ANIMALS AND THEIR ENVlliONMENT 



297 



In the fall the surface water becomes cooler, and the thermocline 

 gradually disappears. When the surface water is of about the same 

 temperature (and hence density) as the bottom water, the wind is capable 

 of stirring the water from surface to bottom. Then the bottom water, 

 which is held captive during the summer, may escape to the surface. 

 In winter the surface water usually freezes to a very slight depth, but 

 the bottom never freezes. Indeed, the bottom may be only 2. or 3° 

 colder than in summer. The sheet of ice, if one forms, prevents wind 

 action; and besides, Avater at or near 0°C\ is not so heavy as that around 

 4°. Consequently there is no intermingling of surface and bottom water 

 during \nnter. In spring, however, as the surface water warms, there is 





■r4 



Fig. 263. — Lake shore kept bare of vegetation by wave action. (Photograph by F. C 



Gates.) 



again a complete circulation of the water under the influence of the wind. 

 As summer advances, the surface water is heated, and the thermocline is 

 again produced. 



Thus, twice a year, spring and fall, the water of moderate-sized lakes 

 in the temperate zones circulates freelj' from surface to bottom; but at 

 other times there is effective stratihcation, and surface and bottom waters 

 do not mix. These facts have an important bearing upon the general 

 ecological features of lakes. A lake is divided into regions whose proper- 

 ties are distinctly different not only in temperature but in light, gas con- 

 tent, and mechanical agents. 



The region above the thermocline in summer is relatively warm, is 

 well lighted near the surface, is subject to mechanical disturbance by 

 waves, has no fixed objects to which organisms may be attached except 

 near the shores, is well svipplied with oxygen from the air (supplemented 

 by that coming from green organisms carrjdng on photosynthesis), and 



