[Chap. XLVI UNDER-WATER ENVIRONMENTS 597 



floating plants, such as elodea, eel grass, pond weeds, water hyacinth, 

 water cress, and willow herb. The margins of rivers rarely have perma- 

 nent plant communities because of deposition or erosion and ever- 

 changing water levels. The flora and fauna most characteristic of streams 

 and rivers are the suspended microscopic or near-microscopic organ- 

 isms known as plankton. The plant constituents of the plankton are 

 largely diatoms and other minute algae. The plankton "harvest" of 

 rivers and streams is surprisingly large. The total mass of plankton 

 organisms of the Illinois River at Havana, Illinois — where it flows 

 slowly — amounted in one year to 200,000 tons live weight, or about 

 10,000 tons of dry matter. In other parts of the same river the tonnage 

 may be greater or much less. 



Plankton develops in the more sluggish and deeper pools of streams, 

 and these areas become the feeding grounds of innumerable animals. 

 Plankton is not to be looked upon as a mass of living organisms floating 

 downstream as fast as the stream flows. It is decidedly localized. 

 Neither is it true that plankton increases in variety and abundance 

 downsteam. Flash floods in small tributaries may increase the plankton 

 in the master stream. Great floods temporarily decrease the plankton in 

 the whole stream system. Compared with lakes, running water is a dis- 

 tinctly different environmental complex. Temperatures are more uni- 

 form, and no periods of winter and summer stagnation occur. In 

 general the concentration of inorganic salts is higher in streams than 

 in most lakes. Effects of pollution with manufacturing and municipal 

 wastes may be much more evident and destructive. Spores and seeds are 

 less likely to lodge on stream bottoms; hence the establishment of new 

 plants by this means is less frequent in streams than in still water. 



MARSHES, SWAMPS, AND BOGS 



As lakes and ponds are gradually filled by partially decayed organic 

 matter and by silt washed from the uplands, rooted vegetation en- 

 croaches upon the area. This filling gradually brings about changes 

 in the nature of the environment. The effects of winds become less, and 

 there is no overturn of water caused by winds in spring and fall. Shading 

 by rooted aquatics becomes important, and organic matter accumulates 

 rapidly. In fact, all stages of transition from a typically acquatic habitat 

 to a land environment may occur as lakes and ponds become bogs, 

 marshes, or swamps through increased drought, through drainage, or 

 through the accumulation of organic matter and soil. 



