ATMOSPHERIC GASES IN SEA-WATER 93 



respiration. In some cases the surface layers have an excess 

 of oxygen over the normal, and this, again, is to be attributed 

 to a biological cause, the liberation of oxygen by the planktonic 

 plants in the process of assimilation. Carbon assimilation takes 

 place in plants provided with chlorophyll, in the presence of 

 sunlight, the atmospheric CO 2 being utilised, the carbon 

 assimilated, and the oxygen liberated. Nitrogen, a relatively 

 inert gas, is much more evenly distributed in the waters of the 

 ocean, although here, again, a variation is met with due to 

 biological influences. Denitrifying bacteria are found in sea- 

 water, and are more active in the higher temperatures of 

 tropical seas. These bacteria break down nitrates, nitrites, 

 and ammonia, and liberate nitrogen, so that their activity leads 

 to an excess of free nitrogen in the sea. On the other hand, 

 bacteria are met with in the sea which are able to fix free 

 nitrogen. On two occasions Knudsen found a great excess 

 of nitrogen present in bottom water from the North Atlantic, 

 a fact which he referred to putrefactive influences. The 

 rapidity with which the atmospheric gases are diffused in sea- 

 water is not yet known, but in pure fresh water the process is 

 very slow. Probably the process of diffusion of gases in the 

 sea is hastened by the constant stream of sinking particles, 

 the shells and skeletons of the minute planktonic organisms, 

 which carry down with them small quantities of gases. There 

 are also convection currents, due to the unequal distribution 

 of temperature, and these doubtless play an important part. 



In the Eastern Mediterranean the deep water contains only 

 from two-thirds to three-quarters of the amount of oxygen 

 which should be present in water of its temperature. This 

 deficiency is undoubtedly due to the utilisation of the oxygen 

 by marine animals. It may be mentioned that the earliest 

 analyses of the deeper waters of the Mediterranean gave such a 

 low percentage of oxygen that it was concluded no animal life 

 could exist. That animal life is found in the deepest Mediter- 

 ranean waters has been proved by later expeditions, so that 

 these earliest analyses must have been incorrect. 



