Durability-weighted crediting of blue carbon in coastal industrial symbiosis

Blue carbon crediting is under scrutiny because prevailing methodologies treat every tonne of fixed carbon as equivalent, irrespective of how long it stays out of the atmosphere. This paper develops TIDE-BC, an accounting framework that discounts fixed carbon by the residence time of the pool it enters, and applies it to a coastal industrial symbiosis loop in which flue gas, nutrient-rich effluent, alkaline steel slag and siliceous ash are routed through a marine microalgal raceway. A coupled model combining depth-averaged photoinhibited growth, cardinal-temperature kinetics, Droop nutrient quotas, seawater carbonate speciation and a closed alkalinity budget is integrated with a durability-equivalence factor derived in closed form as the Laplace transform of an exponential retention kernel. Simulations over six marine species, four process configurations and 512 Sobol draws show that the durability weight of short-cycle biomass products is 0.043, so crediting biomass as permanent overstates removal by 3.8 to 6.1 times. Three of the four configurations are net carbon sources on a grid at 0.71 kg CO₂ kWh⁻¹; the break-even grid intensity is 0.250 kg CO₂ kWh⁻¹. The best configuration delivers 42.8 t CO₂ ha⁻¹ yr⁻¹ deterministically and a tenth-percentile issuance of 25.0 t CO₂ ha⁻¹ yr⁻¹, implying a 30.2% buffer. Two results run against expectation and are reported as such: the diatoms tested fix no carbon at all without a silicate co-feed from ash leachate, and the coccolithophore Emiliania huxleyi is a net source at -5.3 t CO₂ ha⁻¹ yr⁻¹ once calcification is placed on a closed alkalinity budget. The framework is simulation-based and has not been validated against pilot or field data.