The Seaweed Solution to Coastal “Dead Zones”

algaeurope.org   Women of Algae

The second largest dead zone in the world is located in the U.S., in the northern Gulf of Mexico.

Virginia Gewin reports for Bloomberg Green on the excess nutrients from Midwest farm fertilizer and animal waste that rob the waters off Louisiana, Mississippi and Texas of oxygen, fueling toxic algal blooms that cause a “dead zone.” The size varies each year, but this particular patch’s five-year average hovers at about 5,000 square miles.

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.


But there may be a solution on the horizon. A new study makes the case that silky green seaweed, if planted in sufficient numbers, could soak up much of that damaging waste. The concept is in its 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 Ms. 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 potential sites. 

Making it profitable

Making seaweed farms profitable will be a critical consideration to promoting them as a solution to agricultural and urban waste. Asia has high existing demand for seaweed. Human consumption, including everything from sushi rolls to broth to salad, is the biggest market for harvested seaweed. Growing demand can also be seen among the cosmetics and fertilizer industries, as well as feed for farmed seafood. Mass aquaculture, however, 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.  

“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,” said Gretchen Grebe, an aquaculture scientist based at the Marine Biological Laboratory in Woods Hole, Massachusetts.

“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.”

Seaweed has enormous promise, said Dr. 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.”

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