CHAPTER IV 
CONDITIONS OF EXISTENCE OF AQUATIC ANIMALS 
I. INTRODUCTION: COMPARISON OF LAND AND AQUATIC ANIMALS 
The conditions of existence of aquatic plants and animals are very 
different from those of land plants and animals. Some of the most 
important differences are as follows: 
a) Water, the surrounding medium, is about 768 times as heavy 
as atmospheric air at the sea-level. 
b) The necessary gases are in solution in the water and their diffusion 
is much less rapid than in the atmosphere. 
c) The necessary inorganic salts are in solution in the surrounding 
medium. 
d) The necessary organic food substances for plants and some of the 
carbon compounds necessary for animals are in solution in the water and 
are taken directly by the plants and animals (47). 
e) Vegetation rooted to the bottom is important in most bodies of 
water. In large lakes like Lake Michigan, however, there are very few 
attached or rooted plants, and therefore nothing comparable to the 
vegetation of the land, or to the plant-eating animals which live on it, 
is to be found. Most of the plants float freely in the water. Such 
plants are present also, however, where rooted vegetation occurs. 
Il. CuHemicaL CONDITIONS 
I. DISSOLVED CONTENT OF WATER 
In order to support animals and plants, water must contain certain 
minerals and gases in solution (71). Salts (carbonates, sulphates, and 
chlorides) of magnesium, calcium, and sodium and salts of potassium, 
iron, and silicon are practically always present in solution in water, and 
their presence in definite proportions is essential to the life of the animals 
(72). Water without these has been shown to kill fish (71). Dissolved 
gases in definite proportions are also necessary. 
Gases.—The chief facts regarding the occurrence of gases in nature 
and their solubility under experimental conditions are shown in Table II. 
The standard method of expressing quantity of gas in solution is in cubic 
centimeters per liter at o° C. and 760 mm. of mercury (73). All values 
are therefore given in these terms. 
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