Significant carbonate burial in The Bahamas seagrass ecosystem

Seagrass meadows store significant amounts of carbonate (CaCO₃) in sediment, contributing to coastal protection but potentially offsetting their effectiveness as carbon sinks. Understanding the accumulation of CaCO₃ 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, CaCO₃ 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 CaCO₃ banks globally, to evaluate CaCO₃ 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 CaCO₃ burial. CaCO₃ 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 CO₂ sources to sinks. However, the available air-sea gas flux measurement was only 1/5 of the calculated CO₂ emission expected from calcification, suggesting that part of the accumulated CaCO₃ is supported by allochthonous inputs. Furthermore, no perceivable relationship between seagrass density and either CaCO₃ stock or accumulation rate was observed, indicating that seagrass may play a limited role in supporting CaCO₃ production. Further studies on water chemistry, calcification rate, air-sea CO₂ flux, and comparison between seagrass and unvegetated habitats are required to elucidate the carbon budget of this globally significant ecosystem.