Carbon burial in sediments below seaweed farms matches that of Blue Carbon habitats

Seaweed farming has emerged as a potential Blue Carbon strategy, yet empirical estimates of carbon burial from such farms remain lacking in the literature. Here, we quantify carbon burial in 20 seaweed farms distributed globally, ranging from 2 to 300 years in operation and from 1 to 15,000 ha in size. The thickness of sediment layers and stocks of organic carbon accumulated below the farms increased with farm age, reaching 140 tC ha−1 for the oldest farm. Organic carbon burial rates averaged 1.87 ± 0.73 tCO2e ha−1 yr−1 in farm sediments, twice that in reference sediments. The excess CO2e burial attributable to the seaweed farms averaged 1.06 ± 0.74 CO2e ha−1 yr−1, confirming that seaweed farming in depositional environments buries carbon in the underlying sediments at rates towards the low range of that of Blue Carbon habitats, but increasing with farm age.