Date of Award

1-1-2013

Document Type

Open Access Dissertation

Department

Marine Science

First Advisor

Raymond Torres

Abstract

Low tide rainfall-runoff processes have been proposed as a kind of coastal ocean "hot moment" capable of transporting disproportionately large amounts of material from the salt marsh surface to the water column. However, the resulting organic matter cycling has not been well characterized and the cumulative effects of recurring short-term low tide rainfall events are not known, especially in the context of outwelling. The purpose of this Ph.D. research is to quantitatively evaluate the effects of low tide rainfall on particulate organic carbon (POC) cycling in a South Carolina salt marsh.

This dissertation is comprised of four parts. First, a study of metal flux by rainfall-runoff is presented as the preliminary approach of understanding the material cycling processes by rainfall-runoff. Those findings show a heterogeneous distribution of metals in sediment with a concentrated surficial layer leading to higher concentrations of metals in the initial runoff relative to the substrate. Second, the distribution of organic matter in the marsh surface sediment is presented in order to investigate the geomorphic controls on organic matter content and composition. I observe two scales of variability in organic matter characteristics, one at ~101 m and the other at ~103 m and I ascribe this to the geomorphic structure and sediment sources, respectively. Third, I investigate the characteristics and fluxes of rainfall-driven particulate organic matter in tidal creeks. Rainfall-entrained particles contain allochthonous organic matter primarily derived from Spartina alterniflora, which is compositionally different from surface sediment or suspended sediment in no rain conditions. Also, I calculate that rainfall-runoff can

mobilize and transport 0.020±0.002 gPOC/m2/mmRain and annually it can generate up to 13.64±1.36 gPOC/m2/year. This is approximately 19% of tidal current driven POC annually indicating that the previous outwelling estimates taken under calm weather conditions likely underestimated the net delivery of POC from salt marshes to the coastal ocean. Fourth, I review studies related to rainfall-runoff processes from salt marshes and reveal that the intertidal surface has a characteristic response at the tidal creek-subtidal channel interface with an average value of 0.040±0.038 gPOC/m2/mmRain. This is an average response computed from a range of salt marshes and it can be directly applied to any intertidal landscape for evaluating the effects of low tide rainfall on POC flux. I then integrate rainfall-driven POC flux into a box model describing organic carbon cycling through the hierarchical structure of salt marshes. Overall, rainfall-driven flux is 12-73% of tidal current driven POC flux at the tidal creek-subtidal channel interface while at the subtidal channel-coastal ocean interface rainfall is likely ~50% of tidal current driven POC flux averagely. Therefore, previous outwelling studies may have underestimated the ability of salt marshes in providing nutritional material to the coastal ocean. Overall, this dissertation provides both quantitative and qualitative information on rainfall-runoff effects so that future research can integrate the rainfall-runoff effects into studies about intertidal zone material cycling and biogeochemical processes, especially under climate change and sea level rise.

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