Stocking density-driven shift of atmospheric carbon dioxide source-sink functions in mussel culture systems

Bivalve aquaculture has long been excluded from blue carbon frameworks because bivalves release carbon dioxide (CO ₂ ) into seawater during shell formation. However, this ignores the complex roles of bivalves in aquatic ecosystems. Through manipulative experiments with mesocosms, here we identify that CO ₂ source-sink functions of mussel culture systems are stocking density-dependent. At a stocking density of 55.56 g m ^− 3 , mussels suppressed phytoplankton biomass through filter feeding (top-down effect), making bivalve respiration the dominant process governing CO ₂ dynamics and turning the system into a CO ₂ source. When density fell below 27.78 g m ^− 3 , the “bottom-up effect”from bivalve excretion prevailed, enabling net primary productivity to dominate (35.61–40.92%) and turning the system into a CO ₂ sink. The actual stocking densities in coastal bivalve farms considering water renewal are below 27.78 g m ^− 3 , suggesting they can function as CO ₂ sinks. Post-harvest monitoring further indicated that water masses flowing out from the core farming area can absorb more CO ₂ than non-farming seawater. Therefore, this study recommends adopting an integrated, ecosystem-based approach to assess the role of bivalve aquaculture in global climate change.