PLANKTON IN WATER SUPPLY MEIER 397 



mains in gelatinous masses, which seriously reduce the caliber of the 

 mains as well as causing objectionable tastes, odors, and precipita- 

 tion of iron in the water. Its presence can ordinarily be assured 

 when well water containing iron and little or no dissolved oxygen 

 is pumped into the distribution mains. 



ANIMAL FORMS 



Microscopic animal forms flourish among the algae, since the two 

 kinds of life are interdependent in the exchange of gases and other 

 sources of their food supply. The animals consist chiefly of the one- 

 celled Protozoa (pis. 8, 9), which occasionally cause trouble in the 

 water supply. There are also Kotifera, which are common but not 

 troublesome, Crustacea (pi. 7, fig. 3), which include the waterfleas, 

 the Porifera or fresh-water sponges, the Bryozoa, and miscellaneous 

 organisms such as worms and insect larvae. 



FACTORS INFLUENCING PLANKTON GROWTH 



Since algae are plants containing chlorophyll or green coloring 

 matter, they are able to manufacture their own food. The most 

 essential factor of their environment for carrying on the process of 

 photosynthesis or food production is light. Turbid waters allow less 

 light to penetrate, thereby decreasing the amount of algal growth. 

 Ground waters do not permit algal growth unless exposed to light. 

 For this reason, covered reservoirs are effective in algal control. 



Temperature is another important factor in the environment of 

 algae; it may either accelerate or retard their amount of growth. 

 Some algae thrive beneath the ice, while other varieties prefer warm 

 temperatures of 50° to 70° F. Although different kinds of algae 

 grow at different seasons of the year and in different places, there 

 is generally a spring maximum of diatoms, an early summer maxi- 

 mum of green algae, a late summer and early fall maximum of blue- 

 green algae, and sometimes a second wave of diatoms in the fall. 



Recurrences of plankton are not entirely due to temperature 

 conditions. The physical conditions of the stream feeding the water 

 supply may be responsible. The fluctuating levels of a stream, caus- 

 ing changing velocity and turbidity of the water, affect the numbers 

 of the plankton. The velocity of the current is important, as rich 

 growths of plankton are often completely destroyed by passing 

 through a series of rapids or sharp falls. 



Kofoid, after a 5-year study of the Illinois River, decided that 

 the age of the water is an important factor in plankton production 

 in streams. There is very little plankton in young streams. Barren 

 waters impounded for 10 to 30 days develop a large plankton crop, 

 which is affected by the rate of run-off and displacement, the plank- 



