ABSTRACT
THE physical properties of fresh water are largely determined by two variables, temperature and pressure; in the case of sea water, a third variable must be added-the salt content or salinity. Temperature, salinity, and pressure, together determine the density of sea water, and a knowledge of its density is of fundamental importance in dynamic oceanography. Further, bodies of water are characterized by their salinity-temperature properties. In addition, information about temperature and salinity conditions and their variation enables deductions to be made about physical processes taking place at the sea-air interface, processes such as evaporation, precipitation and cooling; all of these are of great importance to physical oceanography. Temperature and salinity are no less important to marine biology. The temperature of the sea changes from the poles to the equator and the annual and seasonal variations differ widely from place to place. Just as on land there are arctic species and tropical species, species with wider and species with narrower temperature limits of distribution-so in the sea, and a knowledge of temperature conditions is required in studying problems of animal distribution. In the open oceans salinity changes are relatively small and their biological influence less marked, but in some regions, for example the Baltic Sea, salinity becomes greatly reduced and decreasing salt content plays an important part in restricting the spread of typically oceanic species into this area.
