Spatial and temporal patterns of nutrient limitation in a western Montana river

Author

Matthew T. NicholsFollow

Year of Award

2025

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Systems Ecology

Department or School/College

Forestry

Committee Chair

Dr. Robert O. Hall, Jr.

Commitee Members

Dr. Matthew J. Church, Dr. Michael D. DeGrandpre

Keywords

rivers, streams, nutrients, eutrophication, Clark Fork

Subject Categories

Terrestrial and Aquatic Ecology

Abstract

Enhanced loading of nitrogen (N) and phosphorus (P) is pervasive in rivers around the world. As a consequence of nutrient-rich conditions, many well-lit rivers experience severe algal blooms, which threaten water quality and aquatic habitats. Nutrient enrichment in the upper Clark Fork River, located in western Montana, has increased the productivity of benthic algae, Cladophora glomerata, which routinely forms extensive blooms during summer growing seasons (July – September). Addressing eutrophication in rivers begins with identifying the nutrients that contribute toward algal productivity. I deployed nutrient diffusing substrates (NDS), collected bi-weekly surface water samples, and conducted algal surveys at four locations during the early, mid, and late-growing season to assay potential limitations to algal growth. NDS growth responses were assessed using a multilevel regression model with Bayesian inference to characterize single and co-limitation. I used model estimates to compare treatment responses, each representing multiplicative differences relative to the control. For all three deployments, each site was co-limited (early: 159 – 349%; mid: 165 – 401%; late: 80 – 282%), with strong P-limitation occurring at locations with relatively high dissolved inorganic N concentrations (267 µg/L ± 22.7) or chronically poor nutrient conditions. N-limitation responses occurred across 6 out of 12 total deployments and was most frequent in lower reaches (late-season: 40 – 100%), as nutrient concentrations decreased further downstream. Algal blooms were seasonally variable across study locations and did not follow patterns of nutrient availability. For instance, algal biomass collapsed at Sites 1, 2, and 4 with each location having depleted or enhanced N and P concentration. Conversely, algal cover at Site 3 increased 8-fold (from 4 to 34%), with declining inorganic and total nutrient concentrations. Changes in nutrient availability did not produce predictable changes in algal succession across my study locations, nor did they elicit consistent bioassay responses. Instead, each site had strong co-limitation responses that were decoupled from trends in algal biomass and N:P availability (i.e., atomic Redfield Ratio of 16N:1P). My results show that consistently strong co-limitation can occur across a wide range of N:P ratios, underscoring the seasonal influence of factors other than nutrient concentrations alone as contributing toward algal growth and senescence.

Recommended Citation

Nichols, Matthew T., "Spatial and temporal patterns of nutrient limitation in a western Montana river" (2025). Graduate Student Theses, Dissertations, & Professional Papers. 12515.
https://scholarworks.umt.edu/etd/12515