CIRCULATION OF THE MACROPLANKTON 377 



high but in the surface waters of both the Antarctic and sub-Antarctic regions the iso- 

 therms are nearly all horizontal. This indicates a stable condition of the upper layers. In 

 September on the other hand the isotherms are nearly all vertical. This indicates con- 

 siderable mixing in the surface layers of both Antarctic and sub-Antarctic water. The 

 conditions in December are intermediate. 



In October the convergence is quite clearly defined at the surface, and the section 

 shows the beginning of a transition from winter to spring conditions. The sinking 

 Antarctic water shows a thrust to the north which is hardly discernible in September. 



Continuous bad weather prevailed in the short interval between the October and 

 November lines, and this is reflected throughout the section in the steepness of the iso- 

 therms which cut the surface in November. Intense vertical mixing has again taken 

 place with a consequent reversion towards winter conditions. The Antarctic convergence 

 was obliterated but it must be considered to lie between Sts. 1475 and 1476. 



VERTICAL DISTRIBUTION OF THE PLANKTON IN 8o° W 



The sections described above, which show the vertical distribution of temperature, 

 are much more accurate and reliable than any sections we can construct to show the 

 vertical distribution of the plankton. The former are based on a large number of tempera- 

 ture readings at accurately located points. Of these there are sixteen in the first 1000 m. 

 at each station. For the plankton sections we cannot determine the concentration of 

 a species at any particular point; we only know its total relative concentration in the 

 six or so columns of water through which the N 70 V is fished. However, the data 

 are sufficient to justify the drawing of a vertical section so long as its limitations are 

 borne in mind. 



The normal series of hauls with these nets is quite appropriate to the positions of the 

 water layers. Thus at any station south of the Antarctic convergence there is no doubt 

 that everything in the samples from 50-0 and 100-50 m. can only have come from the 

 Antarctic surface layer. Hauls from 250-100 m. will usually cover part of the surface 

 layer and part of the warm deep layer, and hauls below this will be exclusively from the 

 warm deep water. The normal series of hauls does not of course reach the Antarctic 

 bottom water. 



The best method perhaps of illustrating the vertical distribution of the plankton is to 

 plot the number counted in each sample in the mean position of the haul. Table III 

 shows the figures for all species of the macroplankton picked out and counted, and there 

 are separate sections for each month in which the line of stations in 8o° W was repeated. 

 The larger hauls are emphasized by heavy type. Allowance has been made for the 

 length of hauls at different depths. Thus a haul from 1000 to 750 m. filters a column of 

 water five times as long as a haul from 100 to 50 m. The numbers of each species have 

 therefore been multiplied by 5 in hauls of 50 m., by | in hauls of 150 m., by 1 in hauls 

 of 250 m., and by h in hauls of 500 m. Each station is plotted in its proper position 

 according to the scale of latitude at the top, but in order to show all the figures clearly 

 it has been necessary to distort the vertical scale slightly in this and the following tables. 



