158 POPULATION'S OF THE SEA 



principal influences that affect the biogeographical characteristics 

 of the organic community or biome, however, are those major 

 external variables that undergo the maximum fluctuation (e.g., 

 Jones, 1950, p. 299). 



External Factors of Existence 



External factors influencing the geographic distribution of the 

 marine biotas are, of course, vastly different from those at work 

 above sea level. Water is heavier, more viscous, a much poorer 

 heat conductor, a far superior heat reservoir and transfer system, 

 and has a far lower gaseous oxygen content and higher freezing 

 point than air. Ocean water is richer in most kinds of dissolved 

 solids and has a far more uniform composition than fresh water. 

 Migration routes for land animals are likely to be barriers and 

 isolating mechanisms for marine ones. 



Let us look at the principal external variables that influence 

 the distribution of marine biotas under given geographic, clima- 

 tologic, and historic configuration. In Fig. 3 the systematic 

 variables define major biogeographical provinces or zones; local 

 variables are shown at the top. Forbes and Godwin-Austen (1859, 

 p. 17) recognized these same variables a century ago when they 

 stated: "The distribution of marine animals is primarily deter- 

 mined by the influence of climate or temperature, sea-composition 

 and depth, in which pressure, and the diminution of light are 

 doubtless important elements." Plants do not fit the same detailed 

 scheme, but they are equally subject to the three basic variables, 

 sunlight, temperature, and salinity. 



Sunlight, of course, is the primary source of radiant energy that 

 impels the whole earthly biosphere and is responsible for all but a 

 minute fraction of its available heat budget. Together with the 

 earth's trajectory and rotation, and the pull of the sun on its 

 equatorial bulge, sunlight determines the latitudinal climatic 

 zones, which designate by other words the amount of local available 

 energy. It determines the vertical division of the hydrosphere into 

 photic, dysphotic, and aphotic zones, which can also be expressed 

 in terms of photosynthesis, oxygenation, and temperature. 



