Zostera capensis Sedimentary Carbon Stock Variability: An Intra-estuarine Site Comparison

Understanding the drivers of organic carbon (C_org) stock variability is key to improving the accuracy of seagrass blue carbon assessments and emphasizing their role as nature-based solutions to mitigate climate change. Using sediment coring (15 cm depth), we investigated C_org variability at fine spatial scales (< 110 m) and large spatial scales (~17 km) between the distributional limits of Zostera capensis at the upper and lower extents of South Africa’s Berg River Estuary. We found C_org stock heterogeneity at large spatial scales, with higher C_org stocks at the lower extent of the estuary (7.18 ± 1.83 Mg C ha⁻¹) compared to the upper extent (3.33 ± 0.70 Mg C ha⁻¹); however, C_org stocks were homogenous at fine scales. We also found that fine particle size proportion and shoot density were not significantly different at fine and large spatial scales, with both factors not significantly correlated to the observed C_org stocks. Future studies should account for intra-estuarine C_org variability, by sampling across the distributional ranges of Z. capensis within individual estuaries, to improve blue carbon assessments for both regional and international carbon stock assessments.