High Temperatures, Excess Nutrients Add Up to Summer "Dead Zones" in Bay

We've all read the stories about the Bay's “dead zones”—areas of the Bay that become devoid of oxygen during the Chesapeake's hot summer months and cannot support most forms of life. But how do parts of the Bay get that way?

Dissolved oxygen, or DO, refers to the amount of oxygen that is present in a given quantity of water. We measure it as a concentration using units of mg/l (i.e., the milligrams of oxygen dissolved in a liter of water). Keeping track of the Bay's oxygen levels is important because everything that swims or crawls in the Bay—from prized striped bass to the worms crawling at the bottom—requires oxygen to live.

Temperature determines the amount of dissolved oxygen that water can hold. Yet, even at the warmest temperatures that we typically see in the Bay—around 91 degrees Fahrenheit—the water is still capable of having DO concentrations of about 6 to 7 mg/l, which is enough oxygen for striped bass and most other Bay species to survive.

On average, the Bay area experiences the warmest weather of the year between mid-July and early August. But high temperatures are only a small part of the reason why oxygen levels drop in parts of the Bay's mainstem each summer.

The causes of the Bay's low DO begin on the land and in the air.

• Excess nutrients (nitrogen and phosphorus) from agriculture, air deposition, septic systems and sewage treatment plants, and runoff from lawns, gardens and paved surfaces reach the Bay and fuel the growth of phytoplankton, or algae.
• While phytoplankton form the base of the Bay's food chain, the amount of nutrients now entering the Bay is overwhelming the system. Oysters and other filter feeders can't consume all the phytoplankton in the water.
• Unconsumed phytoplankton sink to the bottom of the Bay and are decomposed by bacteria in a process that depletes the water of oxygen.
• The decomposing phytoplankton, combined with higher water temperatures, can cause large swaths of the deepest parts of the Bay's mainstem to have little or no oxygen to support marine life.

Residents of the Bay watershed can help give the Bay's crabs, fish and other critters some relief from low DO by taking simple actions to reduce nutrient pollution, including driving less, picking up pet waste and reducing the use of lawn fertilizers.

John Smith Shallop Visits Annapolis to Celebrate Region's History, Heritage and Environment

A replica of the shallop Captain John Smith used to explore the Chesapeake anchored at City Dock in Annapolis on July 14 for two days of festivities that focused on the Bay's history and environment.

The shallop — a 17th century-style boat — and its 12-member crew are retracing Smith's voyages, stopping at various points along the route to showcase their journey and educate the public about the Bay.

Smith's original voyage 400 years ago paved the way for the English colonization of Virginia and subsequent settling of the New World. In December 2006, Congress authorized the nation's first all-water trail, naming it the Captain John Smith Chesapeake National Historic Trail to commemorate the voyage.

Maryland Governor Martin O'Malley joined the shallop crew in rowing the final mile of the leg from Eastport into Annapolis Harbor. Along with the John Smith crew and their exhibit, environmental and historical groups provided interactive displays and activities.

• Several species of wildlife were present, including terrapins and oysters.
• There were ample opportunities to learn about how the Bay has been degraded over the last 400 years and what people can do to help restore it.
• Groups participating in the event included the Bay Program, Chesapeake Bay Gateways, the Chesapeake Bay Trust, the Annapolis Maritime Museum and the Historic Annapolis Foundation.

The modern-day shallop crew began its journey from Jamestown, Virginia, just as Smith did in 1608. From its departure on May 12 to its arrival in Annapolis on July 14, the crew sailed and rowed approximately 700 miles. By the voyage's end, the shallop will have covered 1,500 miles and spent 121 days traveling to the headwaters of almost every tributary of the Bay.

Those interested in following the shallop as it travels the Chesapeake can visit www.johnsmith400.org for daily updates. The shallop's remaining stops are Baltimore and St. Leonard, Maryland, and Tappahannock, Fredericksburg and Deltaville, Virginia, before returning to Jamestown on September 8.

The John Smith project is the work of Sultana Projects, Inc. For more information about the Captain John Smith Chesapeake National Historic Trail, stay tuned to www.nps.gov/cajo.