Date of Award

Spring 3-2023

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

School

Biological, Environmental, and Earth Sciences

Committee Chair

Jake Schaefer

Committee Chair School

Biological, Environmental, and Earth Sciences

Committee Member 2

Brian Kreiser

Committee Member 2 School

Biological, Environmental, and Earth Sciences

Committee Member 3

Ginny Adams

Committee Member 4

Todd Slack

Committee Member 5

Frank Heitmuller

Committee Member 5 School

Biological, Environmental, and Earth Sciences

Abstract

Sediment dynamics are foundational to stream and watershed morphology. Yet aquatic ecologists have relied on an oversimplified model of sediment dynamics characterizing sediments as agents of stream bed burial, and which fails to describe many types of aquatic habitat evolution. In this dissertation I employ both fluvial geomorphic and ecological frameworks to gain a deeper understanding of how sediment dynamics shape stream morphology and fish community evolution at multiple scales. Using a paired historic and contemporary approach, I analyzed geomorphic evolution and fish community change in the Bayou Pierre, Mississippi, from the 1980s to recent. Patterns of erosion due to headcutting have advanced considerably, leading to wide, shallow, homogenous channels with coarser substrates, opposite of stable conditions in the Gulf Coastal Plain. Fish communities homogenized, and community change was tied to geomorphic process change over time. Examining the minnows (a taxonomically and ecologically diverse group), species persisting in the highly unstable mainstem and larger tributaries were characterized by traits indicative of mobility and ecological adaptability. Communities in these reaches had spatial patterns suggesting effects processes. Further examination of the Bayou Pierre showed sedimentary process connectivity at the watershed scale, strong ties between geomorphic activity and aquatic habitat conditions, evidence of an increased pulse of erosion and ongoing relaxation corresponding to the floods of 1983, and evidence that this event led to considerable gravel bar consolidation and habitat fragmentation in the mainstem. I then examined patterns of genetic structure for Fundulus olivaceus, a headwater specialist, in seventeen watersheds throughout southern Mississippi and Louisiana in relation to fluvial geomorphic activity. Planform analysis of the watersheds demonstrated a gradient between stable and unstable, and also detected evidence of a pulse of erosion and relaxation. Analysis of genetic structure and heterozygosity suggested fragmentation of dispersal and potential bottlenecks associated with channel instability. STRUCTURE analysis revealed a complicated pattern of population structure, with five of 82 populations being assigned to a neighboring watershed, potentially either from stream capture or altered extinction rates due to fluvial geomorphic activity. This work demonstrates clear linkages between geomorphic processes, altered channel morphology, and altered local and spatial ecological processes.

ORCID ID

0000-0002-9579-2076

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