76 DISCOVERY REPORTS 



were taken in February 1927: Sts. 160 and 161 on the 7th and 14th ; and WS 67-70 from 

 the 20th to the 23rd. The stations in the Antarctic Zone lie in water of Bellingshausen 

 Sea origin and the phytoplankton belongs to our Group 1. 



PHYTOPLANKTON PRODUCTION IN RELATION 



TO HYDROLOGY 



In Fig. 38 is shown a chart of the phytoplankton production round South Georgia in 

 the December-January survey, 1926-7, as measured by the number of plant cells taken 

 in hauls with the N 50 V nets from 100 m. to the surface. Fig. 40 shows the production 

 as measured by the volume of settled samples. The disadvantages of these two methods 

 of measuring production are discussed in the previous section. The two methods never- 

 theless show a pattern of the same general nature and, as already suggested, a true idea 

 of production probably lies between the two. We see that there are two main regions of 

 more intensive phytoplankton production — one a larger one to the west and south-west 

 and a smaller one to the north-east. 



Discussing this distribution, particularly the larger area of production to the west and 

 south-west, in a paper giving a preliminary account of the work of the expedition in the 

 Geographical Journal, the author (1928) put forward a provisional hypothesis and sug- 

 gested an explanation thus: 



South Georgia is a long narrow island. . .placed almost at right angles to the main westerly drift 

 coming up from the Drake Straits. The currents set up round it will be like those set up round any 

 long object forced sideways through a fluid — the water will be forced in a curve round either end to 

 meet in an eddy some distance behind it, leaving an area of "dead" water immediately against the 

 land. The growth of plant life in the sea is limited, as Atkins has shown, by the quantity of available 

 phosphate. This as the summer advances gets used up in the upper layers. Here, where the main 

 ocean current from the west strikes the continental shelf of South Georgia, there will be an upwelling 

 of water rich in phosphate from the deeper layers on the west side of the island. It is here that we 

 get the densest growth of diatoms, which are carried round either end into the area behind the island. 



This hypothesis was wrong. Whilst the water movements suggested above were 

 correct for the water at a depth of 150 m., see Fig. 171, our hydrologists have shown that 

 in this survey the conditions nearer the surface were not so simple. They have demon- 

 strated, as Mr Clowes has shown in his water- movement chart, Fig. 6, that there are the 

 two currents, one of Bellingshausen Sea and the other of Weddell Sea origin, and that 

 the latter is curved round by the Scotia arc to arrive at South Georgia from the south- 

 east. The Bellingshausen current curves up the western side of the continental shelf and 

 flows in from the north-west. The two water masses mix on each side of the island, and 

 after coming together again to the north are deflected to the eastward by the main west- 

 wind drift. This is described more fully on pp. 8 and 11. There is probably vertical 

 mixing on the western side; but increased phosphates cannot be the cause of the 

 richer phytoplankton on that side, for our hydrologists, those of the Meteor Expedition 

 (Wattenberg, 1926-7) and Ruud on the Vikingen Expedition (1930), have all shown that 

 there is in these waters always a sufficient supply of phosphates and nitrates to support 

 a heavy phytoplankton crop — nowhere would the production of phytoplankton appear 



