Calcification-driven CO 2 emissions exceed "Blue Carbon" sequestration in a carbonate seagrass meadow.

Long-term “Blue Carbon” burial in seagrass meadows is complicated by other carbon and alkalinity exchanges that shape net carbon sequestration. We measured a suite of such processes, including denitrification, sulfur, and inorganic carbon cycling, and assessed their impact on air-water CO exchange in a typical seagrass meadow underlain by carbonate sediments. Eddy covariance measurements reveal a consistent source of CO to the atmosphere at an average rate of 610 ± 990 μmol m hour during our study and 700 ± 660 μmol m hour (6.1 mol m year) over an annual cycle. Net alkalinity consumption by ecosystem calcification explains >95% of the observed CO emissions, far exceeding organic carbon burial and anaerobic alkalinity generation. We argue that the net carbon sequestration potential of seagrass meadows may be overestimated if calcification-induced CO emissions are not accounted for, especially in regions where calcification rates exceed net primary production and burial.