36 Nature of Aquatic Environment ^ 



and further warming induces no descending currents, 

 but instead tends toward greater stability. It some- 

 times happens that in shallow lakes there is little vernal 

 circulation. If the water be warmed at 4° C. at the 

 bottom before the ice is entirely gone, and if a period 

 of calm immediately follow, so that no mixing is done 

 by the wind, there may be no general spring circulation 

 whatever. 



The shallower the lake, other things being equal, the 

 greater will be the departure of temperature conditions 

 from those just sketched, for the greater will be the 

 disturbing influence of the wind. In south temperate 

 lakes, temperature conditions are, of course, reversed 

 with the seasons. In tropical lakes whose surface 

 temperature remains always above 4° C, there can be 

 no complete circulation from thermal causes, and in- 

 verse stratification is impossible. In polar lakes, never 

 freed from ice, no direct stratification is possible. 



It follows from the foregoing that gravity alone may 

 do something toward the warming of the waters in the 

 spring, and much toward the cooling of them in the fall. 

 By gravity they will be made to circulate until they 

 reach the point of maximum density, when going either 

 up or down the scale. Beyond this point, however, 

 gravity tends to stabilize them. The wind is responsi- 

 ble for the further warming of the waters in early sum- 

 mer, and the heat in excess of 4° C. has been called by 

 Birge and Juday "wind-distributed" heat. They esti- 

 mate that it may amount to 30,000 gram-calories per 

 square centimeter of surface in such lakes as those 

 of Central New York, and the following figures for 

 Cayuga Lake show its distribution by depth in August, 

 191 1, in percentage remaining at successive ten-meter 

 intervals below the surface : 



