ZOOPLANKTON ABUNDANCE IN THE CENTRAL PACIFIC 



121 



TREATMENT OF SAMPLES 



The zooplankton collections received the follow- 

 ing treatment in the laboratory : 



1. All fish eggs and larvae were removed. In 

 all cases these have amounted to a negligible frac- 

 tion of the sample and were omitted from the 

 volume measurement. 



2. Several portions of the remaining sample 

 were examined microscopically, and the various 

 groups of organisms were identified at least to the 

 order. "Where possible without great expenditure 

 of time and effort, identifications were made to the 

 genus and species. A list of constituents was thus 

 compiled for each sample. 



3. All organisms whose longest dimension was 

 greater than 5 centimeters were removed from the 

 sample. Such organisms occurred infrequently 

 and were not considered in the analysis. 



4. By means of a splitting chamber the remain- 

 ing sample was then divided into halves; one-half 

 was labeled and placed in a reference collection, 

 the other was used for the organism counts and 

 displacement-volume determinations. 



5. In making the count, a given portion (usually 

 a fourth, occasionally a half or an eighth) of the 

 half reserved for this purpose, was placed in a 

 15 by 20 cm. counting chamber, thoroughly mixed, 

 and distributed as evenly as possible over the 

 entire cell area. Organisms between 2 and 5 cm. 

 in their longest dimension were enumerated in 

 counting-chamber fields 5 cm. square, without 

 magnification. For counting smaller organisms 

 (0.5 millimeter to 2.0 cm. in longest dimension), 

 the counting cell was placed over a Wolffhuegel 

 plate under a binocular microscope. Organisms 

 in this size category were counted in fields 1 cm. 

 square on the Wolffhuegel plate. Organisms less 

 than 0.5 mm. long were identified but not counted, 

 since the mesh of the nets employed was not suffi- 

 ciently fine to catch these forms quantitatively. 

 In counting organisms of all sizes, either (a) 

 enough fields were counted to yield a minimum 

 count of 100 for each type of organism, or (b) 10 

 fields were counted, whichever was reached first. 

 The estimated number of zooplankters of each 

 species or category in the total sample (minus fish 

 eggs and larvae and organisms larger than 5 cm.) 

 was computed by use of the following formula : 



E= 



C-A 



f-a-n 

 where 



E= estimated number in total sample 

 C= counted number 

 J. = area of cell 



/= fraction of total sample in the counting cell 

 a=area of field 

 n= number of fields counted 



The estimated total number of zooplankters in 

 the sample equals the sum of E values, i. e., the 

 sum of the estimated numbers for each type or 

 group of organisms. The estimated number per 

 cubic meter of water was obtained by dividing 

 the estimated total number in the sample by the 

 cubic meters of water strained. 



6. To measure the displacement volume, the half 

 of the sample used for the organism counts was 

 poured into a draining sock of 56xxx grit gauze 

 to filter off the preserving liquid. The drained 

 plankton was then placed in a 50- or 100-milliliter 

 graduated cylinder, depending upon the size of 

 the sample. By means of a burette a known 

 volume of water was added to the drained plank- 

 ton. The difference between the volume of the 

 plankton plus the added liquid and the volume of 

 liquid alone is the displacement volume or "wet 

 volume" of the plankton half-sample. This fig- 

 ure was doubled to obtain the computed volume 

 of the entire original sample (minus fish eggs and 

 larvae and organisms larger than 5 cm.). AVhen 

 divided by cubic meters of water strained, this 

 gave the volume (in cubic centimeters) of zoo- 

 plankton per cubic meter of water. 



The plankton counts given in this report, there- 

 fore, include those organisms between 0.5 mm. 

 and 5 cm., longest dimensions; the volume meas- 

 urements include all organisms in the sample less 

 than 5 cm. in length. 



Almost all stations occupied on the four cruises 

 were in areas where the depth of water is 2,000 to 

 3,000 fathoms (roughly 4,000 to 6,000 meters). 

 By sampling to a calculated depth of 200 meters 

 (the actual depth attained was probably within 

 10 percent of this) we have done little more than 

 "scratch the surface." No information was ob- 

 tained on plankton abundance and distribution 

 below the sampled depth. Therefore, we believe 



