PLANKTON IN WATER SUPPLY MEIER 403 



waters of Lake Ontario. One of the first steps in the purification of 

 "water is its preliminary storage in reservoirs for at least a month 

 before filtration. This gives suspended particles of all kinds time to 

 settle. In addition, the water mixes so that possible variations in the 

 supply are equalized. The numbers of intestinal bacteria decrease 

 markedly. 



The plankton passing from running water into still water tends to 

 increase, and differences in the proportion of the different kinds 

 result. Free-floating diatoms and algae might increase in the reser- 

 voir until it would teem with them but for the animals, especially the 

 Crustacea, which abound in reservoirs and feed upon the plant life. 

 On the sides of a reservoir may be found snails and creeping Crus- 

 tacea, sessile diatoms, and larger attached plants. Fixed animals 

 such as sponges flourish where there is solid support, and worms and 

 duck mussels, as well as diatoms and bacteria, thrive in the accumula- 

 tion of mud and detritus on the bottom. 



All these living things eventually die and decay, to the advantage 

 of the bacteria, but their increase is useful to the water supply. The 

 plants help to aerate the water. Animals use up the oxygen for their 

 respiration. They aid only in the measure that they prevent the 

 algal growth from becoming too excessive. The kind of plankton 

 in the reservoir also serves as an indicator to the engineer of the 

 contamination of the source of the supply, as we have already seen 

 in the case of the presence of Bacillus coli. 



The plants and animals are not evenly distributed in the reservoirs 

 but collect at particular levels. Algae usually abound where the 

 light is most favorable; animals are in the places where there is 

 plenty of food or where the chemical conditions (aeration) are most 

 suitable. This is a useful fact to know, for by constructing the 

 reservoir with outlet valves at different depths it is possible to draw 

 water from the level that, as shown by the examination of the samples, 

 is most suitable for filtration. Bottom water where the sedimented 

 matter accumulates is not desirable, but generally the water should 

 be drawn at a depth of 30 feet in a 40-foot reservoir. This takes the 

 water at a level below that in which the algae are found. Since the 

 levels poor in algae abound in their unused food material, their with- 

 drawal aids in preventing abundant growth of algae. 



These algal foodstuffs, chiefly phosphates and silicates, important 

 as they are to the algae, do not affect the flavor of the water, since 

 the quantities needed are negligible from the human standpoint. A 

 water fairly rich in phosphates from the point of view of the algae 

 may contain only 0.02 parts per 100,000. Such small quantities of 

 phosphates are essential to human nutrition also. The silicates are 

 harmless. 



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