Carbon dynamics under loss and restoration scenarios in the world's largest seagrass meadow.

Seagrass sediments accumulate high amounts of organic carbon, but they are threatened by human activities and their global extent continues to shrink. Simultaneously, there is interest in including seagrass carbon accumulation in countries' Nationally Determined Contributions (NDCs). We used the InVEST Coastal Blue Carbon Model to estimate sediment organic carbon (SOC) accumulation over 100 years in seagrass of the Bahama Banks, the world's largest seagrass meadow. Using seagrass maps and sediment core measurements, we modeled SOC accumulation in two scenarios: (1) 1% seagrass area loss per year, the Business As Usual scenario (BAU); (2) restoration of seagrass extent to that of 30 years prior by 2120, meeting the goals of the Kunming-Montreal global biodiversity framework. With a conservative initial seagrass extent, by 2120, the SOC accumulation was 90.6 Mt CO eq (24.0 autochthonous Mt CO eq) in the BAU and 703.7 Mt CO eq (186.5 autochthonous Mt CO eq) in the restoration scenario, and average additional SOC accumulation was 611.0 Mt CO eq (161.9 autochthonous Mt CO eq). Using a high estimate of initial seagrass extent, by 2120, the net SOC accumulation was 155.4 Mt CO eq (41.2 autochthonous Mt CO eq) in the BAU and 1058.2 Mt CO eq (280.4 autochthonous Mt CO eq) in the restoration scenario, and additional SOC accumulation was 902.8 Mt CO eq (239.2 autochthonous Mt CO eq). The potential for either SOC accumulation or losses to occur if seagrass extent continues to decline highlights uncertainty around whether Bahamian seagrass meadows will remain a net carbon sink. The additional accumulation of autochthonous carbon if seagrasses were restored was comparable in scale to the annual greenhouse gas emissions of The Bahamas, suggesting potential for seagrass restoration to contribute to the country's NDCs and broader climate mitigation strategies.