COMMUNITIES IN RUNNING WATERS 305 



rubble which forms in falls and rapids constitute an additional destruc- 

 tive factor in northern and alpine waters. 5 Fine sand and mud, on 

 the other hand, furnish an opportunity for burrowing and tunneling, 

 and a rich food supply for detritus feeders. 



The supply of oxygen is especially favorable in swiftly flowing 

 streams with strong rapids and falls because of the immense surface 

 area exposed for taking up oxygen, and because of the thorough mix- 

 ing. Even where the current is slow, the water is much better aerated 

 than in standing waters. 



The daily and yearly fluctuations of temperature are much smaller 

 in rapidly flowing mountain streams, often shaded by the mountains, 

 than in the waters of the plains which are exposed to sunshine all day. 

 They are least in the springs which emerge from deep underground. 

 The amount of the annual fluctuation of temperature in streams from 

 glaciers is 0° to 1°; in high mountain springs, 1° to 6°; in lower parts 

 of trout brooks, 15° or more; in barbel rivers (see p. 306) about 19°; 

 in carp rivers, about 24°. 6 Even in smaller streams with little move- 

 ment, the temperature of the water is more uniform than that of the 

 air. Stenothermal animals accustomed to cold environment, as well as 

 eurythermal animals, live in rapidly flowing streams, but only more or 

 less eurythermal animals can live in streams of slow current. 



The fall of a stream and its accompanying environmental factors 

 effect a sorting out which in general is such that the steeper gradients 

 demand more distinct adaptations and thus contain fewer species, 

 which are, moreover, mainly different from those of the slack water. 

 In the river-mouth region of streams of middle Europe, 7 the water is 

 always brackish in the lower and at least occasionally in the upper 

 part; the bottom is soft, the current weak, the water muddy, warm, 

 and deep. The characteristic fish are the smelt and the stickleback, 

 besides the flounder and the sturgeon in the lower regions, and the ruff 

 and the eel in the upper. The carp region, with similar characteristics 

 though without a mixture of sea water, is next upstream. High tide 

 has no effect on this region. It is characterized by weak swimmers with 

 laterally compressed bodies (Fig. 82, e-i) which are not suited for 

 swimming in moving water because, since their muscles are weak and 

 their bodies are flat, they offer too many points of vantage for the 

 rapid? and would be whirled around.* Besides these, however, many 

 fish from the upper region, which has a stronger current, occur here, 

 even the eelpout, Lota lota, which rises to a height of 1800 m. above sea 



* Chief among these are Abramis brama, and Carassius vulgaris, the carp, 

 Cyprinus carpio, and Rhodeus amarus, together with Leucisciis leuciscus, and 

 the Leuciscus erythrophthalmus. 



