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An overlooked opportunity for kelp farms to double as pollution cleanup sites 

Macroalgae get a lot of attention for absorbing carbon. But a new study shows that select species are better at cleaning up nitrogen than carbon.
February 3, 2023

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Some varieties of kelp have an impressive ability to mop up coastal nutrient pollution, finds a new study.

The study team focused on Alaska, a place where kelp farming for the production of fertilizer and food is taking off, but also where nutrient pollution is a recognized problem.

In 2020, the Alaska environment department found that 69 bodies of water were polluted to unhealthy levels with urban sewage, run-off, and fisheries waste. In high amounts, such pollution causes eutrophication in water bodies which creates dead zones, because of the toxic levels of nitrogen it contains. But nitrogen is also a source of food for aquatic plants like seaweeds—and so, there may be an opportunity for the world’s growing number of kelp farms to double up as sites of environmental remediation.

The University of Alaska Fairbanks researchers wanted to find out exactly how much the kelp farms could clean up. Over a period of three months between March and May, they gathered seawater and seaweed tissue samples from two coastal kelp farms, focusing on two species, sugar kelp and ribbon kelp. After drying and grinding up the seaweed tissues in the lab, the researchers analyzed their carbon and nitrogen content, and compared these between the two species, and also with the nitrogen levels detected in the associated water samples. 

Their sample size was fairly small and the study is quite preliminary, the researchers caution—but still, there were some telling results. 

For starters, they found that ribbon kelp is especially receptive to the nutrient, demonstrated also by high amounts stored in the examined tissues. In fact, ribbon kelp seemed to have a much greater appetite than its sugar kelp counterparts—slurping up 87.5% more nitrogen, and 29.8% more carbon. 

This also tracked with nutrient levels in the water: the more nitrogen there was in the seawater samples, the more sharply the ribbon kelp’s intake increased. Sugar kelp, meanwhile, maintained a fairly steady level of nitrogen. 

 

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The differences between kelps suggested that different algae species have different metabolic requirements that affect their ability to soak up nutrients—the ribbon kelp needing more, and therefore more efficiently scrubbing this pollutant out of the water. The researchers point out that their findings add to other research on this point, showing for instance that other kelp varieties such as winged kelp are doubly as hungry for nitrogen as sugar kelp. 

Knowing which species are best at clearing pollution might help farmers choose which ones to grow. However, that’s also slightly complicated by other aspects of seaweed biology and the reasons it’s being cultivated, the researchers point out. Ribbon kelp, for instance, produces almost 50% less biomass over the growing season than the more productive sugar kelp. So, understandably, it may not be the first choice for commercial kelp farmers who sell their product by weight. But, it could for instance be a boon in marine aquaculture. “In the case of Alaska, kelp could be grown next to fish salmon pens and with that reduce potential nutrient excess,” says Schery Umanzor, a marine ecologist at the University of Alaska Fairbanks, and lead author on the study. 

Umanzor has also been working with farmers to develop tools that will help them gather more data from their farms, the researcher explains. “My idea is to continue along these lines to improve our overall understanding of kelp farms in coastal Alaska and elsewhere.”

Overall, the results suggest we’re missing a trick if we treat kelp only as a tool to sequester carbon; as a nature-based solution it has much more to offer, the researchers believe: “Kelp is actually much better at mitigating excessive amounts of nitrogen than carbon. I think that’s a story that’s really overlooked.”

Umanzor et. al. “Nitrogen and Carbon Removal Capacity by Farmed Kelp Alaria marginata and Saccharina latissima Varies by Species.” Aquaculture Journal. 2023.

Image: Seagrove Kelp Co.

Updated 4 February 2023 at 06:00 MST.

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