Alterations of ecosystem processes as a result of anthropogenic modifications to streams and their catchments
Gibson, Catherine Ann
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Ecosystem services are the benefits human societies derive from ecosystem processes. Freshwater ecosystems provide many ecosystem services including clean water and waste assimilation. However, humans are altering freshwater ecosystems through practices such as hydrologic alteration and nutrient loading. The objective of this dissertation is to examine how anthropogenic alterations of streams and/or their catchments affect ecosystem processes and concomitantly the ability of streams to provide ecosystem services. I focus on effects of three human modifications: climate change, dam operation, and urbanization. Future climate scenarios can cause dramatic shifts in flow regimes in the Cle Elum and the Apalachicola-Chattahoochee- Flint River systems. These flow regime changes would result in decreased habitat availability and changes in food resources and demonstrate that these types of analysis are a starting point four understanding the ecological implications of future climate scenarios. The Chattahoochee River below Atlanta, GA is regulated by hydropower dams and receives most of Atlanta’s wastewater treatment plant effluent. Processing of the effluent-derived fine particulate organic matter (FPOM) and nutrients is essential for maintaining downstream water quality. Hydropeaking reduces gross primary production. FPOM and phosphorus (P) loading increases respiration and overwhelms the effects of low flow discharge regime on respiration. Nutrient retention by the river is sporadic and temporary. P retention is controlled by sediment absorption, but retention is temporary as P is readily released from sediments. The study reaches are more likely to be sources of ammonium than sinks. Nitrate uptake is controlled by biological uptake associated with suspended FPOM. Overall, P and ammonium uptake velocities in the Chattahoochee River are lower than those in less modified systems. Urban headwater streams had lower uptake velocities and higher ambient nutrient concentrations than forest streams in the Etowah Basin, GA. P uptake is controlled chemical sorption to sediments. Ammonium uptake is controlled by biological processes. Conserving functioning riparian areas and channels in a conservation subdivison reduces some urbanization effects and maintains nutrient retention. Human alterations have diminished the capacity of these rivers and streams to provide ecosystem services, but reducing hydropeaking, decreasing nutrient loading, and maintaining riparian areas and stream channels could increase the ability of urban streams and rivers to provide the services that human communities value.