, HYDROCLIMATIC FACTORS 295 



water hydroclimatic complexes discloses a set of conditions more ob- 

 scure and complicated than those presented by terrestrial climates. 

 In the main, physical principles governing conditions in air on the one 

 hand and fresh and salt water on the other are the same, but a few 

 differences must be noted. 



HYDROCLIMATIC FACTORS 



Density. The density of water is nearly a thousand times that of 

 air. This increases difference with changes in depth as compared with 

 changes in altitude. The addition of water vapor to air decreases its 

 density, while an increase in salinity increases the density of water. 

 Salts and water vapor are constantly varying in their respective media, 

 and mention of salinity or mineral content is as frequent in literature 

 regarding water as humidity is in that regard to air. The osmotic 

 pressure of sea water is 20 or more atmospheres, while that of fresh 

 water is relatively small. The salt content of the sea is relatively 

 uniform, but that of fresh water may vary to 20 or more times the 

 minimum found in mountain streams in rainy districts. The density 

 of water as compared with air further tends to crowd major differences 

 in conditions into a much smaller space, especially in shallow water. 



Circulation. In the ocean and in large lakes, much of the water in 

 the deeper portions of their basins is comparatively, if not entirely, 

 without currents of measurable magnitude, but even here slow drifts 

 of considerable importance occur. 



In the lesser depths and the more rapidly moving surface, an out- 

 standing general climatic factor in water is movement of the medium. 

 As such, it is concerned with the modification of temperature and salt 

 content. Circulation attains major importance in part on account of 

 its relation to the transportation and deposition of floating materials 

 of various kinds, together with their decomposition products. Where 

 currents are sluggish, deposition occurs, and if the water is not deep, 

 temperatures are higher. The plankton accordingly multiplies, the 

 dead and decaying organisms settle to the bottom in quantity and, in 

 their decomposition and that of the debris resulting from the breaking 

 down of rooted plants, consume oxygen and increase carbon dioxide 

 and sulphur compounds, producing a climate of distinctive character 

 for both benthic and pelagic communities. In Washington Sound 

 these factors differentiate sharply the climates of two major communi- 

 ties (Shelford et al., 1935). 



Suspended Matter and Color. By way of contrast with aerial 

 climates, one of the important general factors that must be consid- 



