ZOOPLANKTON IN THE SOUTHERN OCEAN 199 



As well as removing phytoplankton, it has also been my procedure to pick out any large animals 

 from samples before measuring the volume. 



As Mackintosh (1937, p. 371) has pointed out, the N70V samples only the smaller and medium- 

 sized constituents of the plankton, and thus copepods, chaetognaths, small euphausiids and small 

 amphipods numerically form the bulk of the catch. Often, however, comparatively large inactive 

 animals are caught, such as siphonophores, medusae and salps, which if they were included would 

 increase the volume disproportionately in relation to the number of animals present. For instance, 

 the 500-250 m. samples from Sts. 2379 and 806 had volumes of 0-4 and 3-2 cc. respectively. The sample 

 from St. 2379, however, contained two siphonophore nectophores having a combined volume of 

 2-8 cc, and it is obvious that if this volume were included to bring the total to 3-2 cc. any comparison 

 between the stations would be most misleading. Salps in some cases would influence the volumes to 



Table 2. The number of samples from which particular groups of organisms were removed 

 and the numbers expressed as a percentage of the whole 



* Includes mainly salp tests and siphonophore fragments. 



an even greater extent, since they are animals that occur in swarms and are hence sometimes caught 

 in extremely large numbers. At St. 2585, for instance, in the 50-0 m. haul there were 625 salps 

 having a combined volume of 169-0 cc, and these must obviously be omitted. In addition to these 

 inactive forms there are occasional active swimming animals, such as fish, large euphausiids (e.g. krill 

 above 20 mm. in length) and large amphipods, which are caught by the N70V. These animals I have 

 regarded as atypical of N70 catches and have picked them out. Table 2 gives the number of samples, 

 expressed as a percentage of the total number, from which animals of a particular group and phyto- 

 plankton were extracted before the volume was measured. It is seen that by far the greater percentage 

 picked out were animals of a gelatinous nature, the more active animals being relatively rare. (It 

 should be noted that this table does not give the numbers of animals but only the number of samples 

 from which they had been previously extracted.) 



In view of the particular types of organism removed from the sample before measurement, the 

 volumes adopted in this work may be taken to represent the variations in that part of the total standing 

 crop of plankton composed mainly of copepods, chaetognaths and small euphausiids (which includes 

 krill larvae). The importance of individual species volumetrically is not known. It would obviously 

 depend upon the number and size (i.e. stage of maturity) in each sample and would vary from station 

 to station, depth to depth and month to month. Such variations, however, are only of importance 

 when considering the interrelationships of species, while in this case only gross changes in the bulk 

 of zooplankton are being examined. 



