28 FRESH-WATER BIOLOGY 



sun warms the surface waters they become so much lighter than 

 the deeper colder waters that the currents set up to compensate 

 for the piHng up of the water by the wind can no longer flow to 

 the bottom and a superficial circulation is accordingly set up 

 (Fig. 4 B). A distinct thermocline (T) is thus established. The 

 epihmnion (£) is warm and constantly aerated by circulation and 

 the hypoUmnion (H) is stagnant. In the autumn as the water 

 gradually cools the thermocline gradually migrates to the bottom 

 and the earlier, complete circulation (Fig. 4 ^) is again established. 



In addition to the general circulation, waves and their action 

 must be considered. As was noted in connection with bottom, the 

 shifting of fine bottom materials ehminates most animals from 

 sandy shores. On rocky shores in large lakes are representatives 

 of some of the same animal species found in swift streams. The 

 alternating current does not appear to exclude many such species. 

 In small lakes and ponds the small wave action removes decaying 

 organic matter and thus renders portions of the shores suitable 

 for animals requiring or preferring a terrigenous bottom. The 

 location of such shores which are usually sandy is determined 

 largely by the form of the lake or pond and the direction of pre- 

 vailing winds and inflow of water. 



Currents influence animals directly by bringing pressure against 

 parts. Sessile animals respond to currents by changes in growth 

 form. But few fresh water sessile animals have been studied in this 

 respect, and the exact character of such responses cannot be stated, 

 though sponges and polyzoa are known to vary greatly. Motile 

 animals as a rule turn with their heads upstream and either move 

 against the current, making progress upstream, or remain in one 

 position by swimming enough to maintain themselves. Fishes 

 under experimental conditions will often swim against a current 

 which is stronger than their optimum until they are exhausted. 

 Many fishes orient themselves by visual impressions of the bottom 

 as they float downstream. Others appear to orient by differences 

 in pressure on the two sides of the body or by rubbing against the 

 bottom as they float down. Sight is probably ineffective during 

 floods on account of sediment. Current is essential to the spinning 

 of the characteristic cocoons and cases of some insects Hving in 



