PHYTO- AND ZOOPLANKTON INTERRELATIONS 277 



the region where few E. superba were met with. The average temperatures for the top 

 50 m. at five stations (13-17) passing out from the coast were: 



2-86° C, 2-97° C, 2-51° C, 2-63° C. and 2-58° C, 



or the average for 150 m.: 



2-70° C, 2-25° C, 1-92° C, 1-99° C. and i-8o° C. 



Temperature cannot be a satisfactory explanation of their distribution, nor will 

 salinity explain it. They are not spread throughout the water of Weddell Sea origin; 

 in the tongue of this water to the south of the island they are present at only a few 

 stations and onlv in small numbers. We have seen on p. 71 that we can distinguish two 

 types of Weddell Sea water characterized by different plankton floras: phytoplankton 

 groups 3 and 4. Whilst the water of "group 4" lies largely to the south it also extends 

 up the east of the island, and here the Euphausians are as abundant in this water as they 

 are in the water characterized by "group 3 ". 



One of the striking features of the December-January survey was the dense produc- 

 tion of phytoplankton in a belt along the south-west side of the island and round the 

 southern end, as is shown in Fig. 38 on p. 74. This figure should be compared closely 

 with Fig. 141 showing the distribution of Euphausia superba and whales. The main 

 phytoplankton region to the south-west and south is unoccupied by Euphausians and 

 whales ; also it will be noted that the small dense concentration of phytoplankton centred 

 at St. 1 24 off the north-east coast corresponds to a small number of Euphausians at that 

 station as well as to a break between two main concentrations of Blue whales. Such a 

 gap between these concentrations of whales would seem to indicate a shortage of their 

 food at this particular point. It seemed possible that the distribution of the areas of 

 dense phytoplankton might be linked in some way with that of Euphausia superba and 

 possibly that of other plankton animals. Salpa fusiformis (Fig. 98) and Parathemisto 

 gaudichaudi (Fig. 87) had a somewhat similar distribution to that of Euphausia superba. 



Since the time of the Challenger Expedition a number of naturalists have noted an 

 inverse correlation between phyto- and zooplankton. Castracane is quoted by the 

 authors of the Narrative of the Cruises of H. M.S. ' Challenger ■' (Vol. 1, second part, 

 p. 931, 1885) as having written regarding the expedition's plankton samples as follows: 

 "Another observation, made during the examination of these surface gatherings, is that 

 when the net yielded an abundance of different forms of microscopic animals, Diatoms 

 were extremely rare ; for this I have been unable to suggest any explanation other than 

 that the Diatoms serve as food to the animals, so that where the latter are abundant the 

 former are few in number." 



Herdman (1888) reports that Mr I. C. Thompson found that "as a rule when the 

 gelatinous algae are present very few of the ordinary surface animals such as Copepoda 

 are found in the tow-net". Again, Herdman (in 1894) reports that Mr E. T. Browne 

 found a "great decrease of Copepoda when the sea is full of Diatoms", and (in 1908) 

 Herdman and Scott refer to a "general rule that Diatoms and Copepoda do not abound 

 together". 



DXI 36 



