Seaweed Farming: Could This Carbon-Negative Crop Help Restore Our Oceans?

Scientists and farmers are tapping into the potential of macroalgae to help clean up some of the world’s most polluted marine environments.

Giant kelp (Macrocystis pyrifera) in California

Brent Durand / Getty Images 

China has been cultivating seaweed for about 1,700 years. Coastal populations harvested a wide variety of the algae first as a source of food and animal feed, but later for industrial purposes and nutritional supplements as the practice became more widespread. Today, China remains the world’s biggest producer of farmed seaweed (the country accounted for 60% of the global volume in 2018), but there are plenty of other countries that are starting to realize the potential of this unique marine crop.

Certain red seaweed varieties contain up to 47% protein, but others are also rich in magnesium, iron, and other high-nutrient minerals. Seaweed cultivation is now the world’s fastest-growing aquaculture sector and it shows no sign of slowing anytime soon. In Alaska, where the largest seaweed farm in North America is located, farmers produced over 112,000 pounds of kelp in 2019—a 200% increase from the state’s first commercial harvest in 2017. Using small sites of just a few acres each, farmers grow seaweed in underwater gardens made up of suspended longlines that use the entire water column to save space. It's economical, relatively simple, and comes with a whole host of environmental benefits.

Research shows that seaweed could play a crucial role beyond serving as a source of nutrients and food, but also in the fight against some of the worst issues plaguing our world today: climate change and ocean pollution.

Environmental Benefits of Seaweed Farming

A seaweed farm in Bali, Indonesia
Dallas Stribley / Getty Images 

Seaweed doesn’t need to be fed or fertilized, as the crop gets everything it needs from sunlight and the natural nutrients already found in the ocean water. That means there are no synthetic pesticides, freshwater, or deforestation going into the process, all while providing habitats for local marine life and improving water quality.

More Efficient Carbon Sequestering

Macroalgae have the ability to sequester carbon just like other coastal plants, such as mangroves and seagrass, but with a sustainable twist. Instead of storing CO2 close to shore as the organic materials become buried in underwater soil, seaweed is more likely to move farther into deep sea sediments since its habitat is more rocky and eroded. As the seaweed carbon is stored farther away from shore, it is less likely to be disturbed and returned back into the atmosphere. In fact, macroalgae have the potential to sequester 173 million metric tons of CO2 this way every year, with about 90% of sequestration occurring through export to the deep sea.

Even the Cows Could Benefit

Studies have shown that adding just a small portion of seaweed to cattle feed can reduce the animal’s greenhouse gas emissions by over 80%.

Combatting Ocean Acidification

The ocean is one of the world's largest carbon sinks, absorbing and storing carbon chemical compounds to lower high concentrations of CO2 from the atmosphere. This natural process helps keep the Earth’s carbon dioxide in check, but recent upticks in greenhouse gas emissions (mainly from the burning of fossil fuels) have caused too much CO2. The result is ocean acidification, which causes massive negative impacts on marine species, from mollusks and crabs to fish and coral reefs.

That’s where seaweed comes in. Not only does seaweed grow rapidly, but it also pulls CO2 from the water to do so. A 2021 study comparing three seaweed farms in China found that surface water pH increased by 0.10 within the area, efficient enough to buffer acidification.

Pollution Management

Seaweed isn’t just good at sucking up carbon dioxide, it also acts as a sponge for heavy metals and other coastal pollutants (like those from runoff). Of course, seaweed grown for this reason couldn’t be eaten afterward, but it certainly offers a potentially inexpensive, all-natural solution to help improve the health of marine ecosystems. These types of farms with large, fast-growing kelp also create and restore habitats for fish and other types of ocean life, offering refuge for threatened species.

Runoff is one of the most damaging types of ocean pollution, largely because it is hard to locate the exact source. According to the National Oceanic and Atmospheric Administration (NOAA), 80% of pollution to the marine environment comes from the land, both larger sources like fertilizers and pesticides from industrial-scale agriculture as well as smaller ones from septic tanks and vehicles. Runoff can also pick up other pollutants as it travels to reach a body of water, adding an overabundance of nitrates like phosphorus and nitrogen that cause environmental problems in the form of harmful algae blooms and low-oxygen ocean “dead zones.” Cultivated seaweed can reduce these nutrients while simultaneously producing oxygen, alleviating both the cause and effect of these areas.

One of the world’s worst dead zones is located in the U.S. Gulf of Mexico, which stretched over 6,951 square miles in 2019. A team of researchers from UC Santa Barbara found that 9% of the gulf was suitable for supporting seaweed aquaculture, and cultivating the marine crop in less than 1% of that area could potentially reach the United States pollution reduction goals.

A seaweed farm in China
 Pone Pluck / Getty Images 

Positive Social Impacts of Seaweed Farming

Expanding the seaweed cultivation market could mean supporting more jobs and creating better global food security in the long run.

A Canadian company called Cascadia Seaweed, which is on track to become the largest provider of farmed seaweed in North America, partners with the local First Nations Indigenous group to provide meaningful jobs that align with their cultural traditions.

Limitations to Seaweed Farming

There are, of course, some potential disadvantages to seaweed farming. For instance, large-scale cultivation may have negative ecological implications and alter marine habitats if not performed mindfully; unregulated or overproducing seaweed could impact the amount of natural light available to other sea-dwelling species that rely on photosynthesis.

In addition, technology for transporting, drying, and converting seaweed into biofuels, bioplastics, or food can take up resources and emit CO2 themselves. It’s also possible that the carbon-capturing crops could do their job a little too well and remove too many nutrients from the wild ecosystem.

However, as research continues to delve into responsible seaweed cultivation as an answer to one of our most pressing environmental issues, we may discover that macroalgae’s wide range of applications exceeds any obstacles. The economic value of dealing with nutrient pollution, for example, may mitigate costs for wastewater treatment; the same goes for converting seaweed into biofuel, fertilizer, or fuel depending on water quality.

The balance will come down to a combination of policy, entrepreneurship, and scientific research, but the collaboration is a noble one, as the investment could provide a substantial opportunity to mitigate climate change and help save our oceans.

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