TEMPERATURE, °C 



FIG. 6-3 Vertical temperature distribution throughout a year In a dimictic lake of the second order — Convict Lake, California; 

 elevation 2308 m (from Reimers and Combs 1956). 



at least 1°C per meter of depth (Birge 1904) and 

 occasionally as much as 7°C per meter. The thermo- 

 cline, as here defined, equals the metalimnion of some 

 authorities who have a different conception of the 

 thermocline (Hutchinson 1957). When the thermo- 

 cline forms, early in the season, it is close to the 

 surface. As the season progresses, it sinks lower, in- 

 creasing the volume of the epilimnion and decreasing 

 the volume of the hypolimnion. The temperature of 

 the hypolimnion is fairly uniform, although it de- 

 clines gradually from the lower edge of the thermo- 

 cline to the bottom, where it is seldom below 4°C. 

 During the autumn, the surface water cools and 



the thermocline sinks. The epilimnion increases in 

 thickness until it includes the entire lake. The waters 

 are then uniform in temperature and density, at all 

 depths. Even slight winds produce complete circula- 

 tion. This is the autumn overturn, which may last 

 for the several weeks, until ice forms. 



As surface waters cool below 4°C they no longer 

 sink, and ice may form. Less dense than the underly- 

 ing water, it floats. Immediately below the ice, the 

 temperature of the water is very close to 0°C, but in 

 one or two meters of additional depth it usually rises 

 rapidly to 4°C, although in some lakes temperatures 

 below 4°C occur even at considerable depths. 



62 



Habitats, communities, succession 



