Favorable weather helped limit Chesapeake Bay's 'dead zone' so far

Favorable weather helped limit Chesapeake Bay's 'dead zone' so far (Kim Hairston / June 22, 2012)

After a spate of bad news about the Chesapeake Bay's health, here's an uptick: the "dead zone" that forms every spring is smaller than average so far this year.

Water sampling done in early June by the Department of Natural Resources found dissolved oxygen levels too low to be suitable for fish, crabs and shellfish in just 12 percent of the bay, according to the department's "Eyes on the Bay" web site.

That's well below the long-term average since 1985 of 17.1 percent of the Chesapeake experiencing low oxygen levels.  It's also a dramatic improvement over last year, when fully a third of the bay's waters were starved of the oxygen that fish, crabs and shellfish need to breathe.

Oxygen levels in the bay's deepest waters decline every spring as warming temperatures spur algae to grow, fed by the glut of nutrients in the water from sewage, fertilizer runoff and air pollution.  Those thick algae blooms then consume the oxygen in the water as they die, sink to the bottom and decay.

State scientists say favorable weather most likely is responsible for healthier bay water so far this year, just as unfavorable weather has been blamed for last year's record large dead zone. Drier, warmer conditions from February through April this year meant less pollution washed off the land to feed the algae growth, while wetter, cooler weather in late spring helped keep low-oxygen conditions from setting in.  Last year, by contrast, an extremely wet spring helped flush more nutrients into the water.

For folks in the Baltimore area, the good news about the bay's dead zone this year may come as a surprise, after the recent stinky algae blooms and fish kills that stretched from the harbor south to Annapolis.  But DNR's map of oxygen levels in the bay shows the worst conditions concentrated in our part of the Chesapeake.

Just as the weather can change, so, too can the bay's water conditions. The "dead zone" typically grows through summer and reaches a peak in July or August.  Last year, the winds of Hurricane Irene in late August mixed bay waters and dramatically shrank the the dead zone.  That improvement was short-lived, though, as the torrential rains of Tropical Storm Lee washed millions of tons of mud, sediment and nutrients into the bay, re-forming the dead zone, which then lasted unusually late into the fall.