate is eventually reduced to sulfide by bacteria. Naturally occurring mercury (often present in rainwater) mixes with the sulfide, creating a dangerous cocktail known to scientists as Methylmercury.
“Methylmercury is a problem,” says Naja. “Methylmercury bioaccumulates in the body of an organism, sticking to the cells of those who ingest it.”
According to the United States Environmental Protection Agency, the primary negative health effect of Methylmercury is impaired neurological development, specifically in fetuses. The substance doesn’t always kill directly, but it can drastically affect the human nervous system and also harm vision and speech and cause muscle weakness. In scientific studies, Methylmercury was found to cause kidney tumors in male mice.
Though some forms of sulfur, like hydrogen-sulfide (which is responsible for the rotten-egg smell near a marsh) are regulated by state environmental agencies, there are no limitations on sulfate flowing into wetlands.
Naja and the team at the Everglades Foundation studied the amount of sulfate in the Everglades, comparing actual amounts to the Comprehensive Everglades Restoration Plan target amount. What they found was striking: “The target amount is one milligram per liter. The reality is 30 to 50 milligrams per liter.”
“The phosphorus issue has really taken center stage, and it is a problem. But phosphorus affects 20 percent of an ecosystem. Sulfate affects 60 percent,” says William Orem, a scientist with the United States Geological Survey. “Mercury is rampant.”
According to Orem, the population of wading birds in the Florida Everglades has decreased by 90 percent since 1900, and Methylmercury is a likely culprit. “The decrease isn’t all due to Methylmercury, but it plays a big role over time,” says Orem.
University of Florida Professor Peter Frederick published the results of an in-depth study of one species of wading bird found in the Everglades, the White ibis, in December 2010.
The study found that the ibis population was declining in large part due to altered mating habits, a direct result of mercury consumption. Mercury not only affected the ibis’ courtship habits, but also altered hormones, which led to a high percentage of male birds mating with other males. This particular study was the first that documented mercury’s effects on a bird’s sexual preference.
Frederick’s study followed four different groups of birds, each put on a different diet, with a different level of mercury. Surprisingly, even the group ingesting the lowest levels of mercury (amounts comparable to fish purchased at the grocery store) displayed homosexual behavior and tended to mate earlier in the breeding season.
“The effect was exaggerated the higher the dose, so about 55 percent of those ingesting the highest dosage of mercury had their sexual development affected,” Frederick says. “But even 25-35 percent of those who ingested supermarket-grade levels displayed similar behaviors.”
Even the birds who didn’t display a change in sexual preference showed signs of hormone imbalances. “Even those that did mate male/female didn’t do a good job of parenting,” he says. “They failed to tend eggs or watch their nest. Overall, we found a 35 percent decrease in productivity of the heterosexual nests.” Oftentimes, the males didn’t display good courtship behavior, which led to less female attraction.
Frederick says the ibis case is unlike other homosexual displays between animals, like the famous “gay penguins” of the Central Park Zoo. In some cases, animals mate with their own sex because there are fewer choices of the opposite sex in a captive environment. “That’s not what happened here,” he says. “The ibises had many available females, but the males would court together, build a nest together and stay in the nest for weeks at a time. It was a long-term commitment to the nest.”
Frederick says that mercury is, gram-for-gram, one of the most powerful elements known to man, in how it can alter development.
Mercury is known to cause fetal damage, and pregnant or lactating women are often advised against consuming carriers, such as certain types of fish and shellfish. But it affects more than just fetuses. Frederick says that “even very small amounts of mercury can hinder cognitive development” in “young children.”
The U.S. EPA recommendation’s is to not exceed 0.3 milligrams of Methylmercury per kilogram of fish tissue. Because of the high levels of Methylmercury in fish, the Florida Department of Health regularly issues advisories (.pdf) on consuming fish from the Everglades.
Consumption of Everglades-caught fish by humans isn’t prevalent, but some members of the Haitian community and the Native American Miccosukee tribe are thought to fish in area canals.
But the relevance of Frederick’s study is rooted in wildlife. If mercury leads to hormonal imbalances and a change in sexual preference among birds, it has the power to drastically reduce reproductive output. In short, it is a very serious problem — one that is not being addressed.
“Birds can’t taste mercury, and they can’t avoid it,” says Frederick. “Twenty-five percent of birds just eating supermarket-level mercury experience a change in sexual preference. The message is a very strong one.”
Anywhere from 90 to 98 percent of mercury found in all fish is methylated and, because it is nearly impossible to take mercury out of the Everglades, the natural solution is to get rid of the sulfate. But, like the issue of removing nutrients from Florida waterways, that is easier said than done.
The majority of sulfate comes from Everglades Agricultural Areas, where farmers harvesting products like sugarcane continue using protocol from the 1960s.
In a February 2009 article published in Earth, Orem opined that the Methylmercury problem in the Everglades is still misunderstood:
At the moment, scientists do not know how much sulfate originates from natural sulfur and how much sulfur is locked in the soil due to past agricultural practices. Soil oxidation may be releasing this sulfur, allowing it to run off into the canals that discharge into the Everglades.
According to Orem, recent studies have shown that elemental sulfur is no longer even effective for agriculture because soil is changing so much over time. Even a slight reduction in the use of sulfate could help lower the amount of Methylmercury contamination in the area.
Another solution, says Orem, would be to redesign the Stormwater Treatment Areas, so that they not only filter out phosphorus, but sulfate as well.
One roadblock is the jurisdiction of the mercury problem: The South Florida Water Management District and the Florida Department of Environmental Protection are at odds over who is responsible for the problem.
“It’s really both of their responsibilites,” says Orem. “I think there’s a certain amount of reluctance from upper-level management to even address the issue. The numeric nutrient criteria have caused such heated arguments, and this is potentially an even bigger issue.”