(Inside Science) -- Scientists studying the Chesapeake Bay have found that the dead zones, where a seasonal lack of oxygen kills plants and animals, are getting smaller faster, which may be a sign that the bay is cleaning itself.
“I think you can measure that in a lot of ways, but I think there is no question that over 30 years of data, that assets of the bay improved,” said Jeremy Testa, a researcher at the University of Maryland Center for Environmental Science on Solomons Island. The zones are now breaking up two to four weeks earlier.
The work was published online in May in the journal Limnology and Oceanography. The study was done in the lower part of the bay, near the mouth of the Potomac River.
Oddly, one important cause of the improvement may be from the Clean Air Act, not the Clean Water Act, he said. The air act helps reduce oxidized nitrogen from the air, Testa said.
Dead zones are areas where there is not enough oxygen in the water to sustain the majority of complex life. Excessive nutrients in the water, from wastewater treatment plants, runoff and river flows, cause blooms of algae as the weather warms. The algae eventually die and sink to the bottom.
They are decomposed by bacteria, which release ammonium, a form of nitrogen that accumulates on the bottom. The decomposition also consumes oxygen. As this process intensifies, and where waters are too deep to obtain oxygen from plants or the air, fish and other marine creatures flee or perish.
When oxygen is restored to the deep water as the summer subsides and autumn winds mix the water, the ammonium is converted to nitrogen that can be readily converted into nitrogen gas and released into the atmosphere. This process has always happened in the fall as dead zones break up, but the recent research indicates that the breakup is happening earlier, and that the nitrogen transformation is happening at higher rates.
There are hundreds of dead zones in the seas and oceans of Earth. Robert Diaz, professor emeritus of the Virginia Institute of Marine Sciences at the College of William and Mary, has collected an enormous list of dead zones around the world, filling 485 pages of a spreadsheet.
Dead zones probably occurred before the Industrial Age in ocean areas where there were huge upwellings. In the U.S., they occur more commonly in the Pacific Northwest. They first appeared in the Chesapeake Bay in the 1930s. The largest ever recorded was 2.7 cubic miles in 2010. The average is 1.7 cubic miles, according to the Chesapeake Bay Program.
A 2017 dead zone in the Gulf of Mexico is the largest ever recorded: 8,500 cubic miles, or an area about the size of New Jersey.
Chesapeake Bay, once called an “immense protein factory” by Baltimore journalist H.L. Mencken, is the largest estuary in the United States, getting most of its water from the Susquehanna River that flows down from the mountains of Pennsylvania, as well as the Potomac and about 140 other waterways. Maryland surrounds much of the 200-mile bay, which is 2.8 miles wide at its narrowest and has an average depth of about 20 feet. Virginia wraps around the southern portion, where its mouth opens to the Atlantic Ocean.
The watershed includes Baltimore on the Patapsco River, and Washington on the Potomac, both contributing wastewater into the bay. Baltimore is one of the largest seaports in the U.S., so the bay also is well-travelled and susceptible to pollution.
The bay is famous for its shellfish, the bounty that so entranced Mencken, including crabs, oysters and clams. It’s also home to fish like rockfish, a species of striped bass that’s part of the local cuisine, and Atlantic sturgeon, a fish listed as endangered or threatened.
For most of the 20th century, the Chesapeake was heavily polluted, mostly from chicken farms and corn fields on the Delmarva peninsula, and the pollution killed off much of that protein supply, a serious economic hardship to the people, many of whom live on islands in the bay who harvest the edible creatures. The health of the bay has been restored to the point Mencken’s protein factory is again producing.
Diaz said reducing dead zones can be done. Remediation has worked in San Francisco Bay and the Hudson River near New York City by controlling runoff, but the Chesapeake is still plagued by them.
The best example of how to control dead zones happened inadvertently in the Black Sea, which had one of the world’s largest. When the Soviet Union collapsed in the 1990s, the price of fertilizer soared, making it too expensive for local farmers. The dead zone disappeared. As the price of fertilizer has fallen, it has returned.
The changing climate plays a role, Testa said. “The warmer the water, the less oxygen it holds,” he said.
But further recovery may be challenged.
In February, for the second consecutive year, the Trump administration proposed budget cuts to the Environmental Protection Agency, including funding specifically allocated for restoring the Chesapeake Bay, to the consternation of environmentalists. In the budget, the administration classified the cleanup as a "primarily local effort."
So far Congress has rejected the cuts.