Date of Award

Winter 2017

Project Type

Thesis

Program or Major

Natural Resources

Degree Name

Master of Science

First Advisor

Wilfred M Wollheim

Second Advisor

Anne Lightbody

Third Advisor

Gopal Mulukutla

Abstract

Humans have had a substantial impact on the global nitrogen cycle, releasing large amounts of reactive nitrogen to the landscape. Watersheds have been found to remove substantial quantities of this anthropogenic nitrogen, with aquatic networks preventing much of it from reaching the coast. Within these aquatic networks, channelized streams have been studied extensively. However, in many coastal watersheds, fluvial wetlands are a large component of the aquatic network yet have not been studied as broadly as channelized reaches. As fluvial wetlands are also likely to have a sizeable impact on river network-scale nitrogen removal, a greater understanding of their effect on nitrogen cycling is necessary. We developed a new approach for quantifying fluvial wetland nitrate uptake measured at the whole ecosystem scale, using a method conceived in channelized reaches, in conjunction with new in situ sensor technology. Using this new approach, we found ambient uptake velocities in fluvial wetlands that ranged from -170.6 to 1220.0 m yr-1. These uptake velocities were generally higher than those found for channelized reaches within the same watersheds and in a similar range as uptake velocities found for fluvial wetland surface transient storage patches. Neither gross primary productivity nor ecosystem respiration exerted strong control on uptake rates, however, both discharge and detention time were significantly related to nitrate uptake. The effects of biology and hydrology on nitrate removal were then tested using a reach-scale numeric model. Hydrology was found to have a greater impact on nitrate removal compared to biology however, the cumulative effect of both hydrology and biology had the greatest impact. This is important because the watersheds draining suburban Boston, MA are experiencing ongoing urbanization, leading to greater inputs of anthropogenic nitrogen to the landscape. The beaver population is concurrently experiencing a resurgence, leading to an increase in the number of fluvial wetlands within these watersheds and with it, a greater potential for nitrogen removal. As such, fluvial wetlands may be able to offset the increase in nitrogen loading, preventing additional nitrogen from reaching the coast.

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