High oxygen levels sign of a healthy Chesapeake
Bay’s hypoxic zone 40% smaller than average
Recent reports have shown growing populations of blue crabs, striped bass and anchovies in the Chesapeake Bay, and a fundamental environmental barometer is explaining why: Oxygen levels in the estuary are among the highest in three decades.
Samples taken in late June revealed than the area of water at the bottom of the bay that contains little or no oxygen has shrunk to about 40 percent of its long-term average, at less than half a cubic mile of volume, according to the state Department of Natural Resources. That was the second-smallest volume for late June since scientists started closely monitoring the data in 1985.
“We’re kind of dialing things back about 30 years,” said William Dennison, vice president for science applications at the University of Maryland Center for Environmental Science.
While the abundance of seafood or underwater grasses are visible signs of the bay’s rebounding health, oxygen levels are an unseen yet imperative part of its ecosystem. “Dead zones” devoid of oxygen kill fish, crustaceans, bivalves and even phytoplankton.
Oxygen levels are “a truly integrative kind of measure,” Dennison said. “When that’s getting better, that’s really the ultimate good-news story.”
The air and underwater plants dissolve oxygen in the water, but the gas becomes depleted in a process seen in waterways across the country and globe: Nutrients from farm runoff and sewage leaks fertilize algae blooms, and when the algae blooms die, their decomposition strips the vital element from the water.
Chesapeake Bay oxygen levels are lowest in an area that stretches along the bottom of the waterway from the Bay Bridge south to the mouths of the Patuxent and Potomac rivers, according to the state data.
Scientists collect samples at least once a month, and twice a month during the summer, testing for differences in oxygen levels based on depth, said Bruce Michael, director of resource assessment for the state DNR. During the summer, oxygen levels are lower because warm water holds less oxygen than cold water.
Summertime weather conditions also typically prevent fresher, more oxygenated water near the surface from mixing with colder, saltier water below it.
“Fish can swim away from an area of no
“We’re kind of dialing things back about 30 years.” William Dennison, University of Maryland Center for Environmental Science
oxygen, but clams and oysters, if they are exposed to areas of no oxygen, can’t get away, so they die,” Michael said.
The data collected late last month backs up — and possibly exceeds — a forecast of bay oxygen levels issued last month. The volume of water considered hypoxic, meaning it is low in oxygen, was the second-smallest scientists have measured at that time of year, behind only late June of 1997.
The biggest factors influencing the dead zones are the flow of the Susquehanna River, which carries half of the bay’s fresh water and much of its nutrients, and other efforts to limit nutrient runoff from land and leaks from sewage plants. Susquehanna flow is expected to be average or below average this summer, while efforts to limit farm fertilizer use and modernize sewage treatment continue to ramp up.
The dead zones will continue to grow as the summer wears on. More than 1.5 cubic miles of bay waters was expected to carry low levels of oxygen by late summer, which is about average.
But can the oxygen levels continue to stay better than expected?
“It might, but I can’t really predict it,” Michael said.