Challenges to select suitable habitats and demonstrate ‘additionality’ in Blue Carbon projects: A seagrass case study

Seagrass restoration has been suggested as a Blue Carbon (BC) strategy for climate change mitigation. For Nationally Determined Contributions (NDC) and carbon crediting schemes, BC projects need to demonstrate ‘additionality’, that is enhanced CO₂ sequestration and/or avoided greenhouse gas emissions following management actions. This typically requires determining soil carbon accumulation rates (CAR), which is often done using radionuclides or surface elevation tables to estimate sedimentation rates. Here we undertook a case study, using ²¹⁰Pb and ¹⁴C dating, to detect possible changes in C_org stocks and CAR following the loss and partial recovery of Posidonia seagrass meadows in South Australia since 1980–90s. The ²¹⁰Pb data revealed a lack of accumulation of excess ²¹⁰Pb in most sites, suggesting negligible accumulation of sediments, intense mixing of the upper layers, or accumulation of reworked sediments, precluding the estimation of reliable CAR at decadal time scales. This limitation was also encountered with ¹⁴C. The inability to compare sites over analogous periods of time prevented quantifying differences in soil C_org sequestration, thereby to demonstrate additionality. The lack of significant differences in soil C_org stocks among sites which never suffered seagrass loss, those showing recovery and those with no recovery (5.7 ± 1.2, 4.5 ± 0.7 and 3.3 ± 0.3 kg C_org m^-2 within the top meter, respectively) also precluded estimates of soil C_org gains or losses. Our findings demonstrate that, while ²¹⁰Pb and ¹⁴C provide important information on sediment deposition dynamics, it is not straightforward to demonstrate additionality using radionuclides in low depositional seagrass habitats exposed to hydrodynamic energy, features which may be encountered in seagrass sites. We provide insights for the selection of suitable habitats for seagrass BC projects, suggest possible alternative methods for estimating additionality, and discuss the implications of the findings for the implementation of seagrass BC strategies to mitigate greenhouse gas emissions.