By Virginia Gewin

In May 2019, the Mississippi River dumped a daily average of more than 5,000 metric tons of nitrate and 800 metric tons of phosphorous into the Gulf of Mexico, the highest levels in the last 40 years. These excess nutrients from Midwest farm fertilizer and animal waste rob the waters off Louisiana, Mississippi and Texas of oxygen, fueling toxic algal blooms and causing what’s come to be known as a dead zone.

The size varies each year, but this particular patch’s five-year average hovers at about 5,000 square miles. To date, a U.S. government task force has made little if any progress toward the goal of reducing it to 2,000 square miles.

For environmental experts, the problem seems intractable. Known as eutrophication, dead zones are proliferating all over the globe. There are currently over 700 coastal areas worldwide that are either dead zones or negatively impacted by runoff. While the U.S. suffers mostly from agricultural waste, urban wastewater is the main culprit in South America, Asia and Africa.

Every year, they inflict $3.4 billion in economic damage in Europe and the U.S. alone due to lost tourism and fishing, declining property values, water treatment and adverse health impacts. In the last 10 years, 85 U.S. communities have spent more than $1 billion combined to prevent or treat algal blooms. Among the hardest hit are at the mouth of the Mississippi, where the constant ejection of waterborne waste from America’s heartland decimates local seafood and tourism industries.

But there may be a solution on the horizon. A new study makes the case that the Gulf of Mexico could trade in its slimy algae for silky green seaweed, which if planted in sufficient numbers could soak up much of that damaging waste. The concept is early days and implementation further afield, but given the lack of progress on other fronts, said study co-author Phoebe Racine, “there’s no other option but to consider alternative practices.”

Cultivating multiple species of seaweed in less than 1% of Gulf of Mexico waters could potentially help the U.S. achieve pollution reduction goals that have been out of reach, said Racine, a researcher at the University of California in Santa Barbara. She and her colleagues have already mapped suitable areas for seaweed farms in the Gulf and found more than 24,000 square miles of potentially available sites.

Seaweed aquaculture dates back 1,700 years to China. These days, nations such as Indonesia and the Philippines, along with China and South Korea, lead the world in this arena. In South Korea, where aquaculture has grown 300% over the last 30 years, an intensive effort has demonstrated its usefulness as a waste reduction tool. As one of the biggest producers of seaweed in Asia (where 99% of all seaweed is cultivated), the country has at least 2,144 seaweed farms covering a total of 350 square miles, according to Jang Kim, a marine scientist based at Incheon National University. That’s an area roughly the size of Dallas.

Farms of three major seaweed species and two shellfish species (all filter feeders that require no additional food) took up the equivalent of 5.7% of the carbon dioxide and 8.6% of the nitrogen discharges from all wastewater treatment plants in Korea, according to a study conducted by Kim.

Key to the success of seaweed farming is its increasing commercial potential, and that these aquatic plants can soak up excess nitrogen and phosphorous and turn it into not only human food, but an expanding array of additional commercial uses. The most cultivated seaweeds include red or brown algal species or kelps. Some are used to make culinary thickening agents or agar to culture bacteria in laboratory settings. Others are dried into sheets called nori used to make sushi rolls. While sugar kelp can be used as a sweetener, kelps are also used in toothpastes, shampoos, frozen foods and even pharmaceuticals.

Making seaweed farms profitable will be a critical consideration to promoting them as a solution to agricultural and urban waste. Asia has strong existing demand for seaweed. Human consumption, including everything from sushi rolls to broth to salad, is the biggest market for harvested seaweed, said Kim. But growing demand can also be seen among the cosmetics and fertilizer industries, as well as feed for farmed seafood. “We have more than 50% of kelp production going to abalone feed,” said Kim.

Mass aquaculture faces some significant obstacles, not least of which is its labor-intensive nature. Seaweed cultivation relies on either lab-based nurseries to grow and attach juvenile seaweed to netting or line, or transferring cuttings from mature plants to a submerged line. Harvesting usually involves multiple people on boats cutting seaweed out of the water. 

Globally, demand for seaweed is projected to double to $30 billion by 2025. In the U.S., however, the seaweed market is relatively small. Gretchen Grebe, an aquaculture scientist based at the Marine Biological Laboratory in Woods Hole, Massachusetts, said seaweed farming in American “is still very much in its R&D phase.” She’s been studying coastal seaweed aquaculture in Maine, where in 2020 kelp farmers harvested 482,000 wet pounds worth just shy of $300,000. (Also in Maine, a company received $11 million in seed funding to grow kelp and then sink it into the deep ocean for carbon credits.)  Racine contends that carbon credit markets used to slow pollution “could be used more fully” to encourage seaweed extraction of pollutants—including nitrogen, carbon, and even heavy metals—from coastal areas.

“Using seaweed aquaculture to remediate any substantial quantities of nutrient pollution will require a massive expansion—the current scale of cultivation won’t even make a dent,” Grebe said. 

In the U.S., the states have wildly divergent regulations when it comes to aquaculture permitting. While it’s comparatively easy to get a seaweed farm permit in Maine and Alaska, in California it requires negotiating a maze of overlapping regulations.

“Offshore operations are where the promise lies,” said Bailey Moritz, program director for World Wildlife Fund. “Our goal is to see seaweed grow in a way that will have meaningful impacts [on nutrient depletion], and scale is necessary for that.” WWF invested in Ocean Rainforest in Faroe Islands, an operation that designed rigs to withstand offshore weather conditions.

But large-scale, offshore seaweed operations are still at least a decade away in the U.S., she predicted, and likely only if biofuels, bioplastics, and animal feed create enough demand.

And while the U.S. Department of Energy has invested $35 million into researching seaweed-based biofuel development, with pilot projects starting in Puerto Rico, to date the Army Corps of Engineers has granted only one permit for a seaweed project in the Gulf of Mexico, according to Nakeir Nobles, a spokesperson for the Army Corps of Engineers. Nobels said further hurdles include siting, Endangered Species Act consultations, design plans that prevent entanglement of sea animals and public education.

Seaweed has enormous promise, said Grebe, but scientists are quick to point out that it’s imperative to reduce upstream waste inputs as well. “We’re asking a lot of seaweed aquaculture to take care of the nutrient waste we dump into the Gulf,” she said.

To contact the author of this story:

Virginia Gewin at [email protected]

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