COMMUNITIES IN STANDING WATERS 325 



and of the deep water in Lake Mansfeld, 7 m. deep, is 1.5°, and in 

 many less than 1°. 



Forel 10 has divided lakes into three types with reference to their 

 temperature, according to whether the water is always warm (over 

 4° in tropical lakes), alternately warm and cold (temperate lakes), or 

 always cold (polar lakes). The tropical lakes are always directly 

 stratified. The polar lakes always contain cold water with a tempera- 

 ture lower than 4° and have inverse thermal stratification; during the 

 addition of warmth in summer the temperature of the surface and the 

 bottom becomes equalized and the stratification disappears; during 

 the predominating loss of heat in the cold season it is accentuated. 

 The conditions in the temperate lakes arc stated above; in the summer 

 they resemble tropical lakes, in the winter polar lakes, in spring and 

 fall equalizing circulations appear. 



Lakes of the tropical type are widely distributed throughout the 

 tropics and subtropics. The lakes at the southern base of the Alps, 

 such as Lake Geneva, belong to this group. Polar lakes occur in arctic 

 regions and in high mountains near glaciers; but wherever the ratio of 

 the surface area to the volume of water is small, i.e., in deep lakes in 

 subpolar regions, the characteristics are those of the polar type; even 

 Lake Baikal (51°-55° N. latitude) and Lake Telesky in the Altai 

 belong to this group. In contrast with these, the shallow Lake Enare 

 in Lapland, north of the Arctic Circle, becomes so warm in summer 

 that its waters are directly stratified, so that it belongs to the temperate 

 type. Deep lakes tend to be either polar or temperate, according to 

 the climate. Shallow lakes, on the other hand, are usually of the 

 temperate 11 or tropical type, depending on latitude. 



Oxygenation. — The convection currents are of considerable im- 

 portance in the distribution of oxygen in standing waters. In running 

 waters, even when the movement is slow, constant mixing takes place, 

 which brings water oxygenated at the surface to the bottom. This is 

 not true in standing waters. It is estimated that it would take 42 years 

 for one molecule of oxygen to be transferred by means of diffusion, 

 without any movement of the water, from the surface to the bottom 

 of a lake 250 m. deep (as, for instance, Lake Constance) , 12 Tributaries 

 of lower temperature increase the oxygen content of deep waters in 

 lakes of the tropical type such as Lake Como and Lake Geneva, for 

 their water sinks below the warmer water of the surface. Where there 

 are no such tributaries, the two periods of overturn, in spring and in 

 fall, set the waters of temperate lakes in motion even to great depths. 

 During the summer the surface layers are well mixed and well sup- 

 plied with oxygen, but the oxygen content of the deeper water depends 



