What’s a “dead zone”?

Q: What are the “dead zones” in the Gulf of Mexico and Lake Erie, and what causes them?

The Gulf of Mexico dead zone (red area) during the summer. Photo: NASA

A: When a body of water, such as a lake, stream, or the Gulf of Mexico, receives nitrogen and/or phosphorus nutrients, these nutrients feed the growth of microscopic “plants” in the water called algae. The nutrients may originate from farm fertilizers, manures, or in some cases are present naturally.

As photosynthetic organisms, algae use sunlight, carbon dioxide from the air, and mineral nutrients from the water to make new algal tissue, and this growth can be so great that the water turns green. Then, when the algae die, they sink to the bottom of the water column where microorganisms (mainly bacteria) decompose them. The decomposition process consumes oxygen and when there is an overabundance of decomposing material, virtually all the oxygen in the water can be removed. When this happens, larger organisms that require oxygen to live, like fish, begin to die in large numbers, resulting in what people call a “dead zone.”

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