measured with greatly improved accuracy and frequency. The ability to con- 

 trol environmental conditions and to understand ecological interactions, 

 however, remains a goal of the future. 



Chemical changes —The increase in major ions continued in all the lakes 

 except Superior during the last 25 years. The rates of increase in all the 

 ions except calcium remain high for Lakes Ontario and Erie (Table 9). 

 Population increases also were substantial in the basins of Lakes Ontario 

 and Erie (Table 3) and probably account for the chemical changes. 



TABLE 9. ESTIMATED AVERAGE CONCENTRATIONS OF DISSOLVED CHEMICAL 

 CONSTITUENTS IN THE GREAT LAKES IN 1970 (EXPRESSED IN 

 MG/LITER)a WITH PERCENTAGE CHANGE SINCE 1950 IN PAREN- 

 THESES 



a From Weiler and Chawla, 1969. 



Critically low dissolved oxygen (DO) concentrations had not been re- 

 ported in the open waters of the Great Lakes until 1953. In that year 

 Britt (1955) measured DO concentrations less than 1 mg/liter in the western 

 basin of Lake Erie. Although the low DO levels lasted only a few days, it 

 caused a substantial mortality in the burrowing mayfly ( Hexagenia ) popula- 

 tion. In some areas of the western basin the entire population was killed, 

 where more than 1,000 mayfly nymphs per square meter had previously been 

 found (Britt, 1955). The first extensive zone of low DO (less than 1 ppm) 

 was measured in 1959, in the western portion of the central basin of Lake 

 Erie. An interagency synoptic survey of this basin in 1959 found an area 

 of approximately 1,400 square miles which contained less than 1 ppm of DO 

 in the hypolimnion. These conditions of low DO undoubtedly had occurred be- 

 fore 1959 (Carr, 1962). 



Dissolved oxygen levels of less than 1 mg/liter occur annually in the 

 bottom waters of the central basin of Lake Erie and by 1974 covered several 

 thousand square miles. Oxygen depletion also has been reported in southern 



214 



