Seaweed and macro algae cultivation

Buschmann et al (2017) describe the production and growth of seaweed as a relatively straightforward enterprise. However, despite the long history and experiential knowledge of algal farming around the world, their large scale cultivation for carbon sequestration purposes would likely require significant alterations to be effective. Several factors like the availability of surface area, engineering issues, and market demands, complicate the expansion of the current growing practices (Duarte et al, 2017), and there remain many unexplored areas (Krause-Jensen et al, 2018).

A major issue for such a project would be the durable sequestration of carbon (Ould and Caldwell, 2022). Kelp forests and other macroalgae store their carbon in their biomass, and not, like other blue carbon solutions such as mangroves, partially in the soils, and therefore risk largely decomposing and re-releasing their carbon into the oceans. Ager et al (2023) for instance found that 80 % of the macroalgal biomass studied at a fjord in Greenland stayed in that fjord, and that there is no scientific clarity on what happens to this biomass, and therefore also no certainty about carbon capture potential. Moreover, many of the macroalgal aquaculture projects aim to utilize the algae, and do not provide a carbon sink function (Troell et al, 2022). Another difficulty is highlighted by Rose and Hemery (2023) when they write that ‘methods to assess the permanence of carbon in the natural life cycle of macroalgae and in products following harvest are lacking’, and that it is therefore difficult to measure the carbon sequestration effect of such projects.///There are many operational projects. In Iceland there is for example an experiment by Running Tide (https://www.runningtide.com/), which, with permission of the government, released buoys off the coast in order to stimulate kelp production. The Dutch company North Sea Farmers (https://www.northseafarmers.org) recently brought great encouragement to the sector after it received €1.5m from Amazon for a field trial in the North Sea. NASEM (2022) estimates that $130 million would be needed in initial funding to explore the feasibility of large scale farming and the durability and environmental risks associated. However, despite large investments, and the positive associations around macroalgae as a natural measure that is sometimes even labeled ‘ocean afforestation’ (N’Yeurt et al., 2012), there are still many uncertainties, and the recent news that Exxon as the last fossil fuel company has abandoned their research into algae as biofuels (https://www.bloomberg.com/news/articles/2023-02-10/exxon-retreats-from-major-climate-effort-to-make-biofuels-from-algae) is perhaps indicative that operationalisation is perhaps not as easy as it looks. Ould and Caldwell (2022) therefore specifically warn against 'the hyperbole [around macroalgae potential] that is beginning to permeate the conversation'

NASEM (2022) points out that this measure would have scaling difficulties due to the large amount of required farming area. This difficulty with area requirements is echoed in Ould and Caldwell (2022), as they calculate that if carbon targets of 8 and 12 GtCO2 per year are to be met, ‘this would equate to producing, on an annual basis, between 9.75 and 14.63 Gt of farmed seaweed, which would be a 300- to 450-fold expansion of global seaweed aquaculture.’ Another issue with scaling up would be the required durability of capture. Since it could be possible that biomass would release the captured carbon again after it dies, Hasselström and Thomas (2022) suggest that '[b]lue carbon financing should be directed only to setups proven to lead to additional and permanent carbon storage.' This potential re-release of carbon, and other ecological effects are highly uncertain, and would need to be studied before this measure could be scaled up (Gao et al, 2022).

There are still too many uncertainties to provide a clear answer to this.

Duarte et al (2022) emphasize the potential of the Arctic due to its 'rocky bottoms suitable for macroalgal growth' and the fact that it 'represents 34% of the global shoreline'. The potential for increased macroalgae expansion in the Arctic and Northern regions seems very high, however, the original Arctic cryophilic macroalgal species (see Bringloe et al, 2020) cannot advance further north, and will therefore face habitat loss under warming oceans (Bringloe et al, 2022). The three year long Nordic Blue Carbon project found that of the total 3.9 million tonne CO2 equivalents that were captured in the Nordic region (excluding Greenland) per year, kelp forests were responsible for 69%, or 2.7 million tonne CO2 equivalents. They therefore conclude that apart from artificial farming, 'management measures to protect and restore blue forest habitats will have a wide range of societal and economical co-benefits, therefore making them “no-regret” mitigation options​' (Frigstad et al, 2021). Much remains unknown to this point, and new discoveries, like Krause-Jensen et al’s finding of substantial kelp forests around West Greenland, at relatively extreme depths (2019), continue to advance knowledge about macroalgal importance for the region.

