The construction and operation of over 1 million dams globally has fundamentally altered the way that water and nutrients flow through river networks. The reservoirs behind dams are important sites for nutrient and carbon cycling, but the patterns and controls on this cycling are still being elucidated. In the first chapter of this dissertation, temporal patterns in dinitrogen and nitrous oxide gradients were examined in the bottom boundary layer of a small eutrophic reservoir in southwest Washington. The findings highlight the potential importance of a region called the “internal shoreline” in supporting permanent nitrogen removal (e.g. denitrification). This chapter also lays groundwork towards the development of an in situ method for measuring denitrification rates at time scales of minutes to hours and spatial scales of 10s to 100s of meters. The second chapter examines inter-annual patterns and controls on water column redox biogeochemistry in the same reservoir. Across four years, there was less water column accumulation of reduced iron and methane when spring oxygen and nitrate availability was high. We also report dynamic spikes and drops in reduced solutes and elevated methane ebullition (e.g. bubbling) during an annual autumn water level drawdown event. We suggest that water level drawdowns enhance the transport of sediment pore waters into the water column via either groundwater influx or ebullition-driven exchange. Finally, the third chapter examines the magnitude and controls on global greenhouse gas emissions from reservoirs to the atmosphere. Methane emissions constituted 78-84% of emissions on a CO2 equivalent basis and were best predicted by reservoir productivity (chlorophyll a concentrations). Finally, we estimate that greenhouse gas emissions from reservoirs are comparable in magnitude to between 1 and 4.6% of all global human-caused CO2-equivalent emissions. Given that reservoirs are human-designed and operated systems, we highlight potential management implications associated with water level drawdowns and aquatic eutrophication. The findings presented here, together with projected increases in dam construction, call for further research to better discern the role that reservoirs play in biogeochemical cycling across scales.
|Advisor:||Harrison, John A.|
|Commitee:||Beutel, Marc W., Burgin, Amy J., Henderson, Stephen M.|
|School:||Washington State University|
|Department:||Environmental and Natural Resource Sciences|
|School Location:||United States -- Washington|
|Source:||DAI-B 77/11(E), Dissertation Abstracts International|
|Keywords:||Denitrification, Greenhouse gas, Internal wave, Management, Redox, Reservoir|
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