The Chesapeake Bay has struggled for decades with dead zones — areas in which the water contains little or no oxygen, which sometimes causes massive fish kills. But while the problem has eased here in recent years, it has developed and worsened elsewhere around the globe, researchers say.
Since 1950, the number of coastal ecosystems that scientists say are “suffocating” has grown tenfold. The problem has grown four times over in the open ocean, which is generally a more stable and resilient environment.
Scientists say ocean oxygen content has declined 2 percent over the past 50 years.
Scientists say the trend is another consequence of global warming that threatens to disrupt food chains, destroy habitats and make it harder for some creatures and microorganisms to survive.
The extent of the problem was reported in the journal Science last week by an international team of researchers led by and including Chesapeake-focused scientists in Maryland.
“This is really a global issue — it’s not just a local issue,” said lead author Denise Breitburg, a marine ecologist with the Smithsonian Environmental Research Center in Edgewater. “The severity of the changes that we’re seeing are really worrisome.”
In the Chesapeake and other estuaries, excessive amounts of nutrients such as nitrogen and phosphorus begin the chain reaction that can lead to fish kills. The pollution, which flows into waterways from farms and wastewater treatment plants and urban stormwater runoff, fertilizes growth of algae blooms. When the algae dies and decomposes, microbes use up the oxygen in the water.
Climate change is exacerbating and spreading the problem, the researchers say. Warmer water doesn’t hold as much oxygen as cooler water, so as global temperatures rise, ocean oxygen levels are steadily declining. Rising water temperatures also speed up metabolism in many ocean creatures, forcing them to breathe in more oxygen to survive.
It’s a “double whammy” for ecosystems, said co-author Michael Roman, director of the University of Maryland Center for Environmental Science’s Horn Point Laboratory in Cambridge.
“That has some severe effects for things we care about, like fisheries,” Roman said.
While many fish and other organisms can swim away from suffocating waters, an increase in low-oxygen waters means less suitable habitat is available, potentially making some creatures more prone to predation, the scientists said. Organisms that can’t get away will die.
Fish such as sturgeon and striped bass swim to cooler waters on the bottom of the Chesapeake during the summer. But if those areas have no oxygen, “that habitat is lost,” Roman said.
The research was the product of a United Nations Intergovernmental Oceanographic Commission work group created in 2016. Breitburg is leading the group known as the Global Ocean Oxygen Network, or GO2NE. Its members include Roman and UMCES professor Kenny Rose, along with researchers from Louisiana, Canada, Peru, Kuwait, Sweden, France and the Philippines.
While concerns about low oxygen levels and their consequences are well known among scientists and advocates in many parts of the world, the researchers said their goal was to show the broad and growing scope of the problem and to raise public awareness.
“It highlights the magnitude of the problem,” Breitburg said. “It’s not just upwelling on the Oregon coast; it’s not just low oxygen in Chesapeake Bay because of nutrients. This is truly a global problem, and it’s going to require global solutions.”
Dead zones have steadily decreased in the Chesapeake amid federally coordinated efforts by several states to reduce the nutrients applied to farm fields or released in wastewater. About 13.6 percent of water samples collected by the Maryland Department of Natural Resources last summer contained dissolved oxygen concentrations below two milligrams per liter, second-fewest since 1985, officials said.