MAIN TYPES OF PHYTOPLANKTON 25 



THE PHYTOPLANKTON OF SUB-TROPICAL SURFACE WATER 



The phytoplankton of the sub-tropical surface water on this line of stations was 

 extremely scanty (Table L Sts. 673-7, also Table 2, p. 22). As already explained the time 

 of year was probably responsible for this, and the method of collection for the absence 

 of Coccosphaeriales. In normal circumstances one would expect the phytoplankton of 

 sub-tropical surface water to be intermediate in bulk between the sub-Antarctic and the 

 tropical phytoplankton, apart from local exceptions like that off the south-west coast of 

 Africa. There, in the Benguela current, and also in the Agulhas current on the Indian 

 Ocean side, a moderately rich phytoplankton was found in sub-tropical water ; but the 

 upwelling caused by the incidence of these currents with the continental shelf on their 

 right hand, and by offshore winds, leads to a much richer supply of nutrient ions in 

 the surface water than in the oceanic sub-tropical water we are considering here : the 

 temperature is also relatively lower particularly in the Benguela current. It may be men- 

 tioned in passing that characteristic forms noted in some abundance round the Cape 

 were: small Chaefoceros, notably Ch. furca, Bacteriostrim varians, Stephampyxis sp., 

 Planktoiiiella sol, Corethron pelagiaim, and Dinophysis tripos, Gourret (= D. honnmcidus 

 var. tripos, Paulsen), this last often in catena. Subsequently a species of Stephanopyxis 

 has been found as the dominant form in phytoplankton collected oft" the coast of 

 Patagonia by Mr John, and this was probably in sub-Antarctic water ; but the distinction 

 between sub-Antarctic and sub-tropical surface water between the Falkland Islands and 

 the mainland would seem to be much less well defined than in the open ocean, and it is 

 probable that Stephanopyxis will be found to be a typical dominant form near the 

 southern boundary of inshore sub-tropical waters. 



Among the species of dinoflagellates recorded from sub-tropical water by Mangin in 

 his examination of the material collected during the homeward voyage of the ' Scotia ', 

 Dinophysis tripos (= D. homunctthis) had a very significant distribution. It was found 

 throughout the sub-tropical Zone, but reached its maximum in 39° 48' S, 02° 33' E, 

 probably slightly to the south of the sub-tropical convergence (surface temperature 

 12-1° C). This species was not encountered during our return voyage, but as the route 

 followed by the 'Scotia' was much more easterly (8° W to 16° E longitude) in the 

 southern half of the Atlantic, and as we found it in some abundance off the south-west 

 coast of Africa, it is possible that it is confined to the eastern half of the South Atlantic. 

 Some support for this view is furnished by the fact that these three series of observations 

 were made at the same time of year (autumn). 



The distribution of D. tripos as given by Mangin (1922, p. 72 and tables) in more 

 northerly latitudes is very interesting. He records it as far north as 22° 38' S in 

 long. 02° 08' E (under the synonym D. homunculus). There is then a complete gap in 

 the Scotia material until it turned up again in 29° 54' N, 34° 10' W. It is evidently a 

 temperate and sub-tropical species, and the above distribution is in agreement with the 

 strong northerly trend of the tropical convergence in the eastern half of the South 

 Atlantic which we have already briefly mentioned. 



