Air-Sea CO₂ Transfer in a Macroalgal Habitat and Its Role in mCDR Accounting

Gas transfer velocity (k) parameterisations for air-sea CO₂ exchange are typically derived in the open ocean, leaving coastal habitats like macroalgal coasts poorly constrained. This new study presents the first such parameterisation for a macroalgae-dominated coast, based on direct eddy covariance observations of CO₂ flux at Helgoland, Germany. Here, the gas transfer velocity (k660) followed a quadratic relationship with wind speed, falling between existing (higher) open-ocean and (lower) fetch-limited parameterisations, with a significant non-zero intercept pointing to a suite of non-wind drivers that enhance gas transfer under calm conditions. Using a simulated macroalgal marine carbon dioxide removal (mCDR) intervention, Van Dam 2026 shows that the choice of k660 parameterisation shifts net CDR credits by an amount comparable to a plausible range in operational emissions, implying that gas-transfer should carry equal weight as Life Cycle Assessments when it comes to carbon crediting. Finally, a new framework is proposed to quantify the "risk of incomplete equilibration“, helping to enable a better incorporation of gas-exchange physics into future measurement, reporting, and verification (MRV) systems.

Reference: Van Dam, B. (2026). Parameterizing air-sea CO₂ transfer in a macroalgal habitat and assessing its role in marine carbon dioxide removal accounting. Limnol. Oceanogr. Lett., 11(4), e70139. https://doi.org/10.1002/lol2.70139