Basin-specific pollution and impoundment effects on greenhouse gas distributions in three rivers and estuaries

Large uncertainties exist regarding the combined effects of pollution and impoundment on riverine greenhouse gas (GHG) emissions. It has also been debated whether river eutrophication can transform downstream estuaries into carbon sinks. To assess human impacts on the riverine and estuarine distributions of CO₂, CH₄, and N₂O, two source-to-estuary surveys along three impounded rivers in Korea were combined with multiple samplings at five or six estuarine sites. The basin-wide surveys revealed predominant pollution effects generating localized hotspots of riverine GHGs along metropolitan areas. The localized pollution effect was pronounced in the lower Han River and estuary adjacent to Seoul, while the highest GHG levels in the upper Yeongsan traversing Gwangju were not carried over into the faraway estuary. CH₄ levels were elevated across the eutrophic middle Nakdong reaches regulated by eight cascade weirs in contrast to undersaturated CO₂ indicating enhanced phytoplankton production. The levels of all three GHGs tended to be higher in the Han estuary across seasons. Higher summer-time δ¹³C-CH₄ values at some Nakdong and Yeongsan estuarine sites implied that temperature-enhanced CH₄ production may have been dampened by increased CH₄ oxidation. Our results suggest that the location and magnitude of pollution sources and impoundments control basin-specific longitudinal GHG distributions and estuarine carryover effects, warning against simple generalizations of eutrophic rivers and estuaries as carbon sinks.