Hypoxia in the Gulf of Mexico 07/03/2002. Eutrophication is a natural process taking place in water- characterized by a development towards an environment.

Hypoxia in the Gulf of MexicoHypoxia in the Gulf of Mexico

07/03/2002

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 EutrophicationWhat is Eutrophication

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

What is Hypoxia What is Hypoxia

Dead Zone of the GulfDead 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 GulfHypoxia 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).

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 of the Gulf Water Stratification of the Gulf 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 BasinMississippi 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- NitrogenKey Nutrient to the Gulf- Nitrogen

the most important nutrient leading to hypoxia increased significantly in concentration and loads

in the Mississippi RiverDissolved inorganic nitrogen (nitrate and ammonium): concentration and flux changed greatly and have a larger effect on algal production and hypoxiaDissolved organic nitrogenParticulate organic nitrogen( Total nitrogen: the sum of these three forms )

Other Nutrients to the Gulf Other Nutrients to the Gulf

Phosphorusloads unchanged significantly since the earlier 1970s

Silicarecord 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 )

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 VariabilityNitrogen 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 HypoxiaEcological 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 ResourcesEffects 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

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….

ReferencesReferences 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