26 DISCOVERY REPORTS 



Returning to the scanty phytoplankton taken in 30" W in sub-tropical water, it will 

 be seen that the more important forms were Ceratiiim varians, Bacteriastrum varians, 

 and, to a small extent, Ceratium fusus. The species C. varians was not found south 

 of the sub-tropical convergence, but occurred at several stations in tropical waters. 

 Its maximum for the line was reached at St. 677 in latitude 31° 16^' S. 



The following is a list of all the species recorded in sub-tropical water during the 

 working of the stations in long. 30° W ; those printed in heavy type were peculiar to 

 this type of surface water. 



Planktotiiella sol, Wallich Goniaulax sp. 



Asteromphalus hepactis (Breb.), Ralfs Peridinium tripos, Murr. and Whitt. 



Bacteriastrum varians, Lauder Peridi7uum spp. 



Rhizosolenia alata, Btw. Ceratimn lineatum (Ehrb.), Cleve 



R. ?stylifortnis, Btw. C. fusus, Ehrb. 



Ornithocercus steinii, Mangin C. varians, Mangin 



O. magnificus. Stein Ceratium spp. 



A study of Table I in conjunction with the above list shows that the sub-tropical 

 water passed through was poor as regards both the numbers of individuals and of 

 species of phytoplankton organisms ; on the other hand the greatly increased proportion 

 of dinoflagellates is well brought out by Table 2 (Sts. 673-7, P- 22). 



THE PHYTOPLANKTON OF TROPICAL 

 SURFACE WATER 



The general characteristics of the phytoplankton population of warm seas are clearly 

 demonstrated by the catches obtained north of the tropical convergence in long. 30° W 

 (Tables I and 2, Sts. 679-99). The more obvious features are the scarcity of diatoms, 

 and the large variety of dinoflagellates, coupled with marked paucity of individuals. 



At only three stations north of the tropical convergence did dinoflagellates fail to 

 provide over 50 per cent of the phytoplankton estimated. At the first of these, St. 679, 

 just within the zone of tropical surface water, which is characterized by complete absence 

 of detectable phosphate in the surface layers, the tropical diatom Hemiaidus hauckii was 

 dominant. At the two equatorial stations (690 and 693), the pelagic blue-green alga 

 Tric/iodestnimn thiebautii predominated, but this is not to be taken as indicating that it 

 is confined to tropical seas. On the outward voyage it was met with in vast quantities 

 two days sail south-east of the River Plate in a position which must have been very near 

 the sub-tropical convergence. Darwin (1889, p. 11) describes the appearance of a 

 similar dense swarm of the smaller, closely allied species, Trichodesmium erythraewn, 

 encountered off the Brazilian coast in about 18° S lat. Our finding of T. thiebatitii in 

 quantity in about 40" S on the outward voyage apparently extends its known range 

 considerably to the southward, for Wille (1903, p. 18) quotes C. H. Ostenfeld as giving 

 its range in the Atlantic as from 59° N to 26° S latitude. At the few tropical stations at 

 which it was taken on our return voyage it was very scarce, nothing being found re- 

 motely resembling the dense patches discolouring the seas, and earning the nick-name 



