In a study of chemical transformations of estuarine high-molecular-weight (HMW, >1000Da) dissolved organic matter (DOM) collected over a period of two years along a transect through the Elizabeth River/Chesapeake Bay system to the coastal Atlantic Ocean off Virginia, USA, δ13C values, N/C ratios, and principal component analysis (PCA) of the solid-state 13C NMR (nuclear magnetic resonance) spectra of HMW-DOM show an abrupt change in both its sources and chemical structural composition occurring around salinity 20. HMW-DOM in the lower salinity region had lighter isotopic values, higher aromatic and lower carbohydrate contents relative to that in the higher salinity region. These changes around a salinity of 20 are possibly due to introduction of a significant amount of new carbon (autotrophic DOM) to the transect. PC-1 loadings plot shows that spatially differing DOM components are similar to previously reported 13C NMR spectra of heteropolysaccharides (HPS) and carboxyl-rich alicyclic molecules (CRAM).Applying two dimensional correlation spectroscopy techniques to 1H NMR spectra from the same samples reveals increases in the contribution of N-acetyl amino sugars, 6-deoxy sugars, and sulfated polysaccharides to HPS components along the salinity transect, which suggests a transition from plant derived carbohydrates to marine produced carbohydrates within the HMW-DOM pool. In contrast to what has been suggested previously, our combined results from 13C NMR, 1H NMR, and FTIR indicate that CRAM consists of at least two different classes of compounds (aliphatic polycarboxyl compounds and lignin-like compounds).
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The authors thank Junyan Zhong (COSMIC, Old Dominion University) for his help obtaining 1 H NMR spectra, David Burdige (OEAS, Old Dominion University) and Rachel Sleighter (Chemistry Dept., Old Dominion University) for their valuable comments on the earlier version of the manuscript. We also thank associated editor Peter Hernes and the three anonymous reviewers for their detailed comments and suggestions. A special thanks to the crew of the R / V Slover . This work was supported by the National Science Foundation ( OCE 0453777 to R.F.D. and E.C.M. and OCE 0612712 to P.G.H.).