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Date of Award

Spring 2024

Document Type

Masters Thesis

Department or Program Affiliation

Biology

Degree Name

Master of Science (MS)

Department

Biology

First Advisor

Hooper, David U., 1961-

Second Advisor

Olson, M. Brady (Michael Brady)

Third Advisor

Compton, Jana E.

Fourth Advisor

Behling, Shawn

Abstract

The eutrophication of fresh and coastal waters is a growing global concern. Agricultural best management practices (BMPs) mitigate nutrient pollution, but their effectiveness at the watershed scale is often untested, creating uncertainty around which practices should be prioritized. In the Salish Sea, on the Pacific coast of Washington State and British Columbia, seasonal hypoxia threatens food webs, already-impacted salmon populations, and tribal fishing rights. To what extent can different BMPs alleviate N export in watersheds with heavy agricultural influence? We used the InVEST® Nutrient Delivery Ratio (NDR) model to estimate reductions in N export under BMP scenarios in the transboundary (US and Canada) Nooksack River watershed. In the process, we evaluated several aspects of NDR relevant to assessing watershed nutrient retention, including the addition of a groundwater component. We then calibrated and validated NDR against measured annual fluxes of total N from the whole Nooksack River watershed and multiple subwatersheds. We used the validated model to explore reductions in N export possible from factorial combinations of BMPs: riparian restoration on all waterways, 20% lower agricultural N input, and improving crop nitrogen use efficiency (NUE) to 75%. NDR estimated that lowering agricultural N input was the most effective BMP, followed by riparian restoration and improved NUE, with reductions in anthropogenic export by 22%, 17%, and 14%, respectively. Combining all BMPs yielded a 44% reduction in anthropogenic export from 2,458 to 1,725 (Mg N yr-1), though total N export was still over 2-fold greater than pre-industrial fluxes. Our results suggest that combining management practices is needed to effectively reduce N export from watersheds with heavy agricultural influence. When using default parameters, NDR chronically underestimated measured export, and we found that including subsurface N flux was essential for model estimates to match measured values of export. NDR did not allow detailed analysis of N retention in different land use or vegetation types. After extensive calibration, however, NDR was reliable in estimating the reduction of N export at the watershed level achievable from multiple BMPs. A 20% reduction in N inputs is estimated to be more effective than restoration of 100% of riparian buffers on waterways to reduce anthropogenic N export from the Nooksack River watershed. Future efforts should examine the cost effectiveness of these different practices and incentives available to implement such reductions.

Type

Text

Keywords

nitrogen, watershed, transboundary, biogeochemistry, riparian buffers, ecosystem service model, agriculture, best management practices, eutrophication, hypoxia

Publisher

Western Washington University

OCLC Number

1438955054

Subject – LCSH

Biogeochemistry--Washington (State)--Nooksack River Watershed; Best management practices (Pollution prevention)--Washington (State)--Nooksack River Watershed; Riparian areas--Management; Eutrophication--Washington (State)--Nooksack River Watershed; Hypoxia (Water)--Washington (State)--Nooksack River Watershed

Geographic Coverage

Nooksack River Watershed (Wash.)

Format

application/pdf

Genre/Form

masters theses

Language

English

Rights

Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.

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