THE PLANKTON OF THE SOUTH 



GEORGIA WHALING GROUNDS 



AND ADJACENT WATERS, 



1926-1927 



(Text-figs. 1—193.) 



PART I. INTRODUCTION, HYDROLOGY AND METHODS 



By A. C. Hardy, m.a. and E. R. Gunther, m.a. 



GENERAL INTRODUCTION 



The Blue, Fin, Sei and Humpback whales {Balaenoptera muscuhis, B. physalus, 

 B. borealis and Megaptera nodosa) with which the Discovery Investigations are 

 primarily concerned are well known to be plankton feeders. They come in the southern 

 summer to feed in the Antarctic waters, which are rich in food ; and at the approach of 

 winter return to the warmer and lower latitudes for breeding. A knowledge of the 

 plankton of these southern regions is therefore important in an attempt to understand 

 the natural history of these whales. 



It has been shown by Mackintosh and Wheeler (1929) that the food of at least the 

 Blue and the Fin whales caught at South Georgia consists almost entirely of one species 

 of Euphausiacea : Euphausia superba. 



Whilst particular attention must be paid to the study of Euphausia superba, it is also 

 desirable to investigate the main features of the whole plankton community of which it 

 forms a part. It is important to study the production of the phytoplankton upon which 

 these Euphausians in turn depend for food. It is likewise important to study the more 

 abundant zooplankton forms which may be either competitors with the Euphausians or 

 actually enemies preying upon them. The Hyperiid amphipod, Parathemisto gaudichaudi, 

 has been found at times to swarm in vast numbers in this region, feeding largely upon 

 young Euphausians. The varying conditions found in the plankton community, par- 

 ticularly in the quantities of phytoplankton produced, must if possible be correlated 

 with the changes in the physical and chemical environment as determined by the 

 hydrologists. 



From another point of view, a knowledge of the planktonic changes in time and space 

 throughout the area of investigation is of value, in that it may afford supplementary 

 evidence to that of the physical and chemical observations in the recognition of the 

 main different water masses and their movement. Such knowledge will also be of 

 importance in understanding the migrations of the whales. 



Comparable plankton samples have been collected over a wide area of the Southern 

 Ocean by means of standard closing vertical and horizontal tow-nets of fine, medium 



