Date of Award:

5-2021

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Watershed Sciences

Committee Chair(s)

Janice Brahney

Committee

Janice Brahney

Committee

Bethany Neilson

Committee

Charles Hawkins

Abstract

Human land-use can increase the amount of non-point source (NPS) pollution in a stream, negatively affecting ecosystem health and beneficial services provided by an ecosystem. Unfortunately, NPS pollution remains high in many waterbodies. Beaver dams may be a passive, cost-effective strategy for removing NPS pollution in headwater streams because beaver dams slow stream flow and collect sediments. Impounded sediments can change how nutrients and pollutants are cycled in a stream through multiple pathways. In the first part of our study, we investigated whether beaver activity can reduce nitrogen, phosphorous and heavy metals from otherwise traveling downstream. Results suggest beaver ponds influence the fate of NPS pollutants at intermediate levels of sediment-water interaction, which is defined by the amount of time water spends in the pond, the amount of a given nutrient delivered to the pond from upstream, and the volume of sediment behind the dam. We conclude that under specific conditions, beaver ponds are significant sinks for nitrogen. Further, phosphorous retention or production in beaver ponds is variable and may be related to season or pond age.

Beaver ponds host a mosaic of environmental conditions and as a result can enhance nitrogen cycling within a stream. Characteristics such as valley shape and slope, or sediment-size distribution likely influence drivers of biogeochemical processing, thus the geomorphic composition of a beaver pond may facilitate unique biogeochemical pathways. The second part of our study aimed to identify where and when specific nitrogen reactions occur in a pond. Our study found that while beaver ponds can change how nitrogen is cycled, the net effect of a given beaver pond depends on the distribution of environmental conditions and thus its geomorphic composition. We found that the origin, processing and transformation of nitrogen is different in spring versus summer and is enhanced in sediments that are oxygen-poor and organic-rich. Overall, this research determined the fate and cycling of nitrogen, phosphorous and trace heavy metals in beaver-altered headwater streams.

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