Date of Award

Spring 2021

Project Type

Dissertation

Program or Major

Earth and Environmental Sciences

Degree Name

Doctor of Philosophy

First Advisor

Wilfred Wollheim

Second Advisor

Stephen Frolking

Third Advisor

Erin Hocthkiss

Abstract

River networks play an important role in elemental cycling at watershed, regional, and global scales. They not only serve as pipes that transport elements from land to sea, but also as complex processers that can significantly modify the timing, form, and magnitude of these elemental fluxes. For example, recent studies have shown that river networks contribute significantly to the global carbon cycle by emitting greenhouse gases like carbon dioxide (CO¬2) and methane (CH4) to the atmosphere. However, we currently do not fully understand the controls on GHG emissions from river networks, particularly what drives the variability of emissions across space and time. As such, a robust investigation of the factors controlling carbon delivery to and processing within river networks is critical to improve our ability to predict how changing climate and land use will alter rates of carbon emissions from river networks. This dissertation examines the sources of and controls on CO2 and CH4 emissions from streams and rivers. I used novel field monitoring designs and quantitative syntheses to estimate emissions of CO2 and CH4 from stream ecosystems while considering the factors that control spatial and temporal variability at multiple scales. Regarding CO2, I used a network of ten high-frequency CO2 sensors to explore how flow variability and storms affect CO2 emissions across streams and rivers of varying watershed size and land use. I also explore the relative contribution of diffusion and ebullition to the total CH4 flux in four small streams, and how these emission pathways vary across and within each stream. Combined with isotopic and microbial analyses, I consider how CH4 is produced, oxidized, and ultimately emitted from stream ecosystems and how this differs from other aquatic ecosystems. Results of this dissertation highlight the relevance of stream and rivers as emitters of CO2 and CH4, and highlight the dynamics of these gases as they move through these ecosystems. Storms are not overly important in annual budgets of CO2 emissions from streams and rivers, but do represent moments of heightened transfer of carbon from the terrestrial to aquatic environments that highlights the need to better understand this transfer. For CH4, the shallow, flowing nature of streams favor diffusive emissions and oxidation, which results in a relatively heavy isotopic signature of emitted CH¬4 relative to that produced in the sediments. Collectively, these results continue the work in moving beyond the “passive pipe” model of river networks, highlighting the dynamic nature of how streams and rivers collectively receive, store, transform, and transport carbon across spatial and temporal scales.

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