to compensate for the natural attenuation of water molecules, 

 and the reading in distilled water was established as 100- 

 percent transmission. Measured light loss in the marine 

 environment is due to a combination of absorption and 

 scatter. The instrument did not discriminate between the two. 



TEMPERATURE SENSOR 



Temperature was determined prior to each sampling 

 run by a bathythermograph (BT) lowering from surface to 

 bottom and by a thermistor bead sensor on the cart (fig. 2), 

 with galvanometer readout in the instrument hut. The BT 

 data are considered more accurate than the thermistor 

 measurements, and have been used in this report. 



UNDERWATER PUMP 



Water samples were obtained by a submersible 

 pump. The intake was fitted with plates 1 cm apart (fig. 2), 

 which limited the depth of the stratum sampled to about 10 

 cm, as demonstrated by experiments with dye. Water was 

 delivered to the platform of the tower through 3/4-inch plastic 

 hose. The pump and hydrophotometer were mounted to- 

 gether on the cart. A meter wheel assured sampling from 

 known depths. Two types of samples were taken to insure 

 adequate sampling of variously sized entities of different 

 degrees of motility and population density. 



