Foregone carbon sequestration dominates greenhouse gas footprint in aquaculture associated with coastal wetland conversion.

Coastal wetlands offer large carbon sequestration benefits but their conversion to aquaculture systems could result in substantial carbon losses. Here we show that the conversion of Spartina alterniflora salt marsh to mariculture ponds in China generated a greenhouse gas (GHG) footprint of 20.3 Mg CO2 equivalent per ha per year. Around two-thirds of the footprint can be attributed to foregone salt marsh GHG mitigation capacity, whereas direct carbon dioxide, methane and nitrous oxide emissions in mariculture ponds account for only ~10%, with the remaining ~20% arising from feed, fertilizer and energy consumption. Aquaculture can offer comparatively lower GHG footprints than other animal protein sources, such as terrestrial beef and small ruminants' production on a kg CO2 equivalent per kg protein basis, but this assumption may not be accurate when considering landscape-scale changes in GHG budgets, particularly in relation to the expansion of aquaculture within blue carbon ecosystems.