340 ANIMALS IN INLAND WATERS 



marked variations in the development of nourishing plant plankton, 

 occur. On the other hand, they disappear entirely in constantly warm 

 tropical lakes as well as in the polar or alpine lakes whose tempera- 

 ture never rises above 12°, which appears to be a critical temperature 

 for changes of form in these organisms, 37 



The character of the plankton is further changed by the cyclic 

 appearance of males and sexual females among the Cladocera, and 

 the development of winter eggs in ephippia, whereby the course of 

 parthenogenetic reproduction is interrupted. At such times the surface 

 of a lake may be covered with innumerable ephippia. The various 

 species differ in the appearance of bisexuality; and even the same 

 species behave differently in different lakes. In the lakes of middle 

 Europe dicyclic and polycyclic species are found, i.e., those with one 

 or more appearances of males during the course of the year; farther 

 north, and in the high mountains, on the other hand, a monocyclic 

 condition becomes more and more prevalent. The cladoceran Chydorus 

 sphaericus, for instance, generally has two sexual periods; but in the 

 most favorable waters of central Europe it has become acyclic and 

 consequently perennial. 38 



Bacteria.— Bacteria are the smallest of the nannoplankton or- 

 ganisms, so small in fact that they are not collected by ordinary 

 centrifuge methods and require special techniques for their detection. 

 Of these, the so-called direct-count method is apparently most accu- 

 rate. Using recent modifications of this method, Russian workers 39 

 report from 500,000 to 6,000,000 bacteria per cc. in Lake Glubokoje, 

 and Wisconsin students 40 found from 19,000 to 2,000,000 per cc. in 

 the lakes studied. In Lake Mendota there is an August maximum 

 and a low minimum in late winter; however, such seasonal variations 

 do not occur in all lakes. In Indiana, 41 most bacteria have been 

 reported from the surface, the bottom, and the thermocline, but these 

 results are not confirmed in similar lakes, and no generalization ap- 

 pears to be possible at present concerning the vertical distribution 

 of bacteria in lakes. Below the thermocline, during summer stagna- 

 tion, the number of aerobes decreases. In the bottom deposits of Lake 

 Mendota, of the 100,000 bacteria per cc. about 1% are denitrifying 

 and 0.1% are cellulose destroyers. 42 



Minute as they are, the bacteria of fresh waters form an important 

 food source for other members of the nannoplankton and for some of 

 the mud feeders and hence stand at the very bottom of the food 

 pyramid. The rapid rate of reproduction of bacteria makes their im- 

 portance as food greater than is indicated by the size of the stand- 

 ing crop. 



