294 ANIMALS IN INLAND WATERS 



deeper ones, the shallow shore region, despite its smaller surface area, 

 is far better populated than the deeper water. This comparison is the 

 more striking when considered in connection with similar relations in 

 the sea. Light penetrates to the very bottom of many lakes and makes 

 possible a rich flora rooted in the lake floor. The roots of these plants 

 make use of the mineral substances of the bottom much better than if 

 these were merely dissolved by the standing waters above them ; and 

 the production of organic substances is accordingly increased. Besides 

 this, vascular plants thrive in much greater abundance in fresh water 

 than in the seas; these also are softer and are more easily accessible 

 than land plants as food for snails and insect larvae, because they need 

 no protection against drying, and less mechanical supporting tissues; 

 they therefore disintegrate faster after dying off, and furnish food for 

 the detritus feeders. For these reasons the fresh waters in general are 

 much more thickly populated with living forms than the ocean. Loh- 

 mann, in comparing the value of the catches in the nannoplankton of 

 the tropical oceans, in the cool open ocean, coastal waters of the Bay of 

 Kiel, and in fresh water, got this ratio: 1:10:988:9017. A lake in Hol- 

 stein produced 217.5 cc. of plankton per cubic meter of water, while 

 in the ocean the whole mass of water under 1 sq. m. of surface area 

 produced only 150-180 gm. of organic substance. 



Multiplicity of niches. — The multiplicity of the environmental 

 conditions in inland waters is great. Every gradation in water move- 

 ment is found, from foaming, falling water, tumbling from rock to 

 rock in the mountain torrents, to the lazily flowing streams of the 

 plains. The standing waters, especially those of larger surface area, 

 are often whipped into waves by the wind and beat heavily against 

 their shores while many ponds have placid surfaces and stagnant 

 depths. Tides are practically absent; about 5-cm. tidal differences 

 have been reported for Lake Michigan. Their place is taken to some 

 extent by the seiches,* which also occur along seacoasts. Just as in 

 the ocean, water movement has a decisive influence on living forms. 

 There are rheocolous animals which are adapted to a more or less 

 strong current, and limnocolous, which thrive only in quiet waters. 



The manifold gradations in chemical content, temperature, light, 

 and water currents are again variously combined in the separate bodies 

 of water. There can be no doubt that the far-reaching variation of 

 living conditions in inland waters is primarily a result of the small 

 size of the bodies of water as compared with the ocean. The number- 



*A seiche (sache) is an oscillation of water above and below mean level; it 

 varies from a few centimeters to 1.5 m. and is usually attributed to variation in 

 atmospheric pressure or to the action of a strong wind. 



