Lack of blue carbon recovery in restored tropical seagrass ecosystems

Seagrass ecosystems are vital for coastal resilience, biodiversity, and as critical carbon sinks. With global seagrass declines, restoration has emerged as a key strategy for ecological and carbon recovery. Although through seagrass restoration, various ecosystem services return, there is a lack of information on the return of the carbon sequestration and accumulation. This study aims to assess the potential recovery of blue carbon benefits through seagrass restoration across various sites in Thailand. We analyzed carbon stocks and accumulation rates in restored Enhalus acoroides meadows at four sites, evaluating spatial variability in carbon recovery in restored versus natural meadows and unvegetated sediment. Despite successful seagrass establishment, the organic carbon (OC) content (%) within the surface sediment (top 20 cm) was not significantly different among restored, natural seagrass meadows, and bare sand, averaging 0.8 ± 0.1%, 0.9 ± 0.2%, and 0.9 ± 0.2% respectively. Although significant differences in OC content (%) were observed between sites, no differences were noted between the habitat types within each site. Predominantly sandy sediment (over 90%) with minimal mud content (1% or less) were found at all sites. The highest organic carbon stock in surface sediment was in unvegetated sediment, averaging 16.8 ± 3.4 Mg C ha^-1. Significant differences in OC stocks were also observed across all site comparisons, with higher stocks generally found in bare sand compared to restored and natural seagrass meadows. Sediment accumulation profiles, indicated by the absence of excess ²¹⁰Pb, suggest a lack of net fine sediment accumulation over the past decade or mixing of the upper sediment, precluding reliable sedimentation rate estimation. These findings suggest that these restored meadows are not forming depositional environments contributing to significant additional carbon sequestration, as evidenced by the minimal increase in OC stocks across the sites. Additionally, the low OC content (%) and minimal mud presence suggest overall low sedimentation rates, even in natural seagrass meadows. These results highlight the complexity of achieving carbon sequestration goals through seagrass restoration, emphasizing the need for site-specific restoration strategies that consider local sediment dynamics and ecological conditions to enhance carbon storage capabilities.

Keywords: organic carbon, carbon additionality, carbon accumulation, seagrass, restoration