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3. Quantitative Investigations. 



The second main question which the plankton investigations took up 

 from the commencement, although in a lesser degree, was the question of the 

 closer determination of the quantity of the plankton after H e n s e n ' s method, 

 a question, the importance of which from a fishery-biological point of view is 

 immediately obvious. A large number of catches have been made with nets, the 

 power of filtration of which was known, so that it was possible to calculate the 

 number of plankton organisms present in a unit of water, but the representative 

 character of these results is not universally accepted. Moreover the values ob- 

 tained apply only to such organisms as can be taken in the nets, and the 

 investigations of the last ten years have in particular shown, that there exists 

 in the plankton a great number of very small organisms, which pass through 

 the finest nets and which are of great importance in the economy of the sea 

 as food for the somewhat larger plankton organisms. These very small forms, 

 the so-called Nannoplankton, it is necessary to capture by other means 

 (with the aid of hardened paper filters or centrifugation of water-samples). On 

 the other hand, there are also larger plankton organisms, which, among other 

 things, owing to their power of independent motion, are able to escape the 

 nets, and which are thus wanting, or only present in disproportionately small 

 numbers in the net hauls. The latest investigations thus show that in order 

 to obtain a knowledge of the quantity of the plankton, it is necessary to 

 employ both different nets (wide-meshed for the larger, moving animals, fine- 

 meshed for the smaller animals and for a great portion of the phytoplankton) 

 and other methods of capture, (filter or centrifuge for the smallest organisms). 

 Only by a combination of the results obtained by these methods it is possible 

 to find the total quantity of plankton present in a given place at a given 

 time. This demands the counting of all the organisms in all the samples, to- 

 gether with the calculation of their sizes or of their chemical amount of matter. 

 A German plankton investigator, working independently of the International 

 Investigations, has carried out this toilsome task as far as regards the Kiel 

 Bay, having collected samples every week for a whole year in the various 

 methods just referred to, counted the organisms and calculated their size, thus 

 arriving at fhe quantity of the plankton. Also the international plankton 

 investigations have quite recently commenced a series of investigations as to 

 the quantity of the smallest organisms (Nannoplankton), this being determined 

 by centrifugation of samples of water collected and preserved. 



In order to calculate the total plankton production of a certain part of 

 the sea by means of comprehensive and manysided collections of plankton- 



