Seagrass is important store of organic carbon

Designers of seawater desalination plants should take care their project does nothing to decrease the amount of seagrass in the vicinity, as the plant is an important store of carbon.

In a paper Seagrass ecosystems as a globally significant carbon stock published in Nature Geoscience 5(7): 505-509, the authors from research institutes in the US, Europe and Australia estimate that around 3 tonnes of carbon are stored in living seagrass per hectare covered.

Seagrasses are an important store of ‘blue carbon’, the carbon stored by marine life, and may release an amount of carbon that is equivalent to 10% of that released by land-use changes, according to the study. To reach this figure, the researchers analysed a database containing 3,640 observations of 946 seagrass meadows around the world to try to understand how much carbon is locked away in seagrass plants and soils.

Their calculations suggest that an average of 140 tonnes of organic carbon are stored in the top metre of each hectare of seagrass soil, which is around twice that found in soils on land.

Using only data from sites for which full inventories exist, the researchers estimate that, globally, seagrass ecosystems could store as much as 19.9 petagrams (Pg) organic carbon. According to a more conservative approach, in which they incorporated more data from surface soils and depth-dependent declines in soil carbon stocks, they estimate that the seagrass carbon pool lies between 4.2 and 8.4 Pg carbon.

Around a quarter of the Mediterranean’s seabed, at depths between 1 and 40 m, is covered in seagrass. However, disease, pollution, eutrophication and disturbances, such as dredging and construction, all pose threats to it.

Current rates of seagrass loss could result in the release of up to 299 Tg carbon per year, assuming that all of the organic carbon in seagrass biomass and the top metre of soils is remineralized.