Directional disparity between oysters and mud snails in modulating marine carbon sequestration

Amplified greenhouse effects underscore the importance of coastal shellfish to uptake and sequester atmospheric CO₂ via the biological carbon pump. With continuous allochthonous nutrient input, the filter-feeding oysters enhance the marine carbon sequestration through the biological carbon pump. However, the role of oysters and deposit-feeding gastropod-mud snails without sustained nutrient input remains unclear. This study employed mesocosms to simulate coastal ecosystems for oysters and mud snails, elucidating their disparity on the marine carbon sequestration without allochthonous nutrient input. Results revealed that, regardless of nutrient input, the oysters positively regulate the marine carbon sequestration. Unlike filter-feeders, the deposit-feeders mud snails lack top-down grazing control. Their nutrient excretion caused intense self-shading, hindering atmospheric CO₂ uptake, while their bioturbation promoted sedimentary carbon resuspension and recalcitrant carbon remineralization, jointly negatively modulating the carbon sequestration. These findings elucidated the directionality of shellfish modulating the marine carbon sequestration, providing a theoretical basis for carbon sequestration assessment and insights into the role of intertidal shellfish in marine carbon cycling under climate change.