6 FRESH-WATER BIOLOGY 



isms as are produced at a given point by the constant flow of the 

 water make the river plankton scanty in amount, but many fresh- 

 water lakes produce an immense number of plankton organisms. 

 These have been much studied in recent years and about them 

 alike in ocean and fresh water has grown up a new study, Plank- 

 tology, the Planktonkunde of the Germans. 



Among the forms of the open water are some, primarily the fishes, 

 which manifest indi\idual power of movement adequate to make 

 them independent of water movements, storms, and distances. 

 They can thus determine their own distribution in an active fashion 

 and stand in marked contrast with the plankton, for the latter is 

 unable to regulate effectively its location, and is dependent upon 

 the winds and waves for its dispersal. Typical plankton organ- 

 isms, in fresh water known together as the limnoplankton, are 

 found only in water bodies of some size, whereas in small lakes or 

 ponds the circumscribed open-water area contains life which con- 

 sists of migrants from shore and shallow water regions. Whereas 

 on the land higher forms, especially domestic animals, depend on 

 the higher fixed plants for food, in the water the higher types de- 

 pend upon the smaller floating plant and animal organisms which 

 transform inorganic materials and organic debris into available 

 food substances. 



The floating organisms which taken together constitute the plank- 

 ton are grouped into two purely artificial classes according to 

 methods used in collecting. The constant use of fine nets (cf. p. 74) 

 for collecting plankton organisms led to a conception of this type 

 of life that unconsciously assigned a minimum limit in size. Thus 

 the organisms taken in the plankton net are all that the older 

 authors included under the term plankton, an assemblage which 

 should be termed more correctly the net plankton. It is well known 

 through the work of many investigators during recent years and 

 includes a great variety of Crustacea and Rotifera with many Pro- 

 tozoa and Protophyta, and less regularly some other types. 



Within very recent times there has been obtained by more 

 precise methods of collecting what has been termed by Lohmann 

 the nannoplankton (dwarf plankton) with a size limit he set arbi- 

 trarily at 25/x. It consists of the most minute organisms only, 



