Biogeochemical and Hydrologic Dynamics in Urban Watersheds: Dissolved Organic Matter Quantity and Quality Changes Across an Urbanization Gradient During Baseflow and Stormflow Conditions in Temperate River Networks
Date of Award
Doctor of Philosophy (PhD)
Forestry and Environmental Studies
Dissolved organic matter (DOM) is a pivotal variable in aquatic ecosystems that can influence multiple factors, including pH, light penetration, stream metabolism, contaminant mobility, and water-treatment efficiency. Past studies on headwaters during baseflow have shown that urbanization is leading to DOM compositional changes; however, more research is needed across a range of stream orders with various upstream land influences, including urban point and nonpoint sources, during baseflow and stormflow to better understand urban DOM dynamics. Urban DOM sources can include leaky septic/sewer systems, treated wastewater effluent, and stormwater runoff. This dissertation examined changes in dissolved organic matter quantity and quality across different land uses during baseflow and stormflow conditions within temperate watersheds using a combination of novel techniques, including fluorescence spectroscopy and ultrahigh resolution mass spectrometry. In Chapter 2, our urban vs. forested DOM analyses from sampling multiple watersheds in Connecticut alongside the U.S. Geological Survey during the summer of 2016 for the Northeast Stream Quality Assessment program showed that urban-derived DOM was lower in molecular weight, enriched in protein-like fluorescence, and comprised of more sulfur, nitrogen, and phosphorus-containing compounds. DOM optical indices showed statistically significant differences between urban and forested sites. Compared to forested systems, urban point-source influenced sites had higher fluorescence index (FI) (more microbial-like DOM) and freshness index (more recently produced DOM), but lower humification index (HIX) (less aromatic humic-like DOM). FI not only exhibited statistically significant differences between urban point-source influenced sites and forested streams, but also displayed differences between urban point vs. nonpoint sources, showcasing that spectroscopy can be a useful tool in distinguishing between upstream urban influences. By using spectroscopy in combination with ultrahigh resolution mass spectrometry, DOM composition can be more fully understood across urban systems, elucidating the effects of urban stressors on water quality, carbon bioavailability, and ecosystem functioning to help future watershed management. In Chapter 3, we investigated the effects of urbanization on changes in DOM quality across various hydrologic regimes. By conducting a multi-year study along an urbanized land use gradient in a temperate river network, we found that DOM optical indices showed statistically significant differences between urban and forested sites during both baseflow and stormflow. Compared to forested systems, urban sites generally had higher FI and freshness index, but lower HIX; however, the range of differences in mean values between urban and forested sites during baseflow was greater than during stormflow for each index, exhibiting a variability in the urban effect controlled by hydrology. Our findings shed new light on the integral role that hydrologic events play in regulating urban DOM quality, which, in turn, influences the downstream export and transformation of carbon, resulting in important implications for aquatic ecosystems. In Chapter 4, with carbon and nitrogen intricately connected in ecosystems, we explored the effects of land cover on changes in dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) quantity during both baseflow and stormflow in a temperate river network. The upstream wetland influence had a statistically significant effect on DOC concentrations, exhibiting the highest mean value. The upstream urban point-source influence had a statistically significant effect on TDN concentrations with the highest mean value revealed at the most urbanized site. DOC and TDN exports generally increased with water yields across forested and urban sites. The urban sites exhibited high DOC and TDN variability during storm events. This study performed at a large spatiotemporal resolution enhanced our understanding of these very dynamic systems with a high-frequency dataset spanning headwaters to the mainstem. Lastly, in Chapter 5, we analyzed DOM quantity and quality spanning from headwaters to their corresponding drinking water reservoirs to assess the crucial role that reservoirs play in DOM transport dynamics. This study assessed the variation in DOM quantity and quality across seasons in the two largest reservoirs providing drinking water to Hartford, CT, and its suburbs. This case study highlights an exemplary model of how to sustain high quality drinking water reservoirs for metropolitan use by showcasing the importance of source water protection. With increased urbanization globally and more extreme events (droughts and intense storms) predicted to continue, better understanding the land-water transfer of urban DOM and its fate within drainage networks under varying hydrologic regimes is critical for water quality management. This dissertation fills a previous knowledge gap by linking headwaters to the mainstem in anthropogenically disturbed watersheds during both baseflow and stormflow to provide more insight into carbon and nitrogen changes from urban land cover with further climate change.
Weber, Lisa Christina, "Biogeochemical and Hydrologic Dynamics in Urban Watersheds: Dissolved Organic Matter Quantity and Quality Changes Across an Urbanization Gradient During Baseflow and Stormflow Conditions in Temperate River Networks" (2022). Yale Graduate School of Arts and Sciences Dissertations. 534.