Quantifying coastal blue carbon through primary production in a subtropical bay

Abstract

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 similar to 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.