Seaweed farms cover 3.5 million hectares of ocean and are busy locking away vast amounts more carbon than we realise, say researchers. In a new study they argue that in estimates of their blue carbon potential, we’ve been overlooking the unique way that seaweeds interact with seawater, which makes it absorb more CO2.
“Most discussions about seaweed and climate focus on how seaweed captures carbon as it grows. The concern has been that much of that carbon might be released again when the seaweed decomposes,” says Mojtaba Fakhraee, assistant professor in the department of earth sciences at the University of Connecticut, and lead author on the study. This concern has undermined the idea that seaweed farms can double as carbon capture projects. But Fakhraee and colleague Noah Planavsky say this misses a crucial feature of seaweed farms which connects with the wider ocean chemistry.
As seaweed farms grow, occasional fronds break off and drift down to the sand below. Over time in the shallow coastal waters where most farms exist, these fronds accumulate to create a dense layer, providing a steady supply of biomass that accelerates microbial activity in the sediments below. Their activity drives a chemical reaction that produces bicarbonate and dissolves carbonate minerals in the water column. While these chemical processes are unfolding, the alkalinity levels of the water begin to increase. That’s crucial, because the more alkaline the water, the more capable it is of absorbing carbon dioxide from the surrounding air.
The researchers were aware of this dynamic, but its effects had not been quantified before in seaweed farms. So, using a model that simulated the cycles of carbon in ocean sediment, they explored how this process would play out beneath a patch of seaweed aquaculture.
Their model found that increased biomass from seaweed farms was indeed linked to dramatically enhanced levels of alkalinity. This increased alkalinity would translate to between 0.1 and 2 tons of CO2 removal per hectare of farm area per year. “Higher alkalinity allows seawater to hold more carbon in dissolved form without becoming acidic,” Fakhraee explains. “This shifts the balance so that CO2 naturally moves from the atmosphere into the ocean. Once absorbed this way, the carbon remains stored for thousands of years, governed by slow ocean circulation.”
Scaled up to the 3.5 million hectares of current-day seaweed farms, the researchers calculated that seaweed aquaculture could be locking away between 0.35 and 7 million tons of CO2 annually. This may not initially sound like a gobsmacking figure, but taking the projected future expansion of seaweed farms into account, they calculate that one day this fast-growing agricultural sector could lock away almost 60 million tons of CO2 per year.
Previously, the researchers have shown the similar alkalinity-boosting effect of fish farms. Now this new work shows that by changing ocean chemistry, seaweed farms may be an indirect lever for large-scale ocean carbon capture, too. The next step would be to take direct measurements from seaweed farms to see if this modelling work bears out, the authors say.
The new study brings science a step closer to resolving the question of exactly how much carbon seaweed farms absorb—a crucial bit of information before they can start issuing carbon credits. In the meantime, seaweed farms continue providing other benefits, as increasingly important global sources of food and pharmaceuticals, sites of ocean cleanup, and ocean habitat.
Fakhraee, M & Plavansky N.J. “Seaweed farms enhance alkalinity production and carbon capture.” Nature Communications Sustainability. 2026.
Photo: Ocean Rainforest
