1074 



FRESH-WATER BIOLOGY 



of the blue-green algae have odors suggestive of the pig pen, doubt- 

 less because of their high nitrogen content. 



Prevention of Growths of Algae. Various means have been used 

 to prevent the growth of algae in reservoirs and standpipes. Some 

 of these, such as the exclusion of sunlight, are snccessful but most 

 of them afford only a partial remedy. 



Soil Stripping of Reservoir Sites. The removal of the vegetation 

 and top-soil from the ground that forms the floor of a reservoir 

 tends to reduce the amount of the organic and mineral matter 

 available for the food supply of the organisms and thus tends to 

 diminish their number. In a number of instances, notably the 

 reservoirs that supply the city of Boston, the soil has been carefully 

 removed from the reservoir sites before the reservoirs were filled. 

 This has tended to reduce the growths of algae during the first few 



