Researchers have provided the first direct estimates of how much carbon is stored beneath seaweed farms, a growing industry that now covers roughly 2000 square kilometres worldwide. 

The new Nature Climate Change results show that at the higher end, some farms are locking away up to 140 tons of carbon per hectare. On average, these quantities are smaller, but they still lie within the range of carbon stored beneath mangroves and seagrass meadows—ecosystems where blue carbon credits are already being issued. 

As part of an initiative called the Oceans 2050 Global Seaweed Project, the team which involved dozens of international researchers, spent 15 months taking sediment cores from beneath 20 seaweed farms, scattered across 11 countries including China, Japan, Madagascar and the United States. Seaweed is farmed commercially in many parts of the world for food, fertilizers, fuel, and pharmaceuticals. As they grow, the algal fronds capture large amounts of carbon, and as seaweeds shed bits of biomass, some of this carbon gets buried in soft marine sediments below. The farms the researchers sampled ranged in age from 2 to 320 years old; some were rooted in gravelly ground, and others in softer muddy sediments more able to store carbon. The farms also contained a range of species, from large, tree-like kelps to neon green sea lettuces.

The researchers note in their study that none of the farms they focused on were designed to capture carbon, so they were interested in calculating carbon storage as a potential climate co-benefit of this food system. 

Their sampling revealed a huge range in the amount of carbon locked into farm sediments, which averaged out at 1.06 tons of carbon dioxide equivalent (CO2e) per hectare, per year. That figure counted only the ‘excess carbon’—the sequestered carbon that was attributable to the growth of the seaweed, which excludes the pre-existing carbon stored in the marine sediments below. This is essential to determine the actual environmental benefits of the industry. 

In some farms these estimates were higher, at 8.10 tons of CO2e per hectare per year.  Typically, these richer carbon stores were beneath older farms, in soils featuring deeper, finer sediments, and located in calmer bays, where conditions are optimal for locking in carbon and keeping it that way for longer stretches of time. Sediment cores from one such farm in Japan had stored a striking 140 tons of CO2e per hectare over the years, showing what’s possible under ideal conditions and with time. 

While the average numbers are “relatively modest” as the researchers put it, they still show that the amount of carbon on seaweed farms is comparable to the carbon that’s stored away beneath seagrass meadows and mangroves. The average 1.06 tons of CO2e that’s stored in each hectare of farm sediment is about 33-39% of the amount that these other ecosystems lock away. What’s more, their results suggest that if future seaweed farms were operated at high yield and placed more strategically in locations that optimize carbon sequestration—such as calm water bays, or over fine sediments—then farms could sequester four times that average figure. 

This could put seaweed farms in the running to issue blue carbon credits that are now commonly linked to these other ecosystems. But the researchers warn that we shouldn’t get ahead of ourselves. Plenty more research is needed before the marketplace opens up to seaweed farms they say, not least to determine how long carbon gets stored for on average, or how vulnerable it is to disturbance and release. 

For now, they suggest that the focus remains on carbon sequestration not as the reason for seaweed farming, but as a co-benefit of the many services these farms already provide—such as food and income, the cleansing of marine waters, and boosting biodiversity. The credits may come, but this study is just “a useful first step towards exploring that possibility,” the researchers say.

Duarte et. al. “Carbon burial in sediments below seaweed farms matches that of Blue Carbon habitats.” Nature Climate Change. 2025.

Image: Wikipedia