94 



ANALYSIS OF THE ENVIRONMENT 



COIl- 



the summfir temperature 

 rise, since water is a poor 



water. As 



tinues to 



conductor of heat, a direct stratification is 



established in which the warm upper layer 



of water, the epihmnion, now considerably 



expanded, passes by a narrow transition 



stratum, the thermocline, to the cold lower 



the drop in temperature is at least 1° C. per 

 meter of depth. Above and below, the rate 

 of decrease is less. Within the thermocline 

 it may be much greater. The depth of water 

 down to the thermocline and the depth of 

 this zone of rapid change may vary in the 

 same lake at a given time, both in thick- 



MIDSUMMER TEMPERATURE 

 LAKE GEORGE 



Wind-stirred water 

 Shifting temperatures 

 Plenty of light and air 

 Abundant plankton 



TRANSITION AREA 



65% of fall in temperature is here 



Still water 



No light 



No wind 



Scanty growth of plant plankton 



No weather 



Maximum range of temperature 



for the year, about 5° C. 



Fig. 10. Summer stratification in Lake George, New York. (Redrawn from Needham, Juday, 



Moore, et al. ) 



water, the hypolimnion. The temperature 

 of the epilimnion lags somewhat behind the 

 seasonal march of the temperature of the 

 air. Its waters are kept in a fairly homo- 

 geneous condition by wind action. Their 

 oxygen content is high. The water of the 

 hypolimnion is seldom disturbed by sum- 

 mer winds, and it, too, becomes fairly 

 homogeneous, but with lower oxygen con- 

 tent, frequently very low indeed. 



As becomes a transition zone, the ther- 

 mocline is not sharply marked off from the 

 region above and below, though the transi- 

 tion is usually more abrupt from the epihm- 

 nion than from the hypolimnion (Fig. 11). 

 Birge's arbitrary rule for the location of the 

 thermocline fimits it to the region in which 



ness and in depth with seasonal changes. 

 A thermocline to physical oceanographers 

 means the layer of water in which the tem- 

 perature shows maximum change with 

 depth. They carry this practice over to 

 studies in lakes. Thus, Church (1942, p. 

 14) speaks of a thermocline in Lake Michi- 

 gan in which the mean temperature change 

 was 2.5° C. in 20 meters. This double usage 

 is unfortunate. 



To return to the outline of the seasonal 

 cycle in lakes: The fourth stage comes with 

 autumnal cooling, followed by a complete 

 circulation of the water until shortly before 

 the lake becomes covered Vidth ice. The ver- 

 nal and autumnal overturns not only 

 equalize the temperature relations through- 



