by Alicia Pimental
August 01, 2007
We've all read the stories about the Bay's “dead zones”—areas of the Bay that become devoid of oxygen during the Chesapeake's hot summer months and cannot support most forms of life. But how do parts of the Bay get that way?
Dissolved oxygen, or DO, refers to the amount of oxygen that is present in a given quantity of water. We measure it as a concentration using units of mg/l (i.e., the milligrams of oxygen dissolved in a liter of water). Keeping track of the Bay's oxygen levels is important because everything that swims or crawls in the Bay—from prized striped bass to the worms crawling at the bottom—requires oxygen to live.
Temperature determines the amount of dissolved oxygen that water can hold. Yet, even at the warmest temperatures that we typically see in the Bay—around 91 degrees Fahrenheit—the water is still capable of having DO concentrations of about 6 to 7 mg/l, which is enough oxygen for striped bass and most other Bay species to survive.
On average, the Bay area experiences the warmest weather of the year between mid-July and early August. But high temperatures are only a small part of the reason why oxygen levels drop in parts of the Bay's mainstem each summer.
The causes of the Bay's low DO begin on the land and in the air.
- Excess nutrients (nitrogen and phosphorus) from agriculture, air deposition, septic systems and sewage treatment plants, and runoff from lawns, gardens and paved surfaces reach the Bay and fuel the growth of phytoplankton, or algae.
- While phytoplankton form the base of the Bay's food chain, the amount of nutrients now entering the Bay is overwhelming the system. Oysters and other filter feeders can't consume all the phytoplankton in the water.
- Unconsumed phytoplankton sink to the bottom of the Bay and are decomposed by bacteria in a process that depletes the water of oxygen.
- The decomposing phytoplankton, combined with higher water temperatures, can cause large swaths of the deepest parts of the Bay's mainstem to have little or no oxygen to support marine life.
Residents of the Bay watershed can help give the Bay's crabs, fish and other critters some relief from low DO by taking simple actions to reduce nutrient pollution, including driving less, picking up pet waste and reducing the use of lawn fertilizers.