Year of Award

2020

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Systems Ecology

Department or School/College

W.A. Franke College of Forestry & Conservation

Committee Chair

H. Maurice Valett

Commitee Members

Robert Hall, Payton Gardner

Keywords

nitrate, biogeochemistry, diel, stream, metabolism

Publisher

University of Montana

Subject Categories

Terrestrial and Aquatic Ecology

Abstract

Tight coupling of surface water diel dissolved oxygen (DO) and nitrate-N (NO3-N) signals reflects stoichiometric demand of carbon and nitrogen in stream ecosystems. However, DO and NO3-N can become decoupled due to alternative drivers of diel solutes, resulting in conflicting estimates of stoichiometric and modeled NO3-N uptake. In this study, I measured benthic biomass, hydrology, and dissolved solutes in a montane stream located in western MT over a growing season (June-October 2019). Daily stream metabolism and NO3-N uptake were modeled using a single-station open-channel approach. Timing and amplitude of key diel signals were characterized quantitatively to assess decoupling of DO and NO3-N and investigate diel variation in hydrology. I also analyzed the effect of DO and NO3-N benthic footprint lengths on diel signals. Miller Creek was heterotrophic, with ER ranging from -2.07 ± 0.37 to -5.53 ± 0.11 g Om-2 d-1 and GPP from 0.03 ± 0.04 to 0.82 ± 0.10 g Om-2 d-1. Statistical assessment with generalized additive models (GAMs) tied metabolism to localized channel conditions. Reductions in GPP, water temperature, and hydrology predicted declining ER (R2adj = 0.72, n = 124, 72.8% deviance explained). ER also decreased with benthic standing stocks, as measured with AFDM (r = -0.74, p = 0.034) and chl a (r = -0.92, p = 0.0013), and surface water dissolved organic carbon (DOC) concentration (r = -0.73, p = 0.038). Modeled NO3-N uptake (3.20 ± 1.50 to 12.14 ± 2.16 mg N m-2 d-1) agreed with stoichiometric estimates (0.42 ± 0.05 to 10.10 ± 3.5 mg N m-2 d-1) in magnitude, despite clear decoupling of diel DO and NO3-N signals. GPP, water temperature, surface water flow, and light explained 92.1% of the deviance in modeled NO3-N uptake (R2adj = 0.91, n = 110). Daily benthic footprint of NO3-N ranged from 2.5 to 19.5 km, exceeding that of DO on some sampling days, which varied from 1.2 to 4.1 km. This research provides a model for calculating daily NO3-N uptake comparable to estimates from stoichiometry using a single-station approach, which can be applied in streams and rivers with diel DO and NO3-N decoupling.

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© Copyright 2020 Kimberly Bray