NANNOPLANKTON OF LAKE MEN DOT A 63 



CHAPTER III 

 THE NANNOPLANKTON OF LAKE MENDOTA 



The term nannoplankton, meaning dwarf plankton, was employed by 

 Lohmann 1 in 1911 to designate the very minute plankton organisms. 



He set an arbitrary maximum diameter of 25/u, for the members of 

 this group. Many of the fresh- water organisms which are so small 

 that they readily pass through the meshes of the finest bolting cloth, 

 exceed this dimension, so that the usefulness of the term would be 

 very greatly restricted if it should be applied only to forms whose 

 maximum diameter is not greater than 25ft. Therefore, since these 

 organisms are grouped together on a purely artificial and arbitrary 

 basis, a broader meaning is applied to the term nannoplankton in this 

 report in order to make it more useful to planktologists. As used here, 

 it is applied to the assemblage of plankton organisms which are so 

 small that they readily pass through the meshes of the finest silk bolt- 

 ing cloth and are thus lost regularly by the net. Such an extension of 

 the meaning makes the term much more useful and gives, at the same 

 time, a practical basis for the separation of the total plankton into 

 two definite classes, namely, the net plankton and the nannoplankton. 

 As used in this report, the term nannoplankton is the equivalent of 

 what has been called the centrifuge plankton, but the word nanno- 

 plankton is a more convenient term as well as a more euphonious one. 



In the Wisconsin lakes that have been investigated so far, the assem- 

 blage of organisms which passes through the meshes of the net is made 

 up of rhizopods, flagellates, and ciliates among the animals, and of 

 various algae, such as Ankistrodesums, Oocystis, Sphaerocystis, Aphano- 

 capsa, and species of diatoms belonging to the genera Stephanodiscus, 

 Cyclotella, and Cocconeis. Certain forms are lost by the net acci- 

 dentally, such as rod-shaped organisms which strike the net endwise 

 and so are enabled to pass through, young individuals or colonies, and 

 fragments of the colonial forms. In general, however, the great bulk 

 of the material which was obtained with the centrifuge in these in- 

 vestigations consisted of the minute individuals that were small enough 

 to be lost regularly by the net. 



Quantitative studies on the nannoplankton of Lake Mendota were 

 begun with the large De Laval centrifuge on April 21, 1915, and were 



l Internat. Eevue, Bd. IV, 1911, pp. 1-38. 



