AFTER IRENE: Last week’s hurricane purges Chesapeake Bay’s dead zones

AFTER IRENE: Last week’s hurricane purges Chesapeake Bay’s dead zones

WASHINGTON — Hurricane Irene’s high winds breathed a little life into the polluted Chesapeake Bay, bringing some short-term relief to wildlife by temporarily eliminating the estuary’s vast dead zone, according to the Maryland Department of Natural Resources.

The bay’s dead zone — a stretch of deep water running from Baltimore to Virginia with dissolved oxygen levels too low to support most aquatic life — emerges every summer and naturally fades with the fall. But this year’s dead zone diminished earlier than usual after Irene whipped the area, mixing layers of water containing varying levels of oxygen.

The revived waters could provide habitat for marine life in places that were previously not livable, said Jenn Aiosa, Maryland senior scientist for the Chesapeake Bay Foundation.

"It might open up additional habitat that, three weeks ago, wasn’t hospitable because the oxygen was low," Aiosa said. "A greater volume of the bay with oxygen is good, primarily for the fish that transit the bay, and you know, if there’s more fish available, birds or the wildlife that rely on those fish species would also benefit."

The massive dead zone in the bay is largely responsible for choking out fish, oyster and blue crab populations over the past several decades. Ultimately, similar conditions will return next spring and the hurricane will have likely made little impact on the bay’s long-term status, scientists said.

"Usually this time of year we are seeing some of our poorest conditions in terms of oxygen," Aiosa said. "Maybe for a short period of time we’ve alleviated some of that stress."

After making its way from the Caribbean, Hurricane Irene struck Maryland Aug. 27 as a Category 1 storm that left a trail of flooding, wind damage and power outages in its wake.

Hurricane Irene, and Isabel before it, had a positive effect on bay pollution. But past hurricanes, particularly Tropical Storm Agnes in 1972, set the stage for the bay’s degradation by flooding it with nutrients from rainwater runoff, according to Don Boesch, president of the University of Maryland Center for Environmental Science.

This year’s dead zone would have dissipated by mid-September regardless of the hurricane, DNR Resource Assessment Director Bruce Michael said, but an early flush won’t hurt.

"This will certainly help the bay in the short term; it’ll make conditions a little better," Michael said. "That is a good thing, probably one of the few benefits that you have."

Twin forces cause dead zones to occur — water temperature and nutrient (mainly nitrogen and phosphorus) pollution. The bay’s surface warms during summer, separating it from deep, cool, oxygenated water below. Then nutrients run in, feeding algae blooms that also deplete the oxygen. When they finally die, they sink and decay, further robbing the water of oxygen.

Because fall and winter weather brings higher winds that constantly mix the water column, dead zones disappear during colder months.

Weather will largely determine how long Irene’s positive effect on the bay will last, scientists said. Because the hurricane also brought nutrients flooding into the bay, the right conditions could cause the dead zone to reform.

"My prediction is that you’ll have a small return of low oxygen conditions over the next few weeks," said Boesch, "but it won’t be like the extent or severity that we experienced this summer."

Next year is a different story. Scientists will not be able to assess this year’s dead zone impact until they review more data, but if the nutrients Irene brought remain in the water, Boesch said they could help create a strong dead zone next year.

"If you have an intense event that brings a lot of nutrients into the bay late in the summer, early in the fall, it could very well give the whole process of oxygen depletion a head start for next year," Boesch said.

But Michael and Aiosa said they do not expect Hurricane Irene will play a major role in shaping next year’s dead zone.

"It’s got to be a pretty dramatic storm event to have long-term impacts on the bay," Aiosa said. "I don’t think the impacts that we see from this one are (dramatic), or that Irene was that level of storm."

http://www.delmarvanow.com/article/20110907/NEWS01/110907027