92 



ANALYSIS OF THE ENVIRONMENT 



unchanged by the process. This high latent 

 heat of fusion of water is one of the prop- 

 erties that make it a remarkably fit sub- 

 stance for life and, therefore, a major envi- 

 ronmental factor (see p. 76). 



The amount of heat present affects both 

 living organisms and their environment. 

 While much of the space available for this 

 discussion will be devoted to the more 

 direct effects of temperature on plants and 

 animals, it is useful to remember that the 

 heat relations of the environment are also 

 important ecologically. For example, sur- 

 face layers of suddenly warmed rocks flake 

 off from the cooler inner layers; the flak- 

 ing is often produced by the heat expanding 

 trapped water into steam. Winter cold also 

 disintegrates solid structures, primarily as a 

 result of the force exerted by expanding ice. 

 These matters are commonly treated in 

 physiography. Other aspects of the effect of 

 heat on the physical environment can be 

 effectively summarized and possibihties can 

 be suggested by considering the tempera- 

 ture relations of lakes. A lake affords a 

 rather neat environmental unit, many 

 phases of which, temperature included, 

 have been studied intensively. Good sum- 

 maries of the literature should be consulted 

 for interesting general features and for 

 details.* 



THE HEAT BUDGET 



Outside the tropics, the water of a lake 

 accumulates heat during one portion of the 

 year and gives it off at another. Although 

 the processes involved are complex, they 

 can be summarized in terms of the annual 

 energy budget of the lake. This may be 

 considered as being composed of the energy 

 received from the sun and sky each year 

 and is substantially balanced by the outgo 

 of energy from the lake water. In simplest 

 terms, the annual heat budget of the lake 

 is based upon the amount of energy in 

 gram-calories required to raise the tempera- 

 ture of the water, including the energy 

 used in melting the ice, from the winter 

 minimum to summer maximum. 



Different lakes vary greatly in heat bud- 

 gets. The general principles involved can be 



• Birge and Jtiday, 1911, 1912; Birge, 1915, 

 1916; Needham, Juday, Moore, Sibley, and 

 Titcomb, 1922; Welch, 1935; Hesse, Allee, and 

 Schmidt, 1937, and Juday, 1940. 



illustrated from the data concerning the 

 well-studied Lake Mendota. This lake has 

 an area of about 40 square kilometers, a 

 maximum depth of 25 meters, and a mean 

 depth approximately half of that. The 

 minimum balance occurs late in December, 

 when the lake freezes over, near enough to 

 the end of the year so that, for practical 

 purposes, the fiscal year for Lake Mendota 

 corresponds with the calendar year The 

 mean energy receipts from sun and sky 

 radiation from April, 1911, to March, 1939, 

 inclusive, are shown in Figure 9. The aver- 

 age total of annual receipts is 118,872 

 gram-calories per square centimeter of sur- 

 face, expended as indicated in Table 5. 



Table 5. Estimates of Energy Expenditure of 

 Lake Mendota (see Juday, 1940) 



Gram-calories 



per Square 



Centimeter 



of Surface 



For raising temperature under ice 1800 



For melting ice in spring 3500 



For raising water to summer tem- 

 perature 22,400 



For raising temperature of bottom, 



net 1500 



For evaporation 29,500 



For energy used by organisms, 



maximum 1000 



For surface loss' 28,500 



For loss by conduction, convection 



and radiation 30,000 



Total 118,500 



* Types of surface losses include reflection, 

 upward scattering by particles in suspension, 

 and absorption by snow and ice. 



The bottom of the lake has a heat budget 

 of its own. At four different stations where 

 the depth of the water ranged from 8.0 to 

 23.5 meters, the budget ranged from about 

 3000 gram-calories at the shallower station 

 to about 1100 for the deeper. The mean 

 for the lake is near 2000 gram-calories per 

 square centimeter of surface, of which 

 some 500 are used in heating the water 

 under the ice in winter. 



In general, lakes in eastern North 

 America that do not present imusual 

 features and that lie between 40 and 60 de- 

 grees north latitude have similar heat bud- 

 gets. When lakes are about 10 kilometers 



