As seaweed farming expands, UN report urges more research, ‘cautious optimism’

In a comprehensive assessment of the potential risks and benefits of expanding seaweed farming, the United Nations Environment Programme called this week for “cautious optimism” and a lot more scientific research. Seaweed aquaculture is growing quickly amid enthusiasm about macroalgae’s potential to do everything from mitigating climate change to feeding the world to replacing petroleum-based fuels and plastics. But the potential risks to the environment and to vulnerable communities are still poorly understood, the report found. 

“We recognize that there’s a need to get ahead of this curve so we can ensure that this growth doesn’t pose risks of degradation to nature but rather feeds into a sustainable and equitable blue economy,” said Susan Gardner, director of the UN Environment Programme’s ecosystem division, speaking at a webinar launching the report. 

As FERN recently reported, some experts warn that the rush to scale up seaweed production is getting ahead of critical questions about how industrial-scale farming might affect the environment and how much carbon macroalgae actually sequesters. The UN report, which relied on more than 250 peer-reviewed studies, detailed the state of the science, and the many remaining knowledge gaps about the risks and benefits to the climate, communities, and marine ecosystems. 

Seaweed farming has been proposed as a multi-pronged tool to mitigate climate change because it can sequester carbon and be used to make products that would replace carbon-intensive synthetic fertilizers, fossil fuels, and animal proteins. Some companies are also trying to remove carbon from the atmosphere by sinking seaweed to the deep ocean floor. 

While wild seaweed forests have been shown to absorb carbon dioxide, the amount of carbon that seaweed farms can sequester is not yet fully understood, said Colette Feehan, the paper’s author and a biology professor at Montclair State University. And ventures looking to produce seaweed-based plastics, fuels, and proteins must contend with high production costs, making it hard for them to compete with the large industries they’re trying to disrupt. “There are some growing pains in terms of establishing those economies of scale that can then compete with these larger-scale global commodities,” she said.

Sinking seaweed to remove carbon from the atmosphere is another area of active interest, but research suggests that it’s currently more economically feasible to use seaweed biomass than to sink it, said Feehan. And ethical questions about sinking a crop that could be used to feed people remain unaddressed, she said.

The report also assessed the potential benefits and downsides of seaweed farming expansion for communities. It doesn’t cost a lot to get started as a seaweed farmer, which means barriers are relatively low for smallholders. Some research indicates that poor regions that have begun seaweed farming have seen increased standards of living and food security. But seaweed farmers also face risks — rates of respiratory illness may be higher among seaweed farmers, for example, likely due to inhaling dried seaweed. And while seaweed farming can provide a livelihood to women in poor areas, it also can put them in danger, since women in these communities are often less likely than men to know how to swim or handle boats, the report said. Seaweed shows promise as a nutritious food source, the report said, but it can also concentrate pollutants like heavy metals when it’s grown in contaminated water.

The report presented a mixed and incomplete picture of seaweed farming’s potential environmental risks and benefits. On the one hand, seaweed could be used to combat dead zones, since it soaks up excess nitrogen and phosphorus. But in areas where nutrients are limited, seaweed may outcompete eelgrass and other native species for nutrients. Ultimately, whether seaweed farms increase or harm biodiversity may depend on where they are sited. In less complex ecosystems, like sandy bottom oceans, evidence suggests seaweed farms can boost biodiversity by providing habitat for marine life. But when seaweed farms are put into more diverse ecosystems, like eelgrass beds and coral reefs, these rich ecosystems tend to decline. 

Seaweed farms can also degrade marine ecosystems by spreading invasive species and disease. Non-native seaweed, in particular, has harmed coral reefs, seagrass meadows, and mangroves in places including Hawaii, Panama, and India. And seaweed farms use rope, which poses a risk of entanglement to sea turtles and marine mammals. 

Urging more research into both seaweed’s promise and potential downsides, the report also called for policies that will guide a sustainable expansion. “It’s important that we look at it with cautious optimism,” said Hally Voi Blanchard, one of the report’s editors. “There are incredible opportunities, and we just need to make sure that as we move forward, everybody is aware of the risks.”