302 ANIMALS IN INLAND WATERS 



streams flowing from the melting glaciers. The running waters thus 

 formed a highway by which arctic animals migrated into the lakes of 

 the alpine foothills, and perhaps, vice versa, some of the alpine animals 

 migrated into the Scandinavian lakes. 



The chemical differences among river waters are smaller than those 

 among standing waters. The mingling of waters from the various parts 

 of a more or less large, often also geologically varied, river basin, 

 equalizes chemical variation much more than in standing waters. Even 

 when very salty or boggy streams enter a river, the water contributed 

 by them is as a rule negligible in comparison with that of the main 

 stream. There is the greatest possible difference in turbidity in streams, 

 from the clear water of mountain brooks to the perpetual muddiness of 

 such rivers as the Missouri. At flood, many streams, otherwise clear, 

 become mud-laden torrents. The whole phenomenon of flooding with 

 accompanying frequently drastic changes in level, strength of cur- 

 rent, width of stream, etc., makes another difference between lentic 

 and lotic environments. 



Rivers, furthermore, usually have no very distinct deep stratum; 

 and even in such large rivers as the Mississippi, the Amazon, and the 

 Congo, where the depth is great, we do not know of any deep-water 

 communities. 



Running waters, finally, have no plankton of their own in the sense 

 of typical plankton animals or plankton communities found only in 

 rivers. There is, of course, a floating fauna in rivers; this is not, how- 

 ever, autochthonic, but has its origin in standing waters, in lakes 

 through which the rivers flow, in ox-bow lakes which are connected 

 with the rivers, and in quiet bays and back waters. The river plankton 

 is increased by floods which flush such places, and thus species are 

 added to the river plankton which do not otherwise occur there; in the 

 plankton of some rivers of south Russia, the larvae of the branchiopod 

 Limnetis, which ordinarily occur only in the transient water basins of 

 the steppe, were found after a flood. 1 River plankton, consequently, is 

 sparser than that of the lakes from which it originates. Investigations 

 at the outlet of Lunzer Lake have proved that the stronger swimming 

 plankton organism, e.g., crustaceans and even rotifers, struggle away 

 from the region of outflowing waters and are thus carried into the river 

 in relatively small numbers. 2 The plankton communities of different 

 rivers, however, are distinct, as are those of the lakes through which 

 they flow. So the plankton of the upper Rhine below Lake Constance 

 differs from that of the Aar as the plankton of Lake Constance differs 

 from that of Lake Zurich, and the plankton of the Neva is almost 



