E 

 o 



c 



CO 

 CD 

 CC 



CD 



co 



150 -i 



140 - 



130 - 



a 

 a> 

 a 



g 120 



8 110 



CD 

 CO 



c 



CO 



a> 100 



CO 



CO 



■o 



(!) 



CO 

 CO 



a> 

 i_ 

 a 

 x 

 w 



> 

 o 



c 

 cd 



a 



CO 



c 



CO 



90 



80 - 



60 - 



50 - 



(Darnell 1979) 



(Tarver and 

 Savoie 1976) 



(Stem and Stern 1 969) , . 



(Stone 1980b) 



1953 



— I 



1958 



1963 1968 

 Year 



1973 



1978 



Figure 45. Water transparency (expressed as mean Secchi disc read- 

 ings in cm) as a function of time for Lake Pontchartrain, Louisiana 

 (from Stone 1980a). 



101 



