Theses and Dissertations

More than half of all accessible freshwater has been appropriated for human use, and a substantial portion of terrestrial ecosystems have been transformed by human action. These impacts are heaviest in urban ecosystems, where impervious surfaces increase runoff, water delivery and stormflows are managed heavily, and there are substantial anthropogenic sources of nitrogen (N). Urbanization also frequently results in creation of intentional novel ecosystems. These "designed" ecosystems are fashioned to fulfill particular needs of the residents, or ecosystem services. In the Phoenix, Arizona area, the augmentation and redistribution of water has resulted in numerous component ecosystems that are atypical for a desert environment. Because these systems combine N loading with the presence of water, they may be hot spots of biogeochemical activity. The research presented here illustrates the types of hydrological modifications typical of desert cities and documents the extent and distribution of common designed aquatic ecosystems in the Phoenix metropolitan area: artificial lakes and stormwater retention basins. While both ecosystems were designed for other purposes (recreation/aesthetics and flood abatement, respectively), they have the potential to provide the added ecosystem service of N removal via denitrification. However, denitrification in urban lakes is likely to be limited by the rate of diffusion of nitrate into the sediment. Retention basins export some nitrate to groundwater, but grassy basins have higher denitrification rates than xeriscaped ones, due to higher soil moisture and organic matter content. An economic valuation of environmental amenities demonstrates the importance of abundant vegetation, proximity to water, and lower summer temperatures throughout the region. These amenities all may be provided by designed, water-intensive ecosystems. Some ecosystems are specifically designed for multiple uses, but maximizing one ecosystem service often entails trade-offs with other services. Further investigation into the distribution, bundling, and tradeoffs among water-related ecosystem services shows that some types of services are constrained by the hydrogeomorphology of the area, while for others human engineering and the creation of designed ecosystems has enabled the delivery of hydrologic ecosystem services independent of natural constraints.

Nitrogen is an essential, often limiting, element for biological growth that can act as a pollutant if present in excess. Nitrogen is primarily transported by water from uplands to streams and eventually to recipient lakes, estuaries, and wetlands, but can be modulated by biological uptake and transformation along these flowpaths. As a result, nitrogen can accumulate in aquatic ecosystems if supply is high or if biological retention is low. Dryland and urban ecosystems offer interesting contrasts in water supply, which limits transport and biological activity in drylands, and nitrogen supply that increases with human activity. In my dissertation, I ask: What is the relative balance among nitrogen retention, removal, and transport processes in dryland watersheds, and what is the fate of exported nitrogen? My dissertation research demonstrates that water is a major control on where and when nitrogen is retained and removed versus exported to downstream ecosystems. I used a mass-balance model based on synoptic surveys to study seasonal and spatial patterns in nitrate loading to a dryland stream network. I found that irrigation diversions transport nitrate from agricultural areas to the stream network year-round, even during dry seasons, and are an important driver of nitrate loading. I further explored how seasonal precipitation influences flood nutrient export in an intermittent desert stream by coupling long-term data of flood-water chemistry with stream discharge and precipitation data. I found that higher precipitation prior to a flood fills water storage sites in the catchment, leading to larger floods. In addition, higher antecedent precipitation stimulates biological nitrogen retention in the uplands, leading to lower nitrogen concentration in floods. Finally, I evaluated the consequences of nitrogen export from watersheds on how urban wetlands attenuate nitrate through denitrification that permanently removes nitrogen, and dissimilatory nitrate reduction to ammonium (DNRA) that retains nitrogen in another biologically reactive form. I found that DNRA becomes proportionally more important with low nitrate concentration, thereby retaining nitrogen as ammonium. Collectively, my dissertation research addresses how dryland and urban ecosystems can be integrated into models of watershed nitrogen cycling.