COMMUNITIES IN STANDING WATERS 



327 



below). Where the slope is gradual, the littoral zone is wide; where 

 the slope is steep, as, for instance, in many places in Lake Lucerne, 

 or in the Scottish lochs, it may be entirely absent. 



The littoral region contains by far the richest fauna in the lakes. 

 It furnishes substratum for animals on its floor and on its plant forms; 

 the plants supply shelter from enemies and protection against wave 

 action, offer nourishment, and liberate oxygen in great quantities. The 

 greatest diversity in lake faunae, therefore, appears here. 



The littoral zone may be divided into several parts. First of all, in 

 the shallowest waters, are found the thickly growing emergents, high 

 stemmed grasses and reeds and growths of rushes and sedges, among 

 which in turn grows a mass of swamp plants; then follows a strip of 

 more strictly aquatic plants, such as pondweed (Potamogeton) , water 



Chara 



Najas 



Nymphaea 



Phragmites 



Schoeno- 

 t plectus 



i 



Cerato - 

 phyllum 



F IGt 94 — Schematic diagram of one type of plant growth at the edge of a lake. 



After Brutschy. 



Potamogeton 



Myrio- 

 phyllum 



lilies {Nymphaea, Ceratophyllum, Myriophyllum) , with floating 

 leaves; and finally a zone of submerged plants, Elodea, Isoetes, and 

 Chara (Fig. 94). The gentler the slope of the shore, the wider these 

 zones are found to be. In places where waves driven by prevailing 

 winds break strongly against the shore, there is no flora; there all the 

 loose particles on the bottom are washed away and a surf shore line 

 of gravelly or sandy beach develops which harbors its own peculiar 

 sparse animal life. If the shore line is rocky, the animal communities 

 that develop resemble those of rapid, rock-bottomed streams. 



The environmental factors are subject to greater change in the 

 littoral zone than in other regions of the lakes. Because of the close- 

 ness of the lake bottom the water warms much more quickly here 

 than in the region of open water. This is especially noticeable on 

 shores with southern exposure, where a temperature of 17.2° may exist 

 under the noonday sun while in the open water near the surface the 

 temperature is only 2.5 °. 13 Many insect larvae {Corethra, Libellulidae) 

 congregate for metamorphosis in such warmer littoral regions. Cooling 



