Date of Submission

7-2025

Document Type

Thesis

Degree Name

Master of Science in Environmental Science

Department

Marine and Environmental Programs

Advisor

Sharon Kahara, Ph.D.

Committee Member

Jean-Paul Simjouw, Ph.D.

Committee Member

Nikolas Stasulli, Ph.D.

Keywords

Salt marshes, Phragmites australis, Spartina alterniflora, Microbial diversity, Soil chemistry, Carbon-to-Nitrogen Ratio (C:N)

LCSH

Salt marshes, Phragmites australis, Spartina alterniflora, Microbial diversity, Microbiological chemistry

Abstract

Salt marshes play a vital role in coastal ecosystem health yet are increasingly threatened by spread of invasive plant species, such as Phragmites australis, which can alter native vegetation structure and disrupt underlying ecological processes. While the impacts of such invasions on aboveground biodiversity have been studied extensively, there has been limited research on their effects on soil microbial communities and soil chemistry, leaving much to be understood in this area. This research investigates how Phragmites australis affects microbial diversity and soil chemistry in the Quinnipiac River Meadows by comparing soils dominated by the invasive species to those dominated by the native Spartina alterniflora (aka Sporobolus alterniflora. Using 16S rRNA sequencing, we assessed microbial community structure across six taxonomic levels (Kingdom to Genus), alongside alpha and beta diversity patterns. Clear distinctions in microbial structure emerged at finer taxonomic levels. Spartina soils exhibited greater diversity at broader taxonomic levels, while Phragmites soils supported more diverse and compositional shifts at Family and Genus levels. Additionally, we measured total nitrogen (TN), soil organic carbon (SOC), and C:N ratio to evaluate relationships to plant composition. Mean TN and SOC stocks did not differ significantly between vegetation types. Phragmites soils had TN and SOC values of 0.172 ± 0.017 kg/ha and 2.955 ± 0.285 kg/ha, respectively, while Spartina soils had 0.161 ± 0.011 kg/ha and 3.035 ± 0.208 kg/ha (TN: p = 0.570; SOC: p = 0.821). However, Spartina soils had significantly greater C:N ratio (18.944 ± 1.310) than Phragmites (17.403 ± 1.720; p = 0.002), suggesting slower litter decomposition and limited nitrogen availability. Conversely, Phragmites- dominated soils may foster copiotroph microbial taxa due to higher inputs of labile organic matter and enhanced oxygenation via root activity. These findings underscore the strong role of dominant vegetation on microbial assemblages and nutrient dynamics, with implications for salt marsh function, resilience and management of invasive species in coastal ecosystems.

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