ioS DISCOVERY REPORTS 



temperature was taken only at the surface. However, the mean surface isotherms bear 

 a sufficiently uniform relation to the mean isotherms at 50 or 100 m. and will serve our 

 purpose here. Since the great majority of stations have been taken in the summer 

 months a much more reliable chart can be constructed of the mean summer isotherms 

 than of the mean annual isotherms. Either would suffice, however, for we are seeking 

 a relative and not an absolute correlation. Fig. 22 shows the mean summer isotherms 

 and is a provisional chart derived from temperatures recorded in the three seasons 

 1928-31. 



In Table III the species are arranged in order of their apparent preference for warm 

 or cold water. The figures in the table represent the average number of each species 

 per haul. Samples have been disregarded where the haul was made at a time of day when 

 the diurnal variations of any particular species precluded the taking of a representative 

 sample. Catches in the Bransfield Strait, where the plankton is disproportionately thin, 

 have usually been omitted, and for all but one or two of the rarer species, only one 

 station has been counted for each line of stations in each South Georgia survey. The 

 end station of the line is taken, or the outermost one permitted by the diurnal variations. 

 The actual figures are much influenced by variations in the general richness of the 

 plankton in different localities, or by patchiness of the plankton, and are therefore 

 individually unreliable. Thus the "— i°" column depends almost entirely on a few 

 stations in the Bellingshausen Sea, west of Peter 1st Island, where the plankton was 

 particularly rich, and the figures in this column are consequently deceptively high. The 

 actual figures are therefore used only as the principal basis of a general grouping of the 

 species according to the temperature belt they mainly inhabit. 



The table shows the distinction between the warm-water species, the widespread 

 species, and the cold-water species. The neritic species are unimportant and are shown 

 in a separate category. There is of course no very hard and fast distinction between the 

 main groups or the subsidiary groups, and a larger body of data might bring about some 

 modification of the order in which the species are listed. 



It is interesting to compare this table with the table of mean diurnal variations, for 

 it will be seen that the warm-water species are mostly those which have the most 

 marked diurnal variations and many of the cold-water species have no significant 

 variations. There are exceptions, for Parathemisto shows little diurnal variation, while 

 Salpa and Metridia vary considerably. Hardy and Gunther (1934), however, find a 

 distinct vertical migration in Parathemisto. The phenomenon is no doubt partly con- 

 nected with the reduction or absence of darkness in summer in the high latitudes in- 

 habited by the cold-water species, but the question will not be pursued any further here, 

 since it can be more properly dealt with when the samples from the 24-hour stations 

 have been finally worked out. 



