Multiple Stressor Effects on Urban Aquatic Ecosystem Function: From the Physically Obvious to the Chemically Subtle

One of the major challenges in understanding aquatic ecosystems is teasing apart the interrelated influences of multiple stressors on ecosystem function to determine their relative importance. Urban areas are expanding across the globe at unprecedented rates, and as low-lying areas within landscapes, streams and ponds are particularly hard hit by the multiple stressors associated with the urban stream syndrome. This dissertation investigates the effects of stressors as fundamental drivers of urban freshwater ecosystem function in lentic and lotic systems.

Stormwater ponds and retention basins are ubiquitous lentic features throughout urban landscapes that potentially serve as important control points for nitrogen (N) removal from surface water bodies via denitrification. However, there are possible tradeoffs to this water quality benefit if high N and contaminant concentrations in stormwater pond sediments decrease the complete reduction of nitrous oxide (N₂O), a potent greenhouse gas, to dinitrogen (N₂) during denitrification. Here, I evaluated whether urban stormwater pond sediments from 64 ponds across eight major US cities had elevated potential emissions of N₂O (Chapter 2). I found surprisingly little correlation between surrounding land cover urbanization intensity and pond sediment chemistry. I measured highly variable potential rates of denitrification, but generally low proportions of N₂O relative to total denitrification, allaying the concerns that motivated the study. However, the lack of a relationship between land cover and sediment chemistry within urban ponds calls into question our commonly held assumptions about the relationships between development intensity and the loading, routing, and retention of nutrient and contaminants within urban landscapes.

The typically elevated loading of chemical constituents into urban freshwater lotic ecosystems is due to the efficient routing of water over surfaces heavily modified by human activities (i.e. stormwater and wastewater infrastructure). In this study, I investigated the dominant controls on ion routing and loading within 24 urban watersheds that fell within a narrow range of development intensity but spanned the widest possible range in spatial configuration and connectivity metrics in the Triangle region of the North Carolina Piedmont (Chapter 3). By pairing analysis of land cover attributes with temporal trends in baseflow chemistry and high-frequency data of specific conductance and discharge, I found that increases in watershed road and pipe density lead to increasingly chemically distinct stormflows and baseflows. This enhanced bimodality of both flow and chemistry results in more variable chemical regimes in watersheds where linear urban infrastructure (roads and pipes) connects impervious surfaces directly to streams.

In addition to altered chemical loading, headwater streams draining urbanized catchments are subject to frequent and intense flooding. Here, I investigated how altered hydrologic regimes in urban landscapes affect headwater stream form and function (Chapter 4). I found a surprisingly wide range in dissolved oxygen regimes, ranging from frequent hypoxia to near constant saturation, both of which resulted in net heterotrophic streams with low rates of productivity. This work, paired with work presented in Chapter 3, advances understanding of carbon cycling within streams by documenting a shift in ecosystem dynamics in urban streams towards bimodality between the fast dynamics of advective transport from pavements and hillslopes during storms, and the slow dynamics of redox active zones in eutrophic and organically enriched stream pools at base flow.

Stream ecosystems draining highly urbanized areas are often some of the most extreme cases of altered physical and chemical stressor regimes and provide a testbed for examining the influence of these drivers on ecosystem function. The findings presented improve our understanding of how nutrients and contaminants move through landscapes, undergo biogeochemical transformations, and their ultimate effect on aquatic ecosystem processes.