Date of Award:

8-2018

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Ecology

Committee

Trisha Atwood

Abstract

Due to growing human demands for freshwater within the last century, manmade flow alterations are now a common characteristic of rivers worldwide. Alterations to the volume and timing of flows in rivers are known to negatively impact aquatic biodiversity, biological productivity and ecosystem functions such as nutrient cycling. While previous research has focused on the effects of flow reductions and spates, there is a lack of knowledge on how high flows across longer timespans (i.e. ‘enhanced flows’) impact the structure of river communities and the integrity of ecosystem functions.

The Utah Reclamation, Mitigation and Conservation Commission has expressed interest in reducing enhanced flows in a central Utah river so as to benefit aquatic habitat and native game fish. With their funding support, I collected aquatic invertebrate and ecosystem function data under natural and enhanced flow conditions throughout Sixth Water Creek and Diamond Fork River. These data were supplemented with historical invertebrate data collected by BIO-West, Incorporated. For each river segment, I tested whether flow volume and variation had relationships with the density and health of streambed invertebrate communities. I also sought to determine how these flow metrics affected leaf litter breakdown, primary production and stream metabolism (i.e. the production and respiration of organic matter).

Here, I demonstrate that enhanced flows impact streambed invertebrate communities differently in each river segment, in addition to suppressing stream metabolism system-wide. While streambed communities were mostly unaffected by enhanced flows in Sixth Water Creek, density and community health exhibited negative relationships with flow metrics in Diamond Fork River. In the Lower Diamond Fork mainstem, proportions of pollution-tolerant taxa increased as peak flows increased, while streambed invertebrate density decreased as mean monthly flows increased. Moreover, enhanced flows appeared to severely impact the production of organic matter in Sixth Water Creek and Lower Diamond Fork River. This disproportionate suppression of production over respiration shifted the river towards greater reliance on out-of-stream energy sources. These results demonstrate the importance of considering spatial gradients when investigating community responses to flow alterations, and also reveal how river ecosystems are threatened with losses of in-stream energy supplies under enhanced flows.

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