High algal biomass is decoupled from metabolism in the Gallatin River

Author

• Cora Mae Steinbach, University of Montana, Missoula

Year of Award

2026

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Ecology and Evolution

Department or School/College

Division of Biological and Biomedical Sciences

Committee Chair

Dr. Robert O. Hall

Commitee Members

Dr. Matthew J. Church, Dr. Andrew C. Wilcox

Keywords

algal blooms, filamentous algae, ecosystem metabolism, eutrophication, primary production, ecosystem ecology

Subject Categories

Biogeochemistry | Statistical Models | Terrestrial and Aquatic Ecology | Water Resource Management

Abstract

Assessing eutrophication in rivers is difficult compared to lakes and coastal waters, because most algal biomass occurs on the riverbed and flows interact and co-vary with production (Biggs and Close, 1989; Bernhardt et al., 2018). Riverine eutrophication is typically assessed using algal biomass and water column nutrients (U.S. Environmental Protection Agency, 2000), but biomass is highly variable and labor-intensive to measure, while nutrient concentrations often underestimate enrichment due to rapid biological uptake (Dodds and Smith, 2016). Reach-scale river metabolism can help evaluate long-term functional change in rivers recovering from nutrient enrichment (Arroita et al., 2019; Jankowski et al., 2021; Diamond et al., 2022), but studies directly linking algal structure and biomass to ecosystem metabolism have produced mixed results (Genzoli and Hall, 2025; Carter et al., 2025). For the nutrient impaired Gallatin River (Southwest Montana, USA), we tested how metabolism could detect the establishment of filamentous algal blooms, and more broadly, algal assemblage change. To address this objective, we estimated daily metabolism and measured three algal growth forms for six reaches with and without a history of algal blooms. Within all reaches, algal assemblages shifted from low-biomass biofilms to high-biomass nuisance taxa, yet GPP remained relatively stable throughout the summer. Among reaches, algal assemblages also differed markedly, yet metabolism showed no consistent differences between assemblage types, and cumulative NEP did not cleanly track biomass accumulation. Consequently, metabolism remained a poor proxy for algal biomass despite substantial variation in assemblage structure. We suggest that metabolism was more closely linked to rates of algal growth and turnover rather than standing biomass, underscoring a fundamental limitation of metabolism as a proxy for algal structure. Evidence remains limited that metabolism can detect algal assemblage change in rivers; instead, we conclude that metabolism captures ecosystem function well (Jankowski et al., 2021) but could not resolve underlying biological structure in the Gallatin River.

Recommended Citation

Steinbach, C. (2026). High algal biomass is decoupled from metabolism in the Gallatin River. Master's thesis, University of Montana, Missoula, Montana.