Author

Heather Kish

Date of Award

Fall 2021

Document Type

Open Access Thesis

Department

Marine Science

First Advisor

Claudia Benitez-Nelson

Second Advisor

Erik Smith

Abstract

Coastal wetlands provide numerous ecosystem services, including the ability to sequester and store significant amounts of organic carbon (so-called “blue carbon”). Variability in carbon storage within marshes represents a major information gap in understanding and quantifying the role saltmarshes play in the global carbon cycle. This study quantified decadal and small-scale variability in carbon and nutrient concentrations across tidal inundation gradients in the relatively pristine North Inlet Estuary, South Carolina. Sampling took place within two segments of the marsh platform of Crab Haul Creek, the landward-most basin within North Inlet. In the summer of 2020, a total of 200 sediment cores were collected to a depth of 30 cm (root zone) that spanned the elevation and vegetation community gradient from the creek bank to the upland forest edge. Cores were sectioned in 10 cm intervals and analyzed for bulk density, grain size, percent organic matter, total carbon, total nitrogen, and total phosphorus concentrations. Results from 2020 were compared to similar data collected a decade earlier at the same locations to assess the potential for temporal changes associated with increased sea level. Despite substantial increased tidal inundation over the decade, sediment carbon and nutrient densities were not significant. In contrast, measures of sediment bulk density and carbon and nutrient concentrations had considerable small-scale variability, both between segments and across marsh elevation gradients and vegetation communities. Sediment carbon concentrations were highly variable (0.2 to 17.1%), with younger marsh, upstream marshes characterized by ~ 2 times more carbon relative to the downstream, older marsh region. Similar to previous studies, there was a significant and strong inverse relationship (R2 = 0.86) between sediment dry bulk density and organic carbon concentration (- 0.48ln(OM) + 1.9577), which reduced spatial variability in sediment carbon densities (mean = 0.034± 0.006 g cm-3 ). Carbon to nitrogen density ratios were relatively constant across vegetation type, marsh region, and with increasing depth, and averaged 25.52 ± 2.62.

Combined these results indicate that small-scale spatial variability within a single marsh ecosystem may be as great or greater than across marsh ecosystems. Thus, regional carbon storage requires more detailed measurements in order to scale to global estimates of the role of salt marshes in carbon storage. At the same time, strong relationships between physical parameters and the relatively low range in carbon densities, provides a mechanism for reducing the types of analyses and constraining regional estimates. While spatial variability in carbon storage is relatively stable over the roughly 150 mm of sea level rise the marsh has experienced over the past decade, it is unclear if this stability will remain under current or accelerated rates of sea level rise predicted under most recent climate change scenarios.

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