PHYSICAL OCEANOGRAPHY 257 



where the cold water from below comes comparatively near the 

 surface, it is a little more ; the highest value, over 6 c.c. 

 per litre, is found in high northern and southern latitudes. 

 The second section shows the deficiency from saturation in 

 cubic centimetres per litre at the temperature and salinity 

 ill situ. In the upper 50-100 metres the water is nearly 

 saturated all over the Atlantic, while in greater depths the 

 oxygen is deficient, especially in tropical waters^ at a depth 

 of about 500 metres in lat. 10" N. and S. the deficit amounts to 

 five or six cubic centimetres per litre. This is explained by the 

 abundant supply of oxygen in the surface-layers, through absorp- 

 tion from the atmosphere, and through assimilation by the rich 

 plant life, while the oxygen is being constantly consumed at 

 greater depths, where plant life is scarce and animal life in 

 excess. As a rule, where there is a great deficit of oxygen the 

 water is characterised as " stale," a long time having elapsed since 

 it was aerated at the surface or purified through the action of 

 plants. 



The disappearance of the oxygen is not, however, due only 

 to the respiration of animals, but may also be caused by various 

 hydro-chemical processes. In the Black Sea oxygen is found 

 only in the upper 150-200 metres (about 100 fathoms) of water ; 

 below this it has disappeared totally, whereas sulphuretted 

 hydrogen is present in increasing quantities down towards the 

 bottom. The Black Sea is separated from the Mediterranean Black Sea. 

 by the Bosphorus ridge, so that the water in its deep basin lies 

 stagnant, unrenewed by the influx of other water. Similar con- 

 ditions prevail in several Norwegian " threshold fjords," or on a Norwegian 

 smaller scale in the oyster-" polls." In such places the bottom fj^JS^aSi 

 is thickly covered with organic matter ; a slimy black mud is oyster- ^ 

 formed, swarming with bacteria that produce sulphuretted ^° ^' 

 hydrogen, which spreads through the water, combining with 

 the oxygen to form various sulphates. This causes the oxygen 

 to decrease and finally to disappear altogether, when the 

 sulphuretted hydrogen begins to appear free in solution. It 

 gradually spreads upwards, until the water is devoid of oxygen 

 and contains free sulphuretted hydrogen, at a depth of only 

 100 fathoms in the Black Sea, and in the oyster-basins in 

 autumn often at merely a couple of metres below the surface. 

 In summer the "bottom-water" of the oyster-" polls " lies 

 stagnant, but in the course of the autumn and winter it is 

 generally renewed by the supply of comparatively heavy water 

 from without ; then the sulphuretted hydrogen disappears and 



