278 DISCOVERY REPORTS 



estimations, especially if it is desired to correlate phytoplankton data with that obtained 

 from other lines of research. The numbers of different forms convey very little unless 

 the reader has some knowledge of their shapes and sizes. Counts might well prove 

 misleading to a chemist or zoologist who would perhaps be able to show significant 

 correlation between his observations and those on the phytoplankton, if the quantity 

 of the latter were expressed in a different way. This point is the more important when 

 we bear in mind the tremendously wide range of variation in size and shape which can 

 take place within the limits of many single phytoplankton species. 



An ideal method should provide comparable figures bearing a direct relation to the 

 total amount of organic matter present as phytoplankton. The concept of the biomass, 

 introduced into marine plankton investigations by Russian workers, almost, but not 

 quite, epitomizes this ideal. Zenkevitch (193 1) defines the biomass as 'the quantity of 

 substance in living organisms per unit of surface or volume '. Thus if it were possible to 

 determine this property of the phytoplankton organisms in a unit volume of water, the 

 quantity of inorganic matter in the organisms would be included. This would indeed 

 be necessary and desirable in considering the relation of the phytoplankton to the 

 physical and chemical characteristics of the medium. When we come to consider the 

 possible value of the phytoplankton as food for animals, however, the inclusion of large 

 quantities of an inert substance like silica might well prove misleading. The biomass 

 constitutes the ideal basis for the study of the relation between organism and the 

 physico-chemical factors of the environment, but is not so well suited to the study of 

 biological interrelationships. Moreover, it seems only too obvious that no good routine 

 method of determining this property of the phytoplankton could ever be devised. 



Harvey's method, on the other hand, gives figures that may reasonably be supposed 

 to bear some relation to the total organic content of the phytoplankton captured. 

 It is at least probable that there is a relation between total organic matter and the 

 total amount of assimilatory pigments responsible for the production of that matter, 

 and the arbitrary colour units are a measure of total quantity of pigments. Foremost 

 among the advantages of the method, therefore, we may place this approach to the 

 ideal of comparable figures related to the total quantity of organic matter present 

 as phytoplankton. These can easily be appreciated by workers in other fields without 

 detailed knowledge of the constituent species, and are therefore less liable to mis- 

 interpretation than figures derived from census-taking methods. The great advance on 

 Krep's method of utilizing the pigment extract from a net haul as a measure of phyto- 

 plankton intensity lies in the knowledge of the approximate volume of water from which 

 the catch is filtered. 



I would insist that in the detailed study of the phytoplankton itself census-taking is 

 still very necessary, and likely to remain so; but that Harvey's method has given us a 

 powerful new line of approach, the more valuable when other methods can be used to 

 check and supplement the data. 



The next advance may be expected from simultaneous use of Nielsen's sedimentation 

 methods, and modifications of Harvey's method such as Riley (1938) and Krey (1939) 



