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29 



since they are often found in greatest number in open 

 waters some five to fifteen meters below tlie surface. 

 Some algae are found at all depths, even in total dark- 

 ness on the bottom; notably diatoms, whose heavy 

 silicious shells cause them to sink in times of prolonged 

 calm, but these are probably inactive or dying individ- 

 uals. There are some animals, however, normally 

 dwelling in the depths of the water, living there upon 



UoiMeiers DepiA 

 Fig. 3. Diagram illustrating the penetration of light into the water of a lake; 

 also, its occlusion by inflowing silt and by growths of plants on the surface. 



the organic products produced in the zone of photo- 

 synthesis above and bestowed upon them in a consider- 

 able measure by gravity. To the consideration of 

 these we will return in a later chapter. 



The accompanying diagram graphically illustrates 

 the light relations in a lake. The deeper it is the greater 

 its mass of unlighted and, therefore, unproductive 

 water, and the larger it be, the less likely is its upper 

 stratum to be invaded by obscuring silt and water 

 weeds. 



