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 autumn, 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 life, 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 course 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 living 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 run-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 
