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Abstract Seagrass meadows are important sinks for organic carbon and provide co-benefits. However, data on the organic carbon stock in seagrass sediments are scarce for many regions, particularly The Bahamas, which accounts for up to 40.7% of the documented global seagrass area, limiting formulation of blue carbon strategies. Here, we sampled 10 seagrass meadows across an extensive island chain in The Bahamas. We estimate that Bahamas seagrass meadows store 0.42–0.59 Pg organic carbon in the top-meter sediments with an accumulation rate of 2.1–2.9 Tg annually, representing a substantial global blue carbon hotspot. Autochthonous organic carbon in sediments decreased from ~1980 onwards, with concomitant increases in cyanobacterial and mangrove contributions, suggesting disturbance of seagrass ecosystems, likely caused by tourism and maritime traffic activities. This study provides seagrass blue carbon data from a vast, understudied region and contributes to improving climate action for The Bahamas and the Greater Caribbean region.

This study characterizes the world’s largest seagrass ecosystem in The Bahamas by integrating spatial estimates with remote sensing and performing extensive ground-truthing of benthic habitat with 2,542 diver surveys, as well as data obtained from instrument-equipped tiger sharks, which have strong fidelity to seagrass ecosystems.

Seagrass meadows store significant amounts of carbonate (CaCO _3 ) in sediment, contributing to coastal protection but potentially offsetting their effectiveness as carbon sinks. Understanding the accumulation of CaCO _3 and its balance with organic carbon (C _org ) in seagrass ecosystems is crucial for developing seagrass-based blue carbon strategies for climate change mitigation. However, CaCO _3 accumulation in seagrass meadows varies significantly across geographic regions, with notable data gaps in the Caribbean and Central America. Here, we sampled 10 seagrass meadows across an extensive island chain in The Bahamas, part of the largest seagrass ecosystem and one of the largest CaCO _3 banks globally, to evaluate CaCO _3 stock, accumulation rate, and its balance with C _org sequestration. Seagrass meadows in The Bahamas store 6405–8847 Tg of inorganic carbon (C _inorg ) in the upper meter sediment, with an annual accumulation rate of 38.3–52.9 Tg of C _inorg , highlighting these meadows as hotspots for CaCO _3 burial. CaCO _3 contributes 67 ± 8% (mean ± standard error) of the sediment accumulation, indicating its important role in seabed elevation. Sediment C _inorg showed no significant relationship with C _org , with an average C _org : C _inorg ratio of 0.069 ± 0.002, ∼ 10 times lower than the threshold (C _org : C _inorg ratio of about 0.63) at which seagrass ecosystem transition from CO _2 sources to sinks. However, the available air–sea gas flux measurement was only 1/5 of the calculated CO _2 emission expected from calcification, suggesting that part of the accumulated CaCO _3 is supported by allochthonous inputs. Furthermore, no perceivable relationship between seagrass density and either CaCO _3 stock or accumulation rate was observed, indicating that seagrass may play a limited role in supporting CaCO _3 production. Further studies on water chemistry, calcification rate, air–sea CO _2 flux, and comparison between seagrass and unvegetated habitats are required to elucidate the carbon budget of this globally significant ecosystem.