SUBTROPICAL WATER 63 



Plate IX, Sts. 844-847), between Cape Town and Enderby Land, the subtropical water 

 had a surface content of 260 mg. in the north increasing to a value of 410 mg. in the 

 south of the zone. In the third week of May 1934, i.e. just over two years later, silicate 

 was at zero in the surface water from the Crozets to Durban (Sts. 1 370-1 373), whilst 

 exactly three years later in the second week of April 1935 between Marion Island and 

 Durban (Sts. 1 566-1 569) there was less than 250 mg. of silicate throughout the zone. 

 Evidently although there is not much silicate in the surface water in the subtropical 

 zone in April-May there are indications of an annual variation. 



South of Australia our observations have been restricted to two stations south of 

 Fremantle in the third week of May 1932, when (Sts. 877 and 878, section 8, Plate 

 XIII) the surface value was of the order 160-210 mg. 



East of New Zealand but quite close to the North Island, the surface value in late 

 January 1934 at St. 1281 was zero. There are no observations in subtropical water else- 

 where in the South Pacific Ocean. 



Obviously our present knowledge of the silicate content of subtropical water is 

 insufficient for any real discussion and all we can say is that in the months of January, 

 March, April and May very little silicate is present in this water; even this small 

 quantity may sometimes be absent. 



ANTARCTIC INTERMEDIATE WATER 

 PHOSPHATE AND SILICATE CONTENT 



The Antarctic intermediate current has its origin in the region 100-200 miles north 

 of the Antarctic convergence and consists of the heavier and colder portion of the mixed 

 water found there. It sinks towards the north and can be traced as a poorly saline layer 

 situated below the highly saline surface and subsurface layers and above the highly 

 saline warm deep water. It is characterized by an intermediate salinity minimum and, 

 in certain parts of the ocean, in particular the western part of the South Atlantic Ocean, 

 by an intermediate temperature minimum. This temperature minimum is caused by 

 the bottom part of the current having a lower temperature than the upper layers of the 

 warm deep water. The colder water in the bottom of the intermediate current is a remnant 

 of the cold stratum of the Antarctic surface water. Whether the intermediate temperature 

 minimum is shown or not the boundary between the intermediate and warm deep 

 currents is usually shown by the temperature gradient becoming very much less steep 

 at the boundary zone. 



The Antarctic surface water has a strong northerly component of movement: the 

 phosphate in this layer is contained either in the plankton or in the surface layer itself. 

 The density of the phytoplankton in Antarctic surface water varies enormously with the 

 season of the year, and it follows that the amount of phosphate in the surface layer in 

 the Antarctic zone also varies with the season ; this is because, when the phytoplankton 

 concentration is increasing enormously, as it does at the time of the main outburst, it 

 is using up and withdrawing phosphate, amongst other nutrient salts, from the sur- 

 rounding sea water. Consequently the seasonal effect is of paramount importance to 



