On the Trail of the Chesapeake’s Dead Zone
The evidence leads out of the bay and up the rivers, including the Potomac.
Back home, the Potomac River was pulsing with life. Minnows were darting through the stargrass while mussels serenely filtered water down below. Insect nymphs shucked their shells to emerge at the surface as gossamer mayflies.
But here in the waters off Annapolis, we were about to make acquaintance with a dead zone.
Capt. Paul Bayne plugged one end of a long, black cable into his hand-held dissolved oxygen meter.
“Who wants to read out the numbers?” he asked. A woman stepped forward and took her position.
Bayne lowered the probe on the other end of the cable into the surface water. “What does it say?” he called out. The rest of us edged a little closer.
“10.8,” the woman replied.
This was 10.8 parts of oxygen per million parts of water--or parts per million (ppm)--which is about saturation level. No surprise here: Wave action keeps the water’s surface efficiently oxygenated.
Then things quickly went downhill.
Bayne dropped the sensor deeper and called for another reading. “Five-point-five,” was the reply. At this level, rockfish would begin to gasp. The next reading of 4.3 was marginal for fish of all kinds. The following reading of 3.4 approached the limit for oysters.
Now, the probe was down about 35 feet, nearing the bottom of the channel. The dissolved oxygen had plummeted to 0.3 ppm, well below the 2 ppm that defines a dead zone. Here, nothing can live except bacteria and other very simple organisms. No rockfish, no crabs, no oysters―nothing you would ever find pictured on a placemat in a seafood restaurant.
Our dead zone that day was just a hint of the 100-mile swath in the bay’s main channel that becomes lifeless during the summer. In all, more than 10 percent of the bay’s water is off limits to life.
It was a somber finale to a delightful day on the Stanley Norman. This skipjack sailboat was built in 1902 to dredge for the bay’s once-abundant oysters. Today, the boat is owned by the Chesapeake Bay Foundation (CBF), which it uses as a floating classroom.
Most guests on the Stanley Norman are school children. But others can also book passage, such as our group from the Bethesda-Chevy Chase Chapter of the Izaak Walton League, a national conservation organization.
The Potomac Connection
Could there be a link between our vibrant river and this lifeless spot just an hour away?
The Potomac’s summertime water may seem clear, but it contains a cocktail of chemicals, among them nutrients such as nitrogen and phosphorus. Some of these nutrients come from runoff from farms, particularly in the Shenandoah Valley. A growing share comes from the rapidly growing cities and towns along the river and its tributaries.
Eventually, these nutrients enter the bay where they fuel tremendous biological productivity―but of the wrong kind. Algae thrive on the nutrients, producing great “blooms.” As the algae dies, it is consumed by masses of bacteria, which use up the oxygen.
CBF’s mission is to rally support for restoring the bay’s water quality. It helps to push through critical legislation and forces compliance with existing law. Most of this legislation is aimed directly at reducing pollution in the watersheds of the 150 rivers and streams entering the bay, of which the Potomac is the second largest.
These efforts are paying off. In fact, said Bayne, the Chesapeake is one of the few places in the world where environmental quality is improving even as population increases. Crab numbers are up 66 percent over last year. Nitrogen and phosphorus levels have dropped over the past couple of decades. Rockfish have rebounded from a cataclysmic collapse caused by overfishing.
“This is really good news,” said Bayne. But he also calls these gains “low-hanging fruit.”
“Our big job is to make major improvements in water quality,” he continued. “We already have the technical know-how and the information we need.
“Now we need money and political will to put teeth in the Clean Water Act,” he said (see “Great Reads” column).
Meanwhile, on the deck of the Stanley Norman, water cleanup was already underway. Earlier in the day, the boat’s crew had dredged up some oysters and filled two tanks with bay water, both murky with algae. In one tank they placed the oysters.
Within an hour, the water in that tank had almost cleared. With a filtering rate of 60 gallons per hour, oysters are the cheapest way to get rid of algae. If we can bring back the Chesapeake’s oysters and other creatures, nature can carry on pretty much by itself.