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PhD defense - Joel Singley

Monday, May 24, 2021, 10:00AM - 12:00PM


Joel G. Singley

Dissertation title

Stream corridor connectivity controls on nitrogen cycling


As water flows downstream, it is transported to and from environments that surround the visible stream. Along with surface water, these laterally and vertically connected environments comprise the stream corridor. Stream corridor connectivity influences many ecosystem services, including retention of excess nutrients. The subsurface area where stream water and groundwater mixes—the hyporheic zone—represents one of the most biogeochemically active parts of stream corridors.

The goal of my research is to advance understanding of how connectivity between different parts of a stream corridor controls the availability and retention of nitrogen (N), a primary nutrient that can negatively impact water quality. First, I developed and applied a new machine learning method to objectively characterize the extent and variability of hyporheic exchange using geophysical data. In applying this method to a benchmark dataset, I found that hyporheic extent does not scale uniformly with streamflow and that changes in the heterogeneity of connectivity differ over small (<10 m) distances. Next, I leveraged the relative simplicity of ephemeral streams of the McMurdo Dry Valleys (MDVs), Antarctica, to isolate stream corridor processes that influence the fate of N. Through intensive field sampling campaigns, I found that the hyporheic zone can be a persistent source of N even in this low nutrient environment. Next, I combined historic sample data and remote sensing analysis to estimate how much N is stored in an MDV stream corridor. My results indicate that up to 104 times more N is stored in this system than is exported each year, with most of this storage in the shallow (< 10 cm) hyporheic zone. Lastly, I examined 25 years of data for 10 streams to assess how stream corridor processes control concentration-discharge relationships. I found that in the absence of hillslope connectivity, stream corridor processes alone can maintain chemostasis – relatively small concentrations changes with large fluctuations in streamflow – of both geogenic solutes and nutrients. My analysis also revealed that solutes subject to control by biological processes exhibit more variability within chemostatic relationships than weathering solutes.

Altogether, this research advances characterization of processes that are difficult to measure or are often overlooked in typical studies of temperate stream corridors. These studies provide insight into the surprising ways in which N is mobilized, transformed, and retained due to stream corridor connectivity in intermittent stream systems with few N inputs.


Free and the public is welcome - please email for Zoom link and password.