Steve Pollock has just about given up on farming oysters on the Louisiana coast. Every summer, he says, huge amounts of shellfish on his Grand Isle farm die off, gaping open with hardly anything inside. A former Louisiana State University biologist, he has ideas about possible causes. It could be rising temperatures. It could be pollution from fishing boats. And it could be fertilizer from corn farms in Iowa.

Every spring, rains wash fertilizer off farmland and into the rivers and streams of the Mississippi River watershed. Eventually the polluted water hits the Gulf of Mexico, where the nutrients in the fertilizer feast on heat and sunlight and bloom into green fields of algae. When those blooms die and sink to the ocean floor, they become a hypoxic “dead zone,” depriving marine life of oxygen, and causing huge problems for fishermen like Pollock. Fish, shrimp, and crabs swim away, and shellfish suffocate.

A just-released National Oceanic and Atmospheric Administration (NOAA) forecast anticipates a big dead zone this summer—around 6,700 square miles, an area the size of Connecticut and Delaware combined. If the prediction holds, it would be one of the largest since the agency started tracking the dead zone in 1985. The NOAA says the zone needs to shrink to about 1,900 square miles to avert the death of marine wildlife, but it hasn’t been that small in 20 years.

In May, the volume of nitrogen and phosphorus in the Mississippi and Atchafalaya rivers was about 30 percent higher than average, the agency reported. The Union of Concerned Scientists (UCS), a Cambridge, Massachusetts-based environmental group, says that between 60 and 80 percent of that nitrogen comes from farms and livestock in the Midwest, mostly in the form of synthetic fertilizers and animal manure that runs off corn and soybean fields. Even the average is a staggering load—enough to fill 3,100 standard 40-foot-long shipping containers. 

The effects of climate change make things even worse. Increased rainfall erodes soil and flushes fertilizer into the watershed, and warming temperatures cause algae to flourish. “We’re really seeing dead zones, and algae blooms in general, become bigger and bigger as time goes on, and I think that has a lot to do with climate change,” says Anne Weir Schechinger, an economist with Environmental Working Group, a Washington, D.C.-based nonprofit and lobby that tracks algae blooms across the country.

The annual record was set in 2017, when the dead zone swelled to a New Jersey-sized 8,776 square miles. Last year’s dead zone was just under 7,000 square miles, the eighth-largest ever recorded, after Hurricane Barry swept through and stirred air into ocean bottoms, lessening the impact of the algae.

Even without the hurricane, oyster farmers in the Gulf ended up bearing the brunt of upstream floods. Faced with the prospect of an overflowing Mississippi River, the U.S. Army Corps of Engineers opened a spillway to clear 10 trillion gallons of fresh water into Lake Pontchartrain, then into the briny Mississippi Sound. Salinity levels plummeted, and oyster farms were wiped out. Last month, over $58 million in federal relief funds were allocated to Louisiana fishermen, seafood processors, and other aquaculture businesses—an amount dwarfed by the estimated $338 million the state’s fishing industry lost to the flooding.

The dead zone has devastating effects on fish habitat, which in turn, damages the industries that depend on that marine life, like commercial and recreational finishing, and coastal tourism. Shrimp fishers say that algae blooms result in small, stunted, and less profitable shrimp. That, in turn, knocks out processors. All told, the nitrogen dumped from Midwest farms costs as much as $2.4 billion in damages to Gulf fisheries and marine habitat every year, according to UCS—a big bite out of a regional fishing industry estimated to be worth between $17 and $57 billion every year.

The UCS pushes for federal policies that it says would shrink the dead zone. In its report, it encourages legislators to reward farmers who find ways to keep nitrogen out of waterways, such as planting cover crops that hold soils in place, and establishing buffers between fields and nearby streams. Bills to promote “soil health,” as this suite of conservation practices is called, are gradually picking up steam.

Farm groups, like Wisconsin’s Michael Fields Agricultural Institute, coordinate exchanges with Gulf fishers, as part of efforts to help Midwest farmers understand what’s at stake downstream. In 2017, Lance Nacio, a third-generation shrimper in Montegut, Louisiana, toured fields in Iowa County, Wisconsin, as part of a group that served shrimp and seafood to conservation-minded farmers.

“They’re conscious about their effects,” he says of the farmers. Since then, grain farmers have visited him in the Gulf, where they’ve had the opportunity to see the effects of the dead zone. “It’s the lack of catch. It’s the understanding of what we go through every summer.”

But oyster farmers like Pollock remain pessimistic that the Gulf shellfish industry can ever be saved. Between last year’s opening of the Bonne Carre Spillway, and the coming opening of the Mid-Barataria diversion—another river tributary that would continue to dump Midwest freshwater in the Gulf—and the increasing incidence of massive tropical storms, he’s decided to move his operation up north to Baton Rouge, where he already does his oyster hatching.

“We still have a few down there,” he says, of the Gulf. But in the long run, he says, the industry’s days are probably numbered. “It’s a losing battle. There’s only so much you can do.”