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aquatic ecosystems of the Great Lakes (Griffiths 1993a). Water clarity in some areas of Western Lake Erie increased as much as 85% during May through November, 1988 and 1989, and Chlorophyll A declined 54% during this same period (Leach 1993). Changes in the benthic fauna of the Great Lakes have occurred, including the loss of native mussels (Schloesser and Kovalak 1991; Griffiths 1993a; Stolzenburg 1992), and standing crops of some fishes, including walleye have declined (Griffiths 1993b). If left unchecked, the zebra mussel could become common throughout the temperate zone of North America within the next 5 to 10 years. This single species poses a multibillion dollar threat to industry, governments, and individuals due to its impact on industrial and public water supplies of the Nation, as well as shipping, boating, fishing, and other recreational activities. Its affect on natural systems pH requires _> 4 Months At 13°C Mid Temperature with Monthly Maximum = 20°-25° C requires >_ 3 Months At ?13°C Warm Zone with Monthly Maximum = 25° C EutropFiy Total Phosphorus <0.25 mg/L or Total Nitrogen <4.0 mg/L TABLE 7. L/MNOLOG/CAL PARAMETERS REOU/RED FOR ZEBRA MUSSELS /N NORTH AMER/CA AFTER MCNABB 7993) is more difficult to quantify, but the damage and cost associated with endangerment and loss of species, and the disruption of natural systems could be even greater. Although some waters may not be suitable for zebra mussel proliferation, conditions must be extreme to exclude them (highly eutrophic or oligotrophic, extremely warm, etc.; Table 1). The rapid movement of the zebra mussel from the Great Lakes into connecting waterways and the Mississippi River has left little time to set up programs to evaluate the risks that it poses to natural systems. Although much general information has been obtained about the impacts of zebra mussel on fresh water environ- ments, the potential effects on sensitive western ecosystems are unknown.