Date of Award

2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Natural Resources

First Advisor

Eric D. Roy

Abstract

Wetland restoration has numerous potential ecological and societal benefits, one of which is the retention of phosphorus (P) and consequent protection of downstream water bodies from eutrophication. Past studies focused on influents to and effluents from a variety of wetland types have documented net P retention. However, some wetland systems are less effective at P capture and wetland P retention capacity can change over time. Certain wetland types - especially riparian wetlands restored on former agricultural land - remain understudied. In Vermont, most of the over 4000 potential wetland restoration sites in the Lake Champlain Basin are located on current or former agricultural fields, and little information is available to inform estimates of net P retention (i.e., P balances) for such sites. In this dissertation, I examined various factors affecting P balances in riparian wetlands restored on historically farmed soils of Vermont. P balance in a riparian wetland is largely a function of particulate P capture (e.g., deposition of particle-attached P during floods) and soluble reactive P (SRP) loss (e.g., release of SRP from soils). In Chapter 1, I determined the threshold in P saturation ratio (PSR) for riparian soils in Vermont, enabling calculation of a soil P storage capacity (SPSC) metric. I then quantified soil SRP release using intact soil core incubations with simulated floods for sites ranging from active farms to mature wetlands and confirmed that PSR, SPSC, and other soil parameters were strong predictors of SRP loss during inundation. In Chapter 2, I monitored P dynamics in soil, water, and vegetation at three restored riparian wetlands on former agricultural land in the Lake Champlain Basin, focusing on factors that affect P deposition and SRP release. At wetland sampling plots, observed inorganic sediment gain and decreased water column total suspended solids concentrations relative to the river/inflow indicated that wetlands were effectively trapping particles. Accretion of inorganic P (i.e., best estimate for mineral P deposited during floods) ranged from 0.1 to 1 g P m-2 yr-1 depending on site and elevation. Elevated SRP concentrations in wetland water columns relative to the river sources indicated internal SRP release from soils, and high frequency data indicated that factors such as temperature, dissolved oxygen, and primary production likely influence SRP dynamics. In Chapter 3, I developed a wetland P dynamics model that can generate estimates of net P retention from a simple set of soil and hydrologic inputs, considering both P deposition and SRP release. For proof of concept, I simulated the wetlands monitored in Chapter 2 using two years of monitoring data and a set of model scenarios. I found that net total P balance was typically positive (-0.04 to 0.24 g P m-2 yr-1), with average P retention efficiency of ~40%, though there was substantial variability depending on site and scenario. P retention efficiency was especially sensitive to changes in influent P and total suspended solids concentrations, with the greatest net P retention predicted for systems receiving influent floodwater with high P concentrations. Reduction of influent SRP concentrations promoted SRP release from soils, suggesting that legacy soil P in the wetlands might cause a time lag between the adoption of upstream best management practices and reduction in downstream SRP concentrations. In the future, the model developed in Chapter 3 can be applied more broadly to investigate the potential P retention benefits of wetland restoration at candidate sites across Vermont. Together, the information put forth by this dissertation provides a suite of data and tools that researchers and managers can use to enhance the P retention benefits of riparian wetland restoration.

Language

en

Number of Pages

137 p.

Available for download on Sunday, December 10, 2023

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