RECENT FRESHWATER INVESTIGATIONS 325 



general composition of the plankton may be judged from the 

 statement of Strodtmann (96) that about 80 organisms occur in 

 the plankton of Holstein lakes of which, however, less than 40 

 are usual or important. All authors agree in noting the limited 

 number of species which are found in the plankton and equally 

 regarding the extreme abundance of individuals which make 

 up its volume. These organisms, moreover, are not at all times 

 the same species but manifest certain variations to be noted 

 later. Among the species of animals which the plankton 

 contains the Rotatoria are said to be the most important. 



The total amount of plankton taken in the vertical net or 

 plankton pump and preserved in some suitable fluid is esti- 

 mated in several ways. (1) After settling in graduated tubes 

 for twenty-four hours the volume is read off from the tube, 

 Apstein (94), Reighard (94), Ward (96). Or the volume is 

 measured in a centrifugal machine, Juday (97). (2) Under 

 suitable precautions the entire amount is weighed, Fric and 

 Vavra (94), Zacharias (95, 95b). Or a known quantity of a 

 haul, measured by the first method, is taken, weighed both 

 before and after incineration, and the amount of organic 

 material in the entire haul calculated, Ward (96a). (3) The 

 organisms in a definite portion of a haul are counted under 

 suitable precautions and the number of organisms in the entire 

 haul calculated therefrom. This method, first used by Hensen 

 in the ocean, has been applied to freshwater by Apstein (94, 

 96). Zacharias (94d) employed it in abbreviated form. An- 

 other simplified form is given by Birge (95a). 



There is no known relation between the results obtained by 

 these different methods and consequently no comparison can 

 be made between the results obtained by one method and those 

 obtained by another. Furthermore while the work of one ob- 

 server at a given time is capable of comparison with that done 

 at another, it seems perfectly clear that the work done by one 

 observer can not be directly compared with that done by another 

 even if the same method is employed. Difficulties in this con- 

 nection are noted by Kofoid (97), Reighard (98) and others. 



