Inter-habitat partitioning of estuary blue carbon stocks: A spatial assessment of storage capacity and restoration opportunities

Coastal wetlands, such as salt marsh and seagrass beds, are important sedimentary carbon sinks attributed to their inherent sequestration proficiencies. Their spatial heterogeneity however, introduces uncertainties in sediment blue carbon assessments, requiring explanatory understanding of causal factors, particularly where environmental gradients are evident. This study measured the spatial variability in sediment carbon stocks associated with seagrass, salt marsh, and degraded (disturbed, unvegetated sites) habitats, identifying causal geochemical factors. Significant intra-habitat sediment carbon partitioning was observed (mean SE), where degraded habitats (83.63 9.67 Mg C ha -1 ) and supratidal Salicornia pillansii marsh (65.36 8.62 Mg C ha -1 ) had significantly lower carbon stocks than intertidal salt marsh, Spartina maritima (203.25 35.03 Mg C ha -1 ) and Salicornia tegetaria (243.58 67.84 Mg C ha -1 ), as well as seagrass Zostera capensis (211.4 20.12 Mg C ha -1 ). At the estuary scale statistical correlations were found between sedimentary stocks and several explanatory biogeochemical factors including sediment bulk density (SDB, p < 0.01), particle size ( p < 0.05), total organic matter ( p < 0.001), and salinity ( p < 0.05). Furthermore, a significant inverse association was found between sediment carbon stocks and landward oceanic distance – stressing the effect of geomorphic setting on net estuarine sedimentary carbon accretion. By interpolating the spatial variability in sediment carbon stocks, this study identified opportunities for enhancing salt marsh carbon stock through restoration (385.4ha). The potential carbon emissions, attributed to possible degradation and loss of all habitats from the Swartkops Estuary, was predicted to be 2962.51 Mg CO 2 e ha -1 yr -1 . This further highlights the value of extant carbon stocks, opportunities for restoration of disturbed habitats, and the implications for potential carbon emissions from blue carbon habitats. Furthermore, it reinforces the importance of accounting for local carbon stock variability for blue carbon restoration and/or conservation actions. • Percentage organic carbon partitioned by habitats identified blue carbon hotspots. • Organic carbon decreased landwards and was greater in intertidal marshes. • Carbon stocks were positively correlated with finer grain sediments. • Percentage organic carbon showed strong inter-habitat and spatial heterogeneity. • Salt marsh disproportionately contributes to carbon storage in semi-arid estuaries.