Nature-based solutions: Seaweed
Kelp image: Shane Stagner
Carbon sequestration?
Seaweed is a diverse group of marine macro-algae and plants that, like all plants, uses photosynthesis and carbon dioxide to grow. While it’s well understood that kelp stores far more carbon than trees, and it does so faster, the exact amount hasn’t been quantified.
In Aotearoa one of Blue Carbon Services Ltd research projects in conjunction with NIWA is to quantify how much carbon is in natural kelp beds that grow along the coast, and ends up accumulating in deep-sea (continental shelf, slope, and submarine canyons) sediments. Knowing this would help create a reliable carbon market, one that does not require sacrificing land-based ecosystems for profit, and one that may also enhance seafood production through multi-trophic aquaculture.
However, when seaweed is considered as part of a healthy biodiverse ecosystem, it may in fact lead to higher carbon dioxide emissions due to the way trophic systems work:
“As the surrounding coastal waters wash through the seaweed canopy, they bring in vast quantities of plankton and other organic material from further out at sea. This provides extra food for filter feeders like sea squirts, shellfish living amongst seaweed, and the bryozoan animals which end up coating many seaweed fronds.
As these creatures consume this extra food supply, they breathe out carbon dioxide additional to that produced by eating seaweed. Individually, the amount is tiny. But on an ecosystem scale, their numbers and ability to filter large amounts of water are enough to skew what researchers call the net ecosystem production – the balance between carbon dioxide inflows and outflows. And not just by a little, but potentially by a lot. ” – John Gallagher, University of Tasmania
Secondly, seaweed does not absorb CO2 from the atmosphere, and it also releases CO2 as it grows older (Fig. 2):
“Unlike a tree, seaweed extracts its carbon from the water, so it doesn’t directly affect greenhouse gas levels in the atmosphere. That air-to-water transfer only happens when ocean water depleted of carbon by the seaweed stays at the surface. There, it can interact with the atmosphere and pull CO2 into the ocean to create an equilibrium between the air and water.… Research has also shown seaweed such as kelp isn’t a static vessel filled with CO2. Instead, it’s a leaky one—shedding carbon as it decays. And before an industry can claim credit for sequestering carbon in seaweed, those leaks need to be accounted for. – Science, 29 August, 2024
Integrated multi-trophic aquaculture
The advantages of growing seaweed vs land-based crops:
- Doesn’t need fresh water
- Doesn’t need agrichemicals to grow
- Doesn’t need pesticides
- Doesn’t burn down
- Doesn’t use land
- Has more iron than meat
- Has more calcium than milk
- Serves as a protective nursery for organisms particularly vulnerable to acidification, such as oysters and mussels.
“Seaweeds are able to modify the chemical environment at their surface, in a micro‐zone called the diffusive boundary layer (DBL), via their metabolic processes controlled by light intensity. Depending on the thickness of the DBL, sessile invertebrates such as calcifying bryozoans or tube‐forming polychaetes living on the surface of the blades can be affected by the chemical variations occurring in this microlayer. Especially in the context of ocean acidification (OA), these microhabitats might be considered as a refuge from lower pH, because during the day photosynthesis temporarily raises the pH to values higher than in the mainstream seawater. ” – Noisette & Hurd, University of Tasmania
Videos 3 and 4 explain how kelp is being grown as a food crop for people and animals, as a fertiliser that takes nitrogen from polluted estuaries to grow and is then returned to farms as a natural fertiliser, and how the ‘halo’ effect of kelp reducing the acidity of surrounding waters makes it an ideal co-crop for growing shellfish, given that ocean acidification is already having a measurably bad impact on marine ecosystems. The kelp growing system can be designed to withstand storms by lowering the kelp into deeper waters when necessary.
The videos feature the work being done in the US by GreenWave. Their 3D ocean integrated multi-trophic aquaculture concept is being research and trialled in New Zealand by GreenWave and the Cawthron Institute and by Blue Carbon NZ.
Reduces methane from ruminant animals (cows, sheep)
When fed certain species of seaweed, the amount of methane that cows produce through enteric fermentation (digestion followed by burps) is reduced. At first glance this seems like a game-changer for the New Zealand dairy sector. However, it’s not quite that simple.
Research by the University of California and CSIRO (Australia) shows that Holstein dairy cows fed Asparagopsis armata (the species native to New Zealand) does result in less methane being produced, but with some caveats. Cows given higher doses produced up to 66% less methane but they also ate less, gained less weight, produced more than 10% less milk, and the quantity of protein in the milk fell. They also produced more carbon dioxide and bromoform (which damages the ozone layer in the upper atmosphere). While smaller doses reduced these side effects, the drop in methane emissions wasn’t nearly as impressive. For more information on the pros and cons, see this 2022 research paper: Benefits and risks of including the bromoform containing seaweed Asparagopsis in feed for the reduction of methane production from ruminants.
The Cawthron Institute is expanding its algae research as it offers a luring promise for the agricultural sector if an optimal dosage and accurate delivery system can be developed. The Sustainable Seas Challenge (part of the National Science Challenge New Zealand) is undertaking a range of research projects on seaweed.
“The other major obstacle to using seaweed inhibitors on New Zealand farms is the fact our sheep, beef cattle and dairy cows mostly eat grass. That makes feeding a supplement a potential logistical nightmare. In trials, the seaweed is mixed with a dry food ration. That’s fine for intensively farmed animals fed grain-based diets. But how do you feed it to a sheep grazing on the far flanks of a high country sheep station?” – Stuff, 2020
There is also an argument that funding this research will only offer misplaced hope to an unsustainable ‘sunset industry’ dairy and been sector that also produces large quantities of the greenhouse gas nitrous oxide and is badly polluting our waterways. As the price of carbon continues to climb, growing protein in this way may become prohibitively expensive.
“Methane emissions are only one way that animal agriculture contributes to environmental destruction. Animal agriculture is a major contributor to nitrous oxide emissions and feeding cows seaweed would make no difference to that. Recent advances in precision fermentation technology [protein alternatives] mean that animal agriculture will be obsolete in the next 10 years.” – China Agricultural University former lecturer in environmental management, Michael Morris (Stuff, 2020)
Wild harvest may help improve biodiversity
The seaweed species known as wakame (Undaria pinnatifida) (Fig. 4) is one of the 100 most invasive species worldwide. Unfortunately, it invaded New Zealand waters in the 1980s and eradication programmes have failed. Known as ‘the gorse of the seas’, it’s now commonly found around our coastline, displacing native species.
In a joint Singapore-New Zealand government project and funding from the New Zealand Catalyst Fund, AgResearch is looking at ways to make the proteins in seaweed more digestible and the nutrients locked up in the plant, more accessible. One of the aims is to increase interest in wild harvest of the seaweed from infested coastlines, which might also encourage native seaweed species to re-establish. This is not likely to make a huge difference, if any, to restoring native kelps as the same strategy has been used in terms of hunting possums and other feral pests. We include it here only as a matter of interest.
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