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.

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.”

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.

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.

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.