Silvia Newell, Ph.D. (Advisor); Mark J. McCarthy, Ph.D. (Committee Member); Sarah Tebbens, Ph.D. (Committee Member); Alison Agather, Ph.D. (Committee Member); Carl Lamborg, Ph.D. (Other)
Doctor of Philosophy (PhD)
Mercury (Hg) is a neurotoxin that can have detrimental impacts on the human nervous system and on brain development in infants. Methylmercury (MeHg) is the most toxic form of Hg and can concentrate to potentially harmful levels in higher levels of marine food webs. Production of MeHg in oxic water columns is poorly understood due to lack of knowledge of the mechanisms of formation and distribution. Recent work has reported widespread, putative Hg methylation genes in nitrite oxidizers, but any relationship with nitrifiers is unknown. This work focuses on Hg water column distribution, speciation, and methylation. The aims of this dissertation are to quantify: (1) rates and distribution of HgT and MeHg in the western basin of Lake Erie; (2) Hg methylation and nitrification rates in the Tropical North Atlantic Ocean; and (3) Hg species distribution in the Central Pacific Ocean. In Lake Erie, a Laurentian Great Lake, the mean fraction of total Hg as MeHg in water column measurements from lake areas near three Lake Erie river inputs (summers of 2018, 2019, and 2021) were comparable (~5%), suggesting similar biogeochemical cycling for rivers feeding Lake Erie. Estimated river fluxes of MeHg were an order of magnitude greater than in situ production, and greater from the Detroit River (9.5 kg yr–1) than from either the Maumee River (0.78 kg yr–1) or Sandusky iii Bay (0.03 kg yr–1), as expected since the Detroit River accounts for about 95% of total water discharge into Lake Erie. These results suggest that the Detroit River is a major source of MeHg from the upper Great Lakes basin to western Lake Erie. Overall, Hg concentrations and fluxes observed in this study were up to orders of magnitude less than those reported in previous decades, which may be a positive result of the Ohio Clean Air and Water Act of 2004. In a marine system, potential Hg methylation and nitrification rates were measured in Western Tropical North Atlantic Ocean surface waters (June and July 2019), within and near the Amazon River plume. The Hg methylation rates in this study were correlated positively and strongly to nitrification rates and Nitrospina-specific 16S gene expression. Potential Hg methylation and nitrification rates were highest at the most saline and least turbid stations, indicating that sediment particles and nutrient-rich, Amazon riverine discharges were not the primary factors promoting either process. These novel results in oxic seawater provide further evidence that Hg methylation may be linked to abundant, nitrifying microbes and may help explain marine MeHg distributions. In the open Pacific Ocean, to better understand Hg partitioning and distributions, multiple Hg species were measured during a meridional section from Alaska to Tahiti in 2018. We quantified filtered total Hg (HgT) and MeHg, as well as HgT and MeHg associated with suspended particles, in high-resolution vertical profiles along the meridional transect at 152° W, which included the Alaskan shelf, North Pacific gyre, and the Loihi seamount. All measured Hg species had greater concentrations in the northern than southern Pacific Ocean, consistent with prior measurements, and the Loihi seamount was identified as a major source. Results from this expedition provide useful information vi regarding potential sources and sinks of HgT and MeHg in the North Pacific Ocean, but additional Hg speciation data is needed for water masses south of Tahiti and the Southern Ocean. The overarching theme of this dissertation was to address knowledge gaps in Hg methylation in oxic waters, such as Lake Erie and the Tropical North Pacific Ocean, as well as distribution of total Hg and MeHg in Lake Erie and the Pacific Ocean.
Department or Program
Department of Earth and Environmental Sciences
Year Degree Awarded
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