photograph. These are copepods and are probably the most im- 

 portant group of the zooplankton. Another permanent species is 

 the cylindrical, tube-like worm known as the arrow worm, seen 

 in the lower portion of the photograph. 



To be a bit more specific about the sizes of these organisms, 

 the cell diameter of the phytoplankton ranges from approximately 

 5 U up to 100 p. In some cases, the overall length of a colony 

 of chain-forming diatoms may exceed 100 [}. The zooplankton 

 are considerably larger. Their sizes range from approximately 

 10 p to 10mm. Hence, there is considerable overlap in size 

 between the zooplankton and phytoplankton. The maximal size 

 of zooplankton is difficult to ascertain since some individuals 

 grow from forms which are passively moved by ocean currents 

 to those which are freeswimming organisms. 



These pictures may give a wrong impression with regard 

 to the abundance of organisms in the oceans. They are photo- 

 graphs of concentrates of organisms and do not represent the 

 natural abundance. The concentration of phytoplankton cells in 

 the open ocean ranges from 1 to 10 per milliliter of water, 

 while individual zooplankton organisms are found in numbers 

 of 1 to 10 per liter of water. 



Therefore, whether the initial problem in the study of plankton 

 be taxonomic, chemical, or physiological, the concentration of 

 the orgcLnisnns is almost always a necessary step in their collec- 

 tion. This step is frequently the most difficult one to overcome 

 in the instrumentation of this field. Principally two methods of 

 concentration have been used. These are centrifugation and 

 filtration. Of these, the latter has found most wide use. For 

 exajTiple, phytoplankton biologists frequently measure chloro- 

 phyll by filtering 4 to 6 liters of water through a membrane 

 filter with a pore size of approximately 1 |j . Collection of 

 water in this quantity is not easy, and, in practice, it is pre- 

 sently accomplished by using what is now termed a MODIFIED 

 VAN DORN WATER SAMPLER (fig. 18.3). In addition to having 

 a large water capacity, such a sampler must be non-toxic; the 

 sampler shown in the figure contains no metal, being composed 

 entirely of polyvinyl chloride with the exception of the two 

 plumber ' 8 -helper valves (pure gum rubber). The tension be- 

 tween the valves is applied by gum-rubber tubing, and the 

 valves are kept in an open position by two connected chains 



200 



