نبذة مختصرة : Abstract Coastal wetlands such as mangroves have a great potential in sequestering blue carbon and mitigating future climate change. Yet, these wetlands are being increasingly converted to aquaculture ponds, which could trigger a pulse emission of greenhouse gases (GHGs) from existing carbon stocks, a loss of opportunity for future carbon sequestration from mangroves, and an additional GHG emission incurred from pond establishment and operation. In this study, we determined the magnitude, temporal variations and environmental drivers of ecosystem‐scale carbon dioxide (CO2), methane (CH4) and water vapor fluxes from the subtropical brackish fishponds using the eddy covariance technique, and assessed the net carbon impact arising from the conversion of mangroves to fishponds under three conservation scenarios. Our results showed that the brackish fishponds were significant sources of carbon and water, with a mean annual emission of 687.6 ± 83.1 gC m−2 for CO2, 101.5 ± 2.7 gC m−2 for CH4, and 2422.5 ± 48.0 mm for water vapor. Fishpond CH4 and water vapor fluxes exhibited distinct seasonal patterns with higher fluxes in summer. CO2, CH4, and water vapor fluxes were driven predominantly by shortwave radiation, air temperature, and wind speed, respectively. At the current deforestation rate, the global carbon impact arising from mangrove conversion to fishponds could reach 109 Gt CO2‐equivalent by 2100. Halting global mangrove conversion to aquaculture ponds by 2030 could reduce the net carbon impact by 90.2 Gt CO2‐equivalents by 2100. Thus, preserving coastal wetlands from conversion to aquaculture ponds is among the most effective nature‐based climate solutions.
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