CHAPTER XVIII 

 COMMUNITIES IN STANDING INLAND WATERS 



The absence of current in standing waters acts in several ways to 

 determine the nature of the fauna. In a river the enriching of the water 

 with nutritive materials, which are necessary for plant life, and the 

 quantity of which greatly influences the fruitfulness of the water, is 

 limited. All salts which have been dissolved from the ground, or have 

 been set free by the disintegration of dead plants or animals, or have 

 been brought in by tributaries, are constantly being carried off to the 

 sea. The most favorable conditions are found in the slowly flowing 

 lower course of the rivers, with their decreased velocity, their increased 

 content of dissolved substances and suspended detritus. In standing 

 waters, however, the elements dissolved from the bottom tend to re- 

 main, and those vital elements which have already been used in 

 developing living forms also find their way back into the water at the 

 death of these plants or animals. Standing water may be a segregated 

 and almost self-sufficient habitat. 



The favorable character of the environment for living forms de- 

 creases wherever a river flows through a lake; Kofoid 1 states that, in 

 lakes which have outlets, the production of plankton is in inverse 

 ratio to the time required for renewal of the water; i.e., it is greatest 

 where the renewal is slowest. The current in rivers often produces 

 steep banks; at least at high water, the banks are scoured and their 

 slopes increased by side erosion. In standing waters, on the other 

 hand, the slope of the shores is more often gradual, and a wide border 

 of plant growth develops on them which supports a rich fauna ; through 

 the disintegration of dead plants the rest of the water is well supplied 

 with detritus. 



The fact that the plankton of open water finds more favorable 

 living conditions in standing waters than in rivers also depends on the 

 decrease of current. An autochthonic river plankton can scarcely be 

 said to exist; the river plankters are incessantly being carried to the 

 sea where they perish and serve as nourishment and fertilizer. Aside 

 from fishes, only such animals as are buried in the substratum, or are 

 attached among the plants or stones, can hold their places in the cur- 

 rent. In standing waters, on the other hand, the fauna of the open 



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