DISTRIBUTION OF ANTARCTIC MACROPLANKTON 87 



also normal inhabitants of sub-Antarctic surface waters, while others are sufficiently 

 rare in the latter zone to be regarded as intruders if they are found there. 



DIURNAL VARIATIONS 



During the daytime some species sink to a depth which is beyond the reach of the 

 net, while others do not. Therefore, in order to trace the distribution of the macro- 

 plankton we must have some idea of the hours between which a haul will be indicative 

 of the presence or absence of each species. The following section of this paper is, how- 

 ever, confined to the study of diurnal variations as they affect the catches in the N 100 B, 

 and should not be taken as an attempt to investigate the vertical migrations of the 

 different species. Diurnal variation is, so to speak, an adventitious phenomenon which 

 is caused by vertical migrations, and the proper study of the latter should depend mainly 

 upon hauls taken at different depths with closing nets. Hardy and Gunther (1934) have 

 studied in this way the vertical migrations of certain Antarctic species, and reference to 

 their results is made on p. 96. 



The majority of the N 100B samples from Antarctic water have been collected in- 

 discriminately at all hours of the day and night, so that an estimation of the diurnal 

 variations could be worked out for each species if a comparison were made of the average 

 number per haul for various times of day. The accuracy of the results of this calculation 

 might be disturbed by three factors: (i) the irregularity of distribution of the plankton, 

 (ii) the possible effect of the difference in the period of darkness in different latitudes, 

 and (iii) the varying depths from which the net is fished. These difficulties cannot 

 altogether be disposed of, but the quantity of data is sufficient to swamp any serious 

 error arising from distribution, and we can afford to restrict the estimation almost 

 entirely to samples taken between 52 and 6o° S. This will include the great majority of 

 stations without too great a range of latitude. Errors arising from the different depths 

 at which the net begins its oblique passage towards the surface will also be largely dis- 

 counted through the abundant data. The calculation will of course be rough, but 

 sufficient for our immediate purpose. 



It must be remembered that the diurnal variations revealed by this method are those 

 which result only from the more extensive vertical migrations. There may, for instance, 

 be certain species having a well-defined vertical migration within the limits of the 

 Antarctic surface layer, and these might seem here to show little or no diurnal varia- 

 tion. 



Of previous work on vertical migrations that of Russell (1925-31) is the most im- 

 portant, but this was done on a much finer scale in the shallow water of the English 

 Channel. It revealed movements of a kind which could not be detected in the N 100 B 

 and took into account various subsidiary factors which must be ignored here. 



In working out the variations for each species I have omitted the following stations: 

 (i) All those south of latitude 6o° S. (ii) Those in which it was not possible to make a 

 reliable estimate of the numbers of the species in question (such as sample? which were 



