Aquatic Environment 247 



stroyed, wind stirring may extend sufficiently deep to reach the bottom 

 of shallow lakes and coastal waters, and thus to replenish the oxygen 

 supply. There is a limit, however, to the depth to which turbulence 

 caused by wind is effective. In the open ocean, where the highest 

 winds and largest waves occur, little influence of wave action is felt 

 below 100 m. 



Direction of wind 



Fig. 7.2. Wind-driven currents in a lake with a pronounced thermocline. Circu- 

 lation is limited to the epilimnion. (By permission from Limnologij by Welch, 

 Copyright 1952, McGraw-Hill Book Co.) 



Oxygen may be carried to depths beyond the limit of effective wave 

 action by eddy conduction. Wherever differential movement of cur- 

 rents occurs, eddies are generated along the current margins, and 

 these cause units of water to be exchanged ( Fig. 7.3 ) . The exchange 

 will bring about a transfer of any characteristics of the water that 

 differ in the adjacent water masses. The rate of transfer depends upon 



SURFACE WATER 



Eddy transfer 



O-O-O-O-O-O — 



CURRENT 



■()-()-0-( )-()-()■ 



DEEP WATER 



Eddy transfer 



Fig. 7.3. Diagrammatic vertical section of a body of water, indicating vertical 



transfer by eddies across the current boundaries. Horizontal eddy conduction 



similarly takes place at the lateral margins of the current. 



