Study: Summer Ranked Worst Ever For Oxygen Levels In Chesapeake

Data gathered between September 12-21, 2005 by the Maryland Department of Natural Resources and Old Dominion University for the Chesapeake Bay Program indicate that summer 2005 ranked as the worst summer on record for the volume of anoxic (or oxygenless) water in the Bay, according to an Oct. 3 E-news alert from the Chesapeake Bay Program. A lack of strong winds and warm water temperatures help to combine in creating these conditions.

Scientists found that about 3 percent of the Bay's mainstem was anoxic, and about 21 percent of the mainstem had oxygen levels less than 5.0 milligrams per liter (mg/L).

"May's ecological forecast warned us that the large amounts of pollution washing into the bay this spring were going to lead to a bad summer for oxygen levels in the bay," said University of Maryland Center for Environmental Science data analyst David Jasinski. "But in the end, it looks like high spring pollutant loads, the lack of summer winds and unusually warm water temperatures combined to make it the worst summer on record."

An analysis of wind data for the summers of 1997-2005 indicates that the region had very little wind -- the mean summer wind speed for this year was the lowest recorded during those years.

Strong winds stir the waters of the bay and mix oxygen rich surface waters with the oxygen depleted deeper waters, enabling those deeper waters to absorb oxygen. Without wind to mix the water, the bay's deeper waters sat undisturbed below the pycnocline, allowing anoxia to persist for the entire summer.

Another weather factor that may have contributed to the larger than expected anoxic volume was the above average water temperature in the deep waters later in the summer. Temperature affects the rate at which chemical and biological reactions occur within the water. These higher temperatures may have elevated the consumption of oxygen below the pycnocline.

September data indicate a slight decrease in the amount of anoxic water in the mainstem Chesapeake Bay since the last cruise in August. However, the volume of oxygen-deprived water remained above average for this time of year.

Over the next few months, bay scientists will analyze data gathered over the summer to assess the accuracy of their May ecological forecast. The analysis is expected to be complete by November.

Additional information can be found at

This article originally appeared in the 10/01/2005 issue of Environmental Protection.

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