FIELD BOOK OF PONDS AND STREAMS 



port; in water, even the heavy claws of the crayfish take on 

 a Hghtsome poise (Fig. 133). It is a good transporting agent, 

 too, and even clear riffle- water carries a load of minute organ- 

 isms on which the net-building caddis worm depends for food 

 (PI. XVI). 



Temperature of the water. — Water is safe to live in, summer 

 and winter, for it does not get so hot nor so cold as air. It 

 is a poor conductor of heat and the changes which occur in 

 a pond are far more gradual than the sudden shifts in the air 

 above it. Ponds have fairly equable temperatures even 

 though they be located in temperamental New England. In 

 general the temperature of the water determines the level 

 which it will occupy in the pond. Cold waters tend to drop 

 to the bottom and warm waters rise to the top, shifts of 

 great import to the hosts of little plants and animals which 

 they carry along with them. As the pond surface cools in 

 summer evenings the top waters fall a little, and when it 

 chills in autumn they drop farther, taking myriads of little 

 organisms downward to winter safety. Water is heaviest 

 when it has cooled to 39.2° F. (4°C.), and then it gradually 

 forms a bottom layer. If the surface water grows colder 

 than that, it becomes less dense, lighter in weight, and stays 

 in the upper levels. At 32° F. (o°C.) it freezes, the ice floats, 

 and the pond is covered with a blanket which keeps its warmth 

 from radiating into the colder air above it. In small ponds 

 spring sunshine melts the ice and gradually warms the water 

 through, even in the bottom layers. In large lakes there 

 is a complicated spring overturn and circulation of the water 

 due partly to changes in temperature and partly to the action 

 of the winds which blow across the surface. 



Food content of the water. — The final food supplies of all 

 water organisms are the non-living substances in the water. 

 These are dissolved gases, oxygen and carbon dioxide, and 

 the raw materials of foods, such as carbonic acid and salts — 

 phosphates, silica, calcium carbonate, and nitrates. Their 



8 



