All about “Dead Zones”All about “Dead Zones”

Zones of Oxygen DepletionZones of Oxygen Depletion

What is Hypoxia?What is Hypoxia?

Waters that have a Dissolved Oxygen (DO) Waters that have a Dissolved Oxygen (DO) concentration less than 2mg/L (<3mg/L, some concentration less than 2mg/L (<3mg/L, some systems) are defined as Hypoxia systems) are defined as Hypoxia

If DO= 0 mg/L, it is called Anoxia If DO= 0 mg/L, it is called Anoxia Oxygen Depletion caused by Oxygen Depletion caused by

– Excessive nutrients, intense biological productivityExcessive nutrients, intense biological productivity that depletes oxygen that depletes oxygen

– Decomposition of organic matter consumes oxygen Decomposition of organic matter consumes oxygen – Stratification prevents oxygen refreshmentStratification prevents oxygen refreshment

Gulf of Mexico – Dead ZoneGulf of Mexico – Dead Zone

Dead Zone > 7,000 sq. miles in the Dead Zone > 7,000 sq. miles in the Gulf of Mexico with no marine life in 1999.Gulf of Mexico with no marine life in 1999.

The Gulf of Mexico is the largest fishery in the USThe Gulf of Mexico is the largest fishery in the US

Hypoxia: Low dissolved oxygen (< 3 mg L-1)

“And the fish are the lucky ones…”

Number of fish kill events and number of fishkilled in Gulf of Number of fish kill events and number of fishkilled in Gulf of Mexico coastal waters, 1980 - 1989 Mexico coastal waters, 1980 - 1989

(NOAA/ORCA 1991; USEPA 1999).(NOAA/ORCA 1991; USEPA 1999).

1980 1981 1982 1983 1984 1985 1986 1987 1988 19890

20

40

60

80

100

120

0

20

40

60

80

100

# Fish killed (x 1 million)

Events

Mil

lion

s of

fis

h ki

lled

Mil

lion

s of

fis

h ki

lled

Num

ber of fish kill eventsN

umber of fish kill events

Catch per unit effort in the Gulf of Mexico, 1970 - 2000Catch per unit effort in the Gulf of Mexico, 1970 - 2000

Gulf of Mexico – Dead ZoneGulf of Mexico – Dead Zone

The Size of the Dead Zone Correlates with Mississippi River Discharge Values

Nitrogen inputs to the Gulf since 1910Nitrogen inputs to the Gulf since 1910

20001980196019401920

Nitrogen loading in the Gulf triggers algal blooms, Nitrogen loading in the Gulf triggers algal blooms, which decay and sink to the bottom.which decay and sink to the bottom.

The degradation of this algal biomass results in The degradation of this algal biomass results in oxygen depletion and the hypoxic / dead zones.oxygen depletion and the hypoxic / dead zones.

Sources of nutrients to estuariesSources of nutrients to estuaries Agriculture: FertilizersAgriculture: Fertilizers

Human waste: sewage Human waste: sewage treatment plants, septic treatment plants, septic tanks, landfillstanks, landfills

Atmosphere: Industrial Atmosphere: Industrial emissionsemissions

Annual Nitrogen Inputs to the Mississippi/ Atchafalaya River BasinAnnual Nitrogen Inputs to the Mississippi/ Atchafalaya River Basin

200019801960194019201900

Fertilizer use estimates for the United States, 1991Fertilizer use estimates for the United States, 1991 (USEPA 1999).

Farm fertilizers and othernutrients flushed

into Mississippi River

Discharge of N into GOM Discharge of N into GOM stimulates algal bloomsstimulates algal blooms

Decay of algal blooms removes bottom oxygen

Without oxygen, fish and Shrimp die or leave

the GOM

Simplified Model of Gulf of Mexico Dead Zone Formation

Pacific Northwest Dead ZonePacific Northwest Dead Zone

Primary ProductivityPrimary ProductivityJuly-August 2005 Upwelling SeasonJuly-August 2005 Upwelling Season

Jet Stream shifts to the North

Upwelling of Nutrient-rich Upwelling of Nutrient-rich bottom water bottom water

stimulates algal bloomsstimulates algal blooms

Decay of algal blooms removes bottom oxygen

Without oxygen, fish and crabs die or leave

Simplified Model of Oregon Dead Zone Formation

Northerly coastal winds Northerly coastal winds persist 20-40 days insteadpersist 20-40 days instead

of 2-5 daysof 2-5 days

Persistent winds Persistent winds trigger ‘super-charged’trigger ‘super-charged’

upwellingupwelling

The EndThe End