Sulfur in the South Florida ecosystem: Distribution, sources, biogeochemistry, impacts, and management for restoration
Sulfur is broadly recognized as a water quality issue of significance for the freshwater Florida Everglades. Roughly 60% of the remnant Everglades has surface water sulfate concentrations above 1 mg l-1, a restoration performance measure based on present sulfate levels in unenriched areas. Highly enriched marshes in the northern Everglades have average sulfate levels of 60 mg l-1. Sulfate loading to the Everglades is principally a result of land and water management in South Florida. The highest concentrations of sulfate (average 60-70 mg l-1) in the ecosystem are in canal water in the Everglades Agricultural Area (EAA). Potential sulfur sourcesin the watershed are many, but geochemical data and a preliminary sulfur mass balance for the EAA are consistent with sulfur presently used in agricultural, and sulfur released by oxidation of organic EAA soils (including legacy agricultural applications and natural sulfur) as the primary sources of sulfate enrichment in the EAA canals. Sulfate loading to the Everglades increases microbial sulfate reduction in soils, leading to more reducing conditions, greater cycling of nutrients in soils, production of toxic sulfide, and enhanced methylmercury (MeHg) production and bioaccumulation. Wetlands are zones of naturally high MeHg production, but the combination of high atmospheric mercury deposition rates in South Florida and elevated sulfate loading leads to increased MeHg production and MeHg risk to Everglades wildlife and human consumers. Sulfate from the EAA drainage canals penetrates deep into the Everglades Water Conservation Areas, and may extend into Everglades National Park. Present plans to restore sheet flow and to deliver more water to the Everglades may increase overall sulfur loads to the ecosystem, and move sulfate-enriched water further south. However, water management practices that minimize soil drying and rewetting cycles can mitigate sulfate release during soil oxidation. A comprehensive Everglades restoration strategy should include reduction of sulfur loads as a goal because of the many detrimental impacts of sulfate on the ecosystem. Monitoring data show that the ecosystem response to changes in sulfate levels is rapid, and strategies for reducing sulfate loading may be effective in the near term. A multifaceted approach employing best management practices for sulfur in agriculture, agricultural practices that minimize soil oxidation, and changes to stormwater treatment areas that increase sulfate retention could help achieve reduced sulfate loads to the Everglades, with resulting benefits.