Date of Award

Winter 2020

Project Type

Thesis

Program or Major

Natural Resources

Degree Name

Master of Science

First Advisor

Wilfred M Wollheim

Second Advisor

Anne Lightbody

Third Advisor

Adam Wymore

Abstract

Nitrate inputs pose a threat to aquatic ecosystems, leading to eutrophication, algal blooms, and habitat loss in downstream coastal marine and estuarine habitats. Rivers and streams can attenuate nitrogen between inputs and coastal outputs, moderating ecosystem harm. While nitrogen dynamics in streams and rivers have been studied for decades, less is known about the wetlands through which they flow, namely small reservoirs. Storms can have a large influence on nitrogen processing in reservoirs through hydrologic changes and introduction of new solute sources, but are poorly understood. To understand the spatial and temporal variability of nitrogen processing in a small reservoir, this study made use of high-frequency sensors and spatial sampling within a small coastal dammed reservoir in New Hampshire, USA. This reservoir is not a nitrogen sink, rather acts as a transformer from inorganic to organic nitrogen forms. Inorganic nitrogen is retained temporarily, and exported later as dissolved organic nitrogen, offsetting retention of nitrate. The production of dissolved organic nitrogen and the undersaturation of nitrogen gas indicates that retention in this system occurs via plant and microbial assimilation rather than denitrification. In addition, nitrate is retained during storms due to increased delivery and connectivity to biologically active areas of the reservoir. These areas were found to be responsible for overall reservoir biogeochemical responses for some forms of nitrogen but not for others. Storms had a significant effect on nitrate and dissolved organic nitrogen processing, but there was no evident effect of storm size on nitrogen processing. This works contributes to our need to understand the biogeochemical role of small reservoirs within the landscape in the face of widespread dam removal and land use change.

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