Sulfur is an important water quality issue in the Everglades because of its role in microbial sulfate reduction and the methylation of mercury. Microbial sulfate reduction produces toxic hydrogen sulfide (H₂S) as an endproduct, and accumulation of H₂S in sediment porewater may change sediment redox chemistry and the ability of aquatic macrophytes and other marsh plants to maintain necessary oxygen levels in root systems. Microbial sulfate reduction also produces methylmercury (MeHg), a neurotoxin that is bioaccumulated. MeHg 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 (especially pregnant women) through fish consumption. Based on USGS results, sulfur appears to play a key role in regulating both the magnitude and distribution of MeHg within the ecosystem.

Freshwater wetlands typically have low sulfur concentrations, but we discovered high concentrations of sulfate sulfur in surface water of the northern Everglades. Independent studies conducted by the U.S. EPA and the South Florida Water Management District have also documented high concentrations of sulfate in large parts of the northern Everglades. Marshes in portions of the Water Conservation Areas (WCA’s) have surface water sulfate concentrations that average nearly 60 mg/l, compared to concentrations of 1 mg/l in pristine areas of the Everglades. Areas with high surface water sulfate concentrations are concentrated in the northern WCA’s, and especially near sites of canal discharge and along canal levees. Even higher concentrations of sulfate (average >70 mg/l and sometimes approaching 200 mg/l) were found in canal water draining from the Everglades Agricultural Area (EAA). This canal water appears to be the major source of excess sulfate entering the Everglades. The high loads of sulfate entering the ecosystem stimulate microbial sulfate reduction, and areas with high concentrations of sulfate in surface water also have very high levels of toxic H₂S in sediment porewaters. Porewater sulfide concentrations range from 5,000 ppb at marsh sites near canal discharge, to <0.05 ppb at pristine sites. Porewater sulfide concentrations were highly correlated with surface water sulfate concentrations.

We used sulfate concentration data and the sulfur (d34S) and oxygen (d18O) isotopic composition of sulfate in marsh surface water, canal water, rainwater, and groundwater to trace the source(s) of the excess sulfate entering the Everglades. Although the canals originate in Lake Okeechobee, the lake on average contributes only about 20 percent of the sulfate observed in the canals. Rainwater has too little sulfate to account for the high sulfate concentrations observed in the canals and in large portions of the Everglades. Groundwater beneath the Everglades has either too low a sulfate concentration or a d34S signature that is inconsistent with that of surface water in the Everglades. Both sulfate concentration data and the d34S values of sulfate in surface water from the Everglades and canal water confirm that canals in the EAA are the major source of excess sulfate entering the ecosystem. Furthermore, results showed that canal water with the highest sulfate concentrations had d34S values of +16 per mil, which is consistent with the d34S signature of agricultural sulfur (a soil amendment used in the EAA). Sulfate extracted from the upper 10 cm of soil in a sugarcane field in the EAA also had a d34S value of about +16 per mil. These data suggest that agricultural sulfur is the principal source (but not the only source) of excess sulfate entering the canals and Everglades wetlands.

The high levels of sulfate entering the Everglades from canal discharge have had important effects on the ecosystem. The H₂S buildup in sediment porewater at sites in the northern Everglades has significantly lowered redox potentials in the sediments. The lower redox conditions and high levels of H₂S in the sediments may have an impact on vascular plant growth in the Everglades by limiting oxygen penetration to roots. It is noteworthy that cattails have replaced sawgrass as the dominant macrophyte at sites heavily impacted by sulfur, although factors other than sulfur (e.g. eutrophication and high water levels) may also be influencing this change. Similarly, tree islands have disappeared mostly from the northern Everglades, in areas heavily impacted by sulfur.

The effect of excess sulfur load on MeHg production in the Everglades is complex. Sulfur appears to stimulate MeHg production through increased microbial sulfate reduction, but buildup of sulfide in sediment porewater inhibits MeHg production. The balance between these two effects of sulfur influences both the magnitude and location of MeHg production in the Everglades, and produces the interesting effect that the zone of maximum MeHg production occurs in areas with sulfur loads only moderately higher than natural. This MeHg “Goldilocks” zone (where sulfur levels are just right) occurs in the central portion of WCA 3A, at the front end of the northern Everglades sulfur contamination plume. The complex relationship between sulfur geochemistry and MeHg production in the Everglades was first hypothesized from field studies at various sites in the ecosystem. This hypothesis has been further solidified from experimental studies in the Everglades using mesocosms. These mesocosm studies show that increasing sulfate concentrations increases MeHg production up to sulfate concentrations of about 10 mg/l. At sulfate concentrations >10 mg/l buildup of sulfide from microbial sulfate reduction inhibits the methylation of Hg.

The principal sink for sulfur entering the Everglades ecosystem is long-term storage in sediments. Sequestration of sulfur in sediments typically results from microbial reduction of sulfate to sulfide, and subsequent reaction of sulfide with either organic matter, to form organic sulfur compounds; or metals, to form insoluble metal mono- and disulfides. The dominant sulfur species in Everglades sediments is organic sulfur. Sulfur accumulation rates (gm^-2 day^-1) in Everglades sediments ranges from 9.0 x 10^-3 to 0.38 x 10^-3, a difference of more than 20 fold. The highest sulfur accumulation rates occur in brackish water mangrove (average of 6.0 x 10^-3), and sulfur-contaminated freshwater marsh areas (average of 4.5 x 10^-3). Sulfur accumulation rates average about 1.6 x 10^-3 at pristine sites.

Sulfur in sediments represents a reservoir of reduced sulfur that may be reoxidized and remobilized during drought/fire and subsequent rewetting. Because drought and fire are frequent occurrences in the Everglades ecosystem, the remobilization of sulfur from sediments is an important factor influencing sulfur and mercury geochemistry. Studies of sulfur remobilization were conducted in northern WCA 3A following a burn and subsequent rewetting in that area during 1999. The average sulfate concentration in surface water at 14 sites was 58.6 mg/l immediately after the burn and rewetting of this area, compared to an average sulfate concentration of 5.10 mg/l before and over a year after the burn. The sulfate remobilized after the burn and rewet stimulated sulfate reduction and extreme levels of MeHg production (see abstract by Krabbenhoft et al., this volume). Cores from burned and control (e.g. unburned) sites in northern WCA 3A showed that burned sites had significantly lower total sulfur contents in sediments. A simulated laboratory experiment was conducted in which mini-cores were dried under simulated natural sunlight in controlled laboratory conditions for a specified period of time and then rewetted to further examine the effects of fire/drought on S geochemistry and MeHg production. Results verified the remobilization of sulfate and stimulation of MeHg production following fire/drought events. Ecosystem and water managers must consider the effect of drying and rewetting portions of the Everglades in order to avoid exacerbating the already extensive MeHg problem in the Everglades.

Contact: William Orem, U.S. Geological Survey, 956 National Center, Reston, VA 20192, 703-648-6273 (office); 703-648-6419 (fax); borem@usgs.gov

(This abstract was taken from the Greater Everglades Ecosystem Restoration (GEER) Open File Report 03-54)

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