THE GASES OF SEA WATER The water of the ocean, like any other liquid, absorbs a certain amount of the gases with which it is in contact, and thus sea water contains dissolved oxygen nitrogen (N2), and carbonic acid absorbed from the atmosphere. Great quantities of CO2 are, in addition, present in carbonates. It is important for the sea organisms, particularly for fish which breathe through gills, that the absorbed air, although the amount is very small, contains more oxygen per cent than that of the atmosphere. At the surface one litre of sea water of 35%, salinity and at a temperature of 50° F contains normally 18.7 c.c. of gas, consisting of 6.4 c.c. of oxygen, 12•0 c.c. of nitrogen and 0.3 c.c. of carbonic acid gas. These 6.4 c.c. of oxygen are 34% of the whole absorbed air, while oxygen forms only 21% of the atmosphere. The quantity of oxy gen usually increases with decreasing temperature and decreasing salt content ; it is also greater at the surface than in the depths, where the oxygen is consumed by the breathing of organisms and cannot be directly replaced.
In the last 20 or 3o years the distribution of oxygen in time and place, in the Atlantic and European enclosed seas, at least, has been fairly thoroughly examined. It has been found that in the Tropics, between 20° S. and 20° N., the water between ioo and 2,000 metres deep is very poor in oxygen, having generally only 2 or 3 c.c. per litre, but in the high polar latitudes the water con tains 7 or 8 c.c. of oxygen per litre at the surface and even in the depths. In these high latitudes the water evidently sinks at times from the surface to the sea floor and maintains the oxygen at the bottom. This polar ground water flows away towards the Equator and so the water at the bottom of tropical seas has enough oxygen to enable organisms to exist. Where this vertical sinking of the surface water does not occur, as in the Baltic, the bottom water is very poor in oxygen; often oxygen is absent, e.g., north of Gothland, and animal life is then limited. In the Black sea the deep water is always and everywhere free from oxygen, and organisms are quite absent. On the other hand, one observes in the upper, sun-penetrated layers an abundance of oxygen, when in spring the plankton plants develop and produce oxygen through the so-called assimilation process. Where oxygen is scarce or
absent there is generally a marked increase of CO2, e.g., on the floor of some Baltic hollows. This increase in the amount of CO, up to 12 times the normal amount, is caused by slow accumula tion in the absence of renewal of the water and by the breathing of the sea animals; the water in these hollows is thus in the end poisoned. In contrast with the Baltic the North sea depths are sufficiently supplied with oxygen, and there is no marked increase in carbonic acid. For these reasons conditions are much more favourable for fish in the North sea than in the Baltic.
The gaseous, absorbed nitrogen plays no part here; its sig nificance arises from the fact that nitrogen compounds have a nutritive value for plants, as on dry land. It has already been pointed out that in addition to carbonic acid gas, there is a large amount of present in the dissolved carbonates. The greater part of the occurs as carbonate and bicarbonate.
In the open ocean the alkali proportion of the salt content is set out in the formula A (alkalinity)=o.77 S. (salinity) where S is in and A in c.c. per litre. In this case, where the salt content is the average alkalinity is 27 c.c. But great deviations appear, especially a great increase near coasts, where rivers bring down a large amount of carbonate.
Since 185i it has been known that all sea water has an alkaline reaction. Absolutely pure water, which does not exist naturally, is neutral, it contains an equal amount of hydrogen ions and hy droxyl ions; it does not conduct an electric current and is neither alkaline nor acid. In sea water a considerable part of the water is resolved into the two components, (I) the positive hydrogen ion (H), and (2) the negative hydroxyl ion (OH'), because weak acids, such as carbonic acid, and their salts in aqueous solution are subject to hydrolitic dissociation. The estimation of the hydrogen ions is used as a means of measuring the alkaline reaction of sea water, which depends both on the amount of free CO, and on the alkalinity discussed above. This hydrogen ion concentration of the sea varies around grams per litre. All these factors have fixed relations to one another.