40 



best determine the production of organic matter by a given standing crop? The 

 latter problem is briefly discussed in the section titled "Carbon-14". 



Obtaining reliable estimates of the standing crop of phytoplankton depends 

 only in part on instrumentation. The adequacy of any estimate of a plankton 

 population depends in large part on the intensity of coverage both in space and 

 time. Intensive coverage in space is needed because of the patchiness in the 

 distribution of phytoplankton. C. E. Lucas (1940) in his discussion of the phyto- 

 plankton collected with the Hardy continuous plankton recorder stresses the 

 patchiness in the distribution of phytoplankton. Although he found that some 

 dense patches of phytoplankton occupied areas at least ten miles in extent, 

 smaller patches were observed that were probably less than a mile in extent. 

 Other workers have emphasized the tendency of phytoplankton to clump even with 

 very limited areas. A continuous record, such as that obtained by the continu- 

 ous plankton recorder, would have definite advantages over "spot" sampling if 

 we could resolve some of the limitations of this method (for a discussion of 

 these, see Lucas, 1940). In investigations using "spot" sampling, considera- 

 tion should be given to ways of increasing the intensity of coverage. This prob- 

 ably will involve sampling while underway, as well as at stations. Perhaps a 

 modification of the Spilhaus sea sampler, designed to take larger samples, could 

 be developed for sampling while underway. From a stationary vessel, the 

 problem is simplified, for any of a number of standard water-collecting devices 

 may be used to collect phytoplankton samples at a number of depths. To list a 

 few of these, the Nansen reversing water bottle, the Kemmerer water sampler, 

 or the Allen phytoplankton bottle are all effective for trapping a sample of water 

 at any desired depth. 



Due to the small size of many kinds of phytoplankton, there is a problem 

 in concentrating samples of these marine plants without loss. Nets are unsatis- 

 factory, for there is considerable loss through even the finest mesh used in 

 nets. Although retention is more complete when the concentration is done by 

 vacuum filtering, using filter paper, sand, or adsorptive powders, there is 

 some loss, and the technique is time consuming. Previously, the best retention 

 has been obtained by centrifuging, but with the recent refinement of the molecu- 

 lar filter membranes, discussed below, there is now available an effective 

 method of concentrating phytoplankton samples without loss. The analysis of 

 the samples, once concentrated, usually is accomplished by either chlorophyll 

 measurements or by visual counting of cells. Neither method is completely 

 satisfactory for all purposes. 



The Molecular Filter - The molecular filter developed by Goetz and Tsunei- 

 shi (1951), may prove to be of considerable value in assessing the populations 

 of both phytoplankton and bacteria in marine waters. Molecular filter mem- 

 branes are not new, rather it is the present state of their refinement that war- 

 rants their consideration as recent advances in instrumentation. The mem- 

 branes are built up from a mixture of cellulose esters which form a submicro- 

 scopic sponge -like structure of very uniform interstitial dimensions. It is pos- 

 sible to control these dimensions over a wide range in the manufacturing proc- 

 ess, with the result that filter membranes can be made with an effective pore 

 size ranging from 1 to 5000 millimicrons. Another feature of the membranes 

 is the peculiar pore structure, which is very small and uniformly spaced on the 

 top side of the leaf and much wider on the under side. As a result of the pore 

 ■structure, particle retention is restricted to the surface, and it will be complete 

 if the proper pore size has been chosen. Filtering under vacuum can be ac- 

 complished more rapidly with molecular membranes than with paper filters or 

 adsorptive powders. Several types of filters have been developed for use with 

 the molecular filter membranes. 



