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Climate change is predicted to have many diverse effects on freshwater lakes and reservoirs by increasing both hypolimnetic hypoxia and runoff, which will increase nutrient concentrations and degrade water quality. Hypoxic conditions can trigger the release of metals and nutrients from the sediments, i.e., internal loading, while storms can increase external nutrient loading to a waterbody. One potential solution for combating hypoxia is to use side stream supersaturation (SSS), a novel form of hypolimnetic oxygenation. First, in Chapter 1, I tested the efficacy of SSS operation to improve water quality in Falling Creek Reservoir (FCR), a shallow, eutrophic, drinking water reservoir. I found that SSS operation successfully increased hypolimnetic oxygen concentrations in FCR and suppressed internal loading of iron, manganese, and phosphorus. In Chapter 2, I manipulated inflow volumes to FCR and used SSS as a tool to alter hypolimnetic oxygen conditions in whole-ecosystem manipulations of internal and external nutrient loading. I observed that internal nitrogen and phosphorus loading during hypoxic conditions largely controlled the hypolimnetic mass of nutrients in FCR, regardless of inflow volumes, presumably as a result of the accumulated nutrients in its sediment from historical agriculture. Additionally, FCR consistently functioned as net sink of N and P throughout almost all of the treatments and substantially reduced nutrient export to downstream ecosystems. In summary, my research demonstrates the sensitivity of reservoir water quality to global change. / Master of Science

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.

The National Research Council recently called for a more interdisciplinary approach to drinking water research to address the critical issue of global drinking water supplies. Hypolimnetic oxygenation systems (HOₓ) are being increasingly used to improve water quality in stratified reservoirs by increasing dissolved oxygen (O₂) concentrations and subsequently suppressing the release of soluble species such as iron (Fe) and manganese (Mn) from the sediment into the water. However, while the influence of HOx on the water column has been established, little work has been done on how oxygenation affects sediment O₂ uptake (i.e., sediment oxygen demand) and other sediment-water fluxes. In response to the growing need for alternative approaches for improving water quality, we conducted highly interdisciplinary research to evaluate how O₂, Fe, and Mn cycling at the sediment-water interface is influenced by both natural and HOx-induced variations in water column dynamics, chemical redox processes, and microbial activity within the sediment, all of which may govern sediment-water fluxes. Studies were performed in an alpine lake in Switzerland and in an HOₓ-equipped drinking-water-supply reservoir in Virginia. This research was based on in situ field campaigns paired with laboratory experiments, microbial analyses, and computer simulation to elucidate variable sediment O₂ uptake and corresponding Fe and Mn cycling. This work is unique in that sediment-water fluxes were assessed using in situ data from both sides of the sediment-water interface. Results show that sediment O₂ uptake flux is strongly controlled by both wind- and HOₓ-induced dynamic forcing. Our findings reveal that Fe and Mn fluxes were suppressed from the bulk hypolimnion via biogeochemical cycling in the oxic benthic region. Results also indicate that the sediment microbial community structure may directly respond to HOₓ-induced variation in sediment O₂ availability. Additionally, based on an analysis of the robustness of several commonly used methods for flux calculations, we show that flux estimates are not strongly dependent on the method chosen for analysis. Ultimately, by emphasizing the highly transient nature of sediment O₂ uptake, this research will aid in accurate characterization of various sediment-water fluxes and corresponding water quality. Our results will also directly contribute to the optimization of HOₓ operations and lake and reservoir management. / Ph. D.

The effects of oxygenation and aeration on water quality, rainbow trout (Oncorhynchus mykiss) production, and economics were compared at the Wytheville State Fish Hatchery (WSFH) for 270 days. Mean dissolved oxygen (DO) concentrations and delta DO were significantly higher (P < 0.001) in the oxygenated raceways (9.5 and 2.75 mg/L, respectively) compared to aerated raceways (7.4 and 0.57 mg/L). Total settleable solids loads were significantly greater (P < 0.001) in aerated raceways (10.3 g/L/day) than in oxygenated raceways (8.8 g/L/day). Dissolved nitrogen (%), total gas pressure, and other water quality parameters (CO2, nitrite nitrogen, alkalinity, pH, and TAN) did not differ significantly between the treatments (P > 0.05). Raceway trout production (kg/day), trout growth rates (grams), feed conversion rate (FCR), and fish survival were not significantly different between treatments (P > 0.05). Blood hematocrit (Hct) and percent visceral mass were significantly elevated (P < 0.001) in oxygenated raceways compared to aerated raceways at 46 and 14.4% and 44 and 13%, respectively. Carrying capacity estimates derived from fish loading trials were significantly different (P < 0.001) at 3,355 and 2,217 kg/raceway in oxygenated and aerated raceways, respectively. Estimates of carrying capacity calculated using a fish loading (Ld) equation were also significantly different (P < 0.001) at 1,530 and 990 kg for oxygenated and aerated raceways, respectively. Oxygen injection increased the cost of production by $0.20/kg, however, net present value analysis (NPV) of oxygenated and aerated raceways over 5 years at a 10% discount rate yielded estimates of $50,666.51 and $32,742.15, respectively. Oxygen injection is an effective means of increasing DO concentrations, reducing effluent solids loading, and increasing raceway carrying capacity. / Master of Science

The neurological wake-up test, NWT, is a clinical monitoring tool that can be used to evaluate the level of consciousness in patients with traumatic brain injury (TBI) and subarachnoid haemorrhage (SAH) during neurocritical care (NCC). Since patients with severe TBI or SAH are often treated with mechanical ventilation and sedation, the NWT requires that the continuous sedation is interrupted. However, interruption of continuous sedation may induce a stress response and the use of the NWT in NCC is controversial. The effects of the NWT on intracranial pressure (ICP) and cerebral perfusion pressure (CPP) were evaluated in 21 patients with TBI or SAH. Compared to baseline when the patients were sedated with continuous propofol sedation, the NWT resulted in increased ICP and CPP (p&lt;0.05). Next, the effects of the NWT on the stress hormones adrenocorticotrophic hormone (ACTH), cortisol, epinephrine and norepinephrine were evaluated in 24 patients. Compared to baseline, the NWT caused a mild stress response resulting in increased levels of all evaluated stress hormones (p&lt;0.05). To compare the use of routine NCC monitoring tools, the choice of sedation and analgesia and the frequency of NWT in Scandinavian NCC units, a questionnaire was used. The results showed that all 16 Scandinavian NCC units routinely use ICP and CPP monitoring and propofol and midazolam were primary choices for patient sedation in an equal number of NCC units. In 2009, the NWT was not routinely used in eight NCC units whereas others used the test up to six times daily. Finally, intracerebral microdialysis (MD), brain tissue oxygenation (PbtiO2) and jugular bulb oxygenation (SjvO2) were used in 17 TBI patients to evaluate the effect of the NWT procedure on focal neurochemistry and cerebral oxygenation. The NWT did not negatively alter interstitial markers of energy metabolism or cerebral oxygenation. In conclusion, the NWT induced a mild stress response in patients with TBI or SAH that did not result in a detectable, significant secondary insult to the injured brain. These results suggest that the NWT may safely be used as a clinical monitoring tool in the NCC of severe TBI and SAH in a majority of patients.

Thesis--Copenhagen University. / Summary in Danish. Includes bibliographical references (p. 96-120).