CIRCULATION OF THE MACROPLANKTON 375 



During the summer season, when the vast majority of observations on the Antarctic 

 plankton have in the past been made, the organisms are mostly concentrated in the 

 surface layer day and night. Some of the more abundant species have no very marked 

 diurnal migrations, and many others do not make sufficiently extensive journeys into the 

 warm deep water to counterbalance their northward transport in the surface layer. 

 Observations on the Antarctic plankton in winter have been comparatively few, but it 

 will be shown in the following pages that in the meridian of 8o° W, and in other places 

 where samples have been collected in winter, a large part at least of the plankton at 

 that time of year has descended bodily into the warm deep water. 



HYDROLOGICAL SECTIONS 



Mr A. J. Clowes of the Discovery staff has kindly assisted me in plotting the tempera- 

 ture sections shown in Fig. 3 and has drawn my attention to the more important 

 features in each. 



Fig. 3 a shows the vertical distribution of temperature in 8o° W in December 1933. 

 Half of this line of stations was in Antarctic water and half in sub-Antarctic water, the 

 Antarctic convergence being crossed between stations 1224 and 1225. This convergence 

 may be defined as the point at which the Antarctic surface layer sinks below 250 m. The 

 Antarctic surface layer is seen here as a tongue of cold water in the upper left-hand part 

 of the section. The water in it varies in temperature from —1-5° C. to nearly +2-0° C. 

 It is also seen that the convergence is marked by a sharp rise of temperature at the sur- 

 face. The upper right-hand part of the section is occupied by sub-Antarctic water and 

 some intermediate isotherms are inserted as pecked lines below it to show how the cold 

 Antarctic water pushes on northwards to become eventually the ' Antarctic intermediate 

 layer'. This is a layer which is not always very clearly defined immediately to the north 

 of the convergence, for here considerable mixing takes place with other layers. The 

 ' warm deep ' water occupies the lower left-hand part of the section. Below the Antarctic 

 surface water it varies in temperature from +1-5° C. to 2-5° C. The shape of the 2-0 

 isotherm gives a good idea of the southward, and slightly upward, thrust of this body of 

 water. 



The same layers of water appear in much the same positions in March, but tempera- 

 tures in the surface layers are higher all round, especially on the surface itself where the 

 temperature at any given point is about 2° C. higher that at the corresponding point in 

 December. (Temperatures in the Antarctic are usually expected to reach the maximum 

 late in February.) A peculiarity of this section is that although the true Antarctic con- 

 vergence lies between Sts. 13 16 and 13 17 a shallow layer of sub-Antarctic water has 

 been pushed southwards over the Antarctic surface water a little past St. 13 15. Here we 

 have five stations in Antarctic, and four in sub-Antarctic water. 



In September the pack-ice lay far to the north and only two stations could be worked 

 between it and the convergence. The latter is very poorly defined but it must be between 

 Sts. 1416 and 1417. The sections for March and September show the principal contrast 

 between summer and winter. In March not only are the surface temperatures relatively 



