Export of aged dissolved organic carbon to the Geum and Seomjin estuaries in South Korea

We investigated the source and reactivity of dissolved organic carbon (DOC) in two contrasting Korean estuaries: the Geum, a closed system with a dammed river mouth, and the Seomjin, an open estuary system. The average Delta 14CDOC value in the Geum estuary (-101.8 +/- 38.0 %o) was higher than that in the Seomjin estuary (-176.4 +/- 101.9 %o). The depleted 14C signal in the Seomjin estuary appears to be associated with a high average monthly Q90/Q50 ratio, which is an indicator of base water flow, and a low DOC:sigma B + ratio, which is an indicator of groundwater flow. This suggests that groundwater in the Seomjin watershed passed through a deeper soil column compared with the Geum watershed, transferring older DOC to the river during the base flow season and into the estuary. In contrast, the Geum estuary dam appears to be associated with higher Delta 14CDOC and the export of younger DOC to the estuary due to enhanced primary productivity in the dam reservoir. Our results suggest that input of aged DOC derived from a deeper soil column influenced the estuarine DOC pool in the open Seomjin estuary, and this effect could be dampened by riverine DOC produced in the dam reservoir of the closed Geum estuary.