INTRODUCTORY 13 



reaction and shows a range rarely exceeding pH 8-0 to pH 8-4 in surface 

 waters, although values above and below these limits are sometimes met 

 with under extraordinary conditions. Thus, in tidal pools, the pH some- 

 times rises to 9-6 owing to the activity of plants; and in isolated basins, 

 where decomposition of organic matter is taking place and H 2 S is given 

 off, it may fall to 7-0. Sea water which is in equilibrium with the atmo- 

 sphere has a hydrogen ion concentration of nearly pH 8-1, which can be 

 taken as a normal value. 



Sea water is alkaline as a consequence of excess of cations over anions 

 derived from strong acids. The excess base is equivalent to ions of bicarbon- 

 ate, carbonate and borate, and in consequence, sea water possesses limited 

 buffering power. The removal or addition of 1-25 ml of C0 2 per litre will 

 produce alterations in the hydrogen ion concentration of pH + 0-1. The 

 resistance of sea water to changes in hydrogen ion concentration is a factor 

 of some importance to marine animals, many of which are very sensitive to 

 changes in the pH of the medium. 



Dissolved Gases. In sea water the dissolved gases which are of particular 

 biological interest are oxygen and carbon dioxide. The partial pressures 

 of these two gases in the atmosphere are 20-99 and 0-03 vols% (partial 

 pressures of 159-52 and 0-23 mm Hg). The amount of gas present in 

 aqueous solution is proportional to the partial pressure exerted by the gas. 

 The solubilities of gases and therefore the amounts held in solution decrease 

 with rise of temperature and increase of salinity. Solubility values are 

 given in Table 1.3 for oxygen in sea water of different temperatures and 

 salinities. 



The dissolved oxygen content of ocean waters varies from 0-8-5 ml/1. 

 It is greater in the surface layers, where free exchange with the atmosphere 

 can take place, than in subsurface waters, which obtain their oxygen 

 through mixing, wind action, etc. The oxygen content of sea water is 

 significantly influenced by marine organisms. Both animals and plants 

 consume oxygen, but the latter also release oxygen as the result of photo- 

 synthetic activity. Deeper water masses have derived their oxygen origi- 

 nally from the atmosphere prior to submergence, and this is depleted by 

 the respiratory needs of animals and the oxidation of organic material. 

 In the Atlantic Ocean, for example, maximal values of 8-2 ml 2 per litre 

 are found in surface layers; central water near the equator shows minimal 

 values of 0-5 ml; and intermediate and deep waters have oxygen contents 

 of 4- 5-6 ml/1. 



Although oxygen varies irregularly in distribution, the amounts present 

 are usually adequate for the existence of animal life at all levels. Excep- 

 tions are certain enclosed seas, basins and fjords, in which there is deficient 

 circulation, with the consequence that the bottom layers become stagnant 

 and deficient in oxygen. 



Carbon dioxide varies in concentration from 34-56 ml 1. in sea water, 

 and is not a limiting factor in animal life. Part is dissolved as free C0 2 

 and as H 2 C0 3 , but most is present as carbonate and bicarbonate in 