The expansion of macroalgae in the Northern and Arctic regions could moreover occur partially naturally because of more favorable conditions, especially due to reduced ice coverage leading to greater light availability (Arrigo, and van Dijken, 2015). As Krause-Jensen et al (2014) pointedly summarize in their abstract: this ‘likely expansion of vegetated coastal habitats in the Arctic will generate new productive ecosystems, offer habitat for a number of invertebrate and vertebrate species, including provision of refugia for calcifiers from possible threats from ocean acidification, contribute to enhance CO2 sequestration and protect the shoreline from erosion.'

Recent work by Wright et al (2022) found this climate driven movement could potentially have large effects on macroalgal carbon sequestration potential, for although ‘warm temperate kelp exports up to 71% more carbon per plant, it decomposes up to 155% faster’, and could therefore significantly reduce carbon sequestration potential. Filbee-Dexter et al (2020) study equally show that the carbon storage for global kelp forests reduces with rising ocean temperatures as such biomass degrades more quickly, but emphasize that this should encourage further research into the potential of colder northern regions. Lebrun et al’s overview study of the existing literature on the effect of climate change on Arctic macroalgae (2022) however indicates that there is a lack of complete understanding, and that effects can both be positive and negative. Moreover, adjusting to sea level rises might also impact carbon sequestration potential of macroalgae (Lovelock and Reef, 2020).

Apart from the potential for macroalgae to grow in the Northern regions, this would however not be very likely to make a significant climate impact in the north, as the oceans and atmosphere tend to re-equilibrate their carbon level quickly, and this would largely be effective for global atmospheric carbon levels.

Duarte et al (2017) note that ‘the contribution of seaweed aquaculture to climate change mitigation and adaptation will remain globally modest’.

Duarte et al (2017) state that 'Because of the very low investment required to set up seaweed aquaculture farms, seaweed aquaculture is a particularly sound strategy for coastal developing nations to contribute to climate change mitigation while protecting their shoreline and marine ecosystems from some of the effects of climate change, such as ocean acidification and ocean de-oxygenation.'

Enhanced growth of macroalgae in Northern regions will lead to changes in ecosystems (Krause-Jensen et al, 2014), and the uptake of nutrients by their growth could impact the growth of other organisms. It is also not known what the impact of the sinking of macroalgae would have on the deep ocean (IPCC, AR6, Wg3 p1479). Gao et al (2022) emphasize therefore that the ecological impacts of blue carbon enhancement need to be assessed further if they are to be scaled up.

There could be substantial co-benefits to increased macroalgal growth, both directly related to jobs in the production, maintenance and harvesting, and by providing extra food sources, directly from edible seaweed, or indirectly from increased fish catch as a result of larger spawning areas in kelp forests. Duarte et al (2017) argues that increasing the demand for seaweed and subsidizing farmers could be effective strategies to encourage further local engagement with seaweed aquaculture and provide financial benefits to cultivators.

It is likely relatively straightforward to remove seaweed farms if found undesirable, although care must be taken not to allow local ecosystems to be replaced or overwhelmed by macroalgae.

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Both Krause-Jensen et al (2022) and the Nordic Blue Carbon Project (Frigstad et al, 2021) recommend increased Nordic research and governance collaboration. Most of the potential cultivation areas are inside nations’ exclusive economic zones and can thereby be governed nationally.

Macroalgal stimulation has recently gained large interest from the scientific community and public at large. Importantly, also commercial parties are increasingly getting involved, and funding seems to be increasing. Apart from the already mentioned projects, there are major research projects and organizations that promote macroalgae as blue carbon solutions, like Project Drawdown (https://drawdown.org), Ocean Rainforest (https://www.oceanrainforest.com), and Kelp Blue (https://kelp.blue).