Factors Determining Seagrass Blue Carbon Across Bioregions and Geomorphologies

Seagrass meadows rank among the most significant organic carbon (C-org) sinks on earth. We examined the variability in seagrass soil C-org stocks and composition across Australia and identified the main drivers of variability, applying a spatially hierarchical approach that incorporates bioregions and geomorphic settings. Top 30 cm soil C-org stocks were similar across bioregions and geomorphic settings (min-max: 20-26 Mg C-org ha(-1)), but meadows formed by large species (i.e., Amphibolis spp. and Posidonia spp.) showed higher stocks (24-29 Mg C-org ha(-1)) than those formed by smaller species (e.g., Halodule, Halophila, Ruppia, Zostera, Cymodocea, and Syringodium; 12-21 Mg C-org ha(-1)). In temperate coastal meadows dominated by large species, soil C-org stocks mainly derived from seagrass C-org (72 +/- 2%), while allochthonous C-org dominated soil C-org stocks in meadows formed by small species in temperate and tropical estuarine meadows (64 +/- 5%). In temperate coastal meadows, soil C-org stocks were enhanced by low hydrodynamic exposure associated with high mud and seagrass C-org contents. In temperate estuarine meadows, soil C-org stocks were enhanced by high contributions of seagrass C-org, low to moderate solar radiation, and low human pressure. In tropical estuarine meadows formed by small species, large soil C-org stocks were mainly associated with low hydrodynamic energy, low rainfall, and high solar radiation. These results showcase that bioregion and geomorphic setting are not necessarily good predictors of soil C-org stocks and that site-specific estimates based on local environmental factors are needed for Blue Carbon projects and greenhouse gases accounting purposes.