Date of Award

Spring 2025

Project Type

Thesis

Program or Major

Natural Resources and Environmental Studies

Degree Name

Master of Science

First Advisor

Adam S Wymore

Second Advisor

Wilfred M Wollheim

Third Advisor

Alexandra R Contosta

Abstract

Carbon dioxide (CO2) is a potent greenhouse gas, and surface water concentrations are often supersaturated relative to the atmosphere. To fully quantify CO2 dynamics in stream ecosystems, it is critical to understand sources of stream CO2, particularly the link between terrestrial and aquatic ecosystems. It is unclear how soil CO2 enters streams and to what magnitude. To address this knowledge gap, I analyzed high-frequency CO2 time-series of soil and stream water sensor data to understand source-sink dynamics and time lags between the two pools. I used seven years of hourly sensor data from three soil depths with upscaled 15-minute stream data from the Lamprey River Hydrological Observatory, in New Hampshire, USA. I hypothesized that soil and stream CO2 concentrations would be linked through hydrologic mechanisms that varied by soil depth. To test my hypothesis, I used a reduced major axis regression of the relationship between soil and stream CO2 for each soil depth (0, 15, 30 cm) across multiple discharge bins. The results indicated that soil and stream CO2 dynamics were linked under high flow conditions (75-100% discharge) at each soil depth. Further, I found consistently negative relationships under drought-like conditions (0-5% discharge) suggesting that the streams and soils are not hydrologically linked at low flow and stream CO¬2 is primarily from autochthonous production. I used a cross-correlation function to investigate the time lag of soil and stream water CO2 and found that on the weekly time-step scale, soil-stream CO2 lagged on average 6-7 hours across soil depths. While past studies have found that a percentage of terrestrially-derived CO2 that is exported to aquatic systems, this study leveraged high-frequency datasets to specifically understand how the terrestrial-aquatic links occur in space and time.

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