Accounting for subtidal seagrass can significantly increase carbon stock estimates from South African seagrass meadows

Seagrasses are among the world’s most productive marine ecosystems and are increasingly recognised as natured-based solutions. In South Africa, seagrass ecosystems occupy intertidal and subtidal zones; however, due to logistical and methodological limitations, subtidal seagrass has been largely excluded from blue carbon assessments, thereby potentially underestimating seagrass carbon stocks. Here, we quantify carbon stocks for subtidal meadows including sedimentary and biomass pools and investigate spatio-temporal variability of sediment organic carbon (SOC) and morphometrics (shoot count, leaf count and leaf length) of the Endangered seagrass Zostera capensis for the Olifants River Estuary, South Africa. Subtidal seagrass sediments stored 25.13 ± 7.64 Mg C ha −1 (mean ± SD) to a mean depth of 41.2 cm (2023), with an above-ground and below-ground biomass of 1.09 ± 0.38 and 0.10 ± 0.04 Mg C ha −1 from the upper site (2024), respectively. Our results show no variability of SOC across estuary (1–3 km), site (∼150 m), and meadow (∼5 m) scales, but SOC was significantly different (p < 0.05) when sampled 12 months apart. Seagrass morphometrics were not correlated with SOC storage. Compared to intertidal measurements from the same estuary, subtidal seagrass stored significantly more carbon per unit area. These estimates are preliminary and based on homogenised cores, though they provide valuable insights into the carbon stock potential of subtidal seagrass meadows. Given logistical challenges of working in subtidal environments, we briefly review methods for sampling subtidal meadows and provide some recommendations to help guide future regional seagrass blue carbon assessments.