THE HAUNTS OF LIFE loi 



Thus in true lakes, as in the sea, we have to distinguish 

 a littoral, a pelagic, and an abyssal fauna. 



Physical Conditions.— The physical conditions of 

 freshwater basins are of course very diverse, and they 

 determine noteworthy differences in the fauna and flora. 

 Thus it is well known that certain organisms, such as 

 the stonewort Chara, and the freshwater crayfish, Astacus, 

 require that there be a relatively large percentage of 

 carbonate of lime in the water, while others, like the fresh- 

 water mussels, do not thrive if there is. 



Concerning temperature, it is obvious that in summer 

 that of the surface is higher, while in winter, especially 

 when there is ice, that of the bottom is higher. Averages 

 on the surface for the four seasons read Uke this : Spring 

 6-7° C, Summer 17-8° C, Autumn 11-9° C, Winter 3-9° C. 

 In summer, or indeed for about 280 days in the year, when 

 the warmer water is at the top, there is a decrease down 

 to 4° C, the temperature of water at its greatest density, 

 but the decrease downwards is not uniform — ^there being 

 a strange leap between 5 and 10 fathoms. In wiater, for 

 about 85 days in the year, when the colder water is at the 

 top, there is an increase downwards until 4° C. is reached. 

 For a short time twice a year, the temperature is practically 

 imiform throughout. It should also be remembered 

 that for each 5 fathoms there is almost an additional 

 atmosphere of pressure. 



The degree of illumination is of vital importance as 

 regards the distribution of both plants and animals, and 

 the depth to which light can penetrate varies considerably, 

 especially with the purity and colour of the water. The 

 red rays are lost first, the violet rays go deepest. A 

 common average result with a white plate is that it ceases 



