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* This exhibit is no longer on
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A Legacy of
Issues
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Dissolved Oxygen
For essentially all animals, the level of dissolved oxygen critical for survival. Immense as the Great Lakes are, conditions can develop in certain places that allow depletion of oxygen to the extent that fish and most other animal life cannot survive. Lake Erie is the most impacted. Phosphorus enrichment of the lakes as the result of fertilizer runoff and waste treatment plant discharge stimulate the growth of algae, especially in Lake Erie, Green Bay, Saginaw Bay and other bays and harbors of the Great Lakes. In the summer, the Lakes stratify and dying algae drifts to the bottom to decompose. The decomposition process depletes oxygen |
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in the cool bottom water that,
because of its high density, doesn’t mix with warmer surface
water during the summer. In 1978, the U.S. and Canada agreed to
institute controls on phosphorus discharge. In order to track the
occurrence and rate of oxygen depletion in Lake Erie, EPA
scientists sample the bottom layer of the Lake every 3 weeks
through the summer.
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To Ballast or Not to Ballast Since the St. Lawrence Seaway opened the Great |
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Lakes to deep draft ocean vessels in 1959, thousands of ships have brought and sent billions of tons of cargo between Great Lakes ports and those in Europe, Africa, the Middle East and elsewhere. Sometimes, however, a ship returns from a foreign port lightly laden. Because cargo ships are designed for maximum stability while actually carrying cargo, they must take on ballast water to keep from bobbing about like a cork on the high seas. When the foreign port is in fresh water, such as the Caspian Sea, or brackish water, such as the Baltic, that ballast water may include species of plants and animals alien to North American waters. This foreign ballast water has been the source of the majority of invasive exotic species from foreign ports in the Great Lakes system. Believed among these are the zooplankton species, Bythotrephes and Cercopagis, several fish including river ruffe and round goby. Recent legislation requires ships entering the Lakes to purge, exchange and manage their ballast water. Nevertheless, ballast water continues to present the greatest threat of new invasions to the Great Lakes. |
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Beaches and Human Health With a few notable exceptions, Great Lakes beaches (and water contact in general) present less disease risk than those found on most lakes or the marine coasts of North America. At the end of the decade, just twenty-three beaches accounted for all the significant closures of U.S. Great Lakes beaches - on a coastline of 5542 miles. These were primarily, but not entirely, near metropolitan areas, and are typically caused by storm weather releases from combines sewer overflows. |
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Soluble Reactive Phosphorus Concentrations in the Great Lakes Legend Green: 0.00002-0.0009 ppm Year of Data Collection Lake Superior 1981, Lake Huron 1994, Lake Erie 1995, Lake Ontario 1993, Lake Michigan 1994 |
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Atmospheric Deposition of Contaminants to the Great Lakes Many Contaminants in the air can be deposited into the waters of the Great Lakes through the process of atmospheric deposition. These include organochlorine pesticides, PCBs, and mercury. Some of these contaminants are are in the gas phase and either exchange directly between the air and water or are washed out of the air column into the Lakes in precipitation. Others are adsorbed to dust |
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particles and enter the Lakes as the dust settles or is also captured in precipitation. Some of the contaminants also escape the Lakes into the atmosphere, often to reenter again shortly after. The USEPA and Environment Canada cooperatively operate the International Atmospheric Deposition Network. Using a network of master and satellite stations, the two countries are working to gain a better grasp of the extent and relative loadings of atmospheric toxic contaminants upon the Lakes. |
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Acknowledgments: The exhibit
was written and complied by Robert Beltran -USEPA Great Lakes
National Program Office.
The exhibit was organized and maintained by Barbara Kern - John
Crerar Library, University of Chicago.
For more information about exhibits
at the John Crerar Library,
please contact Barbara Kern at 773-702-8717 or
bkern@midway.uchicago.edu.
| B.Kern, Crerar, 2002 |
Photographs of the exhibit
courtesy: B. Kern
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