This summer turns out not to have been quite as dismal for Bay oxygen levels as previously thought. After monitoring during the month of September, scientists concluded that this summer had the largest volume of anoxic water—areas void of oxygen—observed since Baywide monitoring began 21 years ago.

But a re-examination showed this was actually the third worse summer for anoxia.

After taking a look at the numbers, scientists realized they had defined anoxia this year as water with dissolved oxygen concentrations that were less than, or equal to, 0.2 milligrams per liter of water. That is about the limit of detection for the monitoring equipment used in the surveys.

In previous years, anoxia had been defined as water with less than 0.2 mg/l of dissolved oxygen. While it was only a slight change, it was enough to make a difference.

When an apples-to-apples comparison was made, using the less than or equal to 0.2 mg/l definition, this year’s anoxic “dead zone” was the third largest on record, behind 1993 and 1998.

The total volume of anoxia was 2.67 cubic kilometers, or about 5.1 percent of the mainstem Bay, which does not include tidal tributaries.

Areas of hypoxic water, which contain oxygen, but at lower levels than needed for most species in the Chesapeake, covered a much greater area of the Bay.