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Hypoxia in the Gulf of Mexico. 07/03/2002. 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.

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Hypoxia in the Gulf of Mexico

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what is eutrophication
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
what is hypoxia
What is Hypoxia
  • Waters that have a Dissolved Oxygen (DO) concentration less than 2mg/L (<3mg/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
dead zone of the gulf
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
hypoxia of the gulf
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).
hypoxia time series change
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).

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
stratification and oxygen depletion
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
mississippi river and hypoxia
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
mississippi river basin
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,
    • 21%barren land,
    • 2.4% wetland, and
    • 0.6% urban land

(Goolsby and Battaglin, 2000 )

(Goolsby & Battaglin , 2000)

key nutrient to the gulf nitrogen
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 )

other nutrients to the gulf
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 )

nitrogen from mississippi
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)


N in Commercial fertilizer: Kg/Km2/yr


(Source: Goolsby, 1998)


Nitrate Nitrogen in wet atmospheric deposition into the River Basin

( Annual Average for 1990-1996)

nitrogen variability
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)

Source from:

Goolsby & Battaglin (1997)

potential factors contributing to hypoxia in the gulf
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
ecological analysis of hypoxia
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 system’s energy are diverted from invertebrates to microbial decomposition ( Diaz & Andrew Solow, 1999)
effects on fishery resources
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)
  • 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….
  • 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