28 DISCOVERY REPORTS 



The broad division into four zones based on the hydrology of the surface waters is 

 reflected fairly closely in the quantity of the phytoplankton. The great abundance of 

 the phytoplankton in the Antarctic Zone, with its rich supply of nutrient ions in the 

 surface layers, and diatoms completely dominant over other forms, is well brought out. 

 Proceeding northwards the rapidly diminishing numbers of organisms, running parallel 

 with the average concentrations of nutrient ions in the surface layers, and the increasing 

 proportion and variety of dinoflagellates, is also well shown. It must be borne in mind 

 that the great disparity in bulk between the Antarctic phytoplankton and that of warmer 

 seas is over-emphasized in this material, as the method employed did not permit of the 

 Coccosphaeriales being taken into account, and these nannoplankton forms are un- 

 doubtedly among the most important producers in warm seas. 



The collections from warmer waters did not show any novel features, but they serve 

 to indicate the great richness of the Antarctic Zone, and the fact that in general the 

 quantity of phytoplankton depends on the average nutrient salt content of the surface 

 waters. Certain points of distributional interest also became evident. Among the more 

 noteworthy of these were the facts that Ceratium fusiis appears to find its optimum (in 

 the southern hemisphere) near the northern limit of the sub-Antarctic surface water, 

 and that Dinophysis tripos appears to be confined to the more easterly portion of the 

 South Atlantic. Several dinoflagellates, notably Ceratium gibberiivi f. sinistnmi, were 

 only encountered at the northern end of the line. 



The following list shows the species confined to each type of surface water in the 

 material from the 30° W line. The hauls from the sub-Antarctic and sub-tropical waters 

 were not fully representative, as already explained. An additional reason for the lack of 

 distinctive forms in the sub-tropical water is the considerable overlap of several species, 

 especially to the north of the tropical convergence — the least well defined of the three 

 convergences. 



Peculiar to the Antarctic surface water: 



Astei'oinphalus regularis Fragilaria antarctica 



Chaetoceros curvatus Thalassiothrix antarctica 



Ch. dicliaeta Dinophysis sp. 



Ch. neglect us Peridinium antarcticum 



Peculiar to the sub-Antarctic surface water : 



Rhizosolenia curva Nitzschia sp. 



R. torpedo Goniaidax polygrarnma 



Chaetoceros ? concavicornis Peridinium elegons 



Peculiar to the sub-tropical surface water: 



Asteromphahis Jiepactis Goniaulax sp. 



Ornithocercus steinii 



Pecuhar to the tropical surface water: 



Asterolampra Ch. peruviantim 



Hemidiscus (Eiiodia) cuneiforinis Hemiaulus haucliii 



RJnzosolenia castracanei Gymnodiniiim {Pyrocystis) lunula f. lunula 



Chaetoceros didymum Gymnodinium {Pyrocystis) lunula f . globulus 



