Are Estimates of Blue Carbon Sequestration by Seagrass Sediments Biased High?

Coastal marine sediments bury organic carbon, sequestering atmospheric carbon dioxide. Vegetated coastal ecosystems, including seagrass beds, are thought to sequester carbon disproportionately for their size, making it essential to itemize them in global carbon models, and implying that restoration or protection offers the potential to obtain carbon credits. International protocols have been developed to evaluate carbon capture by seagrass beds, based on local or global studies in semi-terrestrial systems. However, these methods generally do not account for the behaviour of marine sediments. Sedimentation rate, sediment mixing, microbial degradation, and the energy of the environment all affect the burial of organic carbon in the ocean. Neglecting these factors tends to overestimate sequestration by seagrasses, and, in addition, results in an incorrect spatial distribution of carbon burial. We will describe how marine sediments process and bury organic carbon; present a direct and reliable method to determine carbon burial, suitable for international Tier 3 reporting; and discuss the implications for global estimates of carbon sequestration by seagrass beds.