Aquatic Environment 249 



the seasonal circulation does not reach the bottom, with the result 

 that the deepest layers may be completely devoid of oxygen through- 

 out the year. 



In the temperate regions of the ocean the thermal stability built up 

 during the summer is similarly broken down in the autumn. Turbu- 

 lence produced by wind and waves then carries oxygen-rich water 

 downward, and this deep stirring continues all winter. The down- 

 ward extent of the winter mixing is sufficient to reach the bottom in 

 coastal areas but its effect does not go below the permanent thermo- 

 cline of the open sea (Fig. 5.4). Seasonal changes in temperature 

 and wind action thus do not provide for the delivery of oxygenated 

 water to the great depths of the ocean. 



May June July Aug. Sept. Oct. Nov. 



Fig. 7.4. Changes in oxygen diu-ing the warm part of the year at the indicated 

 depths in Lake Mendota, Wisconsin. ( Needham and Lloyd, 1937, after Birge and 



Juday. ) 



Oxygen is supplied to the ocean abyss by deep permanent currents 

 that originate near the surface in high latitudes, sink to intermediate 

 levels or to the bottom, and flow for thousands of miles toward and 

 beyond the equator (Fig. 7.5). This deep flow of water is dynam- 

 ically integrated with the horizontal current systems with the result 

 that the vertical movements of water follow a complicated pattern 

 (Sverdrup et al., 1942). As a simplified generality, however, the 



