292 ANIMALS IN INLAND WATERS 



acids, etc., and oxygen is being given off by green plants, the meso- 

 saprocoles live ; very many protozoans ; numerous worms and rotifers ; 

 the mussel Sphaerium cornewn; the isopod Asellus; and finally a num- 

 ber of insect larvae. Farther down in comparatively clean water some 

 of the oligosaprobiae, e.g., Gammarws, and some lower crustaceans, 

 appear, as well as a number of species of fish. Finally, in practically 

 pure water the may-fly nymphs, stone-fly nymphs, and crayfish are 

 found. The effect of pollution by city wastes in certain American 

 rivers is discussed in the final chapter of this book. 



Oxygen tension. — The oxygen content of inland waters is also 

 subject to many changes. The water of mountain streams is well 

 aerated in the spray of the rapids and waterfalls. In the lower reaches 

 of rivers the amount of oxygen is less and depends upon the number 

 of oxygen-producing plants as well as upon the amount of contamina- 

 tion from tributaries. In the Elbe, near Hamburg, the oxygen content 

 fluctuates between 4 and 8.8 cc. per liter. 8 In standing waters only the 

 relatively small, smooth surface is involved in the absorption of 

 oxygen from the air. On the other hand, the water may be over- 

 saturated with oxygen during the daytime because of the production 

 of this gas by green plants, and at 18° it may contain 9.4 cc. among 

 thick Potamogeton or 9.6 cc. among Spirogyra instead of 6.5 cc. per 

 liter, 9 which is the usual amount at normal saturation at 18°. The deep 

 water of many lakes is as rich in oxygen as the surface water, e.g., 

 in the Alpine lakes and in the clear Eifel crater-lakes; in others, 

 however, the deep water is poor in oxygen content, especially in shallow 

 lakes with muddy bottoms or during the summer, in lakes with a 

 thermocline. This is, of course, of the greatest importance for the 

 animal life on the lake floor. The presence of salmon and lake trout 

 (Salmo lacustris) is also regulated by the oxygen content of the water, 4 

 for they are found only in waters with a high oxygen content. Thus 

 lake Nantua in the French Jura has two tributaries, one of which is 

 much richer in oxygen than the other. The lake trout ascends only 

 the former for spawning. 



Temperature. — The conditions of temperature of inland waters, 

 like their chemical content, are rendered extremely variable by their 

 small size. The shallower depth of many inland waters allows a rapid 

 and intensive heating during the day, but also an equally rapid cooling 

 at night, so that there are great variations in temperature, which are 

 more marked, other things being equal, the shallower the water. These 

 changes are not so great as in the atmosphere, but still much greater 

 than in the ocean. The seasonal variations, as well as the daily varia- 

 tions, are very great, and in extreme cases the waters may evaporate 



