Year of Award

2025

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Environmental Studies

Department or School/College

EVST

Committee Chair

Dr. Robin Saha

Committee Co-chair

Dr. Vicki Watson

Commitee Members

Dr. Vicki Watson, Dr. Robin Saha, Dr. Brian Chaffin, Dr. Samuel Carlson

Keywords

algal biomass, nutrient pollution, hydrologic disturbances, river management

Abstract

Algal communities play a vital role in aquatic ecosystems, but excessive growth can degrade water quality, disrupt ecosystem function, and hinder beneficial uses (Banish, 2017; Davie, 2012; Wang et al., 2007; Wurtsbaugh et al., 2019). Nutrient enrichment, particularly elevated nitrogen and phosphorus concentrations, is a well-established driver of algal proliferation in lentic systems. However, these nutrient–algae relationships are less predictable in lotic systems, where the dynamic effects of flow can modify or obscure these patterns (Bennet et al., 2017; Biggs and Close, 1989; Dodds et al., 2002). The Clark Fork River in western Montana has long struggled with nuisance algal growth, prompting management efforts such as the Voluntary Nutrient Reduction Program (VNRP) and the establishment of the state’s first numeric nutrient and algae standards. Since the development of the VNRP, in-stream nutrient concentrations have declined, and where nutrient standards are met, algae standards are also being met. However, exceedances still occur in the upper river, and nuisance algal blooms persist, suggesting the need for updated management strategies (Hydrosolutions, 2025; Tri-State Implementation Council, 1998; Suplee et al., 2012). To better understand the drivers of persistent algal blooms in the Clark Fork River, this study examined the relationships between in-stream nutrient concentrations, scouring flows, and algal biomass using simple and multiple linear regressions and model comparisons. Total nitrogen emerged as the strongest individual predictor (p << 0.001), explaining 16.5% of the variance in chlorophyll-a concentrations. In multiple regression, the inclusion of an interaction between scouring flows and total nitrogen provided enhanced explanatory power to the model (p << 0.001), explaining 20.2% of the variance in chlorophyll-a. These results indicate that scouring flows act as an additional environmental control on algal growth in the Clark Fork River and highlight the importance of integrating nutrient and flow dynamics in river management. Through revealing additional factors influencing the spatial and temporal dynamics of algal growth, results can be used to provide a basis for updating the aging VNRP/TMDL framework, as well as guiding the development of the Central Clark Fork Watershed Restoration Plan (CCFWRP) to support more targeted and adaptive nutrient reduction strategies.

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© Copyright 2025 Hannah E. Adkins