The Chesapeake is not the only bay with a “dead zone” where oxygen levels are simply too low to support aquatic life.
Dozens of the United States’ best-known bays are starved for oxygen, ranging from Tampa Bay in Florida to San Francisco Bay in California.
A recent national assessment of 138 bays and the northern Gulf of Mexico found moderate high and high eutrophic conditions in 44 estuaries.
The National Oceanic and Atmospheric Administration, which worked with states to conduct the survey, found that these estuaries featured a variety of environmental problems caused by the introduction of excess nutrients, including low dissolved oxygen, reduced sunlight, loss of underwater grasses, growth of seaweed, harmful algal blooms and changes in the kinds of algae present.
Low dissolved oxygen levels, or dead zones, were found in 42 bays, primarily in the Gulf of Mexico, Mid-Atlantic and South Atlantic regions. Moderate or high losses of bay grasses were found in 27 of the assessed bays, primarily the Gulf of Mexico and Mid-Atlantic regions.
This summer, the Chesapeake suffered from the effects of the largest dead zone ever recorded, a 100-mile stretch of low-oxygen water that reached from Baltimore to the Bay Bridge.
Oxygen levels in dead zones fall too low to support marine life, killing some species and dislocating others. Dead zones are caused by an excess of nutrients from a variety of sources, which include runoff from farms, wastewater discharges, fertilizer and air pollution, according to the U.S. Geological Survey and other federal agencies.
Agricultural runoff is the leading pollutant source for dead zones in 13 of the nation’s 17 most eutrophic bays, NOAA and USGS studies show. In each of these bays, agricultural runoff contributes at least one-third of the pollutants that cause low-oxygen levels and toxic algae blooms, and add to the loss of underwater grasses, which provide critical habitat for fish, crabs and other species.
Excess nutrients promote a complex array of problems, beginning with the excessive growth of algae, which, in turn, can lead to more serious symptoms. Nutrients trigger the growth of algae, which reduce the amount of dissolved oxygen in bay water in two ways: by consuming oxygen at night, and through decomposition when the algae die.
When dissolved oxygen levels get too low, the variety of species that can survive dramatically declines.
Although some animals simply leave dead zones, oxygen levels can get low enough to cause fish kills. Many other species, including young fish and shellfish, are unable to escape. In general, fewer long-lived creatures such as clams and crabs live in dead zones, which are more likely to contain small, short-lived species such as worms.
Nutrients also trigger subtle changes in tiny life forms that can, in turn, reduce the variety and productivity of commercial fish populations.
Many of the microscopic organisms that form the base of a bay’s food web face major disadvantages as dead zones develop. For example, zooplankton that graze on algae in surface waters during the night and migrate to a bay’s bottom waters during the day to escape predators may be more vulnerable if the bottom waters have low-oxygen levels.
In some cases, shifts in the makeup of microscopic organisms can cause toxic algae blooms—known as red or brown tides—to be more frequent and extensive.
Among the thousands of microscopic algae species are a few dozen species that produce toxins powerful enough to harm fish and people. Toxic algae contaminate shellfish consumed by people, cause massive fish and shellfish kills, and even the deaths of marine mammals and sea birds.
Although these algae blooms occur naturally, the number of blooms has gone up as nitrogen loadings have increased.
The algae-like organism pfiesteria is one example of a toxic organism that scientists have linked to increased runoff from feedlots. Major blooms of pfiesteria occurred in 1997 in the Chesapeake Bay and the bays of the Pamlico Sound in North Carolina.
The nation’s best-known “dead zone”—the northern Gulf of Mexico—actually shrunk in 2003.
Tropical storms remixed Gulf of Mexico waters and reduced the size of the dead zone from 8,000 square miles—an area roughly the size as Connecticut—to about 3,300 square miles.
Hydrology dramatically impacts the ebb and flow of dead zones, and some bays are far more susceptible to eutrophic conditions than others.
Major factors include the amount and timing of freshwater inflow, tidal flushing, and the degree of stratification between saltwater and freshwater.
In general, susceptibility to nutrients is dependent in large part on the amount of time that nutrients entering a system stay there before exiting, studies show. Estuaries that lack the ability to quickly dilute or flush out nutrients are far more susceptible to nutrient pollution.
“The condition of an estuary has a lot to do with the physical setting of the estuary,” said Rob Magnien, a NOAA scientist who worked for the Maryland Department of Natural Resources for two decades. “The Chesapeake Bay is one of the types of estuaries most susceptible to nutrient pollution.”
The Chesapeake is shallow, “doesn’t flush well,” and features a stratified “wedge” of freshwater on top and saltwater on the bottom that grows more pronounced in the summer and during years of high flow, Magnien said. What’s more, the Bay’s enormous watershed drains into a relatively small waterbody.
In contrast, he said, bays on the west coast, like Puget Sound, are deep and flush well.
Because each bay is different, NOAA works with states and other officials to determine how much nitrogen each waterbody can safely handle.
“Some bays are simply more susceptible to these problems than others,” Magnien said.