Differential river-to-sea transfers and CH4 dominance of greenhouse gas emissions in urbanized and impounded estuaries.

Estuaries are biogeochemical hotspots where greenhouse gas (GHG) emissions are significantly affected by anthropogenic disturbances, such as water pollution and impoundment. To investigate how upstream pollution affects GHG concentrations and fluxes in impounded estuaries, ten seasonal samplings were conducted over two years in three impounded estuaries in South Korea. The highest levels of all three GHGs were observed in the upper Han estuary, which traverses the megacity of Seoul, with an average CO2-equivalent GHG emission of 41 mmol m-2 d-1 (CO2: 38 %, CH4: 53 %, N2O: 9 %). CH4 accounted for 77-79 % of the CO2-equivalent GHG emissions in the other estuaries, which are impounded by a dam to limit saltwater intrusion. Seaward extensions of GHG peaks observed along the lower Han estuary, downstream of a submerged weir, were less pronounced in the other estuaries. Lowered summer-time CO2 and elevated CH4 levels across the Nakdong estuary, which has a partially open dam, indicated inputs from frequent cyanobacterial blooms in the upstream reaches impounded by eight cascade weirs. The influence of high upstream GHG levels was confined to the upper sites of the Yeongsan estuary, where a dam blocks tidal flow. Abrupt declines in CH4 levels across the Yeongsan Dam, combined with summertime increases in δ13C-CH4, imply that warming-enhanced oxidation moderated the level of CH4 produced in the deep freshwater reservoir. Overall, the results suggest that the interaction between upstream pollution and impoundment-induced transformations in estuaries can promote GHG emissions, with CH4 dominance overriding the seasonally enhanced CO2 sinks in impounded estuaries.