COMMUNITIES IN STANDING WATERS 



323 



continues up to the maximum of temperature. This results in convec- 

 tion currents until a uniform temperature of 4° has again been reached. 

 The water is now in unstable equilibrium and wave action usually 

 produces a complete mixing of the waters of the lake, making what 

 is called the spring overturn. Thereupon the heating of the surface 

 water goes on from the end of April or the beginning of May (summer 

 stagnation) until a renewed cooling again brings about equilibrium 

 and the autumn overturn. Water which has a temperature of 4° thus 



Winter 



Spring Summer Autumn Winter Spring Summer 



Xll I II III IV V VI VII VIII IX X XI XII I II III IV V VI VII VIII 



Warming Cooling Warming 



Fio. 92. — Schematic representation of the temperature conditions in a body 

 of water in the temperate zone during the course of the year. The isotherms 

 are continuous lines when the stratification is direct, dotted when it is inverse. 

 Spring overturn at A-B and E-F, autumn overturn at C-D, summer stagnation 

 at B-C and F-G. 



always fills the lower levels of the deeper basins; but twice a year 

 it rises to the surface, in spring and fall. Between these two points 

 the 4° isotherm describes a curve which sinks lower in the summer 

 than in the winter and above which the water is arranged in direct 

 stratification in summer and in inverse stratification in the winter 

 (Fig. 92). 



The change from day to night also sets up convection currents. 

 Warming during the day in summer is followed by a cooling at night, 

 which causes a sinking of the upper strata down to a stratum of like 

 temperature. As a result of this an equalization of temperature among 

 these strata occurs, so that these upper strata differ but little in 



