Date of Graduation

8-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Biology (PhD)

Degree Level

Graduate

Department

Biological Sciences

Advisor/Mentor

Michelle Evans-White

Committee Member

Kristofor Brye

Second Committee Member

Daniel Magoulick

Third Committee Member

Kusum Naithani

Keywords

Aquatic Ecology, Biogeochemistry, Carbon Dynamics, Flow Regime, Riparian Soils, Stream Metabolism

Abstract

The natural flow regime exerts primacy over lotic ecosystem patterns and processes. However, little work has examined the influence of flow regime on instream and riparian carbon (C) dynamics in minimally-impacted, temperate forested systems. To date, most research efforts have focused on characterizing C movement and transformations across biomes and land use categories; however, flow regime represents an overlooked, finer level of detail that may drive differences in ecosystem function. My dissertation objective was to measure C fixation and movement within and across multiple environmental spheres (e.g. within stream channels, between stream surfaces and the atmosphere, and from riparian soils to the atmosphere) to determine whether estimated differences in hydrologic characteristics may give rise to natural variation in ecosystem function. I measured stream C gross primary production (GPP) and ecosystem respiration (ER), stream carbon dioxide (CO2) and methane (CH4) flux rates and sources to the atmosphere, and riparian soil C respiration for study sites classified into the two dominant natural flow regimes in northern Arkansas: Groundwater Flashy (i.e. Groundwater) and Runoff Flashy (i.e. Runoff). Groundwater streams tended to exhibit greater and more variable GPP, ER, and CO2 -evasion and CH4 -evasion across sites than Runoff streams, though no differences between flow categories were statistically significant. Soil respiration differed across seasons and differed between flow regimes during spring (p= 0.01). Runoff sites tended to have greater rates of soil respiration over the year. Hydrologic variables explained up to 88% variation in stream processes even though categorical comparisons between natural flow regimes often did not differ. Field-estimated annual GPP was negatively impacted by the number of no-flow days across flow regimes (r= -0.88, p= 0.02), and annual net metabolism became more heterotrophic with increasing average annual discharge across sites (r= -0.74, p= 0.002). In addition, source composition of C fluxes to the atmosphere depended upon hydrology; isotopic data revealed both biogenic and thermogenic CH4 in Groundwater streams while Runoff streams only evaded thermogenic CH4. My efforts provide evidence that instream and riparian soil processes, such as GPP, ER, and C gas fluxes, are influenced by variation in hydrology within a forested biome.

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