All about

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All about Dead Zones
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Zones of Oxygen Depletion
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What is Hypoxia?

• Waters that have a Dissolved Oxygen (DO)
  concentration less than 2mg/L (lt3mg/L, some
  systems) are defined as Hypoxia
• If DO 0 mg/L, it is called Anoxia
• Oxygen Depletion caused by
• Excessive nutrients, intense biological
  productivity that depletes oxygen
• Decomposition of organic matter consumes oxygen
• Stratification prevents oxygen refreshment

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Gulf of Mexico Dead Zone
Dead Zone gt 7,000 sq. miles in the Gulf of
Mexico with no marine life in 1999.
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The Gulf of Mexico is the largest fishery in the
US
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Hypoxia Low dissolved oxygen (lt 3 mg L-1)
And the fish are the lucky ones
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Number of fish kill events and number of
fishkilled in Gulf of Mexico coastal waters, 1980
- 1989 (NOAA/ORCA 1991 USEPA 1999).
Millions of fish killed
Number of fish kill events
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Catch per unit effort in the Gulf of Mexico, 1970
- 2000
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Gulf of Mexico Dead Zone
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The Size of the Dead Zone Correlates with
Mississippi River Discharge Values
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Nitrogen inputs to the Gulf since 1910
2000
1920
1980
1960
1940
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Nitrogen loading in the Gulf triggers algal
blooms, which decay and sink to the bottom.
The degradation of this algal biomass results
in oxygen depletion and the hypoxic / dead zones.
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Sources of nutrients to estuaries

• Agriculture Fertilizers
• Human waste sewage treatment plants, septic
  tanks, landfills
• Atmosphere Industrial emissions

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Annual Nitrogen Inputs to the Mississippi/
Atchafalaya River Basin
1980
1960
1940
1920
1900
2000
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Fertilizer use estimates for the United States,
1991 (USEPA 1999).
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Farm fertilizers and other nutrients flushed
into Mississippi River
Discharge of N into GOM stimulates algal blooms
Decay of algal blooms removes bottom oxygen
Simplified Model of Gulf of Mexico Dead Zone
Formation
Without oxygen, fish and Shrimp die or leave
the GOM
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Pacific Northwest Dead Zone
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Primary ProductivityJuly-August 2005 Upwelling
Season
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Jet Stream shifts to the North
Northerly coastal winds persist 20-40 days
instead of 2-5 days
Persistent winds trigger super-charged upwelli
ng
Simplified Model of Oregon Dead Zone Formation
Upwelling of Nutrient-rich bottom water
stimulates algal blooms
Decay of algal blooms removes bottom oxygen
Without oxygen, fish and crabs die or leave
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The End