RECENT FRESHWATER INVESTIGATIONS 311 



irregularly, depending upon the action of the wind, and 

 reaching the bottom as a result of the late September gales, 

 disappears,. These conditions are of extreme biological im- 

 portance since below the thermocline the water is stagnant 

 during the entire summer and becomes unfit to support most 

 forms of animal life. The sub-thermoclinal water is reported 

 by Whipple (95a) and others to be malodorous, deficient in 

 oxygen and rich in the products of decay. Its overturning is 

 the occasion of a rapid increase in the diatoms of the plankton. 



Whipple (98) distinguishes three types of lakes, polar, tem- 

 perate and tropical, according to the surface temperature, 

 which in lakes of the first type is never above that of max- 

 imum density (4° C), in those of the tropical type never below 

 that point, and in lakes of the temperate type sometimes above 

 and sometimes below it. He also designates three orders of 

 lakes on the basis of the bottom temperature, which in those of 

 the first order is practically constant at or near the point of 

 maximum density; in those of the second order the bottom 

 temperature fluctuates but never very far from the same point 

 while in lakes of the third order the bottom temperature rarely 

 varies from that of fch 3 surface. With regards to periods of 

 circulation which are so important for the development and 

 distribution of the plankton, he says: " Speaking in very gen- 

 eral terms, we may $i:y that lakes of the first order have no 

 circulation; lakes of the third order no stagnation (except in 

 winter); and lakes of the second order have both circulation 

 and stagnation." According to Birge (98) the thickness of the 

 surface stratum of warm water depends on the wind, the ex- 

 posure of the lake, and among those similarly located in these 

 particulars, upon the area of the lake, being less in a lake of 

 smaller area. The bottom temperature of a small lake is likely 

 to be lower than one would expect from the depth merely and 

 that of a large lake higher. Here also Ule (93), Langenbeck 

 (93) and Dolan. 



The amount of oxygen present in various parts of a water 

 basin and the dissemination of gases through the water is of 



