Does eutrophication enhance greenhouse gas emissions in urbanized tropical estuaries?

Estuaries are considered as important sources of the global emission of greenhouse gases (GHGs). Urbanized estuaries often experience eutrophication under strong anthropogenic activities. Eutrophication can enhance phytoplankton abundance, leading to carbon dioxide (CO) consumption in the water column. Only a few studies have evaluated the relationship between GHGs and eutrophication in estuaries. In this study, we assessed the concentrations and fluxes of CO, methane (CH) and nitrous oxide (NO) in combination with a suite of biogeochemical variables in four sampling campaigns over two years in a highly urbanized tropical estuary in Southeast Asia (the Saigon River Estuary, Vietnam). The impact of eutrophication on GHGs was evaluated through several statistical methods and interpreted by biological processes. The average concentrations of CO, CH and NO at the Saigon River in 2019-2020 were 3174 ± 1725 μgC-CO L, 5.9 ± 16.8 μgC-CH L and 3.0 ± 4.8 μgN-NO L, respectively. Their concentrations were 13-18 times, 52-332 times, and 9-37 times higher than the global mean concentrations of GHGs, respectively. While CO concentration had no clear seasonal pattern, NO and CH concentrations significantly differed between the dry and the rainy seasons. The increase in eutrophication status along the dense urban area was linearly correlated with the increase in GHGs concentrations. The presence of both nitrification and denitrification resulted in elevated NO concentrations in this urban area of the estuary. The high concentration of CO was contributed by the high concentration of organic carbon and mineralization process. GHGs fluxes at the Saigon River Estuary were comparable to other urbanized estuaries regardless of climatic condition. Control of eutrophication in urbanized estuaries through the implantation of efficient wastewater treatment facilities will be an effective solution in mitigating the global warming potential caused by estuarine emissions.