186 THE ANIMALS OF LAKES AND RIVERS 



deeper, colder layer, whose oxygen is only renewed 

 very slowly. This relative paucity of oxygen naturally 

 checks life in the deeper parts of the lake. 



In tropical lakes the fact that the surface-water does 

 not cool in the same way in autunm, and that there is 

 no spring melting of ice, makes the vertical circulation 

 much less marked, and leads to a virtual absence of 

 a deep-water fauna. In brief, then, the deep-water 

 fauna of lakes is always poor, because of the absence 

 of a well-marked circulation of the water, and the defi- 

 ciency in the amount of oxygen, with the resultant 

 paucity of Ufe, increases towards the Equator. 



The second unfavourable condition in fresh water is 

 its frequent turbidity. Rivers are powerful eroding 

 and transporting agents, lakes are great filter-beds on 

 the cotirse of rivers ; necessarily, therefore, the waters 

 of both must often contain fine particles in suspension, 

 which would form a deposit on the surface of the 

 breathing organs of animals Hving in them, and greatly 

 retard respiration. The importance of this may be 

 illustrated by an example. The Firth of Forth is 

 a great estuary swept by the tides, once inhabited by 

 great numbers of marine animals tolerant of estuarine 

 conditions, notably by oysters. There is much evidence 

 to show that its waters are becoming muddier year by 

 year, apparently because drainage of the * mosses ' in 

 the upper regions of the river has made the nm-off 

 there more rapid. As a result of the increasing muddi- 

 ness many animals have disappeared — the oyster-beds 

 long ago ceased to be productive, and other forms also 

 are diminishing. Now if an increase in the amount of 

 mud in the water kills estuarine animals, which are 

 naturally tolerant of some suspended matter, even 

 a small amount of suspended matter will prevent the 



