UNDER-WATER ENVIRONMENTS 



595 



[Chap. XLVI 



lowered through the water. This layer in which differences in tempera- 

 ture are comparatively abrupt is known as the thermocline. 



Since many organisms, both plant and animal, move about only within 

 certain temperature limits, and since their growth is also limited by the 

 available CO2 and O2, it must be evident that these organisms in deep 

 lakes are distributed in strata in summer, and to a less extent in winter. 

 During an exceptionally warm winter in a mild temperate climate the 

 surface water may not be cooled below A" C; no spring overturn oc- 

 curs, and there is no renewal of oxygen in the lower layers of water. 

 During the following summer those fish that can live only in layers of 

 cold water may suffocate. Similar consequences may also follow when- 

 ever there is insufficient wind to cause the spring overturn. 



Z|0 



-21° 



21° 



THERMOCLINE 

 6° 



> .. 



SPRING 

 OVERTURN 



SUMMER 



AUTUMN 

 OVERTURN 



WINTER 



Fig. 270. Diagram of seasonal changes in temperature and stratification in cer- 

 tain deep lakes. Also usual effects on distribution of oxygen and carbon dioxide. 

 Data from Paul Welch. 



The data given in Fig. 270 should help one to understand ( 1 ) why in 

 some lakes the thermocline in summer is a potent barrier between the 

 many organisms that live above and below it; (2) why the greatest pro- 

 duction of plankton, both plant and animal, occurs in the spring and 

 autumn; (3) why certain fishes in the summer time live only in deep 

 water and others only in shallow water. 



Violent winds may at any time cause the mixing of water to con- 

 siderable depths. When the mixing results in carrying oxygen to greater 

 depths, it may be followed by an enormous increase of certain species. 

 Violent winds may also increase wave action and destroy the plants 

 in shallow water or wash them up on land. Wave action may also stir 



