66 DISCOVERY REPORTS 



current, after the manner of certain better known water movements in the northern 

 hemisphere,^ may be found to oscillate considerably from year to year in correlation with 

 extensive climatic changes farther south, where this current has its origin. The operation 

 of some such factor, perhaps augmented by strong south-easterly winds, appears to 

 afford the most reasonable explanation of the unusual northerly extension of the dense 

 belt of phytoplankton up the western side of the island at this time. 



On this survey the total number of phytoplankton organisms was estimated by 

 Hardy at 1,930,948,760. From his lists it would appear that the principal species were: 



Chaetoceros socialis Fragiloria a?itarctica 



Nitzschia seriata Rhizosolenia styliformis 



Halosphaera viridis (at three stations only) Chaetoceros atlanticus 



Corethron valdiviae Ch. criophilum 



in that order of importance. All these, with the exception of Halosphaera viridis, were 

 also abundant in the spring of 1930-1, and generally speaking the first survey appears 

 to have been worked under more normal conditions than the later summer survey 

 of 1929-30. The major differences between the three surveys are summed up in 

 Table 13, in which the estimated numbers of the twenty leading forms upon each of 

 them have been tabulated. The species here have been arranged systematically, in order 

 to avoid repetition, and the dashes do not necessarily indicate that a species was entirely 

 absent, but merely that it was not one of the twenty leading forms on the given survey. 



From this list it will be seen that at mid-season 1926-7, most of the species were con- 

 siderably less abundant than in the spring of 1930, but that two — Nitzschia seriata and 

 Halosphaera viridis — were more so. Of the eight principal species encountered by 

 Hardy, all except the two just mentioned show some faUing off in numbers compared 

 with the catches obtained in spring. This was doubtless due to the normal seasonal effect, 

 most of the southern forms showing a marked spring maximum. 



On the other hand several forms encountered in abundance on the spring survey were 

 definitely rare on the mid-season survey, notably Thalassiosira antarctica. Hardy (in 

 press) has already shown that this species was abundant early in the season he studied 

 and the collections here described fully confirm his suggestion that it is very definitely 

 a spring form. In this respect its behaviour resembles that of Th. nordenskjoldii in 

 north temperate latitudes (Johnstone, Scott and Chadwick, 1924, pp. 62, 138). 



The differences in both the quality and quantity of the phytoplankton obtained on 

 the January-February 1930 survey, and on that described by Hardy are so great that 

 there can be no doubt of the abnormality of the conditions during the latter part of the 

 1929-30 season. We have already remarked on the unusual mildness of this season, 

 which was accompanied by marked poverty of the phytoplankton, in which dino- 

 flagellates were much more numerous than is usual in Antarctic surface waters. The 

 1929-30 season presented conditions clearly constituting one of the major fluctuations, 

 as distinct from the normal seasonal variations. The most probable explanation of the 

 unusually poor phytoplankton and high surface temperatures that prevailed over the 



1 E.g. the incursion of Atlantic water into the North Sea. See Hardy, Publ. de Circ, No. 78, 1923. 



