42 The Medium 



plankton in relation to flotation the reader may refer to Sverdrup 

 et al. (1942) and Welch (1952). 



TRANSPORTATION BY MEDIUM 



Another significant action of the medinm is its action in providing 

 ti'ansportation for plants and animals. Certain requirements such as 

 light can often reach the organism without the movement of either 

 the organism or the medium. Most necessities, however, are not 

 adequately provided for unless either the organism or the medium 

 moves. Put in simplest terms the needs for mobility are: (1) to 

 provide materials for metabolism and growth; (2) to remove waste 

 products; (3) to bring male and female elements together; (4) to 

 distribute progeny; (5) to avoid unfavorable conditions. 



Either the organism must be able to forage and to distribute itself 

 effectively, or else some mobile agent must be available. Fortunately, 

 both media are mobile, but their potentialities for providing transpor- 

 tation differ considerably. Air moves much more readily and much 

 faster than water, but water can carry heavier objects in suspension. 

 The composition of air remains relatively constant, but the water 

 medium contains widely varying amounts of ecologically important 

 substances. 



Movement of the air medium ranges from local wind currents of 

 beneficial nature to violent and destructive gales. Animals and 

 plants in some areas are subject to strong winds which vary in direc- 

 tion from day to day, whereas in other regions, such as the trade- 

 wind belts, the wind blows almost constantly from one quarter with 

 corresponding unidirectional influence. Air movement also indirectly 

 affects other factors such as temperature, rainfall, and evaporation. 

 The circulation of the atmosphere controls the weather and provides 

 for the transport of moisture from the sea to the land. 



Movement of the water medium similarly varies from small-scale, 

 sporadic circulation, generated by local wind and waves, to larger 

 and stronger currents. Plants and animals living in ponds and lakes 

 are regularly subject to wind-driven currents. In fresh- water streams 

 the one-way transport of water is a matter of vital concern to all the 

 inhabitants. Along the sea coast, on the other hand, the tidal cur- 

 rents are characteristically reversible or oscillatory. Farther offshore 

 the permanent currents are encountered, and these are parts of the 

 great current systems of the ocean which include a rotary circulation 

 in each of the ocean basins (Fig. 2.10). Two well-known oceanic 

 currents of great ecological significance in the northern hemisphere 



