Year of Award

2021

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Systems Ecology

Committee Chair

Dr. H. Maurice Valett

Commitee Members

Dr. Robert Hall Dr. Matthew Church Dr. Michael DeGrandpre

Publisher

University of Montana

Subject Categories

Environmental Sciences | Water Resource Management

Abstract

Studies relating ecosystem energetics to nutrient uptake in streams have generally found respiration to be a more dominant control on nutrient dynamics relative to primary production due to light limitation in most low-order systems. In this study, I measured biomass, hydrology, metabolism, and dissolved solutes in an open-canopy mid-order river located in western MT. Daily stream metabolism was modeled using a modified single-station, open-channel method, and measures of dissolved solutes were evaluated at both local and reach scales to examine how point-measures and those integrated through space represent biogeochemical behavior. Metabolic rates and standing stocks of benthic organic matter were greater at Site 2 despite greater nutrients loads delivered to Site 1. Benthic organic matter ranged from 1.11-60.4 g OM m-2 and chlorophyll a from 40.9-150.9 mg Chl a m-2 between sites. GPP ranged from 1.89 to 11.14 g O2 m-2 and ER from -0.93 to -8.66 g O2 m-2 d -1 with greater rates observed at Site 2. The fraction of GPP consumed as autotrophic respiration (ARf) was high at both sites, 76 and 66% for Sites 1 and 2, respectively. Mean loads for NO3-N at Site 1 (61.29 ± 4.92 kg N d-1 ) were greater than loads delivered to Site 2 (27.02 ± 3.36 kg N d-1 ), and an effective solute flux (Ueff) calculated from mass-balance that the reach between sites is a biological sink for NO3-N. Compared to hydrologic losses, biological uptake (Ueff-bio) accounted for nearly 90% of the reduction in material loads. Correlation between Ueff-bio-NO3 and NEP (r2 = 0.22, p < 0.01), as well as calculated autotrophic N demand (Udem) derived from NPP (r2 = 0.29, p < 0.01), suggests that autotrophic assimilation is a major control on changes in material load. Average Udem (147.7 ± 6.06 mg N m-2 d -1 ), however, was nearly triple the mean value for Ueff-bio-NO3, suggesting unmeasured sources of DIN linked to autotrophic assimilation via other processes such as Nfixation, nitrification, and N mineralization.

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