Edaphic and vegetative controls on mercury cycling in oligohaline wetlands

Abstract

With the expansion of the human population and associated industries there is a concomitant increase in both resource utilization and the production of waste and deleterious by-products. Mercury is a naturally-occurring toxic metal with a complicated and unique biogeochemical cycle, and is often a contaminant of ecotoxicological concern in unindustrialized aquatic habitats. The research described herein was designed to elucidate multiple aspects regarding the behavior of mercury in oligohaline wetland habitats, particularly with respect to edaphic and vegetative effects, through a monitoring and characterization study as well as a series of manipulative experiments. The observational study, conducted in the wetlands surrounding Lake Maurepas, Louisiana, indicated that during the time of the study, little reduction of sulfate to sulfide was occurring, and thus soil methyl mercury concentrations were quite minimal. Further, the total mercury concentrations in local vegetation were found to be typical of uncontaminated environments in the United States. The first experimental study was a hydroponic greenhouse study that evaluated the capacity of several species of local wetland plants to function as phytoremediation agents for aqueous mercury contamination. Several of the species tested demonstrated the ability to reduce aquatic inorganic mercury concentration, but generally not to a greater extent than has been shown for other plant species. The second experimental study assessed vegetative response and partitioning of elevated surface water inorganic mercury into the soils and vegetation in a wetland mesocosm. The concentration of mercury in the surficial soil became elevated, but mostly occurred in biologically unavailable forms, while substantial mercury uptake by plants occurred only in belowground tissues. The final experimental study investigated the potential stimulation of soil methyl mercury concentration via elevated surface water nutrients in a benchtop soil core incubation study. No evidence of significantly increased soil methyl mercury concentration was found.