2i8 DISCOVERY REPORTS 



Long after he produced this interesting hypothesis the author (1957) referred to it in the following 

 passage. ' We have believed . . . that the absence of large quantities of zooplankton often observed in 

 areas with mass "blooming" of phytoplankton, is a result not of the frightening or "exclusion" of 

 animals but of the seasonal plankton development. If this is true, the plant community is, naturally, 

 first to develop '. 



The same concept has recently been invoked by Beklemishev (19586) to explain direct and inverse 

 relationships between diatom and copepod abundance in the Okhotsk and West Bering Seas, the 

 major factors involved ' being the movement of biological seasons from the south northward and the 

 difference of seasonal states of adjacent plankton communities '. 



Many years ago Fish (1925) wrote, 'the great numbers of diatoms filling the water apparently cause 

 conditions unfavourable for animal life of any sort. The macroplankton seems to be literally choked 

 out '. He was not, however, altogether convinced that such a phenomenon could in fact be taking place 

 and merely offered this as a possible explanation of the inverse relationship between density of plant 

 and animal plankton he had observed in the Woods Hole region. Although possibly unfavourable for 

 some animal life, dense concentrations of diatoms may not, however, be unfavourable for euphausians. 

 Lasker (i960), working with the swarming Euphausia pacifica from the Californian coast, finds that if 

 healthy specimens of this species be put into a heavy suspension of algae (enough cells to tint the 

 sea- water green), they ' quickly filter out enough algae to completely fill the intestine, digestive gland, 

 and crop ', suggesting rather strongly that a rich phytoplankton need not be harmful to the southern 

 krill, but rather the reverse. 



Effect of pack-ice 



Dense concentrations of larval and adolescent krill are frequently encountered at the ice-edge, 

 particularly in winter and spring, and Fraser (1936, pp. 158-9) suggests two explanations to account for 

 this phenomenon. First, he says, the cutting down of the sun's rays by the ice, and the interruption 

 of the rhythmic vertical movements of the larvae which follows, lead to a concentration of young krill 

 at the surface whatever the hour of the day or night. Secondly, he suggests that the state of the 

 illumination of the winter sea, already poor and now aggravated by the presence of the pack, and the 

 pronounced scarcity of diatoms that goes with it, will result in a movement of larvae from deeper 

 inside the ice-field to areas of richer grazing at its periphery. Such a movement, he says, to some 

 extent at any rate, would explain the dense concentrations of young krill so often met with there. 

 Whether this or the interruption in diurnal rhythm does in fact explain them is very hard to judge, 

 for no one can yet say for certain what effect, if any, the pack-ice may have on the horizontal or 

 vertical movements of the larval and adolescent krill. I believe it has nothing to do with the forma- 

 tion of these dense concentrations whatsoever, since there now, it seems, can be little doubt 

 (p. 75) that the apparent congregating of both larvae and adolescents at the ice-edge is simply 

 due to the periodic freezing over of the open water of regions already rich in larval krill. 



It has sometimes been suggested that the krill in an ice-infested sea might congregate underneath 

 the floes, subsequent melting of the ice leaving the sea covered with a haphazard system of surface 

 patches reflecting the patchwork pattern of the open pack. Whether they do in fact so congregate is 

 difficult to ascertain. So far as can be seen, however, it is to the cracks and open spaces between 

 the floes that the krill prefer to keep rather than to the poorly illuminated conditions directly below 

 them. However, neither congregation between, nor under, the fioes would provide a satisfactory 

 explanation of patchiness since in the end a system of patches would be formed far more closely 

 knit than direct observation (p. 148) and the evidence provided by towed nets (p. 148) have shown 

 it to be. It would be difficult, too, to explain the widespread patchiness that has been encountered 

 hundreds of miles from the ice in terms of a patchiness having its origin in the pack. 



