Bay scientists report smallest July ‘dead zone’ on record
Unseasonably cool temperatures and an Independence Day hurricane teamed up to produce the the smallest July “dead zone” in 30 years of Chesapeake Bay monitoring, according to the Maryland Department of Natural Resources.
Department scientists reported earlier that the amount of oxygen-starved water they observed in early July was the smallest seen during the first half of the month.
They attributed the better-than-expected conditions to Hurricane Arthur, which passed through the region during the 4th of July weekend and produced winds that helped mix oxygen-starved bottom waters with oxygen-rich water on the Bay’s surface. At the time, scientists warned that oxygen conditions could worsen later in the month as impacts from the hurricane receded.
But late July monitoring found less than half a cubic mile of hypoxic water in the Bay’s mainstem — the smallest hypoxic area observed in the second half of the month during three decades of monitoring. The volume of the mainstem — which does not include the tidal portions of tributaries — is about 31 cubic miles. (Hypoxic water has less than 2 milligrams of dissolved oxygen per liter of water.)
Scientists credited cool summer temperatures for continued good conditions because they reduced thermal stratification in the Bay.
When there is a large difference in temperature between surface and bottom waters, it contributes to stratification that prevents the mixing of water. When surface and bottom water cannot mix, the bottom areas of the Bay typically become oxygen-starved, or hypoxic, as algae die, sink to the bottom and are decomposed by bacteria that remove oxygen from the water.
The monitoring results were a great improvement from the slightly worse-than-normal oxygen conditions that scientists has predicted for the summer. That prediction was based on higher-than-average spring water flows from the Susquehanna River, which can contribute to strong stratification, accompanied by high nitrogen levels which generate algae blooms.
DNR scientists said if cool temperatures and relatively low rainfall persists, good dissolved oxygen conditions could remain into August. But they also said a number of large algae blooms had been observed this summer which could fuel a larger dead zone if conditions change and stratification increases in the Bay.
For the DNR’s late July hypoxia report, click here.
More water quality data can be found on the DNR’s Eyes on the Bay website.