1076 FRESH-WATER BIOLOGY 



catchment area is desirable not only for sanitary reasons but also 

 for lessening the growths of algae. 



Aeration. One of the elements of food supply required by algae 

 is carbonic acid, which is present to some extent in all surface 

 waters but is likely to be especially abundant in swampy and pol- 

 luted waters, and wherever organic matter is undergoing decay. 

 The stagnant water at the bottom of a reservoir, for example, usu- 

 ally contains large amounts of carbonic acid. The amounts of 

 carbonic acid may be considerably in excess of saturation, so that 

 when the water is exposed to the air the gas escapes. Thus the 

 process of aeration tends to reduce the likelihood of the occurrence 

 of heavy growths of algae. Aeration also tends to reduce the odors 

 of the water as the exposure of the water to the air gives opportunity 

 for the escape or volatilization of the essential oils. Sometimes 

 natural conditions of aeration exist and are very beneficial, when 

 water flows rapidly over the rocky bed of a stream. 



Chemical Treatment. With our present knowledge little can be 

 done in the way of treating the water chemically to prevent the 

 growth of algae. It is possible that the application of lime to reduce 

 the free carbonic acid in the water would be of some benefit but 

 this has never been practically used. Chemical treatment has been 

 successful in destroying organisms as referred to below. 



Exclusion of Light. The exclusion of light from a reservoir is 

 an effective remedy in preventing the growth of algae. This can- 

 not be done in large reservoirs but in small reservoirs and in stand- 

 pipes it has proved very successful. In cases where ground water 

 that contained large amounts of plant food has been exposed in 

 open reservoirs algae growths have been very troublesome, and it 

 has been found that covering the reservoir or standpipe in which 

 the water is stored completely prevents the trouble. It has 

 become an axiom, therefore, among waterworks men that ground 

 waters should not be stored in the light. 



Methods of Killing Algae. Various methods have been suggested 

 for killing algae in reservoirs, such as copper sulphate, bleaching 

 powder, ozone, and creosote. Of these copper sulphate has proved 

 to be by far the most effective. Quantities as small as one part in 

 one million by weight, and sometimes even smaller quantities, have 



