Energetic Opportunities and Chemical Risks in Urban and Forested Stream Ecosystems

Abstract

Urban watersheds are dynamic ecological systems shaped by social, chemical, physical, and ecological forces. Urban stressor and disturbance regimes alter the availability and incorporation of energetic resources and chemical risks into stream ecosystems, and their ultimate transport and fate to downstream or paired terrestrial ecosystems. In this dissertation, I investigate how human-driven stressors (land-use, organic and inorganic chemical contaminants, heat, and hydrologic "flashiness") alter the energetic and chemical regimes of each system. Study systems include stream ecosystems that drain urban (Ellerbe Creek) and forested (New Hope Creek) watersheds in the Piedmont of North Carolina. In Chapter 1, I quantified the concentrations of chemical indicators in a novel approach to characterize the timing and spatial distribution of three common mixtures of contaminants (and their chemical indicator) in our urban watershed (Ellerbe Creek): treated and untreated sewage (sucralose, artificial sweetener), lawncare runoff (DPG and 6PPD-Q, automobile tire additives), and road runoff (Glyphosate and AMPA, active ingredient of herbicide RoundUp® and its major degradation product). We demonstrate that the concentrations of these chemical indicators, and by extension the mixtures of contaminants they represent across the heterogenous urban watershed, are highly variable across the landscape, seasons, and discharge regimes. In Chapter 2, I measured rates of ecosystem metabolism (gross primary production and ecosystem respiration), macroinvertebrate secondary production, and aquatic insect emergence to assess the efficiency at which energy moves through two types of urban stream ecosystems (wastewater and stormwater dominated) in Ellerbe Creek and our forested watershed in New Hope Creek. Urban disturbance regimes alter the composition, magnitude, and timing of energy availability. In turn, the efficiency at which energetic inputs were converted into primary production did not readily predict macroinvertebrate secondary production and emergence in our urban sites. This is the first known investigation of primary and secondary production in an urbanized watershed, and it provides compelling evidence that energetic regimes in two common types of urban watersheds (wastewater and stormwater dominated) are notably distinct from one another. Finally, in Chapter 3, emergent insects collected for Chapter 2 were assessed for trace metals to investigate how distinct communities of emergent winged insects altered the timing, magnitude, and composition of metals flux. I demonstrated that the transport of positive (energy) and negative (trace metals) subsidies is driven by distinct communities of organisms, which are simplified and constrained in our wastewater and stormwater dominated sites, rather than total emergent biomass or metals loading alone. Families of aquatic insects were differentially efficient at moving metals (Zn, Cu, Se) out of the stream food web and into the terrestrial food web. The biological community dynamics—composition, diversity of traits, and timing—play an equally if not more important role in the magnitude, timing, and composition of metals flux relative to metals inputs alone. The availability, assimilation, and transformation of energetic and contaminant inputs in-stream will ultimately enable (or suppress) export into paired riparian and terrestrial ecosystems. In human impacted systems, such as urban watersheds, analysis requires methods, models, and concepts in ecosystem and community ecology, urban ecology, environmental chemistry, and biogeochemistry. By further exploring the heterogeneity of urban watersheds, as demonstrated in this dissertation, stakeholders can better manage these ecosystems to support wildlife communities and human societies.