This spring, scientists predicted a smaller than average low-oxygen “dead zone” in the Chesapeake Bay this summer. But that forecast was based on rainfall and pollution from January through May. Then, in June, the Bay region was drenched with wettest June on record since 1972. Stormwater pollution often plays an important role in the size of the dead zone.
Meanwhile, a relatively cool spring and early summer turned into blazing heat this week. The National Weather Service is predicting that the next three days will be the hottest of the year, with temperatures hitting 95 to 98 degrees Fahrenheit -– combined with humidity that will make it feel like 105 degrees in some areas.
What does all this rain and heat mean for the Bay? Chesapeake Bay Foundation Senior Scientist Dr. Beth McGee said that the heavy rains in June could make the “dead zones” in the Bay worse and more persistent, especially when combined with a lack of strong winds that stir oxygen in surface water down into the oxygen-deprived depths.
Warm fresh water deposited by rain tends not to mix with the colder, heavier, salt water on the Bay’s bottom that is often starved of oxygen in the summer months, Dr. McGee said. “The potential impact of the rain is to increase the stratification of the Bay,” Dr. McGee said.
This stratification can reduce the mixing of waters that circulates oxygen into the Bay’s depths. With more low-oxygen “dead zones,” more fish, crabs, and bottom-dwelling clams and worms suffer from a lack of oxygen and die.
The good news is that, in general, “dead zones” in the Bay have been slightly declining over the last quarter century. The “dead zone” last summer, for example, was the smallest on record since 1985, according to the Maryland Department of Natural Resources.
Monitoring by the state agency from June 3 to June 5 of 2013 (before the big rain storms that month) showed that the “dead zone” in the Bay was smaller than average. But then a second round of monitoring, from June 24 to June 26 (after the rains), showed that the “dead zones” had “expanded and intensified,” and were the 10th highest for June over the past 29 years, according to the Maryland Department of Natural Resources.
More rain also tends to lead to more closures of swimming beaches because of high bacteria levels, with stormwater flushing waste from dogs and other animals into local streams, as well as bacteria from overflowing sewage systems and failing septic tanks. Anne Arundel County, Maryland, for example, closed three beaches because of unhealthy bacteria levels during parts of June.
Heat can increase the size of low-oxygen “dead zones” in the Chesapeake Bay because warmer water holds less oxygen than cool water, making it harder for aquatic life to breathe. Spikes in water temperatures, especially in late summer, can kill a species of underwater vegetation called eelgrass that is a critical habitat for juvenile blue crabs. Huge swaths of eelgrass died in the southern Bay last summer and in 2005 in part because of late-summer heat waves.
Earlier this month, the Maryland Department of the Environment warned swimmers in the Bay to take precautions against infection by the vibrio bacteria, which is more prevalent in hot conditions. “Vibrio infections are rare. However, when Vibrio comes into contact with an open wound,
it can cause serious infections,” MDE officials said in a brochure.
The Virginia Department of Health on July 12 closed shellfish harvesting areas off Fisherman’s Island because three people had become sick from a form of vibrio they contracted from eating raw oysters.
In general, however, vibrio remains uncommon -- and 2013 has not been an unusually hot year, on average, in the Chesapeake Bay region, according to Greg Schoor, Meteorologist with the National Weather Service.
“This year has been as close to average as you can get for this time of year,” Schoor said. By contrast, last year's heat broke records in some areas.
I'm not sure that will make you feel any better as you sweat in the brutal heat today, but at least you'll know the big picture.
By Tom Pelton
Chesapeake Bay Foundation
(Photo from iStockphoto)