296 



PRINCIPLES OF ANIMAL BIOLOGY 



() 



gradually filled in and converted into dry land; but that fact is of n 

 importance in relation to the community of organisms existing in them 

 at any one moment. 



Animals of ponds are of too many kinds to l^e named with any com- 

 pleteness, but very characteristic ones are many protozoa; the fairy 

 shrimp (particularly in temporary ponds) ; the immature stages of May 

 flies, dragonflies, stone flies, and midges; mites or water spiders; snails 

 and small bivalve mollusks; and often frogs, toads, and salamanders. 

 Fish are uncommon; and occasionally muskrats build their houses in and 

 over the water. Visitors are ducks, grebes, and other wading birds 

 AA'hieh feed upon the pond animals. Pond animals must produce many 

 offspring, for the environmental toll is especially heavy. 



Lakes. — Lakes differ from ponds chiefly in size, but this difference 

 carries Avith it profound changes in all the principal factors of environ- 

 ment — light, temperature, and dis- 

 solved gases, with their effect upon 

 nutrition. Lakes vary so much in 

 their qualities, depending largely on 

 size and geographic position, that what 

 is said here will be limited chiefly to 

 those of moderate size in the temperate 

 zones. Two-thirds of the light falling 

 upon a lake is absorbed by the flrst 

 meter of water, and almost none pene- 

 trates farther than 3 or 4 meters. The 

 bottoms of most lakes are therefore in 

 total darkness. The heat received 

 from the sun and from contact with 

 warm air in summer aftects only the 

 surface water. The water near the 

 A\'ind, and a layer of water of nearly 

 down as far as wave action I'eaches. 





 5 



10 

 15 



Vi 



? 20 



X 



S 25 

 0.30 



0) 



a 

 35 



40 



45 



50 





8 10 12 14 16 



18 20 2? 

 Degrees Centigrade 



Fig. 262. — A thermocline; curve 

 of temperature at different depths in a 

 typical lake in summer. 



surface is stirred up by the 

 uniform temperature extends 

 In a lake of moderate size this surface layer is apt to have a temperature 

 around 2()°C'. in late summer, and to be 10 or 12 meters in thickness. 

 Below this depth the water becomes rapidly coldei' with increasing 

 depth, as shown in Fig. 2G2. This layer of rapidly falling temperature 

 is known as the thermocline, and in the lake represented in the figure it 

 extends from about 11 meters to about 20 meters in depth. Ji(>low the 

 thermocline the water continues to become colder at lower depths, but 

 at a very much slower rate. Since the warm water above the thermocline 

 is less dense than the cold water below it, there is practically no inter- 

 mingling, and the water lielow is rather completely cut off from an>' com- 

 munication with th(» world above. 



