Biologically Mediated Carbon Cycling and Budget in the Yangtze River Estuary and East China Sea

Abstract The East China Sea (ECS), receiving substantial terrestrial inputs from the Yangtze River, serves as an important carbon sink strongly influenced by biological processes. Although carbon fixation and p CO 2 dynamics have been extensively studied, the biological mechanisms governing organic carbon partitioning, sequestration and their environmental drivers remain unclear. This study integrates spatiotemporal field surveys and incubation experiments to investigate biologically mediated carbon cycling across the Yangtze River Estuary and ECS continuum. Field data highlight that salinity and nutrient‐driven biological activity collectively regulate dissolved organic carbon (DOC) concentration and properties. Incubation experiments further revealed how these factors shape bioavailable organic carbon (BOC) and microbial metabolism. Spatially, BOC exhibits a bimodal distribution along salinity gradients, with terrestrial‐derived BOC peak in turbid low‐salinity zones accelerating mineralization and CO 2 efflux, while plankton‐derived BOC maxima in nutrient‐rich mid‐high salinity regions enhance carbon turnover rate through autotrophic‐heterotrophic metabolic coupling, sustaining seasonal DOC accumulation. The ECS exhibited enhanced biological carbon sequestration (including the biological and microbial carbon pump) efficiency compared to oligotrophic oceans, jointly sequestering 8.17 ± 3.19 Tg C a −1 , equivalent to 40% of the annual net air‐sea CO 2 influx. Moreover, the observed temperature dependence of microbial carbon pump efficiency suggests that ongoing climate change may shift microbial carbon partitioning patterns in marginal seas. This study advances our understanding of carbon transformation and sequestration mechanisms in marginal seas, highlighting the necessity to incorporate microbial feedbacks to improve coastal carbon‐climate projections.