Organic carbon transfer across the open and closed estuary systems: a case study of Geum and Seomjin Riversystems, South Korea

Abstract

To understand river damming impact on the transport of riverine carbon, we investigated spatial and temporal variations in organic carbon (OC) concentration and their stable and radio carbon isotope. The surface water samples were collected in two contrasting Korean river systems (Geum and Seomjin) across the river-sea interfaces along a salinity gradient in August and December 2016 to analyse OC concentrations and carbon isotopes. The Geum River flows into the Yellow Sea which has a dam at the river mouth while the Seomjin River flowing into the South Sea of Korea has an open estuary. The riverine total organic carbon (TOC, dissolved OC (DOC) and particulate OC (POC)) fluxes were much larger (4236.9 and 963.1 g/s in August and December, respectively) in the Geum River than those (51.5 and 38.5 g/s in August and December, respectively) in the Seomjin River. The DOC concentrations in the Geum River showed similar range between August (2.0-3.8 mg/l) and December (1.3-5.3 mg/l), while the POC concentrations in August (0.2-12.7 mg/l) showed much large variation than in December (0.3-2.6 mg/l). In the Seomjin River, the DOC and POC concentrations were 1.9-2.4 mg/l and 0.8-1.0 mg/l for August and 1.1-1.5 mg/l and 0.3-0.5 mg/l for December, respectively. The DOC and POC concentrations showed decreasing trends from river to sea in the Geum River. In the Geum River, the δ13C POC values were -21.1+/-2.5 ‰ before the dam and -22.4+/-1.5 ‰ after the dam in August, while they were -30.61+/-2.7 ‰ before the dam and -27.17+/-2.4 ‰ after the dam in December. We observed a large algal bloom before the dam during the sampling in August, which resulted in heavier δ13CPOC values in the Geum River. In the Seomjin River, the δ13 CPOC values were -29.1 to -21.1 ‰ in August and -29.0 to -26.6 ‰ in December, showing a decreasing contribution of terrestrial organic matter from river to sea. The D14C POC values in the Geum River were -51.1 ‰ before the dam and -98.2 ‰ after the dam in August and -87.0 ‰ before the dam and -221.8 ‰ after the dam in December. In the Seomjin River, the D14C POC values were much lower with -186.7+/-2.2 ‰ in August and -214.2+/-38 ‰ in December. Accordingly, our results show that the two contrasting river and sea interface systems differently influence OC sources and transport from river to sea.