Date of Award

Summer 2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean & Earth Sciences

Committee Director

David J. Burdige

Committee Director

Elizabeth C. Minor

Committee Member

Robert F. Dias

Committee Member

Patrick G. Hatcher

Committee Member

Margaret R. Mulholland

Committee Member

Desmond Cook

Abstract

This dissertation used Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (13C-NMR) data to quantify the changes of major chemical compound classes in high molecular weight (HMW, >1kDa) DOM isolated along a transect from Great Dismal Swamp through the Elizabeth River/Chesapeake Bay system to the coastal Atlantic Ocean off Virginia, USA. Results show that both carboxylic acids and aromatic compounds are lost along the transect, while amide, and carbohydrate moieties could have a mid-transect source.

Addressing the seasonal and spatial changes in the chemical composition of high molecular weight DOM using C/N ratio and δ13C signatures indicates a dramatic shift in the relative importance of the processes affecting the HMW-DOM as it moves from fresh water to the marine end member. Sorption and flocculation and reworking by heterotrophic bacteria seem to be the major players in the lower salinity region, but at the higher salinity regions the introduction of new carbon sources by primary production seems to be the major process.

Applying principal component analysis (PCA) and two dimensional correlation spectroscopy to the 13C-NMR spectra of the HMW-DOM shows that HMW-DOM consists of three major components that have different biogeochemical reactivity. The first component appears to be composed of a heteropolysaccharide (HPS) component and it increases as I move to the marine end member, while the second component appears to be composed of carboxyl-rich compounds (CRC) and its carbon percentage decreases as we move away from the fresh water end member. The third component contains the major functional group of amide/amino sugar (AMS) and its carbon percentage stays almost constant regardless of the seasonal and spatial changes along the salinity transect.

It seems that the HPS and CRC components are present in many aquatic environments at different relative ratios. Across aquatic environments the components contain compounds that share similar backbone structures although there is significant variation in some of their functional groups as a function of aquatic system.

The 13C-label method presented here for determining the enhanced aqueous solubility of organic compounds by natural aqueous DOM is a promising new tool for investigating the reactivity of DOM. Applying the method to Dismal Swamp DOM shows that the reactivity differences between high molecular weight, low molecular weight, and total DOM samples are consistent with potential variations in their higher order structures. However, coupling the method with FTIR analysis indicates that ultrafiltration is not merely a pure physical separation but involves a chemical separation as well.

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DOI

10.25777/thrc-c573

ISBN

9781109404203

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