Everglades Mercury Debate Environmental

Rebecca Renner / Science and Technology v.35, i.3 1feb01

Mesocosm studies in the Everglades may help resource managers determine the most effective strategy for limiting mercury methylation. Cynthia Gilmour

Sulfur-based fertilizers used by Florida sugarcane farmers may be increasing methylmercury production in the Everglades, according to preliminary results from the U.S. Geological Survey’s ongoing Aquatic Cycling of Mercury in the Everglades (ACME) project. 

In the Everglades’ marshes, methyl mercury concentrations in fish, alligators, wading birds, and fish-eating mammals are among the world’s highest. Fish consumption advisories for mercury cover nearly the whole Everglades ecosystem, which stretches south from Lake Okechobee to the Florida Keys and includes the Everglades National Park.

Over this 1-million-acre area, mercury deposition rates are fairly constant, but methylmercury levels vary significantly. Levels in the north and south are relatively low, but some of the highest methylmercury levels have been found in the center.

The ACME scientists discovered a sulfur gradient in 1998, which they believe explains the methyl mercury distribution in the Everglades. Freshwater marshes such as the Everglades usually have low sulfate levels of ~1 mg/L. Sulfate levels in the northern Everglades are about 100 mg/L, according to USGS scientist William Orem. Orem has used stable sulfur isotopic ratios to rule out groundwater as the source of sulfur and to trace the source back to the use of sulfur-based fertilizers in agricultural areas.

The ACME researchers hypothesize that sulfur, which is released from fertilizers as sulfate, dramatically affects the Everglades mercury cycle by stimulating and controlling the location of maximum methyl mercury production and bioaccumulation. But the relationship is complex, and the details are still being researched.

Sulfur’s role in the mercury problem is coming to light as federal and Florida state agencies embark on the $8 billion Everglades’ restoration project, a vast replumbing scheme to reverse decades of environmental degradation and ensure adequate water supplies for cities and farms. The 30-year plan includes constructing a series of interlocking projects to restore the Everglades’ natural wet–dry cycles and reestablish the natural flow. The ACME scientists believe that these changes are likely to affect the mercury problem and may even make it worse.

In particular, the restoration project’s plan to increase wet–dry cycles could increase methyl mercury production, according to USGS scientist David Krabbenhoft, one of the project’s principal investigators. Following the drought and fires of 1999, methyl mercury production skyrocketed when the waters returned, he says. “The levels were an order of magnitude higher than our previous highest levels.” They eventually returned to normal. Krabben hoft thinks that increased sulfate supplied by reoxidation of reduced sulfur within the sediments is the explanation.

Microbiologists have known for some time that sulfate-reducing bacteria are important mercury methylators in many aquatic sediments, according to Cynthia Gilmour, a microbiologist with the Academy of Natural Sciences, who is one ACME’s principal investigators. Net methylmercury production seems to be optimized when the sulfate levels are just right. Too little sulfate and the bacteria do not go into action; too much sulfate and the bacteria produce excess sulfide, which inhibits mercury methylation, says Gilmour.

The sulfide role appears to involve novel chemistry. Rather than the bacteria taking up free ions, Gilmour thinks they prefer neutral compounds. “Sulfide influences the speciation of mercury and therefore its bioavailability to methylating bacteria,” she says. When bacteria generate high levels of sulfide, charged mercury sulfide complexes form. These charged complexes do not diffuse rapidly across cell membranes. 

This model fits with broad field observations, according to USGS’s Orem. The “Goldilocks” area, where the sulfate levels are just right, is Water Conservation Area 3, he says. There, methyl mercury levels in sediments, gambusia (also known as “mosquito fish”), and chick feathers are the highest in the Everglades. Despite the good fit, the model still requires confirmation, according to Gilmour.

Last summer, Orem, Gilmour, and Krabbenhoft started a series of field experiments in the Ever glades to do just that. They are adding stable isotopes of mercury and sulfur into small areas corralled in plastic rings (see photo). By measuring methylation rates and determining into what substrate the isotopes are incorporated, they will be able to trace the system’s response to each chemical.