In the 1970s, the Chesapeake Bay was found to contain one of the planet's first identified marine dead zones, where waters were so depleted of oxygen that they were unable to support life, resulting in massive fish kills.
Today the Bay's dead zones are estimated to kill 75,000 tons of bottom-dwelling clams and worms each year, weakening the base of the estuary's food chain and robbing the blue crab in particular of a primary food source.
Crabs are sometimes observed to amass on shore to escape pockets of oxygen-poor water, a behavior known as a "crab jubilee".
Hypoxia results in part from large algal blooms, which are nourished by the runoff of residential, farm and industrial waste throughout the watershed.
One report in 2010 criticized Amish farmers for having cows that "generate heaps of manure that easily washes into streams and flows onward into the Chesapeake Bay".

The runoff and pollution have many components that help contribute to the algal bloom, which is mainly fed by phosphorus and nitrogen.
 This algae prevents sunlight from reaching the bottom of the Bay while alive and deoxygenates the Bay's water when it dies and rots.
 The erosion and runoff of sediment into the Bay, exacerbated by devegetation, construction and the prevalence of pavement in urban and suburban areas, also blocks vital sunlight.
 The resulting loss of aquatic vegetation has depleted the habitat for much of the Bay's animal life.
 Beds of eelgrass, the dominant variety in the southern Chesapeake Bay, have shrunk by more than half there since the early 1970s.
Overharvesting, pollution, sedimentation and disease have turned much of the Bay's bottom into a muddy wasteland.

One particularly harmful source of toxicity is Pfiesteria piscicida, which can affect both fish and humans.
Pfiesteria caused a small regional panic in the late 1990s when a series of large blooms started killing large numbers of fish while giving swimmers mysterious rashes; nutrient runoff from chicken farms was blamed for the growth.

The Bay has improved slightly in terms of the overall health of its ecosystem, earning a rating of 31 out of 100 in 2010, up from 28 in 2008.
An estimate in 2006 from a "blue ribbon panel" said cleanup costs would be $15 billion.
Compounding the problem is that 100,000 new residents move to the area each year.
A report in 2008 in the Washington Post suggested that government administrators had overstated progress on cleanup efforts as a way to "preserve the flow of federal and state money to the project."
In January 2011, there were reports that millions of fish had died, but officials suggested it was probably the result of extremely cold weather.
In the 1970s, the Chesapeake Bay was found to contain one of the planet's first identified marine dead zones, where waters were so depleted of oxygen that they were unable to support life, resulting in massive fish kills. Today the Bay's dead zones are estimated to kill 75,000 tons of bottom-dwelling clams and worms each year, weakening the base of the estuary's food chain and robbing the blue crab in particular of a primary food source. Crabs are sometimes observed to amass on shore to escape pockets of oxygen-poor water, a behavior known as a "crab jubilee". Hypoxia results in part from large algal blooms, which are nourished by the runoff of residential, farm and industrial waste throughout the watershed. One report in 2010 criticized Amish farmers for having cows that "generate heaps of manure that easily washes into streams and flows onward into the Chesapeake Bay".
The runoff and pollution have many components that help contribute to the algal bloom, which is mainly fed by phosphorus and nitrogen.  This algae prevents sunlight from reaching the bottom of the Bay while alive and deoxygenates the Bay's water when it dies and rots.  The erosion and runoff of sediment into the Bay, exacerbated by devegetation, construction and the prevalence of pavement in urban and suburban areas, also blocks vital sunlight.  The resulting loss of aquatic vegetation has depleted the habitat for much of the Bay's animal life.  Beds of eelgrass, the dominant variety in the southern Chesapeake Bay, have shrunk by more than half there since the early 1970s. Overharvesting, pollution, sedimentation and disease have turned much of the Bay's bottom into a muddy wasteland.
One particularly harmful source of toxicity is Pfiesteria piscicida, which can affect both fish and humans. Pfiesteria caused a small regional panic in the late 1990s when a series of large blooms started killing large numbers of fish while giving swimmers mysterious rashes; nutrient runoff from chicken farms was blamed for the growth.
The Bay has improved slightly in terms of the overall health of its ecosystem, earning a rating of 31 out of 100 in 2010, up from 28 in 2008. An estimate in 2006 from a "blue ribbon panel" said cleanup costs would be $15 billion. Compounding the problem is that 100,000 new residents move to the area each year. A report in 2008 in the Washington Post suggested that government administrators had overstated progress on cleanup efforts as a way to "preserve the flow of federal and state money to the project." In January 2011, there were reports that millions of fish had died, but officials suggested it was probably the result of extremely cold weather.
Chesapeake Bay