446 Ecology 



The plankton of rivers, as clearly stated by Kofoid, is subject to more 

 catastrophic changes than that of lakes. The quality of the plankton and 

 the number of the constituents depend upon the rate of flow of the stream 

 (Zacharias, '98 ; Zimmer, '99). It has been repeatedly shown that the more 

 rapid the stream the fewer the individuals in the plankton. In the rivers 

 with a slow current the multiplication of plankton-organisms, provided the 

 conditions are favourable, goes on while they are being carried down in the 

 stream. 



Zimmer ('99) suggested that there were three types of plankton-organisms 

 in rivers: (1) eupotamic planktonts, which thrive and multiply both in the 

 flowing water of the stream and in the backwaters, ponds, etc.; (2) tycho- 

 potamic planktonts, which only multiply in still water and when carried into 

 the river-current live for a time, but do not reproduce themselves ; (3) auto- 

 potamic planktonts which have adapted themselves to a life in flowing water. 



As previously stated diatoms are the most abundant organisms of 

 potamoplankton, and in discussing this point Schmidle ('02) remarks that 

 the more delicate Algae and animals withdraw themselves from the society 

 of the siliceous Bacillariea3 owing to the fact that they are incapable of 

 withstanding the buffeting they would be subjected to in the main stream. 

 The Centric diatoms are fairly well represented by the genera Cyclotetta, 

 Stephanodisciis and Melosira, and more rarely by Rhizosolenia and Attheya. 

 Species of the three first-named genera are all common, although it is 

 probable that Melosira varians is the most abundant Centric diatom. This 

 diatom, and also others, may have both a spring and an autumn maximum, 

 so that the diatom-phases of the potamoplankton are, as a rule, the most 

 important. The genus Synedra is much more important than in lake-plankton 

 and is sometimes entirely dominant. Fragilaria capucina, F. virescens and 

 F. crotonensis are all important constituents in temperate climates. Asterio- 

 nella gracillima also occurs in abundance in many of the rivers of north 

 temperate areas. In contrast to lake-plankton the genus Nitzschia is often 

 of great importance. 



The Green Algre are chiefly represented by certain of the Protococcales 

 of which members of the Volvocineae are especially conspicuous. Various 

 species of Chlamydomonas are frequent, Pandorina Morum, Gonium pectorale 

 and Eudorina elegans are fairly general, and both Pleodorina illinoisensis 

 and Volvox aureus sometimes occur. Platydorina caudata is one of the 

 characteristic constituents of the plankton of the Illinois River. Several 

 species of Pediastrum and Scenedesmus are sometimes abundant, and Actin- 

 astrum Hantzschii, although never occurring in great quantity, is a typical 

 constituent of river-plankton. Species of Kirchneriella, Ankistrodesmits and 

 Micractinium also occur. Botryococcus Braunii sometimes occurs, but never 

 in the quantity in which it may occur in lakes. Ophiocytium may also be 



