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.