138 Temperature 



ice has formed across the surface, no vertical stirring by the wind can 

 take place. Although the air temperature may go far below zero, 

 heat can be lost from the deeper layers of the pond only very slowly 

 because of the insulating action of the ice and the upper strata of 

 water. This is the chief reason why most ponds do not freeze to the 

 bottom during the winter. 



Stirring is also inhibited when a sharp density gradient has been 

 developed between the upper strata and the deeper layers. Such a 

 condition arises in situations in which the surface water is warmer or 

 less saline, and hence lighter, than the deeper water. Under these 

 circumstances the vertical stability of the water tends to resist any 

 efforts on the part of the wind and waves to produce stirring, and heat 

 exchanges are consequently limited to the surface. When no impor- 

 tant density gradient exists, vertical stirring can become effective and 

 heat will be carried down. This condition commonly occurs during 

 the spring and during the autumn in waters of temperate regions. 

 Under some circumstances the density of the surface stratum may be- 

 come actually greater than the underlying layers resulting in a gravita- 

 tional overturn of the water. 



Seasonal Changes in Vertical Temperature Structure. The typical 

 temperature cycle in a lake during the course of a year is indicated 

 diagrammatically in Fig. 5.5. Since the maximum density of fresh 

 water occurs at 4°C, any warmer or colder water will float on top of 

 water of this temperature. If the underlying water has a temperature 

 lower than 4°C as it may in the early spring, or higher than 4°C, as it 

 may in the autumn, then a top-heavy condition will develop. 

 Archimedian forces aided by strong winds will then bring about the 

 spritig overturn and the fall overturn. Winter stagnation occurs when 

 the lake is ice covered and summer stratification is found after a dis- 

 tinct density gradient has been produced by the warming of the sur- 

 face layers. The layer of rapid vertical temperature change is termed 

 the thermocline and in lakes is defined as having a thermal gradient 

 of at least 1°C per m. In lakes the wind-stirred and largely homog- 

 enous water layer above the thermocline is known as the epilimnion 

 and the relatively stagnant water mass below the thermocline as the 

 hypolimnion. The very considerable changes in temperature and the 

 occurrence of stagnation and of circulation which they control exert 

 a profound effect upon the biota of the lake as will be discussed in 

 later sections. For further details on the thermal cycles of lakes the 

 reader should consult a textbook of limnology such as Welch (1952) 

 orRuttner (1953). 



A similar seasonal cycle of temperature occurs in the ocean but the 



