Farm Findings: Cultivated seaweed sheds back into the sea, removing carbon dioxide in the process.
- Anjali Krishna
- Sep 19
- 3 min read
As producers, seaweeds photosynthesize, absorbing carbon dioxide from the atmosphere and growing plant matter that feeds marine life. Wild seaweed is also one of the most productive producers on the planet, growing quickly to cover large areas of ocean.
By examining sea beds from all over the world, scientists have discovered that seaweeds have another potential function. By absorbing carbon dioxide and storing it in their “leaves” – also called “blades” – they can contribute to carbon sequestration. When the fragments of these blades naturally fall, they settle to the seabed where they integrate with sediment and may remain there for hundreds of years. This decreases the quantities of carbon dioxide in the atmosphere, potentially offsetting some of the effects of global warming through the reduction of a key greenhouse gas.
Cultivated seaweeds follow the same seasonal patterns as their wild counterparts, and shed their fragments when they mature in the spring. This introduces the question of whether seaweed cultivation could contribute to the removal and long-term storage of carbon dioxide.
In order to estimate the scope of this carbon storage, we collected seaweed samples from Car-Y-Mor’s Porthlysgi seaweed farm throughout the cultivation season (Jan - July 2022) and measured the biomass (weight of seaweed per meter of rope) and blade length over time.
What we found?
We measured a peak in blade length in mid-May, followed by a slight decline in June. The speed of blade growth was highest between March and April, reaching 2 cm/day, before decreasing through May and June.
By looking at changes in biomass, we calculated “shedding rate,” or how much biomass was lost per day before harvest. Seaweed shedding surged to 2 cm/day in May and stabilised at a lower level in June and July. These measurements highlight the rapid growth and subsequent shedding of seaweed in this high-energy coastal environment.
Although the exact fate of shed seaweeds is not known, recent studies predict that between 10-20% of the carbon from seaweed fragments either settles on the seabed or converts into other long-term stored forms of carbon, offering a long-term storage solution for the absorbed carbon dioxide (Filbee-Dexter et al. 2024). Our shedding measurements, therefore, highlight the potential for seaweed farming to contribute significantly to carbon capture. Furthermore, with 2,900,000 hectares suitable for farming sugar kelp and oarweed in English waters alone (MMO 2019), the maximum hypothetical carbon sequestration is far higher than what we estimate here (Canvin et al. 2024)
There is, of course, uncertainty in the pathway from cultivated seaweed to stored carbon dioxide. One example is that local current patterns may shift the seaweed fragments to areas of seafloor that aren’t stable enough for long-term storage, or even wash them onto land, where they may decompose and release greenhouse gases back into the atmosphere. Another is that seaweed farms may harvest their crop before the maximum amount of carbon has been shed and stored.
Nevertheless, as the effects of climate change become ever more apparent, the importance of finding sustainable mitigation methods is only increasing. As such, while seaweed farming grows in popularity, research should continue into its potential for carbon storage.
References
Filbee-Dexter, K., Pessarrodona, A., Pedersen, M.F. et al. Carbon export from seaweed forests to deep ocean sinks. Nat. Geosci. 17, 552–559 (2024). https://doi.org/10.1038/s41561-024-01449-7
Canvin, M.C., Borrero-Santiago, A.R., Brook, T., Gupta, M., Knoop, J., Menage, G., Moore, P.J., O’Connor, N.E., Ricart, A.M., Smale, D.A., 2025. Can the Emerging European Seaweed Industry Contribute to Climate Change Mitigation by Enhancing Carbon Sequestration? Reviews in Aquaculture 17, e70004. https://doi.org/10.1111/raq.70004