4 FRESH-WATER BIOLOGY 



the only ones that are present. But in a pond one can usually 

 determine the existence of a third zone in which the fixed vegeta- 

 tion is lacking. 



With increase of the water body in size or more especially in 

 depth, new conditions are presented. The littoral region passes 

 over insensibly into a deeper bottom region with its own biological 

 series and to a free open-water area known as the limnetic region. 

 The corresponding region in the ocean is designated the pelagic 

 and this term is also used by some for the fresh-water area. The 

 plants and animals in this region are characteristic; they constitute 

 what is called the plankton, the floating life of the water. Such 

 organisms remain suspended in water during their entire existence; 

 they live and die "on the wing." In the larger lakes the shore 

 zone loses in prominence whereas the pelagic and bottom regions 

 gain in distinctness and relative importance. 



Lakes vary widely in character and abundance in different 

 regions. They are infrequent in areas that are physiographically 

 old and most abundant in glaciated territory, where they occur 

 in eroded rock basins, in partially filled rock valleys, in hollows 

 over the moraine, and more rarely at the margin of the ice sheet. 

 Sometimes lakes are found in old volcanic craters, in the depres- 

 sions of a lava-covered area, or behind a lava flow dam. They 

 occur regularly in streams as mere expansions in the course or are 

 formed by the inflowing delta of a lateral tributary or when the 

 stream breaks through a narrow neck and leaves an ox bow or cut- 

 off lake at the side. One finds them often on low coastal plains 

 some distance from the shore, more commonly close to the sea 

 and even on the same level with it. Old lakes without an outlet 

 become strongly alkaline or saline and develop aquatic life of a type 

 peculiar to each. Most lakes, however, are fresh and shelter organ- 

 isms of the same general type. 



Taken together lakes compose one-half the fresh water on the 

 surface of the globe. They present an infinite variety of physical 

 features in rocky, sandy, swampy margins, in steep and shallow 

 shores, in regular and broken contours with no islands or many, 

 with shallow water or depths that carry the bottom far below the 

 level of the sea. 



