THE LARGER AQUATIC VIXETATTON 207 



The natural conditions of our lakes and rivers are undoubtedly 

 changing from decade to decade. The extensive destruction of 

 forests that has occurred in the lake region and along tributary 

 streams has certainly indirectly affected the plant and animal life 

 in the lakes, although at present it cannot be stated in any detail 

 how such influences have operated. The same may be said in 

 regard to the vast volume of organic matter that constantly comes 

 to the lakes and rivers as sewage from the cities. The influences 

 which operate to regulate or determine the food supply of the 

 water are numerous. Adequate knowledge is only possible by a 

 thorough study of the food relations among the animals themselves, 

 between the animals and plants and between the plants and the 

 soil. 



Cycle of matter. — Animals cannot organize food from inorganic 

 substances but must use, as food, matter that is already organized, 

 either as vegetable or animal substance. Plants, however, can 

 and do organize food, using both the mineral salts occurring in the 

 soil or water, and the gaseous carbon dioxid which occurs in the 

 air and in the water. The dependence of the animals upon the 

 plants is at once apparent and the dependence of the plants upon 

 the earth and air is also apparent. The most important influence 

 exerted by plants in fresh-water biology is undoubtedly concerned 

 with their nutrition but they are also important in other ways. 



As geological agents in the formation of marl. — The formation of 

 lime incrustations by water plants has already been considered, so 

 that it is only necessary to state here that considerable e\idence has 

 been brought forward to show that the large marl deposits in the 

 marl lakes have accumulated as already described through the ac- 

 tivity of plants, especially Chara. 



As aerating agents. — Both animals and plants must have air to 

 breathe, and in running water or open lakes there is usually suffi- 

 cient oxygen dissolved in the water to support the respiration of 

 the organisms present. In the still waters of coves and bayous, 

 however, conditions are different. It is in such places that organic 

 debris tends to accumulate, and, in decay, overcharges the water 

 with the gases of decomposition, especially that of carbon dioxid. 

 Of course, whatever animal life is present under such conditions 



