igo DISCOVERY REPORTS 



Stood, though it seems fairly certain that the continual mixing of the surface layers 

 during the more than usually violent gales of the spring equinox, and the fact that the 

 northward moving pack-ice does not begin to melt and disperse much before November, 

 will be largely responsible. It is a notable fact that the great differences in climate of 

 the two hemispheres, with the Antarctic summer very much colder than the Arctic, and 

 the south polar influence extending much farther towards the Equator, are closely re- 

 flected in the hydrological and plankton conditions. The commencement of the spring 

 increase off South Georgia in November corresponds exactly with Vanhoffen's (1897, 

 pp. 264, 289) observation that the main diatom increase in Karajak Fjord, North 

 Greenland, some fifteen degrees farther from the Equator, occurred in May. 



The details of the influence of light on phytoplankton production have been in- 

 tensively studied by Marshall and Orr (1928 and 1930), who have succeeded in estab- 

 lishing the compensation point (balance depth of Gran), or depth at which gain by 

 photosynthesis is balanced by loss in respiration, for the latitude of the Clyde at different 

 times of year. By happy chance this latitude is almost directly comparable with that 

 of the southern end of South Georgia, so that from their findings we may assume 

 that production in the northern part of the Antarctic Zone proceeds actively only above 

 a depth of some 35 m. in summer and considerably less in autumn and spring. In 

 making this estimate due account has been taken of the facts that the almost continuous 

 rough weather lessens the penetration of light, but that conversely, in the Southern 

 Ocean, owing to the inconsiderable extent of the land, the adverse factor of inorganic 

 particles decreasing the penetration of light will only be felt in extremely localized areas 

 (e.g. Bays of South Georgia, see Appendix I). The above authors have also shown that 

 at the surface in winter, and to a somewhat greater depth at midsummer, direct sunlight 

 slows down synthesis in diatoms, inducing systrophe, or contraction of the endochrome 

 (1928, p. 325). Proceeding southwards from South Georgia the level of both these 

 critical points will obviously rise with the increasing obliquity of the sun's rays. With 

 the narrowing of the depth range over which diatoms can actively reproduce and the 

 more stable conditions prevailing in the far south after mid-season we have a plausible 

 explanation of the observed differences in the phytoplankton from the more northerly 

 and the more southerly parts of the area. To the south it was moderately rich and uni- 

 form, while to the north it was sometimes extremely rich and much more variable. It 

 may be noted that there is some ground for believing that the phenomenon of systrophe 

 near the surface is due not so much to the total light intensity being harmful to the 

 diatoms, as to the action of the shorter ultra-violet rays.^ When the sun is at a low altitude, 

 the proportion of long to short wave-length light falling upon the surface of the water 

 is increased (Harvey, 1928, pp. 155, 160), and hence we may conclude that if systrophe 

 is induced by the shorter uhra-violet rays this adverse effect will be much reduced in 

 high latitudes. This would further accentuate the value of shallow discontinuity layers, 

 such as are found near melting ice, and may be the explanation of the preponderance of 



1 The shorter ultra-violet rays are absorbed rapidly, the longer penetrate farther than light of any other 

 quality. 



