The distribution and δ13C of dissolved organic carbon and its humic fraction in estuaries of southeastern USA

In this study, we examine the distribution and carbon stable isotope signature of dissolved organic carbon (DOC) and humic substances (HS) along a salinity gradient in the Altamaha and Satilla River estuaries. The maximum DOC concentrations in the Altamaha and Satilta were 10 and 29 mg Cl-1, respectively, though concentrations were similar at the mouth of both estuaries. There was a decrease in HS content of DOC from 50 to 80% at the head of the estuaries to 10% at salinities higher than 30%.. The delta(13)C DOC varied between -25.5 and -19parts per thousand and between -27 and -21%o in the Altamaha and Satilla estuaries, respectively. The tendency towards more depleted delta(13)C DOC in the Satilla, especially in the lower salinity portion of this estuary, suggests greater terrestrial inputs in the Satilla. than in the Altamaha. Seasonal fluctuations were observed in the form of increased (two to three times) range in DOC concentration, heavier 813C DOC and increased proportion of estuarine marine-derived DOC (average enrichment of delta(13)C DOC from + 1 to 4- 2) during low river flow (July-October). The delta(13)C HS in both rivers showed a similar trend, but was consistently more depleted than DOC, with an average range from -28 to -24.5parts per thousand. This suggests that HS have larger proportions of terrestrial components (a maximum of >60% at the mouth of the estuary) than DOC. The less depleted delta(13)C values of DOC in comparison with HS indicate a different source for the non-humic (non-HS) component of DOC (range in delta(13)C non-HS, -22 to -16parts per thousand). That source could either be the decomposition of detrital material derived from saltmarsh environments or microalgalderived DOC of estuarine or marine origins. (C) 2003 Elsevier Science B.V. All rights reserved.