334 ANIMALS IN INLAND WATERS 



cola is a variety of L. palustris, for their descendants raised in an 

 aquarium revert to the original species and again make periodic excur- 

 sions to the surface for breathing. L. ovata and L. palustris are eury- 

 thermal; deep-water forms of the stenothermal warm water species 

 L. stagnalis and L. auricularia do not exist. In lakes with a thermo- 

 cline the hypolimnion typically becomes oxygen-deficient and even 

 oxygen-free, at least in limited regions during the summer stagnation. 

 This produces a severe selection, and only facultative anaerobes such 

 as some protozoans and chironomid larvae can survive as permanent 

 residents (see p. 291). 



The fauna of the deeper pedon evidently originated in the littoral 

 region and probably is constantly restocked from the same source. 

 Most of the abyssal animal species also occur in the littoral region. 

 For this reason the amount and the composition of the deeper pedonic 

 fauna depend on the type of littoral fauna of the lake in question. 

 Thus the fauna of the deeper waters of Scottish lochs, like that of 

 their littoral regions, is very sparse. Those species, however, which 

 are found in the deeper regions but are lacking in shallow waters of 

 the same lakes are not specifically abyssal animals; in mountain lakes 

 they are also found in the littoral regions. They formerly inhabited 

 the shores of lowland lakes also, and only because of a change in 

 environmental conditions, especially because of the rise in temperature, 

 have they been crowded into the depths. The conditions of the lake 

 abysses, consequently, have seldom proved productive of new species, 

 since even the deeper regions are relatively short lived. Special abyssal 

 forms exist only in the very old, very deep, inland lakes such as Lake 

 Baikal and Lake Tanganyika (see Chapter VI). 



The limnetic fauna. — The fauna of open water is especially 

 characteristic of lakes, for it is just this constant existence of a pelagic 

 region which distinguishes them from the ponds in which perhaps small 

 areas of open water without vegetation exist occasionally, especially in 

 the spring, but in which there is no continuous widespread region with- 

 out plant growth. Imperceptible transitions occur here also, which 

 make a sharp demarcation impossible. Although there is a pond plank- 

 ton, the communities of the open waters of bodies of fresh water find 

 their typical development in the lakes. 



The plankton of inland waters consists of plant and animal forms, 

 the former furnishing the foundation for the existence of the latter. 

 As in the ocean, some of the plankton animals here are so tiny that 

 they escape being caught with ordinary gauze nets and can be sepa- 

 rated only by filtering or centrifuging the water. Some of these organ- 

 isms, the dwarf or nannoplankton, are partly photosynthetic and 





