Hypoxia in the Gulf of Mexico
07/03/2002
What is Eutrophication

Eutrophication is a natural process taking place in watercharacterized 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

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
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

( Source: Goolsby & Battaglin, 2000 )
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

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
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
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

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)
(Goolsby & Battaglin , 2000)
18% woodland,
21%barren land,
2.4% wetland, and
0.6% urban land
(Goolsby and Battaglin, 2000 )
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
 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

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 )
Major Nitrogen Inputs to The Mississippi-Atchafalaya Basin
(Source: Goolsby, 1998)
N in Municipal and Industrial Discharge : Kg/Km2/yr(1996)
(Source: Goolsby, 1998)
N in Commercial fertilizer: Kg/Km2/yr (1992)
(Source: Goolsby, 1998)
Source from: Goolsby & Battaglin (1997)
Nitrate Nitrogen in wet atmospheric deposition into the River Basin
( Annual Average for 1990-1996)
(Source: Goolsby,1998)
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
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
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
 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)
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….
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