Seasonal variations of organic carbon fluxes and sources in Geum and Sumjin rivers in South Korea

Abstract

As a pathway of terrigenous carbon to the adjacent seas, the river-sea interface is a key place for better understanding regional as well as global carbon cycles. Dam constructions exert strong impacts on riverine carbon fluxes and sources. The water flow of the Geum River is controlled by a dam in its estuary while the Sumjin River has an open estuary. In this study, we analyzed concentrations and stable carbon isotopes of particulate organic carbon (POC) and concentrations of dissolved organic carbon (DOC) to investigate variations in fluxes and sources of riverine OC in two contrasting Korean river systems (Geum and Sumjin). The surface water samples were collected at a site located in the lower part of each river between May 2016 and March 2018. The TOC (POC + DOC) fluxes ranged from 1755 to 7451 gC/sec and 58 to 228 gC/sec in Geum and Sumjin rivers, respectively. Interestingly, the TOC flux in the Geum River drastically increased up to 7451 gC/sec in August 2016 with 77% of the POC flux which was larger than the average TOC flux in other periods (1907±248 gC/sec, 50% of the POC flux). The stable carbon isotope values of POC varied between -28.7 and -19.4 ‰ in the Geum River and between -29.1 and -25.2 ‰ in the Sumjin River. The δ13CPOC in August 2016 (-19.4 ‰) was enriched more than 5 ‰ in comparison to that of other periods (-27.2±1.4‰) in the Geum River. Our results suggest that the TOC fluxes were in general controlled by the water discharges in both rivers and the major source of POC was derived from surrounding lands. However, in the Geum River, the major source of POC was changed in August 2016 due to a heavy algal bloom. Accordingly, our study shows that the damming affects the fluxes and the compositions of riverine OC by lowering the water flow rate, increasing residence time, and thus promoting primary productivity in the reservoirs.