Dead Zones

As the levels of nitrogen and phosphorus pollution in the Bay rose, the first areas with insufficient dissolved oxygen to support life were discovered in the 1930s.

What causes dead zones?

Being fertilizers, nitrogen and phosphorus stimulate the growth of excessive amounts of algae.  In the upper Bay, the density of algae cells increased 250-fold from the 1950s to the 1980s.  As algae die, they sink to the bottom of the Bay where they are decomposed by bacteria.  The decomposition process consumes oxygen, depleting the level of dissolved oxygen in the water.

Different species need different levels of dissolved oxygen in order to thrive.  Worms and oysters, which have adapted to the lower levels of oxygen on the Bay’s muddy bottom, need 1 milligram of oxygen per liter of water (mg/L).   Blue crabs need 3 mg/L and fish need up to 6 mg/L of dissolved oxygen, particularly during sensitive periods of their life cycles.

Source: The Chesapeake Bay Foundation, cbf.org

Water containing low levels of dissolved oxygen (< 5 mg/L) is called hypoxic.  Even worse is anoxic water, which contains almost no dissolved oxygen (0 – 0.2 mg/L) and is incapable of supporting life.

Appallingly, the volume of anoxic and hypoxic water in the Bay has more than tripled over the last four decades.  In many years, up to one-quarter of the entire water in the mainstem of the Chesapeake Bay is hypoxic or anoxic by the middle of the summer.  In July 2007, monitoring showed that 40% of the water in the Bay was hypoxic or anoxic.

Source:  The Chesapeake Bay Foundation, "The Chesapeake Bay's Dead Zones" Fact Sheet, cbf.org.

Anoxic water in the deep waters of the Bay’s mainstem kills clams, oysters and worms living on the floor of the Bay.  When algae blooms occur in shallow, contained areas, dead zones can occur so rapidly that fish and crabs are unable to escape.