Theses and Dissertations at Montana State University (MSU)
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Item Daily signals in nitrate processing provide a holistic perspective on stream corridor hydrologic and biogeochemical function(Montana State University - Bozeman, College of Agriculture, 2023) Foster, Madison Jo; Chairperson, Graduate Committee: Robert A. Payn; This is a manuscript style paper that includes co-authored chapters.Understanding interactive pathways of biogeochemical reaction and water movement in stream corridors is critical given the role stream corridors play in mitigating nitrate loading from agricultural watersheds. However, few studies consider the interactive effects of nitrate loading, riparian processing, and stream ecosystem processing, which may limit abilities to predict downstream nitrate delivery. Riparian groundwater inputs and stream ecosystem processing may vary due to daily cycles in evapotranspiration or stream ecosystem primary production. Recent advances in high-frequency monitoring of stream chemistry throughout the day exhibit potential to explore both hydrologic and biogeochemical influences on nitrate attenuation. In this thesis, I explore how diel variations in stream reach nitrate processing can provide holistic perspectives on the attenuation of nitrate along stream corridors within a watershed that is heavily influenced by agricultural land use. Nitrate processing is defined as the evident changes in nitrate concentration in parcels of water as they travel along a given reach of a stream, as measured from nitrate sensors located at the head and base of ca. 0.5 km reaches. To understand controls on diel variation in nitrate processing, we measured diel processing signals in agricultural headwater reaches in Central Montana, USA spanning variable atmospheric and flow conditions from March through August in 2020-2022. Across 168 days with valid data, most signals exhibited little diel variation (n = 106) and this lack of variation occurred most frequently during cooler and shorter days. In contrast, signals with greater variation were common during longer days, warmer temperatures, and lower flows (n = 62). This seasonal shift in patterns suggests that solar radiation and stream flow are primary controls on diel nitrate processing signals in these low-order reaches. In addition to diel variation, less overall nitrate attenuation in the study reach with direct inputs of high-nitrate upland waters suggest that the degree of hydrologic connection to upland aquifers influences apparent reach nitrate processing. This work highlights how understanding the drivers of diel processing signals may lead to a more holistic understanding of how multiple interacting processes in stream corridors influence nitrate delivery to downstream ecosystems.Item Understanding hydrogeomorphic influences on stream network denitrification and temperature dynamics(Montana State University - Bozeman, College of Agriculture, 2020) Carlson, Samuel Paul; Chairperson, Graduate Committee: Geoffrey PooleThe removal of nitrate from stream networks through the process of denitrification is an important component of local and regional nutrient cycles, but the controls on stream network denitrification rates remain poorly understood. Previous work has demonstrated general effects of stream channel size and nitrate loading rates on network-scale denitrification rates, but has been unable to elucidate connections between the complex environmental template of streams, and resulting denitrification rates. Understanding links between land use and management practices, physical characteristics of streams, and stream denitrification rates is critical to interpreting observed patterns of nitrate in freshwater systems and forming holistic management strategies for reducing the negative effects of elevated nitrate concentrations. To address these critical uncertainties, I developed a stream network simulation model that incorporates the effects of whole-stream aerobic respiration on biotic denitrification demand. This model is applied to a small, subalpine stream network under scenarios designed to explore: 1) the implications of temperature-controlled, network scale patterns of respiration rates on the distribution and overall magnitude of stream network denitrification, and 2) the effect of logging-induced channel simplification on whole network denitrification rates. The first analysis is complimented by an evaluation of controls on stream temperature across this network, revealing the spatially and temporally variable influence of in-network lakes on stream temperatures. Results from the first analysis suggest that reach- and network-scale denitrification rates are strongly influenced by respiration rate and temperature when nitrate supplies are high relative to removal rates, indicating an increased contribution of lower, warmer streams to whole-network denitrification. The second analysis reveals that historical logging has caused a ~30% loss of stream network denitrification capacity, which is manifested as a corresponding reduction in whole-network denitrification rates when nitrate supplies are not limiting. In sum, this work emphasizes the diverse set of factors that influence reach- and watershed-scale biogeochemical characteristics and processes, and suggests that land management actions which influence stream morphology may also alter stream denitrification rates.