CIRCULATION OF THE MACROPLANKTON 37t 



before analysis (St. 1445, 1000-750 m.) and one attempted haul at St. 1417, 1500-1000 m. 

 failed through premature closing of the net. 



The N 70 V is designed to catch the medium and smaller sized constituents of the 

 animal plankton, and although the average volume of the samples is small in comparison 

 with that of a sample from, say the oblique i-m. net, the number and diversity of the 

 organisms may be very large. A complete analysis of these catches is a very laborious 

 process, and a single sample requires rather more than a full day's work. It occurred to 

 me however that some useful information on the vertical distribution of the macro- 

 plankton might be obtained from an abbreviated or preliminary form of analysis, and I 

 found that it was possible, by picking out and counting only the larger organisms, to 

 work through six or seven samples quite easily in one day. A disadvantage of this 

 method of course is that it may be difficult to define the difference between those 

 organisms which should be picked out, and those which should be disregarded. In the 

 samples at least from this particular region the difficulty was not so great as might be 

 thought. It was found that if those species were picked out which we are normally 

 accustomed to find in the i-m. nets the residue generally consisted of smaller species 

 and larval forms which could be confidently disregarded. The result is very much the 

 same, with the exception perhaps of one or two species, if one picks out only those 

 organisms which can be identified with the naked eye or a low-powered lens. Acting 

 on these criteria I have had no difficulty in estimating the numbers of the more import- 

 ant species of the macroplankton. One or two genera such as Metridia and Limacina 

 were represented in some samples by large numbers of immature specimens which 

 were difficult to identify with certainty and would probably not have been retained in 

 the i-m. net, and these species I have disregarded in the subsequent treatment of the 

 analyses. Of such species as Rhincalanus gigas however, which are easily identifiable, I 

 have counted every specimen recognizable by the naked eye, and a small proportion of 

 these would probably have been too small to be retained in the i-m. nets. 



It has frequently been pointed out that in plankton work of this kind which involves 

 the examination of large quantities of material it is impossible to attain complete ac- 

 curacy in the analysis of the samples. Mistakes in identification may occur now and then 

 and small inaccuracies in counting the individuals are liable to occur, especially if sub- 

 samples are taken. The figures quoted in Table II for instance might require some cor- 

 rections if the samples were subjected to a more careful and prolonged analysis, but there 

 could be no very important change in the order of abundance of the species. Samples 

 from these vertical closing nets however give a much more reliable indication of quanti- 

 tative distribution than the samples from the towed nets such as the i-m. and 70-cm. 

 oblique nets (N 100 B and N 70 B). Differing weather conditions cause much variation 

 in the depth to which the towed nets sink and to the speed at which they move through 

 the water, but the vertical nets can always be hauled at the correct speed through the 

 required depths and closed at the right moment. 



