CHICAGO - Just off the coast of Louisiana, where the Mississippi River lets out into the Gulf of Mexico, an enormous algae bloom, fueled by fertilizer from Midwestern farm fields and urban sewage, creates an area so devoid of oxygen it's uninhabitable to most marine life every summer.
Nutrients like nitrogen from fertilizer and phosphorus from sewage act as a catalyst for algae growth. While algae are the base of the food chain for some fish, when these green plumes proliferate beyond what fish are capable of eating, their decomposition consumes much of the oxygen in the water.
This year, historic rains and flooding in the Midwest have roiled farm fields and overwhelmed sewer systems, flushing a tremendous amount of nutrients into the Mississippi River and into the Gulf, spurring a remarkable amount of algae. While the agricultural runoff from farms — exempted under the Clean Water Act — is the main driver of the Gulf dead zone, Chicago's sewage is the largest single source of phosphorus pollution.
The Stickney Water Reclamation Plant, which handles the waste for Chicago and some suburbs, is the biggest single source in the entire region and drains into the Mississippi River. According to the U.S. Geological Survey, agricultural sources in the watersheds of the Mississippi River basin contribute more than 70% of the nitrogen and phosphorus, versus about 9% to 12% from urban sources.
"It's amazing how big the Illinois impact is on something that's 1,100 miles away," said Josh Mogerman of the Natural Resources Defense Council, a nonprofit with offices in Chicago.
Climate scientists say the problem is expected to get worse in the future as a wetter climate in the Midwest — specifically one characterized by heavy rainfall in the winter and spring — creates more runoff.
"From a runoff point of view, it's actually the worst-case scenario to get more heavy rain," said Jim Angel, former Illinois state climatologist. "Those are the ones that really flush out the system.''
Scientists with the National Oceanic and Atmospheric Administration and several research universities announced in mid-June that the dead zone in the Gulf of Mexico is projected to be around 7,800 square miles, roughly the size of Massachusetts, the second largest on record behind 2017.
As in past years, the dead zone is expected to result in widespread die-offs and migration, influencing the fishing and shrimping industry.
"The fish that can move leave the area," said Don Scavia, an aquatic ecologist at the University of Michigan and collaborator on the NOAA forecast. "The organisms that live in the sediment — one of the main food supplies for the fish — can't. They die.''
Globally, dead zones have been growing in ocean waters since the 1950s. Since then, these oxygen-diminished areas have collectively expanded by an area about the size of the European Union, according to researchers. The Gulf of Mexico dead zone is considered the world's largest.
From June 2018 to May 2019, the United States experienced the wettest 12 months on record, with many of the hardest-hit areas in the Corn Belt: Iowa, Illinois, Nebraska and Missouri.
Environmental groups have tried to tackle the issue of water pollution and its effect on the Gulf. A coalition sued the Metropolitan Water Reclamation District of Greater Chicago over its share of phosphorus pollution in local waterways, citing its faraway impacts on the Gulf. Environmental advocates have also worked to incentivize the use of cover crops, plants like cereal rye that soak up water and nutrients during the off-season.
"Just like climate change, there's no one thing that is going to fix the algae apocalypse," Mogerman said.