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What Is the Coastal Dead Zone?


Water pollution often causes fish death.

Water pollution often causes fish death.

Doug Menuez/Getty Images
Question: What Is the Coastal Dead Zone?
One of the environmental issues that may be related to pollution and global warming is the 'dead zone'. This is a region of water that lacks the oxygen necessary to support animal life. Have you heard of the dead zone before?
Answer: In 2007 the oxygen-depleted 'dead zone' off the coast of Louisiana was the largest it had been in 22 years (when the mapping started), with an estimated area of 8,543 square miles. The dead zone forms when oxygen becomes depleted in the coastal waters. To some extent, oxygen depletion occurs naturally off the coast of all of the major basin systems as water warms and nutrients become washed out to sea from the seasonal turnover of lakes and rivers. Fertilizers and pollution greatly add to the natural nutrients, causing algae to bloom. The algae dies and oxygen is consumed by microorganisms as part of the decay process. Shrimp, fish, and other animals move out of the oxygen-depleted region or die. However, it's difficult to determine if fish death is due to lack of oxygen or temperature or some other factor associated with the phenomenon. The dead zone starts forming in the spring and recedes in autumn. The area of the dead zone sometimes shrinks from one year to the next, though it is considerably bigger now than when the mapping began.

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