ECOLOGICAL GROUPING OF SPECIES 283 



The classic foundation for such a subdivision of the phytoplankton into mainly 

 ecological, part arbitrary, ' plankton elements ' is the binary system introduced by Gran 

 (1902) for the description of conditions observed in the north Atlantic. Using the 

 terms introduced by Haeckel (1890), he divides the phytoplankton species into three 

 main groups: 



Oceanic species — entirely holoplanktonic. 



Neritic species — mainly meroplanktonic. 



Tychopelagic species — essentially bottom forms of littoral waters. 



Each of these groups he again divides into arctic, boreal and temperate elements, 

 according to their temperature requirements. It is here that some arbitrary distinctions 

 have to be drawn, owing to the overlapping caused by the variation of temperature with 

 the seasons and the existence of cosmopolitan and other more or less eurythermal species. 



In an attempt to arrive at a satisfactory 'division of the flora', Hendey (1937) has 

 attempted to apply essentially similar concepts. His system, however, which is intended 

 to include all southern seas, is not of much help in considering conditions within the 

 Antarctic zone, where the temperature gradient is very slight and the annual range does 

 not normally exceed 5° C. 



As already noted (p. 269), Hendey experienced difficulty in applying the terms 

 'oceanic', 'neritic', 'holoplanktonic', etc. in the strict connotation originally intended 

 by Haeckel, which we also have found. It is especially pronounced within the Antarctic 

 zone. The reason is not far to seek. Our evidence as to whether the vast majority of 

 marine plankton diatoms are holoplanktonic or meroplanktonic is entirely circum- 

 stantial, and based mainly upon the general distributional data available. The occurrence 

 of resting spores which tend to sink may be regarded as strong evidence that a species 

 should be regarded as neritic, meroplanktonic. The resting spores of comparatively few 

 species are known, however, and it is by no means certain that all must inevitably sink 

 to the bottom. It is conceivable that some might be of such a density that they could 

 be returned to the surface layers in the normal course of the water movements of the 

 regions they inhabit without sinking to the bottom. 



In the northern hemisphere, where much intensive plankton work has been carried 

 out for some seventy years, it is probable that the circumstantial evidence as to whether 

 a given diatom species is holo- or mero-planktonic is usually sufficient to be conclusive. 

 In the Antarctic zone it is not so, and there are two peculiar features of phytoplankton 

 communities in the far south which add to the difficulty of arriving at a clear-cut 

 decision in the matter. First, many undoubtedly holoplanktonic species, to be found at 

 all seasons at the greatest possible distance from land, reach their greatest abundance in 

 regions subject to neritic influence. Secondly, some of the almost certainly mero- 

 planktonic species are able to use pack-ice when they require a solid substratum, and so 

 are able to flourish for a short period in the open ocean at the greatest possible distances 

 from land, for a short period after the pack-ice reaches its northern limit and disperses. 

 The situation is still further complicated by the presence of living diatoms in the pack- 

 ice, which from their general space/time distribution would be classified as holo- 

 planktonic, oceanic species without hesitation, for they are to be found in the open 



