Cultivated kelp and blue carbon – hype or opportunity?

Maybe both, but certainly a growing opportunity. Sometimes hypes can be useful. In this case, it brought much needed attention to the topic and to seaweeds in general, leading to more projects and more high-level discussions from local networks to UN panels.

Yet, it should not be all about huge numbers and over-promised potential, rather, about understanding the ecological and biogeochemical processes that lead to carbon sequestration in the oceans. Which of these we can quantify and where are the uncertainties? Japan has included blue carbon from seaweed in their national GHG accounting, could we do the same…?

The role of macroalgae as a natural sink for carbon dioxide (CO₂) was first recognized over 40 years ago. It is more recent that researchers have been discussing the carbon flows and sequestration opportunities from cultivated macroalgae. Will kelp farming solve climate change and all our problems? Well, no. But as a crop we are still learning to use, it has a good head start.

It’s a fascinatingly versatile biomass with a multitude of applications from food to feed, nutraceuticals, biostimulants, bioplastics, carbon dioxide removal (CDR) products and more. It is an excellent crop which grows rapidly with no fertilisers, pesticides or fresh water, and provides a variety of ecosystem services while growing at sea.

High biodiversity, including fish and seabirds are attracted to the farms. Kelp filters out excess inorganic nutrients from the seawater, and takes up carbon through photosynthesis, releasing oxygen. But the carbon that is lost is difficult to study in open sea, so in a carbon mitigation sense, it is extremely difficult to link carbon from seawater or sediment samples to having originated from biomass grown in the kelp farms. A lot of it floats away with ocean currents into the unknown, but it is possible, especially with new techniques being developed today.

In my PhD I aim to quantify how much organic carbon is naturally lost from a seaweed farm, in what form – particulate or dissolved, labile or recalcitrant – and how we can incorporate this into cultivation strategies as a value-add to any other target market.

Growth versus erosion

We think of the kelp on a seaweed farm as having a growth period then an erosion period after being overgrown by bryozoans. But what we see is the net growth, as the kelp is constantly growing and eroding throughout the entire season.

There are pulses of small biomass fragments and biomass-bound carbon being lost to the surrounding environment continuously. This is hard to measure unless we do hole-punch measurements. It allows us to calculate an “expected” kelp length, based on the added growth increments between the holes, and compare this to what we measure on-site. It is interesting data in the end, but very time consuming to obtain and time consuming to analyse, as it is based on individual kelp, several holes per kelp, and 200 tagged kelp in the full study over two years.

Dissolved organic carbon

The biomass-bound carbon is what we call particulate organic carbon or POC. There is also dissolved organic carbon, or DOC, which are even smaller particles exuded by the kelp actively or leaked out passively. Kelp releases DOC when stressed, cut, is exposed to non-optimal environmental conditions, and perhaps as a defence mechanism against fouling organisms.

According to scientific literature, 14- 62% of fixed carbon is released as DOC from various species. To date, however, there were no available numbers for Saccharina latissima (sugar kelp). We found that 12% of fixed carbon is released from a winter deployment after 24 h and 29% from an autumn deployment, before the onset of fouling organisms. The leaked DOC can be seen in the intense brown-orange colour in our experiment incubators.

How much is lost

An autumn and a winter deployment of S. latissima were monitored at SINTEF Ocean’s research facility in Skarvøya, Hitra for two consecutive years. The results show that by June-July, as much as 45-65% of the CO₂ taken up by the kelp was passively released into the ocean as kelp particles and dissolved organic compounds. POC and DOC losses depend on the deployment time of seedlings and biomass harvest times.

On average, for each tonne of kelp harvested in April, June and July 8, 18 and 28 kg C has been lost to the environment, respectively. DOC release rates were 4.1–7.6 mg C g(dw)^-1 h^-1 in the first 4 hours, perhaps due to handling stress, then lowering to 0.3–3.1 mg C g(dw)^-1 h^-1 after 24 hours. These numbers may inform accounting methodologies for carbon removal, as much of this carbon can eventually be stored in sediments beneath farms, exported to the deep sea and transformed into recalcitrant dissolved organic carbon (RDOC), leading to carbon sequestration.

RDOC and sequestration

Sequestration happens when carbon is removed from the system for hundreds or thousands of years. Carbon that cycles and returns to the atmosphere once products are consumed for example, is not sequestered.

Microbes break down the POC and DOC into more recalcitrant dissolved forms, which resist further degradation. We can think of it as apples and coconuts: labile carbon is the apple (more easily transformed) and recalcitrant carbon is the coconut, very hard to chew. Once in this resistant form, the carbon may be carried to the deep sea, remaining in the water column or incorporated into sediments, staying away from atmospheric exchange for hundreds to thousands of years. So, for carbon sequestration, RDOC is my holy grail.

And how do we measure RDOC? This began by taking water samples and letting it biodegrade naturally in the dark cold (10°C) laboratory for 180 to 210 days. Then, at every sampling time, extracting the dissolved organic matter with a 4-hour long intricate protocol and analysing the composition of this extract with Fourier Transform Ion Cyclotron Resonance Mass Spectrometry, or FT-ICR MS for short – the Rolls Royce of mass spectrum analyses. The resulting data is used to calculate specific ratios and chemical characteristics for each molecular formula identified and then categorize them into groups according to their level of recalcitrance (resistance to further degradation).

The scientific literature reports that 5-78% of the DOC released by macroalgae was refractory (RDOC), and an additional 0.12 to 0.3% of POC may be converted into RDOC. It is a large variation, and methods are not fully standardised. Again, there is no DOC composition data for Saccharina latissima, one of the most commonly farmed seaweeds in Europe. This is the subject of my next two papers, one lab-based long-term experiment and one at a commercial seaweed farm.

Why does this matter?

For knowledge of natural processes. For monitoring environmental impact. For science. For informing decision makers. For recognising the active contribution of the seaweed farming industry to society and the environment. It is not all about carbon either. As mentioned, cultivating seaweeds bring an array of positive environmental effects, none of which are valued as a provided service.

In Japan, seaweeds are officially incorporated into national greenhouse gas accounting, with J-Blue credits in the order of 400 USD per ton CO₂. This is not exclusively a carbon credit, but stackable, to account for all the other ecosystem services we know exist but are difficult to quantify. For this, you would need another 15 or so types of credits, according to Brian Takeda from the Secretariat of Foreign Affairs, at the Japan Blue Economy Association (JBE).

Japan is using carbon as a “common language” but recognising the wider importance of seaweeds to the environment, both natural and cultivated. Can Norway use this as an example to create a coherent and credible carbon(+) accounting system? Will we see financial incentives cycle back to seaweed farmers and those who restore natural forests, helping these activities to grow sustainably? Can the political will to support this materialise?

We believe in a bright future for seaweeds!

Read more

Neves, L., Smeby, K., Broch, O. J., Johnsen, G., Ardelan, M. V., & Skjermo, J. (2025). Particulate and dissolved organic carbon losses in high latitude seaweed farms. Science of The Total Environment, 982, 179677

Acknowledgements

This work was funded by the Research Council of Norway as part of the PhD thesis Seaweed Cultivation as a Climate Positive Solution (RCN 323324).

Other projects contributed with funding and supervision:

• Seaweed CDR (SINTEF’s global climate fund)
• JIP Seaweed Carbon Solutions (Industry funded)
• The Norwegian Continental Shelf: A Driver for Climate-Positive Norway (NCS C+, RCN 328715).

The research site in Skarvøya, Hitra was funded by the Norwegian Seaweed Centre 2021-2031 (National Research Infrastructure, RCN 322258).