165 



conditions o The personal aptitude of the operator is in no 

 way involved. On the other hand, with the instrument used 

 during this survey, many peculiarities were encountered 

 which could not be explained and which led to light trans- 

 mission percentages that could not be rectified. There were 

 occasions when the instrument was in obviously clear sub- 

 surface waters and values higher than those of the 100% 

 calibration value were obtained. There were several stations 

 occupied in sewage fields when no transmitted light was 

 received, although ambient light was present at that depth „ 

 An instrument check showed no irregularities and subsequent 

 lowerings repeated the performance o On other occasions, when 

 in water with strata of sewage debris, the motion of the ship 

 was such that wide fluctuations allowed no reasonable average 

 reading to be obtained „ Thus, despite its theoretical pre- 

 cision, the photometer has inherent failings which may lead 

 to spurious results o These, coupled with the fact that the 

 meter determines the horizontal transparency of discrete 

 layers rather than the net vertical transparency of a water 

 column, a factor of considerable importance when one must 

 relate the transparency to the non-scientist, leads one to 

 suspect the immediate value of the hydrophotometer in- its 

 present form in a monitoring program such as that required in 

 Santa Monica Bay, 

 A Comparison of Hydrophotometer and Secchi Disc Readings 



In an attempt to relate simultaneous hydrophotometer and 

 Secchi disc readings, two graphs were drawn using the same 

 Secchi disc readings, but different averages of those obtained 



