Blue carbon stocks in southern Africa's Endangered seagrass Zostera capensis

Abstract

Blue carbon research, although rapidly expanding, still requires site- and species-specific assessments to inform decision-making and policy formation. In this study, C stocks were assessed in the Endangered seagrass Zostera capensis. in the Knysna, Swartkops and Nxaxo estuaries, South Africa. Mean biomass C storage was 2.39 ± 1.42 Mg C ha−1 (similar to the global seagrass average), while 177.65 ± 122.29 Mg C ha−1 was estimated in the top meter of sediment. Sediment organic C content was 0.99 ± 0.45% — markedly lower than the seagrass global average but within the range reported for other Zostera species — and displayed significant spatial variability within and between estuaries. Sediment C was particularly high in the eutrophic Swartkops, likely due to excessive wastewater discharge introducing organic matter to the estuary's seagrass habitats. Based on the C stocks determine in this study, it is estimated that approximately 315 970 ± 217 111 Mg C could be stored in South Africa's 1755 ha of Z. capensis habitats. However, these important blue carbon habitats face multiple threats, particularly climatic events like droughts and floods, and anthropogenic eutrophication. This study has provided the first comprehensive C stock assessment for Z. capensis and contributes to global blue carbon datasets. Furthermore, this work has reinforced the importance of protecting the Endangered seagrass by highlighting the important ecosystem service it provides through climate change mitigation and adaptation.