i 5 o DISCOVERY REPORTS 



are very uniform, and the Chaetognatha increase towards the convergence, but the 

 commonest copepod at Sts. 657, 658 and 659 is Calanus propinqnus, Rhincalanus being 

 dominant only at St. 656. Apart from this, however, the plankton is similar in its make- 

 up to the plankton found on other lines in this area. 



Enough has been said now to show that the northern zone contains, during the 

 summer, a plankton population sufficiently characteristic to form the basis of a separate 

 faunistic area. The most important features of this plankton may be summed up as 

 follows. 



It is a population of homogeneously distributed typically warm-water species. 

 Rhincalanus is generally the most abundant species and almost always has a large 

 majority over other copepods. The Chaetognatha are nearly always present in large 

 numbers near the convergence and in steadily decreasing numbers to the south or 

 south-east of it. Calanus simillimus, Pareuchaeta sp. and Euphausia frigida are nearly 

 always present in small or moderate numbers in the night hauls. Owing to their marked 

 diurnal variations they are rarely taken in day hauls, but it is evident that they are 

 usually quite evenly distributed over the whole area. The occurrence of Calanus pro- 

 pinquns and Thysanoessa is a little irregular, but if they occur in large numbers at one 

 station in a line across the northern zone, it is probable that they will be taken also in 

 large numbers at other stations in that line. 



In general, it can be said that, in the northern zone, the summer plankton is both 

 uniform and stable. That is to say that, compared with other parts of the area in- 

 vestigated, excepting perhaps the eastern Weddell Sea, a uniform type of plankton 

 community is to be found everywhere at a given time, and that the plankton does not 

 alter very much from month to month or from year to year. The northern zone seems 

 to be a little more uniform and stable in the Drake Passage than it is between the Falk- 

 land Islands and South Georgia. In the latter region, however, there is known to be an 

 eddy in the currents near the convergence, and this might be expected to cause some 

 irregularities in the distribution of the plankton. 



FAUNISTIC DIVISIONS 



We are now in a position to map out the Weddell sector of the Antarctic in respect 

 of the different types of plankton which have been found there, and it is possible, in 

 my opinion, to distinguish seven separate divisions. These are shown in Fig. 48 and they 

 may be defined as follows. 



(1 ) The northern zone. This zone is bounded to the north by the Antarctic convergence, 

 and to the south by the region of scarce plankton in the vicinity of the South Shetlands, 

 and by the beginning of the transition zone in the vicinity of South Georgia. It is 

 characterized by a uniform and stable plankton, in which Rhincalanus gigas is generally 

 predominant ; the Chaetognatha and Limacina balea are plentiful, especially near the 

 convergence, and the typically warm-water species are fairly evenly distributed. 



(2) The Graham Land area. This includes the whole of the region of scarce plankton 



