Date of Award

2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biological Science

First Advisor

Serena Moseman-Valtierra

Abstract

For centuries, coastal marshes have been subjected to anthropogenic stressors. Great expanses of coastal marshes were drained and filled to make way for development, and those that remained were diked and ditched, encroached upon by upland development, and used for agricultural purposes such as livestock grazing. Today, as the values and services coastal marshes provide to human society are understood, marshes are protected from direct degradation. However, especially in developed and densely populated estuaries such as Narragansett Bay, coastal marshes are subject to impacts including nutrient pollution and introduction of invasive species. Global climate change and associated sea level rise further threaten coastal ecosystems. As marsh vegetation community structure, biogeochemistry, and microbial and faunal assemblages shift in response to anthropogenic impacts and global change, ecosystem function is likely to be altered as well. Since coastal marshes provide highly valued services such as coastline protection, wildlife habitat, nitrogen (N) transformations and carbon (C) sequestration, understanding the outcomes of these functional shifts is an important research concern. Of particular interest is the potential for impacts to coastal marshes’ important ecosystem service of C sequestration, since perturbations to this function could result in climate change-exacerbating feedbacks.

Coastal marshes are such effective C sinks due to their high productivity and associated carbon dioxide (CO2) uptake, slow decomposition, and minimal emission of climate-altering greenhouse gases (GHGs). However, emission of GHGs may be stimulated by several of the global change drivers coastal marshes face. These potential drivers include N pollution, which can stimulate emission of the potent GHG nitrous oxide (N2O) from coastal marshes, and invasion of the aggressive introduced grass Phragmites australis, which may stimulate emission of methane (CH4). Testing how these impacts may interact to alter fluxes of GHGs in coastal marshes is important for a clear understanding of the role that coastal marshes play in global climate and whether this role is likely to be affected by a changing climate.

Very recently, development of novel technologies for measuring GHG concentrations in situ in real time have made simultaneous measurement of the GHGs CO2, CH4, and N2O a possibility, and have opened the door to experiments that will improve understanding of coastal marsh GHG flux dynamics and their response to changes to the coastal marsh ecosystem.

The objective of the research projects presented in this dissertation was to elucidate responses of coastal marsh GHG fluxes to drivers of global change including climate change, N pollution, and invasion of Phragmites australis. Four research projects employing mensurative and manipulative experiments incorporating cavity ringdown spectroscopy (CRDS) technology for GHG flux measurement were conducted. First, GHG flux dynamics in native vegetation and Phragmites-dominated coastal marsh zones along a salinity gradient were characterized to determine whether Phragmites invasion may potentially affect marsh GHG fluxes. Next, the effect of vegetation presence on GHG fluxes and their diurnal variability in a coastal marsh were tested with the aim of better understanding mechanisms underlying coastal marsh GHG fluxes. Since Phragmites removal as part of restoration activities is a commonly employed management activity in coastal marshes, the third experiment tested effects of coastal marsh restoration activities and Phragmites removal on GHG flux dynamics. Finally, to examine potential interactive impacts of climate change, N pollution and Phragmites presence on GHG flux dynamics, a multifactorial greenhouse experiment was conducted in chambers that simulated elevated atmospheric CO2 and temperatures expected to occur by the end of the century.

Results of these experiments revealed a potentially complicated role of Phragmites in mediating GHG fluxes from coastal marshes under conditions of global change. While Phragmites-dominated marsh zones consistently emitted more CH4 relative to native vegetation marsh zones, they also had substantially greater CO2 uptake per unit area. Within Phragmites stands, clearing vegetation resulted in an increase in CH4 emissions that was exacerbated by the loss of photosynthetic CO2 uptake. Testing effects of N pollution and climate change on GHG fluxes revealed that Phragmites-dominated marshes might emit more CH4 under conditions of climate change. While further research is required to determine the spatial and temporal consistency of the effects and to continue clarifying mechanisms, results presented in this dissertation make clear the potential for Phragmites australis invasion to alter marshes’ role in a changing global climate.

Available for download on Friday, November 03, 2017

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