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Hypoxia
Hypoxia means "low oxygen." In estuaries, lakes, and coastal
waters low oxygen usually means a concentration of less than 2 parts per
million. In many cases hypoxic waters do not have enough oxygen to support
fish and other aquatic animals. Hypoxia can be caused by the presence
of excess nutrients in water. Excess nutrients
can cause intensive growth of algae. The consequences of this enhanced
growth are reduced sunlight penetrating the water, a decreased amount
of oxygen dissolved in the water, and a loss of habitat for aquatic animals
and plants. The decrease in dissolved oxygen is caused by the degradation
of dead plant material (algae), which consumes available oxygen. The overall
effect is called eutrophication. Nutrients can come from many sources, such as fertilizers
applied to agricultural fields, golf courses, and suburban lawns; deposition
of nitrogen from the atmosphere; erosion of soil containing nutrients;
and sewage treatment plant discharges.
What is the Gulf of Mexico Hypoxic Zone?
The Gulf of Mexico hypoxic zone is an area along the Louisiana-Texas
coast where water near the sea floor has hypoxic conditions. Hypoxia can
cause fish to leave the area and can cause stress or death to bottom dwelling
organisms that can't move out of the hypoxic zone. The hypoxic zone is
thought to be caused primarily by excess nitrogen delivered from the Mississippi
River in combination with seasonal stratification (layering) of Gulf waters.
Nitrogen promotes algal and attendant zooplankton growth. The associated
organic matter, such as dead algal cells and other debris from the algae,
sinks to the bottom where it decomposes, consuming available oxygen. Stratification
of fresh water from the Mississippi River and saline water prevents the
mixing of oxygen-rich water on the surface with oxygen-depleted water
on the bottom. This lack of mixing limits the replenishment by oxygen
and sustains the hypoxic zone.
Information on Hypoxia
Information on Hypoxia in the Gulf of Mexico
- Hypoxia in the Gulf of
Mexico, U.S. Geological Survey
- Mississippi River/Gulf
of Mexico Watershed Nutrient Task Force
- Science
Assessment of the Causes and Consequences of Gulf of Mexico Hypoxia,
National Ocean Service, National Oceanic and Atmospheric Administration
- Hypoxia Watch System
for the Gulf of Mexico, National Oceanic and Atmospheric Administration
- Gulf of Mexico
Ecosystems and Hypoxia Assessment, Center for Sponsored Coastal
Ocean Research, National Oceanic and Atmospheric Administration
- Northern Gulf of Mexico Ecosystem Description, Hypoxia
in the Gulf of Mexico (by Nancy N. Rabalais, Louisiana Universities
Marine Consortium), National Oceanic and Atmospheric Administration
- Hypoxia
In The Gulf Of Mexico and Nutrients in the Midwest, Illinois State
Water Survey
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