Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Geography

Supervisor

Yates, Adam G.

Abstract

Agricultural development of lands in Southern Ontario, Canada, have resulted in many headwater streams being sourced by agricultural tile drains instead of wetlands. Tile drainage inputs can influence stream conditions (i.e., temperature, hydrology, and water chemistry) that are important drivers of ecological function. To assess the influence of agricultural tile drainage inputs on stream ecosystem function, I compared heterotrophic activity (i.e., organic matter breakdown and benthic respiration) in forested streams originating from wetland and agricultural tile drainage sources over four seasons. I found a reduction in heterotrophic activity in the tile-sourced stream, particularly in the summer, that appeared to be due to reduced stream temperatures from influxes of groundwater. Reduced heterotrophic activity was also evident in downstream network segments. My findings suggest there may be a widespread reduction in heterotrophic activity in streams across agricultural regions where tile drainage is prevalent.

Summary for Lay Audience

In Southern Ontario, Canada, many headwater streams were historically sourced by wetlands. However, with agricultural developments, numerous wetlands were drained and consequently many streams are now sourced by agricultural tile drains. Agricultural tile drainage is a subsurface drainage system that removes excess water from soils, through the use of underground pipes, for improved crop production. Although tile drainage has agronomic benefits, tile drainage inputs can influence stream conditions (i.e., temperature, hydrology, and water chemistry) that are important in maintaining ecosystem function; the natural processes that control the movement of energy and matter through an environment. To assess the influence of agricultural tile drainage inputs on stream ecosystem function, I used a stream network within the headwaters of the Thames River Basin, where adjoining streams had different sources (i.e., wetland versus tile drainage). I compared consumption rates of carbon and oxygen by microbes (heterotrophic activity), largely bacteria, fungi, and archaea, between the wetland-sourced and tile-sourced streams over four seasons. Heterotrophic activity was assessed through rates of microbial respiration and organic matter (OM) breakdown. Respiration is a metabolic process that breaks down organic carbon, while consuming oxygen, to produce carbon dioxide and energy. OM breakdown is the process of breaking down complex organic matter (e.g., leaves) into simpler inorganic matter (e.g., carbon dioxide, inorganic forms of nutrients) to be cycled back into the environment. OM breakdown was measured using the cotton strip assay, which is a method to compare streams’ capacities to process organic matter by assessing cellulose breakdown. I found a reduction in heterotrophic activity in the tile-sourced stream, particularly in the summer. This reduction in heterotrophic activity appeared to be due to colder stream temperatures from increased groundwater inputs, as tile drainage pipes intersected the water table and extracted more groundwater. Reduced heterotrophic activity was also evident in downstream network segments. Additionally, I found less variation in heterotrophic activity along the tile-sourced stream. Therefore, if my findings are representative of how tile drainage has affected headwater streams more broadly, there may be a widespread reduction in heterotrophic activity in headwater streams across agricultural regions where tile drainage is prevalent.

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