94 Water 



to Iceland without encountering a salinity, lower than 35% c or higher 

 than 37%o (parts per thousand). Ocean areas with unusually high 

 evaporation, like the Mediterranean Sea and the Red Sea, exhibit 

 salinities that run up to 40%o. On the other hand, the salinity of the 

 Baltic Sea is reduced to 8%o or less due to the large inflow of fresh 

 water. 



In river estuaries and in coastal areas receiving large amounts of 

 run-ofi^, salinitv may vary over the whole range from nearly fresh 

 water to full sea water. In smaller habitats, such as rock pools filled 

 by ocean spray, the salinity may be typical of ordinary sea water on 

 one occasion, may be greatly increased by evaporation a few days 

 later, and may be reduced almost to zero by rainfall on another day. 

 However, for the marine environment as a whole, such situations are 

 definitely exceptional. The great length and breadth of the ocean 

 maintains a nearly constant salinity of about 35 to 36%o. 



Typical Values for Osmotic Pressure Expressed As Freezing Point 



Depression (= A) 



AWater AInvertebrates ATeleost Fishes 

 Mediterranean 2.14 2.2 0.8-1.0 



Coastal Atlantic 1.79 1.8 0.7-0.8 



Fresh water . 03 0.1-0.8 0.5-0.6 



The osmotic pressure of a molar solution of a non-electrolyte is 22 

 atmospheres or 1700 cm of mercury. The freezing point of such a 

 solution is depressed to — 1.84°C, and this fact provides a convenient 

 method for measuring osmotic pressure. In a solution of electrolytes 

 with the same number of molecules the osmotic pressure would be 

 higher because of the dissociation. Sea water is a 0.55 molar solu- 

 tion, and, since the salts are dissociated, sea water has an osmotic 

 pressure almost equal to that of a molar solution of a non-electrolyte. 



