326 ANIMALS IN INLAND WATERS 



upon the consumption of oxygen in that region. In waters whose sub- 

 stratum is mainly composed of mineral elements the oxygen content 

 near the bottom does not sink below 70% of saturation (Fig. 93, Lake 

 Cayuga) . Wherever the bottom is covered with ooze, which is rich in 

 disintegrated organic material, the oxygen content, especially in shallow 

 depths, is much reduced (Fig. 93, Lake Madue), so that often only 

 40%, or less, of normal saturation content is left, or, in the deepest 

 layers just above the floor, none at all (Fig. 93, Lake Keller). The 

 decrease of oxygen content begins at the thermocline (Fig. 93) . 9 In 

 such waters the bottom fauna is greatly influenced. In shallow lakes, 

 in which wave action extends throughout, there is no lack of oxygen at 

 the bottom. 



Animal communities in lakes.* — Standing waters fall naturally 

 into two groups — the deeper and more permanent lakes, on the one 

 hand, and the smaller, shallower, more evanescent ponds and pools on 

 the other. There are, of course, no sharp demarcations between these 

 groups or their component bodies of water. Our general discussion of 

 the fauna in standing waters will be based primarily on that of lakes. 



A lake may be defined as a continuously closed, stagnating mass of 

 water found in a depression of the earth, never directly connected with 

 the ocean (Forel), in which the shore vegetation does not extend to 

 the greatest depth, and in which, therefore, there is a central area 

 free from plant growth. A lake, therefore, in contrast with a pond, 

 contains open water and has a limnetic region. In this respect the 

 conditions in a lake are more like those in the ocean ; but the limnetic 

 region in general does not exceed the pedonic region nearly so much 

 as in the ocean. 



The shore zone. — The pedonic region is composed of a shore zone, 

 which extends as far as the green plant growth on the lake floor, and 

 the deep pedonic zone beneath the limnetic region. The depth to 

 which shore vegetation extends varies in different lakes, sometimes 

 down to about 30 m., but usually not exceeding 6 to 12 m. This depends 

 primarily upon the transparency of the lake water, and this is the 

 greater the smaller the amount of plankton present in the lake (see 



* Communities of organisms which are free from direct dependence on bottom 

 or shore are called pelagic if in the ocean and limnetic if in lakes. Communities 

 dependent on the bottom are said to belong to the benthos in oceans and to the 

 pedon (pedonic) in lakes. The communities related to the shores are said to 

 belong to the littoral community in both oceans and in lakes. If need is felt for 

 further sharpening of this terminology, littoral may be retained for such com- 

 munities along the shores of the ocean, and the new term, paralimnic, may be 

 used for the corresponding community in lakes. The region would be that of 

 the paralimnion. 



