Date of Award

2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Biological and Environmental Sciences

Specialization

Environmental and Earth Sciences (EVES)

Department

Natural Resources Science

First Advisor

Yeqiao Wang

Abstract

Salt marshes are a frontline of climate change providing a bulwark against sea level rise, an interface between aquatic and terrestrial habitat, important nursery grounds for many species, a buffer against extreme storm impacts, and vast blue carbon repositories. Since the 1700s salt marshes have been in flux due to anthropogenic actions, such as reclamation for development causing loss and an influx of sediment from land clearing leading to marsh expansion. The Clean Water Act of 1972 provides legal protections for wetlands, limiting wetland reclamation and requiring that impacts be offset. However, salt marshes continue to change rapidly due to anthropogenic stressors including elevated rates of Sea Level Rise (SLR) due to climate change, herbivory driven by overfishing, droughts, and eutrophication. Salt marsh monitoring across large spatial extents requires remote sensing. This dissertation’s objectives include: Developing methods for monitoring how mid-Atlantic salt marsh ecosystems are changing and where, determining how restoration and Hurricane Sandy affected Jamaica Bay’s salt marshes, and quantifying the effect of the tidal stage at the time of acquisition on very high spatial resolution (<1 m) salt marsh mapping.

This dissertation is composed of three chapters in the format of published and prepared manuscripts for professional journals. In chapter/manuscript 1, a methodology for monitoring salt marsh with very high resolution imagery was developed and applied to the Jamaica Bay Unit of Gateway National Recreation Area. Jamaica Bay’s salt marshes were mapped using object-based image analysis (OBIA), random forest classifier, and a diverse set of data including high spatial resolution (<1 m pixel size) satellite imagery. Change analysis was conducted at Gateway National Recreation Area with satellite imagery collected in 2003, 2008, 2012, and 2013. All classifications achieved >85% overall accuracies. In Jamaica Bay, from 2012 to 2013, restoration efforts resulted in an increase of 10.6 ha of salt marsh. Natural salt marshes within the Bay demonstrated a decreasing trend of loss. Larger salt marshes in 2012 tended to increased vegetation extent in 2013 F(4, 6) = 13.93, p = 0.0357 and R2 = 0.90).

In chapter/manuscript 2, the effect of the tidal stage on salt marsh mapping was modeled using topobathymetric LiDAR and VDatum. Verification of the tidal effect on very high resolution imagery was explored within Jamaica Bay using bathtub models derived from topobathymetric LiDAR and imagery data collected at a range of tidal stages. The effect of the tidal stage was minimal at 0.6 m above MLW, only 3.5% of S. alterniflora was inundated. This varied greatly between salt marsh islands within the Bay.

In chapter/manuscript 3, salt marshes change across seven HUC-8 mid-Atlantic watersheds was mapped from 1999 to 2018 using time series analysis of the Landsat 7 and 8 archives with Google Earth Engine. Back-barrier salt marshes are integral to the barrier systems function and their long-term resilience in the face of SLR and future extreme storms. This analysis included watersheds across Maryland, Delaware, northern North Carolina, Virginia, New York, and New Jersey. Aboveground green biomass across the mid-Atlantic declined by an average of -68 g m-2. The Landsat derived estimates of aboveground green biomass were an indicator of salt marsh vegetation extent within a pixel (F(1165,1)=1316, p < 0.001) and R2 =0.53

Salt marsh environments along the mid-Atlantic coast are in decline and projected to suffer more losses due to SLR. These changes are evident with both localized mapping and regional assessments. Satellite remote sensing monitoring provides the spatial context necessary for successful salt marsh management. The response of salt marshes to SLR is uncertain, where will migration, persistence, and loss occur? Satellite remote sensing of salt marsh change is necessary for the appropriate management of these ecosystems. The synergistic stressors that are driving loss require both in situ monitoring to determine change and remote sensing to expand these analysis beyond a singular location.

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