Ecology of Animals 661 



Within the lake certain physical factors control the distribution of 

 the fauna and flora. These factors include the amounts of the dis- 

 solved gases such as oxygen and carbon dioxide, the dissolved salts, the 

 pH, wind action, temperature, turbidity, and the nature of the bottom. 

 Other major stresses faced by the animals dwelling in standing water 

 are the freezing of winter and the drying of summer. Many of the 

 smaller bodies of water dry up completely while even the larger lakes 

 have areas that become dry along the shores. 



These numerous stresses have been met by invertebrates in several 

 different ways. For many it means a reduction of the more delicate 

 larval stages and the production of special encysting forms which can 

 withstand desiccation and freezing. Other species produce special re- 

 sistant eggs. These special resistant eggs or cysts can be blown long 

 distances, resulting in the widespread distribution of certain species. By 

 such means, the smaller animals and plants of lakes throughout the 

 world are often the same. The presence or absence of certain species 

 within any one lake is dependent more upon immediate physical condi- 

 tions than proximity to any other body of water. 



Lakes within the temperate zone have an unusual cycle of tem- 

 perature relationships during a year's time. These unusual relation- 

 ships are fundamentally due to the peculiar properties of water ; that is, 

 water is heaviest at 4° C, just above the freezing point — both above 

 and below this critical temperature the water is much lighter. During 

 the winter season, the surface of the water is ordinarily covered with a 

 layer of ice ; below this, the water is of a comparatively uniform tem- 

 perature and with little or no thermal circulation. At this time, the 

 oxygen is depleted ; but inasmuch as the metabolism of the poikilothermic 

 animals is low, such depletion has little deleterious effect. As the ice 

 melts in the spring, the water gradually becomes heavier. Also the winds 

 blow this surface water against the shore where it tends to "pile up" 

 and circulate to the bottom. This eventually results in a complete turn- 

 over of the water, with an accompanying replenishment of the oxygen 

 supply. As the spring weather continues and the surface water becomes 

 warmer and lighter, the circulation continues but is no longer complete 

 owing to the greater density of the lower layers of water. As a result, 

 there is marked thermal stratification of the water. In a relatively 

 narrow band known as the thermocline, the temperature changes 

 abruptly. It is above this narrow band that the surface water circulates 

 due to wind action. As a result, the layer above the thermocline, the 



