352 DK MUKKAY ON THE DEEP AND SHALLOW-WATER MARINE FAUNA 



over the ocean's floor to make good the deficiencies caused in the tropics l)y surface 

 currents and evaporation, and there can be little doubt that it also flows southward to 

 supply the place of the ice and cold surface water drifted northward. The compara- 

 tively warm water which reaches the Antarctic Circle at depths greater than 200 fathoms 

 probably comes also from this zone between 42° and 56° S. lat., its temperature being, 

 of course, lowered by being drawn into polar areas. 



If there were in the Antarctic Ocean basins like that of the Norwegian Sea, cut off 

 by submarine ridges from general ocean circulation, the water in these basins would have 

 the same low temperature of 29° F. from surface to bottom. The brisk superficial 

 circulation which is kept up in the Arctic Ocean by the extension of the Gulf Stream 

 waters along the coast of Norway, and the return of the cold jjolar currents removing 

 ice by the eastern coast of Greenland and Baffin's Bay, results in that ocean being 

 comparatively open to a very high latitude, and produces at the same time a great 

 depth of relatively warm and saline water along the northern coasts of Norway, A 

 similar circulation appears to be entirely wanting in tlie Antarctic Seas ; hence their 

 ice-bound character. 



The mean of the eighteen surface salinities observed by the Challenger to the south 

 of lat. GO" S. is 1 '02475 (Sg—), and of the six furthest south of these the mean is 1'0245. 

 This is the same as Mohn's line of lowest salinity between Greenland and Spitzbergen. 

 Nothing was observed, however, so low as 1'0240 found by Admiral Makaroff in the 

 southern part of the Sea of Okhotsk. 



We have already pointed out that, owing to the mixture of cold and warm waters in 

 the upper layers of the Southern Ocean, large numbers of pelagic marine organisms arc 

 killed, and their bodies, falling to the bottom, afford abundant food for the deep-sea 

 animals of this region, and thus have a determining influence on the richness of the 

 Benthos fauna. The abundance of food, however, is not the only condition favourable 

 to animal existence, for the physical condition of the w^ater renders it peculiarly fit for 

 the support of animal life. Owing to the low temperature of the surface, the dissociation 

 tension of the atmospheric gases in solution is very low. In order, therefore, to come 

 into equilibrium with the atmospheric pressure, the water takes up a correspondingly 

 large quantity of atmospheric gases, particularly oxygen. • This cold, and therefore 

 heavy, water sinks down, canying with it to the lower strata its store of oxygen. This 

 unusually copious supply of oxygen cannot but have an important influence on the life 

 at great depths in these regions. 



Colour of the Water. — The colour of the sea varies considerably throughout th(( 

 Southern Ocean, sometimes being a deep blue, but in the neighbourhood of the ice it is 

 often green or brown-coloured, due to the presence of Diatoms or other marine Alga;. 

 The lower forms of Crustacea are occasionally so numerous as to give the water a 

 reddish tinge. 



