COMMUNITIES IN RUNNING WATERS 303 



identical with that of Lake Ladoga, of which it is the outlet, while its 

 companion river, the Tosna, contains a wholly different plankton, 

 which is explained by its bog origin. 



That rivers do not develop their own plankton is due to the current. 

 The Neva, which extends for 60 km. from its source in Lake Ladoga 

 to the ocean, flows at a speed of 1.0 m. to 1.7 m. a second; a portion of 

 water with its plankton, therefore, reaches the ocean in 12 to 14 hours; 

 this time is too short for any considerable multiplication, even among 

 animals which reproduce very rapidly. A portion of water in the upper 

 Rhine, flowing at a rate of about 2.5 m. a second, covers the distance 

 between Basel and Mannheim in V/4 days. The conditions become 

 more favorable with the reduction in their gradient, which occurs in 

 most rivers near their mouths, and in some much sooner. The waters 

 of the Moskva cover only between 14 and 30 km. in 24 hours; conse- 

 quently short-lived animals which reproduce rapidly can greatly 

 increase their numbers on their way to the ocean — rotifers, for exam- 

 ple, whose numbers tend to be much greater than those of Crustacea in 

 slowly flowing rivers. For the same reasons there is seldom a monot- 

 onous plankton in a river, such as would result from mass development 

 of any one species. 



In addition to their quickened run-off, rapids often serve to destroy 

 plankton organisms. In the Mississippi River, the amount of plankton 

 immediately below Rock Island Rapids is less than half that just above. 

 The passage of these rapids requires about 8 hours; the cause of death 

 is thought to be the violent impacts against suspended sand grains or 

 against the bottom. 3 Mats of submerged vegetation also remove plank- 

 ton; a decrease of as much as 50% has been observed within 20 m. in 

 dense aquatic vegetation. 4 



A peculiarity of river mouths is the piling up of the water at high 

 tide and its mixture with salt water. Because of this, the outflow of 

 otherwise slowly moving rivers is temporarily stopped and the condi- 

 tions resemble those in standing waters. The remarkable richness of 

 the river plankton in such regions is no doubt connected with this fact. 

 Thus a cubic meter of water in the Elbe above Hamburg contains a 

 few thousand cladocerans, but below Hamburg the number rises to 

 many millions, in the India Harbor, for example, to 11,040,000 Bos- 

 mina longirostris. Most important of all, forms adapted to brackish 

 water become mingled with the water of the river mouth, and some of 

 these increase in great numbers. In the plankton of the lower Elbe 

 below Hamburg, Coscinodiscus, a marine diatom, and Eurytemora 

 affinis, a copepod of brackish water, are very prevalent. The copepod 



