166 



from the meter = In Figwre 79,, the Secchi disc depths are 

 plotted against the per cent of light transmission at a depth 

 of five feeto Assuming the Secchi disc readings to be accurate 

 with the limitations imposed 5 and the photometer readings 

 accurate s. within their limits 9 then it is apparent that a low 

 light transmission at 5 feet is effective in limiting the 

 depth to which the Secchi disc can be seen. 



In Figure 80, the average of the hydrophotometer readings 

 at five-foot intervals to the depth of the corresponding 

 Secchi disc is plotted against the transparency as determined 

 by the Secchi disc. For examples if the Secchi disc was 

 visible to a depth of 15 feetp the photometer readings at 5, 

 10, and 15 feet were averaged amd • plotted against the trans- 

 parency of 15 feeto Of course o oae m^iist recognize that since 

 the transparency of the water usually improves with depthj 

 then if completely erroneotts Secchi disc readings were taken 

 and plotted in this method;, a similar curTe would result » On 

 the other hand, one aeeds to assume the Secchi disc observer 

 was competent and the readings as noted are as nearly accurate 

 as possible. Also., one must assume that the measurements 

 made with the meter are true 9 for just as there is a spread 

 in Secchi disc readings » there is one for the hydrophotometer. 

 With these assumptions in mind„ a similar conformity to each 

 mode of measurement as is shown in Figure 79 exists with this 

 plot. 



A relationship does exists therefore « between the trans° 

 parency as determined by the meter and that by the Secchi disc. 

 In some instances the relationship is not close and without 



