CONDITIONS OF EXISTENCE 



33 



circulation in summer are cold at the bottom, being heated at the 

 surface only (Fig. 4 B). Lake Michigan is a large deep lake and 

 none of the seasonal temperature changes extend to the deepest 

 parts (Table III). In summer the water of the surface is warmed, 

 but if the vertical circulation is complete all the heat in the waters 

 flowing downward at the leeward side (Fig. 4 B) must be absorbed 

 above no meters (Table III) when the temperature of maximum 

 density is recorded. These are chiefly bottom records and do not 

 therefore represent the temperatures at the same level in the open 

 water, especially in the shallower situations where the sun's energy 

 is distributed through a thinner layer of water.^ 



TABLE III 



Most fresh-water animals are poikilothermic or cold-blooded and 

 their temperature varies with the surrounding temperature. IMam- 

 mals and birds with the exception of the manatee and rare fresh- 

 water dolphins and seals are not truly aquatic. Truly aquatic 

 warm-blooded animals usually have a thick covering of fat which 

 is a poor conductor of heat. A few fishes maintain 10° C. or more 

 above the surrounding medium, but for most fresh-water animals 

 0.1° to 5.0° C. are reported. Rogers recently reported only very 

 minute difference for goldfish. This heat is due to metabolism. 



^ Temperatures below the surface may be taken with a thermometer in a two-gallon 

 bottle filled at the desired lev^el or better with a Negretti-Zambra reversing ther- 

 mometer. For devices making continuous records of temperature, the thermophone 

 i of Whipple or Friez's soil and water thermograph may be used. 



