Graduation Year

2017

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Biology (Integrative Biology)

Major Professor

Thomas Crisman, Ph.D.

Committee Member

Susan Bell, Ph.D.

Committee Member

Philip Motta, Ph.D.

Committee Member

Mark Rains, Ph.D.

Keywords

trajectory, mitigation, landscape development intensity

Abstract

Urbanization and wetland mitigation are increasingly common in coastal watersheds with expanding populations. These mitigation wetlands are intended to offset the functional and structural losses experienced when natural systems are degraded or destroyed. In the Tampa Bay watershed, urbanization is both expanding into the upper reaches of the watershed and intensifying in previously-developed areas, resulting in the creations of hundreds of freshwater mitigation wetlands. This dissertation utilized an existing database of mitigation wetlands, publicly available data, and field surveys to investigate the relationship between constructed wetlands and their surroundings and also determine how design affects wetland condition over time. The overarching goals of this dissertation were to evaluate the geospatial distribution and areal extent of constructed freshwater mitigation wetlands in Hillsborough County and determine how they influence the landscape; evaluate design variables and environmental factors influencing constructed wetland trajectories; and determine how future changes to the landscape will likely affect constructed wetland systems.

The goals of Chapter 2 were to evaluate the relationship between mitigation wetland construction and total freshwater wetland area; determine if forested and non-forested wetlands are being impacted/mitigated at similar rates; determine if wetland mitigation is offsetting impacts from increased urbanization at the landscape scale. This study concluded that since 1985, permitted impacts of non-forested wetlands have occurred at a significantly greater annual rate than forested systems, despite their smaller regional footprint. Interestingly, this increased impact frequency, combined with mitigation ratios greater than 1:1 (mitigation to impact area), have helped decrease proportional difference in area between forested and non-forested wetlands in the region. Over the period of the study, mean LDI scores for drainage basins across watershed have increased, with those containing mitigation projects significantly increasing compared to those without. Changes in drainage basin LDI were significantly correlated to the number of mitigation projects per basin, total impact area, and total mitigation area. Upward shifts in drainage basin LDI categories have been documented in 25 of Hillsborough County’s 184 basins, however no significant connection to permittee-responsible freshwater wetlands was established. These results imply that current mitigation practices are failing to ameliorate increasing development intensity at the landscape scale.

The goals of Chapter 3 were to determine the current condition of created freshwater mitigation wetlands in Hillsborough County, Florida; determine if forested and non-forested wetlands maintain similar trajectories after release; and evaluate how design and changes in the landscape influence created wetland condition over time. Original wetland engineering plans and historical data were used to establish baseline conditions at the time of wetland “release” and track wetland changes over time. A chronosequence approach was utilized to determine wetland trajectories and analyze potential differences between forested and non-forested systems. This study found that surveyed freshwater wetlands had decreased in size from their intended area by a total of approximately 18%, but due to increased mitigation ratios, were likely still producing a net gain in total wetland area and meeting the goals of “no net loss”. On average, wetland condition (as determined by WRAP scores) decreased by 9% from the time of release to the time of survey. Few differences were observed between wetland types with the exception of canopy richness and wetland trajectory, although correlations between wetland condition and time were non-significant. From the regression optimization analyses, it appeared that wetland location (as measured in the design WRAP score) was one of the most important factors contributing to surveyed wetland condition.

In Chapter 4, future land use data was used to determine predicted anthropogenic pressure on these urban wetland systems and evaluate changes to the overall landscape. GIS based analyses on landscape development intensity (LDI) determined that significant changes are not expected at the landscape scale by the year 2025, however drainage basins that possess mitigation wetlands are anticipated to increase in development intensity. Predicted LDI scores for constructed freshwater wetlands is predicted to increase significantly, which could have detrimental impacts on wetland condition.

This dissertation highlights the significance of wetland design and location on wetland condition. From this research, it is apparent that consideration of site placement is the most important design variable for small (3 hectares or less) freshwater wetlands; and that understanding of future conditions may promote long-term success. Long-term studies such as this are valuable tools for understanding how specific ecosystems respond to changing landscapes and should be used to help shape policies that reflect these ecological advancements. Understanding the past and preparing for the future is the only way to foster restoration success.

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