Impacts of seaweed-bed restoration in southern Osaka Bay coastal waters on climate-change mitigation functions and biological productivity: Prediction and evaluation using EMAGIN-B.C

Restoration of seaweed beds is expected to contribute to biodiversity conservation, biological productivity, and climate-change mitigation in coastal ecosystems. However, relatively few studies have evaluated these effects while explicitly considering benthic–pelagic coupling, carbonate chemistry, and physical carbon transport. In this study, the benthic–pelagic coupled ecosystem model EMAGIN-B.C. was applied to shallow coastal waters of southern Osaka Bay, Japan, including an OECM-recognized coastal area and blue-carbon-credit-certified seaweed habitats. A Control case and two restoration scenarios with 50% and 100% prescribed seaweed standing biomass were compared. Nutrient concentrations changed by <0.001 mg L⁻¹, while dissolved oxygen increased by approximately 0.01 and 0.02 mg L⁻¹ in the 50% and 100% scenarios, respectively. Phytoplankton production decreased by approximately 0.6% and 1.2%, whereas deposit-feeder production increased by approximately 2.8% and 5.7%, indicating a shift in carbon transfer from pelagic to benthic pathways. Atmospheric CO₂ absorption increased by only 0.02% and 0.04%, whereas net carbon fixation increased by approximately 78% and 154%. The sedimentary carbon-storage function, evaluated as the sum of separately quantified organic-carbon and CaCO₃ burial fluxes, increased by approximately 14% and 30%, respectively. Carbon-budget analysis showed that the large increase in fixation was balanced predominantly by increased DIC inflow across the system boundary rather than by a proportional increase in local atmospheric CO₂ uptake. Physical DIC replenishment therefore buffered the seaweed-induced decrease in local DIC and pCO₂. Carbon storage (③) was resolved into organic-carbon burial and CaCO₃ burial. Organic-carbon burial increased from 96.58 ton C km⁻² yr⁻¹ in the Control to 110.09 and 125.37 ton C km⁻² yr⁻¹ in the 50% and 100% scenarios, while CaCO₃ burial increased from 3.80 to 4.30 and 4.66 ton C km⁻² yr⁻¹. In EMAGIN-B.C., the effects of CaCO₃ formation and dissolution on DIC, TA, pCO₂, and air–sea CO₂ exchange are already represented through carbonate chemistry and are therefore reflected in the carbon-absorption function (①). CaCO₃ burial is reported separately as an inorganic component of sedimentary carbon storage and should not by itself be interpreted as equivalent net atmospheric CO₂ removal. These results demonstrate that seaweed-bed restoration affects coastal carbon cycling through interconnected biological, physical, and sedimentary processes, and that its ecosystem effects cannot be evaluated solely from local atmospheric CO₂ absorption.