Honors Theses and Capstones

Date of Award

Spring 2019

Project Type

Senior Honors Thesis

College or School

COLSA

Department

Natural Resources

Program or Major

Environmental Science: Ecosystems

Degree Name

Bachelor of Science

First Advisor

Wil Wollheim

Abstract

Reactive nitrogen (N), which harms ecosystem health, has been increasing in the biosphere, leading to higher N export to coastal ecosystems. Although man-made reservoirs can be significant sources of greenhouse gases, they can also retain N, thus reducing N export. Because many dams are relics from industrial hydropower, their removal is becoming increasingly common. It is therefore crucial to understand the ecological tradeoffs of man-made reservoirs. While previous studies have examined nutrient budgets and denitrification at inputs and outputs of large reservoirs, small reservoir dynamics remain understudied. In this study, we measured inputs and outputs of NO3 and N2 at two small coastal reservoirs and assessed reasons for changes by sampling internally within the reservoirs. We hypothesized that denitrification is high in small reservoirs due to lower dissolved oxygen. While we found evidence of denitrification in one reservoir the second reservoir showed evidence of N fixation. Fixation was evident within the reservoir where low NO3 concentrations and high algal growth occurred, suggesting that NO3 was being assimilated, limiting algal growth, and allowing the occurrence of N fixing algae. As a result, reservoirs may not always remove N, but may at times be a source of additional N. As dam removal decisions continue, the role of reservoirs in N export should be carefully considered.

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