LAKE ERIE OFFSHORE IN 1990 - RESTORATION AND RESILIENCE IN THE CENTRAL BASIN

The Great Lakes Water Quality Agreement (GLWQA) of 1972 resulted in large expenditures to build and operate treatment plants to remove phosphorus from sewage. Technology is available to remove even more phosphorus than is customary now. Thus, it is important to the eutrophication issue to document and understand the effects of management in order to diagnose the efficacy and appropriateness of present and future controls. The ground-breaking Canada-U.S. ''Project Hypo'' study of 1970 resulted in phosphorus reduction targets for Lake Erie in the GLWQA. Research cruises in the offshore of the central basin during 1979, 83, 84, and 85 were complemented by a series of cruises in 1990. Chemical and biological water quality variables were measured in the relatively stable months of July and August. Chlorophyll and phosphorus concentrations and secchi disk readings suggest that exemplary surface water quality was already apparent in 1979. Based on scaling arguments, relatively small chemical changes are expected in the off-shore and these are consistent with observations. Sediment phosphorus regeneration and dissolved oxygen depletion were similar in 1970 and 1990 despite 20 years of nutrient reductions. The delayed improvement in the hypolimnion may be consistent with delayed or avoided damage due to mechanisms of system resilience. The GLWQA may have prevented a sediment response that would take many decades to reverse.