ZOOPLANKTON IN THE SOUTHERN OCEAN 201 



The meridian of 0°, 1938-39 season (Fig. 1) 



The April line of stations started much farther south than those in the following months and all 

 stations were in Antarctic water. From the diagram it is apparent that while most of the standing crop 

 of plankton was concentrated in the top 50 m. of the Antarctic surface water large volumes occurred 

 in hauls below 250 m. at the two northernmost stations. 



There were no observations in May and July, but in June (winter) the shallow hauls were relatively 

 poor in plankton compared with those in April. The concentration of plankton in the deeper hauls, 

 however, had increased, particularly at Antarctic and southern sub-Antarctic stations. 



In August the volumes of catches at all levels were poorer than those of the previous month, but 

 the bulk of the plankton was below 250 m. in the warm deep current, except at Sts. 2389 and 2391 

 where the hauls from 50 m. to the surface had large volumes consisting mainly of small euphausiids 

 and copepods. 



In September (late winter) the standing crop was still concentrated in deep water, and samples 

 from the 750-500 m. hauls consistently gave the largest volumes. As in August there are small 

 concentrations in the northern Antarctic surface water. 



The October (spring) results show that there was a rise in the vertical distribution of the bulk of the 

 plankton, which was concentrated above 250 m. at sub-Antarctic and Antarctic stations. This apparent 

 upward migration of plankton seems to have proceeded further at St. 2463 which was just south of the 

 Antarctic convergence. 



In December the concentration of plankton had increased in the top 100 m. at the three southernmost 

 stations, although at one of them (St. 2496) there was a large volume at the 500-250 m. level due to the 

 presence of many Rhincalanus gigas in the sample. At two of the other sub-Antarctic stations the 

 plankton did not appear to be concentrated above the 250-100 m. level. 



In January the observations extended farther south than in the previous months owing to the 

 recession of the ice edge. The vertical distribution was similar to that in December with the bulk 

 of the plankton concentrated in the top 100 m. in Antarctic surface water. St. 2535 just south of the 

 Antarctic convergence had large volumes at nearly all depths, but at the other Antarctic stations the 

 volumes were on the whole less than in the previous month. In sub-Antarctic waters the largest con- 

 centrations were also between 100 m. and the surface compared with 250-100 m. in December. 



In late February and early March the Antarctic surface layer appeared to be poor in plankton 

 except in the region of the convergence, at St. 2586. The volumes at the sub-Antarctic stations were 

 similar to those in the previous month. 



The meridian of 20 ° E, 1938-39 season (Fig. 2) 



In April the volumes of samples from Antarctic stations were not as great as those in o°. The vertical 

 distribution of the plankton, however, was basically the same, with the largest concentrations in the 

 Antarctic surface layer but with large volumes also in the warm deep current. The sub-Antarctic 

 surface waters appeared to be poor in plankton, while the volumes of the deeper hauls, particularly 

 at St. 2348, suggest that the largest concentrations were deeper than in the Antarctic. 



In July both sub-Antarctic and Antarctic surface waters were poor in plankton except at St. 2374. 

 The volume of plankton in the deeper hauls down to 1000 m., however, had increased. 



In August the concentration of surface plankton had decreased still further except for the 50-0 m. 

 haul at St. 2412, which contained many small euphausiids. The bulk of the plankton, as in o°, is 

 concentrated in the warm deep layer, and in the Antarctic the largest volumes occurred at the 1000-750 

 and 750-500 m. levels as compared with the 500-250 m. level in July. 



