Characteristics of particulate orgarnic carbon in the Geum and Seomjin estuaries, South Korea

Abstract

In this study, we investigated spatial variations in particulate organic carbon (POC) concentration and carbon isotope composition to understand estuary damming impact on the transport of riverine POC. We collected river and seawater samples across a river-sea interface with a salinity gradient in two contrasting Korean estuary systems (Geum and Seomjin) in August 2016. The Geum River flowing into the Yellow Sea has a dam at the river mouth while the Seomjin River flowing into the South Sea of Korea has an open estuary. The POC concentrations in the Geum estuary were 0.2-12.7 mg/L, while 1.9-2.4 mg/L in the Seomjin estuary. An abrupt decrease in POC concentration occurred after the dam in the Geum estuary. The δ13CPOC values were -19.4 to -22.9 ‰ before the dam and -21.0 to -23.9 ‰ after the dam in the Geum esturary and -29.1 to -21.1 ‰ in the Seomjin estuary. The Δ14CPOC values were -51.1 to -81.7 ‰ before the dam and -98.2 to -48.1 ‰ after the dam in the Geum estuary and lower with the value of -132.7±50.2 ‰ in the Seomjin estuary. We calculated contribution of fossil and modern carbon using a simple binary mixing model. The contribution of fossil OC was on average 11.1 % and 12.0 % before and after the dam in the Geum esutuary, while it was on average 17.4 % in the Seomjin estuary. Lower proportion of fossil OC in the Geum estuary than in the Seomjin estuary indicates that higher contribution of newly fixed modern OC in the Geum estuary. We observed an algal bloom before the dam during the sampling period which resulted in higher δ13CPOC value (on average -22.0 ‰) in the Geum estuary. Accordingly, our study shows that an estuary damming affects composition and reactivity of OC by promoting primary productivity in the reservoirs and thus OC transfer in river-sea interface regions.