10 THE BIOLOGY OF MARINE ANIMALS 



Since the major constituents of sea water retain the same relative pro- 

 portions wherever the sample is taken, it is possible, by determining the 

 concentration of any one of them, to estimate the concentrations of the 

 others. Because of this constant composition the relations between chlorin- 

 ity, salinity, density and temperature are fixed, and interconversion of 

 values is readily carried out. Finally, the biologist recognizes, in the cons- 

 tant ionic composition of sea water, a stable environmental factor of the 

 utmost importance in the physiology of marine animals. 



The constancy in composition of sea water is due to the system of 

 oceanic circulation and to the continual mixing which occurs. This soon 

 equalizes any local variations resulting from the discharge of rivers, the 

 activity of living organisms, formation and melting of sea ice, the inter- 

 action of suspended material with dissolved substances and exchange with 

 bottom deposits. There are, however, restricted areas, such as the Black 

 and Baltic Seas and the mouths of large rivers, where dilution and pecu- 

 liarities of circulation bring about changes in the relative concentrations 

 of dissolved substances. In such regions modifications of the chlorinity 

 ratios with respect to sodium, potassium, calcium and sulphate ions may 

 be encountered. 



Salinity. The salt concentration of sea water is known as salinity. 

 Salinities are always expressed as grammes per kilogramme of sea water 

 (parts per mille, % ) and, in practice, are usually obtained by measuring 

 the chlorinity, using argentometric titration. The reader will find a simpli- 

 fied procedure in Harvey (25). The relationship between the two quantities, 

 salinity and chlorinity, is given by the expression — 



Salinity (% ) = 0-03 + 1-8050 x chlorinity (% ) 



For most biological purposes, the concentrations of substances in 

 solution are usually expressed on a volume basis as percentages or grammes 

 per litre. It is convenient, therefore, to have corresponding values for 

 chlorine content, and this is available in the use of the term chlorosity, 

 which is the equivalent of chlorinity expressed as grammes per litre at 

 20°C. The calculations involved are — 



(vol AgN0 3 ) (molarity AgN0 3 ) 



molarity of CI in sample = : : 



vol sample c.c. 



chlorosity = 35-5 (molarity CI) 



chlorosity 

 chlorinity 



density of sample at 20°C 



The chlorinity may be read from the graph shown in Fig. 1.4 

 relating it to chlorosity, or may be calculated with greater accuracy from 

 the data given in Knudsen's hydrographical tables. 



The salinity range in the open oceans is rather small, and usually lies 

 between 33% and 37% in surface waters, with a mean of nearly 35% . 

 Marked deviations from these values are due to peculiar conditions. In 

 regions where there is much dilution by heavy rainfall, discharge of rivers 



