Hypoxia in the Gulf of Mexico

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Hypoxia in the Gulf of Mexico

• 07/03/2002

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What is Eutrophication

• Eutrophication is a natural process taking place
  in water-characterized by a development towards
  an environment rich in nutrients and increased
  primary productivity.
• Human activities have greatly increased the rate
  of the process of eutrophication-excessive
  discharge of nutrients like phosphorus (P) and
  nitrogen(N).
• Eutrophication results in
• an increase in primary productivity ( in form of
  algal blooming )
• algal blooming may shade out plants in lower
  water and cause a loss of biodiversity
• development of hypoxia conditions

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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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Dead Zone of the Gulf

• Large zone of oxygen-depleted water extends
  across the Louisiana continental shelf and on to
  the Texas coast in most summers
• The Gulf of Mexico is the largest hypoxia zone
  in coastal waters of Western Hemisphere

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( Source Goolsby Battaglin, 2000 )
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Hypoxia of the Gulf

• Hypoxia occurs from Feb. through early Oct., but
  is most widespread, persistent, and severe in
  June, July and August.
• Hypoxic waters can include 20-80 of the low
  water profile between 5 and 30 meters water
  depth, and waters can extend as far as 130km
  offshore (Rabalais, 1999).

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Hypoxia Time-Series Change

• Gradual decline in oxygen in the spring with
  interruptions due to wind-mixing events
• Persistent hypoxia and often anoxia for extended
  parts of the record from May through September
• Occasional summer upwelling of oxygenated water
  from the deeper shelf waters
• Seasonal disruption of low oxygen in the fall by
  tropical storms or cold fronts (oxygen restored )
  (Rabalais, 1999).

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Stratification of the Gulf Water

• Warm and fresher water that forms a high
  production layers over the deeper salty, cold
  waters
• The greater the density difference between the
  layers, the more stable the stratification
• In order to mix the very strongly stratified
  systems, it requires a lot of wind energy, like
  tropical storm
• If storms do not mix the waters, they will remain
  in layers, isolating bottom waters from aeration
  until fall brings cooler surface temperature and
  the density of the surface water approaches that
  of the bottom water

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Stratification and Oxygen Depletion

• Layer separates bottom waters from the
  atmosphere and prevents re-supply of oxygen from
  the surface
• Oxygen deficits created by decomposing organic
  matter in the deeper waters remain
• The extent of hypoxia is determined by the
  balance between the rate of delivery and
  decomposition of algae and other organic matter
  and the rate of oxygen re-supply

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Mississippi River and Hypoxia

• High stream flow in Spring and Summer produces a
  large freshwater flux to the Gulf, which promotes
  stratification
• Nutrients to the Gulf produce algae in the
  surface water. Organic material from algae and
  other organisms settles into the bottom water,
  then decomposed by bacteria which consume oxygen

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Mississippi River Basin

• Largest river basin in North America
• Third largest basin in the world
• Including 70 million people, 30 states
• One of the most productive farming regions in the
  world
• 58 of the basin is cropland ( corn, soybeans,
  wheat)
• 18 woodland,
• 21barren land,
• 2.4 wetland, and
• 0.6 urban land
• (Goolsby and Battaglin, 2000 )

(Goolsby Battaglin , 2000)
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Key Nutrient to the Gulf- Nitrogen

• the most important nutrient leading to hypoxia
• increased significantly in concentration and
  loads in the Mississippi River
• Dissolved inorganic nitrogen (nitrate and
  ammonium) concentration and flux changed greatly
  and have a larger effect on algal production and
  hypoxia
• Dissolved organic nitrogen
• Particulate organic nitrogen
• ( Total nitrogen the sum of these three forms )

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Other Nutrients to the Gulf

• Phosphorus
• loads unchanged significantly since the earlier
  1970s
• Silica
• record shows decreased between 1950s and 1970s
  and have not changed significantly since then
• ( Goolsby and Battaglin, 2000 )

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Nitrogen from Mississippi

• Every year 1.57 million metric tons of nitrogen
  (nitrate and organic nitrogen) into the Gulf of
  Mexico from Mississippi
• The primary sources include
• Soil mineralization
• Fertilizer, the largest annual inputs
• Legumes and pasture
• Animal manure
• Atmospheric deposition
• Municipal and industry point pollution
• (Goolsby and Battaglin, 2000 )

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Major Nitrogen Inputs to The Mississippi-Atchafala
ya Basin
(Source Goolsby, 1998)
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N in Municipal and Industrial Discharge
Kg/Km2/yr(1996)
(Source Goolsby, 1998)
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N in Commercial fertilizer Kg/Km2/yr
(1992)
(Source Goolsby, 1998)
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Source from Goolsby Battaglin (1997)
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Nitrate Nitrogen in wet atmospheric
deposition into the River Basin
( Annual Average for 1990-1996)
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(Source Goolsby,1998)
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Nitrogen Variability

• Loads and freshwater discharge are usually
  highest during the late Winter, Spring, and early
  Summer when runoff is highest
• Precipitation leaches the highly soluble nitrate
  from the soil into streams via
• Ground-water discharge
• Agricultural drains
• Overland runoff
• Nitrate is subsequently transported into the
  Mississippi River and eventually discharges to
  the Gulf of Mexico ( Goolsby Battaglin, 2000)

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Source from Goolsby Battaglin (1997)
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Potential Factors Contributing to Hypoxia in the
Gulf

• Landscape changes in the drainage basin
• Channelization of the delta and loss of coastal
  wetlands
• Intrusion of deeper offshore waters
• Short-or-long-term climate changes

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Ecological Analysis of Hypoxia

• Loss of bottom and near-bottom habitat through
  the seasonal depletion of oxygen levels
• Alternation of energy flows
• During hypoxia, significant amounts of the
  systems energy are diverted from invertebrates
  to microbial decomposition ( Diaz Andrew Solow,
  1999)

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Effects on Fishery Resources

• Reduce food resources for fish and shrimp
• Reduced abundance of fish and shrimp
• Decline in shrimp catch and catch efficiency
  since hypoxia expanded
• Loss of production potential due to the blocked
  migration of juvenile shrimp offshore by the
  presence of hypoxic zone (Diaz Solow, 1999)

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Conclusion

• Hypoxia in the Gulf of Mexico can be reduced by a
  cut in the nutrient loading
• System management of the entire Mississippi
  watershed plays a key role in reduction the
  problem in the Gulf of Mexico
• Coastal ecosystems recovery may be slow, but
  improvement is achievable.

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References

• Nancy N. Rabalais.1999. Hypoxia in the Gulf of
  Mexico
• Robert J. Diaz Andrew Solow. 1999. Ecological
  and Economic Consequences of Hypoxia in the Gulf
  of Mexico (Topic 2 Report for the Integrated
  Assessment on Hypoxia in the Gulf of Mexico)
• Donald A. Goolsby William A. Battaglin. 1997.
  Sources and Transport of Nitrogen in the
  Mississippi River Basin. http//wwwrcolka.cr.usgs.
  gov/midconherb/st.louis.hypoxia.html