ECOLOGICAL 87 



insects. Many lakes have been provisionally surveyed, and a number 

 of freshwater biological stations have been founded. 



Physical Conditions. — There is considerable diversity in the 

 chemical substances dissolved in the water, and this sometimes 

 exerts a limiting influence on the distribution of organisms. Thus 

 freshwater crayfishes do not survive in water that has a very low 

 proportion of lime salts; though freshwater mussels, with much 

 heavier calcareous shells, seem to be strangely indifferent to this. 



Of great importance is the fact that in the cold and temperate 

 zones the surface temperature in lake and pond is highest for about 

 280 days in the year, and lowest for the remainder. As fresh water 

 has its maximum density at 4° C. and expands below that till it 

 freezes, there is in winter a rise of the expanding bottom water to 

 the surface, where it adds to the thickening of the sheet of ice. This 

 sheet retards further cooling, and in most cases prevents the total 

 freezing of the lake or pond. Thus, for about 85 days of the year, 

 the bottom has the highest temperature, and this helps greatly 

 towards the survival of the freshwater fauna in north temperate 

 countries. The intricate rhythmical fluctuations in the temperature 

 of lakes and large rivers have been much studied, especially in the 

 United States; and correlated with waxing and waning in the 

 density of the aquatic population. 



There is rarely much penetration of light below six fathoms, but 

 the sensitive photographic plates show that the chemical rays are 

 still appreciable at depths of fifty to ninety. Blue water, which is 

 purest, is the most penetrable; green water, with its innumerable 

 microscopic and ultra-microscopic particles, is less penetrable; and 

 least conducive to life is yellow-brown water, which is darkened by 

 much humic acid, from peaty soil and the like. The degree of illumina- 

 tion is of obvious biological importance, since it determines the 

 depth-range and rate of multiplication for minute green Algse, like 

 Diatoms and Desmids, on which minute Crustacea and the like in 

 turn depend. 



Zones of a Lake. — The littoral lacustrine zone includes all the 

 well-illumined shallow water near shore. Among its familiar plants 

 are duckweed (Lemna), pondweed (Potamogeton), water-buttercup 

 (Ranunculus), water-lilies (Nymphsea), mare's-tail (Hippuris), bog- 

 bean, and bulrushes. The land-building by the growth and soil- 

 formation of such vegetation is often so marked as to make notable 

 extensions into an old lake area, apart from the delta-like deposits 

 from its tributary streams. There is often an abundance of fila- 

 mentous and single-celled Algae ; and of much importance as a food 

 for animals is the minute detritus broken from the shore vegetation. 



The littoral fauna includes birds like waterhens and grebes, in 

 some countries turtles and terrapins, amphibians like newts and 

 frogs, in their young stages especially, small fishes like sticklebacks 



