flow through a plankton net. At the same time the water 

 flow was recorded in liter/min. However, it was evident 

 that a large number of particles went through the bolting 

 silk. The finest netting available was No. 2 5 with an 

 aperture of 65p., and the average diameter of Gonyaulax 

 polyedra and Peridinium are 50|_i. Moreover, there was 

 no assurance that the plankton count in the pump water 

 would remain constant, since the plankton-rich layer was 

 known to move up and down due to the surface wave action. 

 A count accomplished by this means would therefore not 

 necessarily be representative of the water in the cavity. 



Effect of Oxygen Content 



Examination of figures 16, 21, and 2 6 shows that the 

 high attenuations are intimately connected with a high 

 content of oxygen. The highest concentration of phytoplank- 

 ton was usually found in a well defined layer which varied 

 between the surface and a depth of 2 feet. The oxygen 

 produced by photosynthesis of the phytoplankton can be at 

 saturation under a total pressure well above the atmospheric 

 pressure. It is therefore understandable that the water 

 samples can be supersaturated with oxygen when placed in 

 the cavity at atmospheric pressure. No determination of 

 the nitrogen content was attempted since there is no simple 

 chemical procedure available. A gasometric method must 

 be used and the equipment required was not available, nor 

 could it have been utilized due to the limited space available 

 on the tower. 



Dissolved gas has very little effect on the sound propa- 

 gation in water, as already noted. 33 ' 37 Gas in the form of 

 bubbles does, however, have a marked influence on the 

 sound propagation. Accordingly the high oxygen content 

 indicates that bubbles clearly must be considered in the 

 evaluation of the data. 



Free bubbles have a short life in a small sample of a 

 liquid which contains no particulate matter. 36 ' 38 Never- 



89 



