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 Satilla were 10 and 29mgCl 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 d13C DOC varied between 25.5 and 19& and between 27 and 21& in the Altamaha and Satilla estuaries, respectively. The tendency towards more depleted d13C 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 d13C DOC and increased proportion of estuarine–marine-derived DOC (average enrichment of d13C DOC from +1 to +2) during low river flow (July–October). The d13C HS in both rivers showed a similar trend, but was consistently more depleted than DOC, with an average range from 28 to 24.5&. 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 d13C values of DOC in comparison with HS indicate a different source for the non-humic (non-HS) component of DOC (range in d13C non-HS, 22 to 16&). That source could either be the decomposition of detrital material derived from saltmarsh environments or microalgalderived DOC of estuarine or marine origins.