Seagrass Carbon Stock Calculator

A seagrass carbon inventory separates a large, old, slow pool from a small, fast one. The sediment term multiplies effective meadow area by core depth to get a soil volume, then by an organic carbon density in kilograms per cubic meter; the biomass term multiplies leaf and rhizome standing crop in grams dry weight per square meter by a carbon fraction near 35 percent. In most surveyed meadows the sediment dominates, often supplying well over 90 percent of the stock, because waterlogged and largely anoxic sediments slow decomposition enough for organic matter to accumulate for millennia. Effective area is the mapped extent scaled by percent cover, since a meadow at 60 percent cover does not store what a continuous one does. Conversion to CO2 equivalent uses the factor 3.67, the ratio of the molar mass of carbon dioxide to that of carbon. Sequestration rate and stock are distinct quantities and should not be added. The species rates offered here, from roughly 45 grams of carbon per square meter per year for small fast-turnover Halophila up to about 138 for Zostera marina, describe annual burial, while the stock figure describes what centuries of burial have already built. Reported top-meter stocks typically fall between 100 and 500 tonnes of carbon per hectare, with exceptional Mediterranean Posidonia oceanica mats far above that range. The main limitations are that carbon density varies by a factor of several within a single meadow, that a 1 m core truncates deposits which often continue several meters down, and that not all buried carbon is seagrass-derived, since meadows trap allochthonous particles from the water column. Stable isotope work is needed to apportion that fraction before claiming it as a project benefit.

Seagrasses were long treated as a fisheries habitat rather than a carbon reservoir, and the shift dates to the late 2000s. The term blue carbon was introduced by a 2009 report issued by the United Nations Environment Programme with FAO and IOC-UNESCO, which argued that mangroves, salt marshes and seagrass meadows warranted accounting alongside terrestrial sinks. Earlier Mediterranean work had already hinted at the scale: research on Posidonia oceanica showed that its refractory fibre mats accumulate over thousands of years, with some deposits radiocarbon dated to the mid-Holocene, several meters thick and effectively locked away. The quantitative turning point was a 2012 Nature Geoscience paper led by James Fourqurean of Florida International University with Carlos Duarte and other coauthors, which compiled global core data and estimated seagrass sediment stocks of several billion tonnes of carbon, establishing the per-hectare ranges this calculator uses. Methods were standardized soon after through the Blue Carbon Initiative manual coordinated by Conservation International, IUCN and IOC-UNESCO, and through the IPCC 2013 Wetlands Supplement, which brought coastal wetlands into national greenhouse gas inventory guidance. Market machinery followed: Verra approved a tidal wetland and seagrass restoration methodology in 2015, allowing verified credits, though restoration cost per hectare has generally exceeded credit revenue, which is why conservation of existing meadows remains the stronger economic case.