| Introduction
 In
June, 2002, the U.S. Environmental Protection Agency in cooperation
with many universities and other agencies in the United States and
Canada, began an intensive special study to determine the cause of
the oxygen depletion in Central Lake Erie. This effort has received
Congressional interest and lots of
media coverage. In this photo, Dr. Glenn Warren (right) of
EPA's Great Lakes National Program Office explains water sampling
procedures to Sen. George Voinovich (center) of Ohio aboard EPA's
Research Vessel Lake Guardian.
Video: Lake Erie
Oxygen Depletion
USEPA Environmental Scientist, Paul Bertram, Ph.D. explains
oxygen depletion in Lake Erie and the actions that USEPA is
taking understand the problem.
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Animation: How Oxygen
Depletion Develops
This animation gives a simplified explanation of how oxygen depletion
develops.
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The bottom waters in the Central Basin of Lake Erie become anoxic (without oxygen) in the late summer. Aquatic creatures need oxygen in the water to live. Sometimes, however, conditions
exist in which the dissolved oxygen in the water is used up by organisms
faster than it can be replaced from the air. If all the oxygen gets used up,
the organisms will suffocate. The configuration of the Central Basin of
Lake Erie is partly responsible for the problem, but too many nutrients (especially phosphorus) from human activities is also a major factor. 
The amount of oxygen in the bottom waters of Lake Erie
becomes smaller over the summer. In June, oxygen concentrations are
relatively high, but by late August or mid-September, many
locations are nearly without oxygen. The severity of oxygen depletion
varies from year-to-year, depending on water temperature and the
thickness of the bottom layer.
The rate at which oxygen is used up over the summer lets us know if
conditions are getting better. Progressively lower rates over time will
eventually lead to oxygen remaining in the water all summer. These rates
are calculated with adjustments for annual differences in water
temperature and bottom layer thickness. Although the long-term trend is
downward, there hasn’t been any improvement since the late 1980s (