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TEXTBOOK OF BOTANY 



individual periodicity determines what species will be found associated 

 at any time of the year. Ponds that regularly dry up in late summer ap- 

 pear to have a greater variety of species than permanent ponds. 



The situation in large lakes is somewhat unlike that in ponds because 

 great bodies of water neither warm up quickly in the spring nor cool 

 rapidly in the autumn. In Lake Erie, for example, the diatoms are likely 

 to be abundant in May and June, the green algae in July and early 

 August, the blue-greens in late August and September, and the diatoms 

 again in October and November (Fig. 277). A few species of diatoms 

 are more abundant in winter than at anv other season. 



MAY JUNE JULY AUGUST SEPTEMBER 



Fig. 277. Seasonal abundance of some plankton algae in Lake Erie. 



Water blooms are just as characteristic of large lakes as of ponds. 

 At certain seasons of the year diatoms, blue-greens, or greens may 

 within a few days become so abundant as literally to cover acres of 

 water to the depth of an inch or two. This colored soup-like layer may 

 be objectionable to bathers, and it may be the source of offensive odors; 

 but it contains an abundance of food available to the smallest aquatic 

 animals. The life cycles of the algae composing the bloom are soon 

 completed, and after several days the plants may disappear as sud- 

 denly as they appeared. Most plankton algae are found within a few 

 inches of the surface of the water, although one may usually find them 

 in collections made at depths of 10 to 50 feet. 



