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 CO2 mitigation disregard the co-occurring inorganic carbon cycle, whose antagonist effect on CO2 sequestration can buffer and potentially outweigh the effects of Corg production on net carbon exchange with the atmosphere. This study examines the extent of both organic carbon (Corg) and inorganic carbon (Cinorg) 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 CaCO3 soil content. Seagrass soils in SE Brazil contain an average of 67.6 ± 14.7 Mg Corg ha−1 in the top 1 m, compared to an average of 175.0 ± 20.4 Mg Corg ha−1 for their counterparts in Florida Bay. Cinorg as CaCO3 in SE Brazil averaged 141.5 ± 60.0 Mg Cinorg ha−1 in the top meter of soil while the warmer, calcification-promoting waters of Florida Bay had higher soil Cinorg areal stock, averaging 754.6 ± 26.7 Mg Cinorg ha−1. When the CO2 evasion related to CaCO3 production is considered, seagrass ecosystems with high CaCO3 content may have CO2 sequestered via Corg accumulation negated by CO2 produced by calcification. These findings prompt the reconsideration of carbon inventory methods and encourage regionally- and community-specific assessments of CO2 sequestration abilities of seagrass ecosystems