196 DISCOVERY REPORTS 



From the work of Deacon (1937), Clowes (1938) and Mackintosh & Herdman (1940) it is clear that 

 the physical and chemical features of the environment are disposed in relatively uniform zones all 

 round the Southern Ocean, and Baker (1954) has shown that the distribution of the species of Antarctic 

 plankton are correspondingly continuous in a circumpolar direction. It may be expected, therefore, 

 that the seasonal cycle and latitudinal succession observed in even one transverse (meridional) section 

 are broadly representative of the whole of the Antarctic and sub-Antarctic zones. 



It is desirable to check this, however, in so far as the available data will allow. Repeated lines of 

 stations of the kind worked in 8o° W, o° and 20 E, are not available from other parts of the Southern 

 Ocean, but there are other lines running approximately north and south which can be used at least 

 to show whether or not the vertical and meridional distribution of plankton is similar at a given time 

 of year to that found in 8o° W, o° and 20 E. I have grouped together the following stations and for 

 convenience will refer to them as the lines in 20 W (804-817 and 1990-2004), no° E (877-887, 

 1720-1735 and 2152-2168) and 160 E (912-928, 1675-1684 and 2201-2209). These lines of observa- 

 tions cover two or three different months in each particular area, but from different years. Experience 

 in 8o° W, o° and 20 E (see p. 205), however, has shown that year to year differences in a given area 

 are less than the month to month variations. 



All these lines of stations run approximately in a north-south direction, in which there is the 

 greatest change in normal hydrological conditions. I have made use of stations selected from 

 circumpolar cruises (845-875, 2213-23 10 and 2838-2881), to ascertain whether or not, in comparable 

 months, there is any great variation in standing crop from east to west, in which direction there is 

 least hydrological change. This material also provides additional observations for the study of seasonal 

 and regional variations. 



Although this work is concerned principally with variations in the standing crop of zooplankton 

 in the Southern Ocean it seemed worth while to extend the observations as far north as possible to 

 gain some comparison with conditions in sub-tropic and tropic oceanic waters. For this reason 

 I have included a line of stations across the Indian Ocean in 32 S (1736-1766), and a line of stations 

 running north from 30 S nearly to the equator, in the meridian of 90 E (2886-2895). 



In the initial planning of the work at sea complications due to diurnal migration were mitigated by 

 working full stations at a fixed time of day (20.00 hrs., ship's time) whenever practicable, but in this 

 paper some series of data including both day and night hauls have had to be considered. The relevant 

 diagrams and text (p. 209) show that their comparability is unaffected by diurnal movements of the 

 plankton. 



METHODS 

 Before describing the method of volume measurement used in this work it is worth while to consider 

 briefly the methods used by other workers. 



Measurement of the volume of a sample as a rough estimate of relative plankton concentration was 

 one of the earliest techniques used in the quantitative approach to plankton, and in papers by Schiitt 

 (1892) and Hensen (1895) two methods, usually termed the settlement and displacement methods, 

 were described. 



The settlement method, in which the volume is obtained by allowing the sample to settle in a 

 graduated cylinder, has been used with success by many workers including Huntsman (1919), Hardy 

 (1928) and Gunther (1936). The settlement volume, however, includes the volume of the liquid in the 

 interstices between the organisms which, since this varies with their shape and compactness, makes 

 the method very inaccurate (Hardy & Gunther, 1935, p. 27; Sheard, 1947, p. 10) and particularly 



