The biogeochemical cycles of iron (Fe) and manganese (Mn) in lakes and reservoirs have predictable seasonal trends, largely governed by stratification dynamics and redox conditions in the hypolimnion. However, short-term (i.e., sub-weekly) trends in Fe and Mn cycling are less well-understood, as most monitoring efforts focus on longer-term (i.e., monthly to yearly) time scales. The potential for elevated Fe and Mn to degrade water quality and impact ecosystem functioning, coupled with increasing evidence for high spatiotemporal variability in other biogeochemical cycles, necessitates a closer evaluation of the short-term Fe and Mn cycling dynamics in lakes and reservoirs. We adapted a UV-visible spectrophotometer coupled with a multiplexor pumping system and PLSR modeling to generate high spatiotemporal resolution predictions of Fe and Mn concentrations in a drinking water reservoir (Falling Creek Reservoir, Vinton, VA, USA) equipped with a hypolimnetic oxygenation (HOx) system. We quantified hourly Fe and Mn concentrations during two distinct transitional periods: reservoir turnover (Fall 2020) and initiation of the HOx system (Summer 2021). Our sensor system was able to successfully predict mean Fe and Mn concentrations as well as capture sub-weekly variability, ground-truthed by traditional grab sampling and laboratory analysis. During fall turnover, hypolimnetic Fe and Mn concentrations began to decrease more than two weeks before complete mixing of the reservoir occurred, with rapid equalization of epilimnetic and hypolimnetic Fe and Mn concentrations in less than 48 hours after full water column mixing. During the initiation of hypolimnetic oxygenation in Summer 2021, we observed that Fe and Mn were similarly affected by physical mixing in the hypolimnion, but displayed distinctly different responses to oxygenation, as indicated by the rapid oxidation of soluble Fe but not soluble Mn. This study demonstrates that Fe and Mn concentrations are highly sensitive to shifting dissolved oxygen and stratification and that their dynamics can substantially change on hourly to daily time scales in response to these transitions. / Master of Science / Iron and manganese are chemical elements that occur in many freshwater systems. Although they are naturally-occurring, high concentrations of iron and manganese can have negative effects on drinking water quality as well as the health of aquatic ecosystems. In temperate regions, iron and manganese can accumulate in the bottom waters of lakes and reservoirs during the summer months, but generally remain at low levels during the fall through spring. This seasonal cycle has been previously documented, but few studies have investigated the ways in which iron and manganese concentrations in a lake or reservoir change over shorter periods of time, such as hours or days. Recent advances in technology to measure chemical elements in the environment have allowed scientists to observe chemical fluctuations of other elements over relatively short time periods, which suggests that iron and manganese could potentially exhibit similar trends. In this study, we used an advanced sensor system to make hourly measurements of iron and manganese concentrations in a drinking water reservoir and observe how they changed during two time periods: in the fall of 2020, as the reservoir was transitioning from summer to winter, and in the summer of 2021, when oxygen was added to the bottom waters to improve water quality. Our observations indicate that iron and manganese concentrations in the reservoir waters were highly variable over short time scales and that they can change dramatically in as little as 24 hours, especially during transitional periods. We also successfully demonstrated the ability of our advanced sensor system to monitor these hourly changes, which could have many benefits for drinking water management and understanding metals cycling in freshwater systems.

京都大学 / 新制・課程博士 / 博士(人間健康科学) / 甲第23828号 / 人健博第99号 / 新制||人健||7(附属図書館) / 京都大学大学院医学研究科人間健康科学系専攻 / (主査)教授 椎名 毅, 教授 杉本 直三, 教授 辻川 明孝 / 学位規則第4条第1項該当 / Doctor of Human Health Sciences / Kyoto University / DFAM