47 



For example, with some reduction in the quantity of water used but avoid- 

 ing micromethods, it would be possible to determine salinity, phosphate, nitrate, , 

 nitrite, silicon and oxygen with 400 milliliter of water and have 100 milliliter left for 

 a cursory examination of phytoplankton. In this connection I should mention a 

 recent paper by Jerlov (1951) on the measurement of particle distribution in the 

 sea, based on the photometric determination of the intensity of a Tyndall beam. 

 I think this method could be used in oceanic waters well away from shore as a 

 gross indicator of the quantity of plankton, since Jerlov has shown that the 

 scattering is mainly from particles of greater than 3 microns. The method 

 would replace chlorophyll determinations and other such methods that ordinarily 

 require large quantities of water and a time-consuming filtration process. Jer- 

 lov's discussion and work that I have recently done on the relation between plank- 

 ton and extinction coefficients indicate that the phytoplankton primarily scatters 

 light rather than absorbs it. The relation with the volume of plankton therefore 

 is of a peculiar form, but the deviation from the mean curve is no greater than, 

 for example, in comparisons of chlorophyll determinations with estimated phyto- 

 plankton volunnes as determined from cell counts and measurements. 



Thus the phytoplankton work on such a cruise would comprise (a) a gen- 

 eralized estimate of the quantity of particulate matter; (b) the same water could 

 then be preserved with formalin for later microscopic examination. Complete 

 counts are neither feasible with such large numbers of samples nor possible with 

 such small volumes of water. But a mere 15 minutes examination would be suf- 

 ficient to reveal major aspects of species composition and population size. 

 More complete work could perhaps be done on surface bucket samples. 



DISCUSSION: George L. Clarke 



PLANKTON SAMPLING EQUIPMENT 



Dr. Ahlstrom has given proper emphasis to the unavoidable selectivity 

 of plankton nets. Comment could also be made on the fact that some organisms 

 show a differential avoidance of the net according to whether the hauls are made 

 during the day or during the night. During the daytime there is a difference in 

 avoidance between nets hauled near the surface and those hauled at such depths 

 that the nets cannot be seen due to the lack of light. It should also be mentioned 

 that the effective pore size of the nets changes with age during the life of the net 

 and also changes during the course of each haul as clogging progressively oc- 

 curs. 



Hardy Plankton Recorder - We tested two of these instruments at Woods Hole 

 and found that the conical timing device was rather inconvenient. Perhaps this 

 part of the instrument has been improved. Once the instrunnent is assembled 

 and put in running order it can be handled by the crews of merchant ships but 

 the servicing of the instrument before and after each run must be done by ex- 

 perienced technicians. As Dr. Ahlstrom has pointed out the copepod plankton 

 and smaller forms were so badly crushed by the rolled up filtering band that 

 many of them were impossible to identify. The identification of the plankton is 

 extremely laborious. However, the fact that so many of these instruments are 

 being used from the British laboratories at the present time indicates that this 

 objection must be sufficiently overcome. Technicians who are either exceed- 

 ingly skillful, exceedingly patient, or both, must be employed in making the 

 counts and identifications. This instrument is designed for towing at a single 

 depth only. However, the recorder could perhaps be arranged with an oscillat- 

 ing paravane causing the instrument to change its level periodically and thus 

 sample a stratum of water. Several of these instruments could be towed at 



