DNR reports bay dead zones smaller than average

Dissolved oxygen conditions in Maryland waters of the Chesapeake Bay fared better than average in early August, the Maryland Department of Natural Resources reported this week.

Areas with less than 2 milligrams per liter oxygen, also known as the hypoxic water volume or dead zones, was 0.91 cubic miles, considerably smaller than the early August three-decade-average of 1.31 cubic miles, according to DNR.

Dead zones are caused by excessive nutrient pollution, primarily from human activities such as agriculture and wastewater, according to the National Oceanic and Atmospheric Administration.

Excessive nutrients can stimulate an overgrowth of algae, which then sinks and decomposes in the water. That process uses oxygen in the water, leaves areas of water with low oxygen and causes fish and other marine life to die or leave the area.

Crabs, fish, oysters and other marine life require oxygen to survive with levels higher than 5 milligrams per liter to be considered optimal to support aquatic health, according to DNR.

In a release, DNR attributed the better-than-average results this August in part to lower-than-average temperatures in the week leading up to the sampling.

“Higher temperatures generally cause more stratification of the water column, which inhibits oxygen from mixing into deeper waters,” DNR said in a release. “Waters with higher temperatures also hold less oxygen.”

Earlier in June, scientists had predicted the size of dead zones in Chesapeake Bay to be higher than average this summer due to higher spring flows and nitrogen loading from the Susquehanna River.

The anoxic portion of the zone, which contains no oxygen, was expected to be 0.35 cubic miles in early summer, growing to be 0.49 cubic miles by late summer in the bay.

In the August report, DNR said no anoxic zones, namely areas with less than 0.2 milligrams per liter oxygen, were detected in Maryland’s waters.

Beth McGee, Chesapeake Bay Foundation’s director of science and agricultural policy, said in a statement the August report is good news because it is another sign that shows the Bay may be becoming more resilient.

“All the cruises this summer have found no anoxic conditions, areas of the bay with virtually no oxygen,” McGee said. “If the trend continues, it will be the third year in a row that scientists have found no anoxic conditions, something that hasn’t happened since water quality monitoring began in 1985.”