Large variability in organic carbon and CaCO3 burial in seagrass meadows: A case study from three Australian estuaries

Blue carbon refers to the carbon accumulation capacity of vegetated coastal habitats, including salt marshes, mangroves forests and seagrass meadows. Here we present estimates of organic carbon (Corg) and calcium carbonate (CaCO3) burial rates from 4 seagrass species (Halophila ovalis, Posidonia australis, Ruppia megacarpa, Zostera muelleri) in 3 temperate estuaries on the east coast of Australia. The Corg burial rates (mean ± SE) varied by an order of magnitude across the seagrass communities (16 ± 3 to 130 ± 40 g m−2 yr–1). The δ13Corg and Corg:N ratios suggest that the seagrass communities buried variable mixtures of seagrass, algal and mangrove/ terrestrial material. CaCO3 burial rates ranged from 15 ± 11 to 188 ± 122 g m−2 yr−1, which, if precipitated by calcifying organisms in these or nearby habitats, may offset up to 89% of the Corg burial across the 8 seagrass communities. Our results highlight a large range in both Corg and CaCO3 burial rates, and the provenance of the carbon sequestered in seagrasses, factors that need to be considered when assessing the role of seagrasses in blue carbon and climate change mitigation strategies.