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.

Automated summary

Seagrass meadows are important organic carbon sinks, with variability in soil C-org stocks and composition observed across Australia. Large seagrass species meadows have higher stocks compared to meadows formed by smaller species. Soil C-org stocks are influenced by environmental factors, highlighting the need for site-specific estimates in Blue Carbon projects and greenhouse gases accounting.