Potamoplankton 445 



any seasonal form -variations. In certain of the plankton-diatoms seasonal 

 form-variation does occur, but it is in the colony and not in the individual. 



Wesenberg-Lund ('08) in commenting upon the cosmopolitanism of the 

 plankton-community stated that ' freshwater plankton-communities, in con- 

 trast to all other communities on land or water, everywhere contain the same 

 types, nearly everywhere the same species.' We now know this statement 

 to be erroneous (W. & G. S. W., '09 B). It does not hold good for the 

 desmid-flora of the plankton and in a less degree it is not true of the diatom- 

 flora. The geographical peculiarities of the desmid-flora are especially well 

 marked in the plankton. 



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As an appendix to the general account of limnoplankton it is necessary 

 to mention the NANNOPLANKTON, a name first used by Lohmann ('11) to 

 embrace all those organisms which are so minute as to pass easily through 

 the meshes of the finest plankton-nets. The organisms are best obtained by 

 centrifuging the water and they consist of very minute Green Algae and 

 Flagellates. Pascher ('11) has investigated a number of the freshwater forms 

 and Scourfield ('11) has written a good account of the use of the centrifuge 

 in pond-life work. 



POTAMOPLANKTON. 



Slow and moderately slow rivers possess a plankton which differs much 

 from that of lakes. It is more mixed in character and its constituents are 

 largely recruited from the backwaters of the rivers, which are the breeding- 

 grounds of the organisms which become mingled to form the river-plankton. 

 Diatoms are the most abundant organisms of the phytoplankton and as a 

 rule the dominant forms are not those which are so conspicuous in lakes. 

 The most complete investigation of river-plankton yet published is that of 

 the Illinois River by Kofoid ('08), but there are numerous observations of 

 importance on the plankton of other rivers: notably those of Schroder 

 ('97 ; '99) and Ziimner ('99) on the Oder ; Zacharias ('98) ; Brunnthaler ('00) 

 on the Danube ; Zykoff ('00) on the Volga ; Fritsch ('02 ; '03 ; '05) on the 

 Thames, Trent and Cam; Volk ('03; '06) on the Elbe; and Lemmerrnann 

 ('07 A and B) on the Weser and the Yang-tse-Kiang. 



Climatic conditions have a profound influence on the phytoplankton of 

 rivers. In countries with a comparatively mild winter living plankton occurs 

 all the year round [vide Fritsch's observations ('03) on the Thames-plankton] ; 

 but, in countries with a severe winter, plankton, at any rate in a living condi- 

 tion, is not found in the winter months (Schroder, '99; Brunnthaler, '00). 

 The periodicity of potamoplankton is also somewhat variable, since this type 

 of plankton is subject to extreme fluctuations in quantity and composition. 



