Using Mercury Stable Isotopes to Identify Sources and Track Biogeochemical Cycling of Mercury in Freshwater Ecosystems

Abstract

Mercury is a trace metal toxin that is harmful to the health of humans and wildlife. Anthropogenic mercury released directly into waterbodies during mining activities or industrial processes may remain in the sediment and soil for decades or longer as legacy mercury. Mercury in the environment may also be converted into methylmercury, a more toxic and bioaccumulative form of mercury that biomagnifies in food webs. Throughout this dissertation, I applied measurements of mercury stable isotope ratios in sediment, fish, and aquatic invertebrate samples to study mercury biogeochemical cycling within East Fork Poplar Creek, a point-source contaminated stream ecosystem. In Chapter 2, I investigated the potential mechanisms by which legacy mercury within streambed sediment may be remobilized to the surface water as dissolved mercury. This was done by combining sequential extractions and mercury isotope analysis, through which I observed isotopic differences between various pools of mercury within the sediment. These isotopic signatures, along with previous measurements of the isotopic composition of surface water, pore water, and suspended particulates, suggested that small amounts of the largely-recalcitrant legacy mercury in the sediment is contributing to the flux of dissolved mercury along the flow path of the stream. In Chapter 3, I developed a method of extracting and isolating methylmercury from fish and aquatic invertebrates for isotope analysis. This method involved a novel combination of nitric acid digestion and anion-exchange resin separation, and successfully achieved high methylmercury recovery and purity. This method allows for direct determination of the isotopic composition of methylmercury within a variety of organisms, which enables the sources of methylmercury to the food web to be identified. In Chapter 4, I applied this new methylmercury separation method to directly measure both the total mercury and methylmercury isotopic compositions of a range of aquatic organisms from East Fork Poplar Creek and a regional reference stream, Hinds Creek. This allowed for the identification of sources of methylmercury to the food webs of these streams, as well as a comparison of biogeochemical processing of methylmercury within freshwater stream ecosystems containing either natural background or highly elevated levels of mercury. I found that the two streams each contain multiple sources of methylmercury, with organisms from Hinds Creek ultimately deriving methylmercury from a combination of precipitation and dry deposition, and organisms from East Fork Poplar Creek ultimately deriving methylmercury from a combination of legacy and newly released mercury originating from an industrial point source. I also found that for both streams, multiple basal resources deliver methylmercury to the food web, including streambed sediment as well as biofilm and/or suspended particulates. Finally, I found that microbial demethylation tends to be an important reaction within streams containing either natural background or moderately elevated levels of mercury contamination, including Hinds Creek, but that isotope fractionation induced by microbial methylation tends to be more dominant relative to that induced by microbial demethylation within the most highly contaminated stream ecosystems, including East Fork Poplar Creek.