64 BIOLOGY OF THE SEAS OF THE U.S.S.R. 



The rise of the mass of abyssal forms to depths unusual for them (80 to 

 100 m), which does not occur anywhere in the southern part of the Arctic 

 basin (at the outlet into the Atlantic) may be explained by the following four 

 causes : (/) low temperature of surface water of high Arctic; (2) small annual 

 temperature fluctuations ; (3) comparatively low transparency of water ; (4) 

 obscuration caused by the ice cover which lasts almost all the year round. 



Hence the deep-water fauna with a sharply expressed cold-water steno- 

 thermy and a negative phototropism finds no obstacles here for expanding 

 into comparatively higher levels. 



III. ZOOGEOGRAPHICAL ZONATION OF THE ARCTIC 



REGION 



All our Arctic seas, except for the most southwestern corner of the Barents 

 Sea, belong to the Arctic region, which is limited by about 70° N latitude and 

 only comes down to 60° N in the Norwegian and Greenland Seas. The 

 boundaries of the Arctic and boreal regions in the North Atlantic are not the 

 same for the bottom and the pelagic fauna. 



Ortmann noted this general phenomenon as early as 1896. Pelagic organ- 

 isms, easily carried around by currents and with life cycles shorter than those of 

 the bottom organisms, form more mobile zoogeographical boundaries than 

 the slowly growing benthos organisms liked with the bottom. The boundaries 

 given in Fig. 16 refer mainly to benthos. Sea currents would widen these bound- 

 aries more for plankton than for benthos. The boundary between the Arctic and 

 boreal plankton along the shores of Norway and in the western part of the 

 Barents Sea would (in relation to the boundary for benthos) therefore lie con- 

 siderably farther north and east, possibly as far as the central parts of the 

 Barents Sea. Along the eastern coasts of Greenland, on the other hand, this 

 boundary would be found such farther south, towards Newfoundland. In 

 exactly the same way fresh river waters carry fresh-water plankton out into the 

 sea and, in spreading outwards, move the boundary between the sea and 

 brackish plankton away to the north. Conversely, in the near-bottom layers, 

 the saline waters together with the bottom population move towards the 

 shore, often entering the estuarial zones, so that the surface layer frequently 

 has a completely fresh-water fauna, and the near-bottom one a sea fauna. 

 This can be seen by comparing the boundaries in Figs. 13 and 14. 



The littoral fauna provides another case of the boundaries for different 

 groups of the population of the northern seas not being coincident. Owing 

 to a number of conditions which have already been mentioned the boreal 

 littoral fauna has been moved far to the east, covering all the Murman coast 

 and White Sea, i.e. regions where the fauna of the sublittoral, and the plank- 

 ton too, have a true Arctic character. 



The Arctic region may be divided into three sub-regions (Fig. 16). First of all 

 there is the abyssal Arctic sub-region, embracing the three depressions 

 (Norwegian, Greenland and Central Arctic) of the Arctic basin and separating 

 them from the abyssal of the Atlantic Ocean proper. Species of the genus 

 Themisto can serve as an excellent example of the sharp fauna distinction 

 between the Arctic and Atlantic abyssal forms. 



