Challenges Part 4: Sulfur and Mercury
Water quality conditions in the Everglades are affected not only by the input of single pollutants like phosphorus or nitrogen, but also by the inputs of other elements that alter their behavior. An example, which has received increasing attention, is the relationship between sulfur and mercury and its accumulation in fish and other wildlife as well as humans. This accumulation appears to be controlled not simply by inputs of inorganic mercury, but by the interaction of mercury and supplies of sulfate and dissolved organic carbon. 
Sulfur per se, while it is not recognized as a water pollutant, has a particularly important role as a contaminant in the Everglades, which evolved not only as phosphorus limited but also as a sulfur-limited system. Sulfur is usually introduced into The Everglades from agricultural areas under oxidizing conditions in the form of sulfates (SO42-). Under the reducing conditions found in any wetland environment, sulfates cycle back and forth into sulfides (S2-), which can damage the ecosystem. Elevated concentrations of sulfide are often toxic to plants; and sawgrass is three times more sensitive than cattails to sulfide concentrations. Thus, inputs of sulfate to the Everglades alters the distribution of plant species in favor of cattails. The EPA water quality standard for sulfide is 2 ppb, which is exceeded in porewaters in parts of the Everglades that receive high inputs of sulfate, from canal drainage. Sulfur applied in the Everglades Agricultural Area for agricultural purposes is a major contributor to the excess sulfate concentrations in the Everglades .
The interaction between sulfur and mercury is even more deadly. Since the early 1990s, mercury contamination has been recognized as a critical health issue for humans and wildlife that consume fish from the Everglades. The state of Florida has advisories that either ban or restrict consumption of nine species of fish from more than 3,000 square miles (65 percent of the total area) of the Everglades. Advisories include a ban on consumption of largemouth bass that exceed 14 inches, and fishing for consumption is not advised in the Everglades. In addition to concerns related to human health, there is evidence that elevated exposure to mercury is harming piscivorous birds like the White Ibis and mammals like the Florida panther, due to an impact on breeding success.
As research has been carried out over the past 10 to 15 years, it has become clear that the major cause for dangerously high concentrations of mercury in the form of methyl mercury is linked to the way sulfates move through the everglades. Even though mercury in its inorganic form is reasonably plentiful in the soils of the everglades, this is ionic mercury (Hg2+), which cannot enter plant or animal tissues easily. In the presence of the oxidation-reduction reactions between sulfates and sulfides, carried out by sulfate-reducing bacteria, Hg2+ is transformed into methyl mercury (HgMe), a neurotoxin. methyl mercury becomes increasingly concentrated as it moves up Everglades food chains resulting in dangerous exposure to humans and wildlife.
Since it is the process of sulfate reduction that stimulates the biological activation of Mercury, methylation takes place when the concentration of sulfates is neither “too low” nor “too high”. Concentrations of sulfates, normally very low for the undisturbed everglades (less than 10 parts per million or ppm), are too low for methylation to occur. At high concentrations of surface-water sulfate (> 20 ppm) and/or high concentrations of sulfide (> 0.3 ppm), production of methyl mercury becomes curtailed because of immobilization of mercury by sulfide.   Thus peak methylation occurs at sulfate concentrations in surface waters of 10-20 ppm, which coincides with sulfide concentrations of 0.2-0.3 ppm in sediment porewaters.
In the northern Everglades the high supply of sulfate from the agricultural areas, results in high concentrations of sulfide, which may limit methyl mercury concentrations.  With decreases in sulfate and sulfide concentrations as the sulfates move south into the Water Conservation Areas, there is an increase in methyl mercury production and an uptake of mercury by fish. As sulfate concentrations become even more reduced through Everglades National Park toward the south, methyl mercury formation declines once more.   However, increases in water discharge since the mid-1990s appear to have increased sulfate transport southward, resulting in mercury contamination in the southern portions of The Everglades