Abstract The hypothesis that seaweed farming contributes to carbon burial below the farms was tested by quantifying burial rates in 20 seaweed farms distributed globally, ranging from 2 to 300 years in operation and from 1 ha to 15,000 ha in size. This involved combining analyses of organic carbon density with sediment accumulation rate in sediments below seaweed farms relative to reference sediments beyond the farm and/or prior to the farm operation. One in every four farms sampled was set over environments that export, rather than retain materials. For the farms that were placed over depositional environments, where sediment accumulation could be quantified, the thickness of sediment layers and stocks of carbon accumulated below the farms increased with farm age, reaching 140 ton C ha -1 for the oldest farm, and tended to exceed those in reference sediments beyond the farm and/or prior to the operation of the farms. Organic carbon burial rates in the farm sediments averaged (± SE) 1.87 ± 0.73 ton CO 2 equivalent (CO 2-eq ) ha -1 year -1 (median 0.83, range 0.10 – 8.99 ton CO 2-eq ha -1 year -1 ), twice the average (± SE) burial rate in reference sediments (0.90 ± 0.27, median 0.64, range 0.10-3.00 ton CO 2-eq ha -1 year -1 ), so that the excess organic carbon burial attributable to the seaweed farms averaged 1.06 ± 0.74 ton CO 2-eq ha -1 year -1 (median 0.09, range −0.13-8.10 ton CO 2-eq ha -1 year -1 ). This first direct quantification of carbon burial in sediments below seaweed farms confirms that, when placed over depositional environments, seaweed farming tend to sequester carbon in the underlying sediments, but do so at widely variable rates, increasing with farm yield.

Carbon Burial in Sediments below Seaweed Farms