CO₂ released by carbonate sediment production in some coastal areas may offset the benefits of seagrass “Blue Carbon” storage

Seagrass ecosystems have been identified as long‐term carbon sinks whose conservation could serve as a tool to mitigate carbon emissions. Seagrasses alter landscapes in a way that stimulates carbon biosequestration, but discussions of their role in atmospheric CO 2 mitigation disregard the co‐occurring inorganic carbon cycle, whose antagonist effect on CO 2 sequestration can buffer and potentially outweigh the effects of C org production on net carbon exchange with the atmosphere. This study examines the extent of both organic carbon (C org ) and inorganic carbon (C inorg ) stocks as proxies for long‐term production and calcification in the poorly studied seagrass meadows of southeastern (SE) Brazil and compares values to Florida Bay (U.S.A.), a well‐studied system known for both high autotrophy and calcification, representing extremes of CaCO 3 soil content. Seagrass soils in SE Brazil contain an average of 67.6 ± 14.7 Mg C org ha −1 in the top 1 m, compared to an average of 175.0 ± 20.4 Mg C org ha −1 for their counterparts in Florida Bay. C inorg as CaCO 3 in SE Brazil averaged 141.5 ± 60.0 Mg C inorg ha −1 in the top meter of soil while the warmer, calcification‐promoting waters of Florida Bay had higher soil C inorg areal stock, averaging 754.6 ± 26.7 Mg C inorg ha −1 . When the CO 2 evasion related to CaCO 3 production is considered, seagrass ecosystems with high CaCO 3 content may have CO 2 sequestered via C org accumulation negated by CO 2 produced by calcification. These findings prompt the reconsideration of carbon inventory methods and encourage regionally‐ and community‐specific assessments of CO 2 sequestration abilities of seagrass ecosystems.