WATER QUALITY ISSUES IN THE EVERGLADES: II. SULFUR CONTAMINATION AND LINKS TO METHYLMERCURY PRODUCTION

Sulfur is an important water quality issue in the Everglades because of its role in microbial sulfate reduction and the methylation of mercury. Methylmercury (MeHg), a neurotoxin that is bioaccumulated, has been found in high concentrations in freshwater fish from the Everglades, and poses a potential threat to fish-eating wildlife and to human health through fish consumption. Sulfur appears to play a key role in regulating both the magnitude and distribution of MeHg in the Everglades.

Freshwater wetlands typically have low sulfur concentrations, but marshes in portions of the northern Everglades have average surface water sulfate concentrations of 60 mg/l, compared to 1 mg/l or less at background sites. Canal water originating from the Everglades Agricultural Area (EAA) and discharged into the marshes appears to be the major source of excess sulfate entering the Everglades. Sulfur isotope (ƒÔ34S) studies of sulfate in sulfur-contaminated areas of the ecosystem suggest that agricultural sulfur, used as a soil amendment in the EAA, is a principal source of the excess sulfate entering the Everglades.

The excess sulfate entering the Everglades from canal discharge stimulates sulfate reduction and sulfide buildup in the sediments. This lowers redox potentials in sulfur-contaminated areas to values more reducing than natural, and may adversely impact macrophyte growth in the Everglades by limiting oxygen penetration to roots. Mesocosm studies are currently underway to test the effects of sulfur toxicity on macrophyte growth in the Everglades. Excess sulfur has two differential effects with respect to MeHg production: (1) stimulation through increased sulfate reduction, and (2) inhibition through buildup of excess sulfide in sediment pore water. The balance between these two effects influences the magnitude and distribution of MeHg production in the Everglades. Field studies and mesocosm experiments in the ecosystem suggest that the MeHg problem in the Everglades results largely from two factors: (1) increased fallout of mercury on the ecosystem, and (2) sulfur contamination of the ecosystem from agricultural runoff. Current restoration plans calling for diversion of more water into the Everglades may have the unwanted effect of increasing the load of sulfur into the ecosystem, thereby exacerbating an already serious MeHg problem.