ENVIRONMENTAL FACTORS 293 



or freeze. Such conditions again naturally bring about a definite selec- 

 tion of fauna. 



Of considerable influence for the conditions of temperature in in- 

 land waters is the fact that pure water is densest at 4°. This determines 

 a condition in the temperate and frigid zones which is of greatest 

 importance for the survival of fresh-water animals during the cold 

 season. When the surface waters cool they become heavier than the 

 warmer waters beneath them, and sink down, and thus warmer waters 

 rise to the surface; this continues until the whole mass is cooled to 

 4°. Further cooling makes the water less dense, so that the surface 

 water does not sink as it cools below 4°. Finally a covering of ice 

 forms, in contact with which somewhat colder strata of water are 

 found, but the temperature of the deep water remains at 4°. The 

 ice covering, which is confined to the surface by the fact that it 

 is less dense than water, is in itself a shelter which slows down further 

 cooling. It is because of these facts that water a few meters deep, 

 even in the polar regions, seldom freezes to the bottom. The peculiar 

 temperature stratification in lakes, which results from the sinking of 

 the cooled surface waters, will be discussed in more detail below. 



Heat budgets of lakes. — The amount of heat necessary to raise 

 the water of a lake from the winter minimum to the summer maximum 

 temperature is called the annual heat budget. 10 With many temperate 

 and subarctic lakes, this is the amount of heat necessary to raise the 

 water of the lake from 4° to the summer maximum. 



Lakes of eastern United States about 10 km. in length by 2 or 

 more km. wide and with a mean depth of about 30 m. have heat 

 budgets of the order of 30,000 to 40,000 gram-calories per sq. cm. of 

 the lake's surface. These lakes of approximately the latitude of Chicago 

 tend to have a somewhat higher and more uniform heat budget than 

 those of Europe, and there is no evidence of an increase in the annual 

 heat budget from latitude 40° to 60° N. For most lakes, the majority 

 of the annual heat budget is distributed as a result of wind action. 11 



Light penetration. — As in the ocean, many factors influence the 

 penetration of light into fresh waters; of these the amount of suspended 

 matter is particularly effective in excluding light. Crystal Lake, Wis- 

 consin, with highly transparent water, has approximately the same 

 light penetration as Puget Sound. In Wisconsin lakes, the depth at 

 which light is reduced to 1% of that at the surface was found by Birge 

 and Juday to vary from 1.5 to 29 m. 11 



The amount of light in inland waters is markedly influenced by 

 their shallowness. Only a few of these waters are more than 300-400 m. 

 deep; most of them are shallower than 30 or 40 m. But even in the 



