Quantifying coastal blue carbon through primary production in a subtropical bay

Coastal ecosystems play a disproportionate role in global carbon sequestration, yet conventional assessments often underestimate contributions from phytoplankton and benthic macroalgae. In this study, we quantified the carbon sequestration capacity of Maoao Bay, a subtropical embayment in the northwestern Pacific, by partitioning fluxes into phytoplankton-mediated biological carbon pump (phyto-BCP), phytoplankton-associated microbial carbon pump (phyto-MCP), and benthic macroalgal export production (benthic EP). Year-round sampling revealed strong seasonality in both pelagic and benthic primary production: phytoplankton production peaked during winter under enhanced nutrient availability, whereas subtidal macroalgal production was highest in summer under elevated light conditions. Total carbon sequestration averaged 88.3 kgC d -1 (1.62 tCO 2 e ha -1 yr -1 ), with phyto-BCP contributing the largest annual fraction (68.3%), followed by benthic EP (17.5%) and phyto-MCP (14.2%). Despite occupying only ∼12% of the bay, macroalgal habitats exhibited higher per-area carbon removal efficiency than phytoplankton, highlighting their disproportionate role in coastal carbon sequestration. These findings underscore the ecological complementarity of pelagic and benthic primary producers and suggest that targeted restoration or expansion of macroalgal beds could significantly enhance nearshore blue carbon pathways, supporting regional climate mitigation strategies and broader sustainability objectives. Coastal ecosystems disproportionately contribute to carbon sequestration, yet phytoplankton and macroalgae are often underrepresented. In subtropical Maoao Bay, phytoplankton-driven carbon export peaks in winter under nutrient-rich conditions, subtidal macroalgal production peaks in summer under high light, and intertidal macroalgae reach maximal productivity in spring and autumn due to moderate irradiance and reduced stress from aerial exposure. Total sequestration averaged 1.62 tCO 2 e ha -1 yr -1 , with phytoplankton dominating annually. The results highlight the complementary roles of pelagic and benthic producers and the potential of macroalgal habitat restoration to enhance coastal blue carbon and support climate mitigation. • Developed a productivity-based framework for assessing coastal blue carbon, using net primary productivity (NPP) multiplied by an export ratio to more accurately estimate carbon sequestration. • In Maoao Bay, phytoplankton production is primarily limited by seasonal nutrient availability, while subtidal benthic macroalgae remain largely light-limited; intertidal macroalgae exhibit intermediate responses influenced by both irradiance and periodic aerial stress. • Subtropical Maoao Bay shows exceptionally high macroalgal NPP, exceeding values from temperate and polar ecosystems, underscoring the substantial potential of subtropical macroalgal habitats for coastal carbon sequestration and climate mitigation.