Dead zones a coastal threat


A dead zone in San Diego Bay is part of a new national assessment of coastal waters suffering from low levels of oxygen. It shows areas of hypoxia have increased rapidly over that past 30 years.
Earnie Grafton
A dead zone in San Diego Bay is part of a new national assessment of coastal waters suffering from low levels of oxygen. It shows areas of hypoxia have increased rapidly over that past 30 years.

Dead zones

The term refers to areas of estuaries and coastal waters where the level of dissolved oxygen is too low to sustain most forms of animal life, notably fish, oysters and mussels. The condition is called hypoxia, which is generally defined as dissolved oxygen levels that are less than 2 to 3 milligrams of oxygen per liter of water. Sensitive organisms can be affected at higher thresholds.


The condition can be caused by a variety of factors, including excess nutrients, primarily nitrogen and phosphorus, and waterbody stratification due to saline or temperature gradients. High concentrations of nutrients can lead to eutrophication, which promotes the excessive growth of algae. As algae die and decompose, high levels of organic matter and the decomposing organisms deplete the water of available oxygen.


The criteria for the health of various marine and aquatic species varies. But scientists say the criteria used for Chesapeake Bay represents a good example of the impact hypoxia can have. Here are the minimum dissolved oxygen requirements of eight marine species: Striped bass, 5-6 milligrams per liter (mgL); American shad, perch and hard clam, 5 mgL; blue crab and bay anchovy, 3 mgL; spot fish, 2 mgL; worms, 1 mgL.

Dead zones increased dramatically in U.S. waters over the past 50 years, threatening ecosystems and fisheries nationwide, according to a sweeping report Friday by the federal Office of Science and Technology Policy.

The multiagency assessment said that incidents of hypoxia — a condition in which oxygen levels drop so low that fish and other animals are stressed or killed — have risen nearly 30-fold since 1960 due in part to man-made pollutants.

It called for renewed efforts to reduce water pollutants that lead to low levels of dissolved oxygen and improve strategies to protect marine food sources.

A dead zone in San Diego Bay, first documented in the 1980s, was part of the analysis. Scientists have used it for years as an example of an oxygen-starved area where runoff from cities contributes to hypoxic conditions.

“There are reasons to worry about San Diego Bay, but hypoxia hasn’t been studied as rigorously here as it has in other places like the Chesapeake Bay, where it is a much bigger problem,” said Brian Hentschel, a biology professor at San Diego State University who studies bottom-dwelling organisms such as worms, clams and shrimps.

He hopes Friday’s national assessment spurs more funding for local research that tracks dissolved oxygen and related factors across San Diego Bay over time.

“That report should trigger some alarm bells because it’s pretty clear that the human impacts that create hypoxic conditions have been increasing,” Hentschel said. “More detailed data now will make it easier 15 or 20 years from now to know how the bay is changing.”

The new analysis, billed as the most in-depth on the topic, follows a string of other dour assessments about the health of the world’s oceans. They are under pressure from fishing, harmful algal blooms, toxic contaminants and other factors.

Hypoxia can be caused by natural processes, but the dramatic spread of low-oxygen areas was linked by scientists to nutrients such as nitrogen and phosphorus from human activities. Sources of enrichment include wastewater discharges, air pollution and fertilizer-laced runoff from croplands and urban areas.

Dead zones were detected in nearly half of the 647 waterways assessed, including the Gulf of Mexico , home to one of the largest such zones in the world. The impact of the BP Deepwater Horizon oil spill on oxygen levels in the gulf was not assessed because it occurred after the report was written.

On the West Coast, federal researchers found a sixfold increase in the number of dead zones over the past 20 years, with 37 areas now suffering from low oxygen. A region off the coast of Oregon and Washington has become the second-largest seasonal hypoxic region in the United States and third largest in the world.

“These growing dead zones endanger fragile ecosystems and potentially jeopardize billions of dollars in economic activity,” said Lisa P . Jackson, administrator of the Environmental Protection Agency.

The trends highlighted in Friday’s report are part of a growing global problem, said Tony Koslow, who studies low-oxygen areas at the Scripps Institution of Oceanography, part of the University of California San Diego in La Jolla.

He said researchers around the Pacific Rim have noticed decreasing oxygen levels over the past decade.

“This is a large phenomenon not due to nutrient outflows” from land, said Koslow. “This big question is, ‘Is this due to climate change?’ ”

As the top oxygen-rich layer of the ocean warms, Koslow said it mixes less with the colder, oxygen-poor layers of the deep. Global climate models predict that the oxygen levels in deep oceans will decline 20 to 40 percent the next century.

“There are substantial ecosystem concerns,” Koslow said. “A number of species that live in the deep ocean are very sensitive to changes in oxygen levels. These species — although they are not of commercial interest — are prey to squid, fish, marine mammals and seabirds so changes in oxygen will have repercussions throughout the food web.”

Friday’s report said work to study and control pollutants are advancing but management efforts to stem the tide of hypoxia “have not made significant headway” in part due to increased development and population growth in coastal watersheds.

“If current practices are continued, the expansion of hypoxia in coastal waters will continue and increase in severity, leading to further impacts on marine habitats, living resources, economies, and coastal communities,” the report’s authors said.

Mike Lee: (619)293-2034; Follow on Twitter @sdenvirobeat.