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REDFIELD, KETCHUM AND RICHARDS 



countercurrent mechanism on the distribution of a nonconservative property 

 is found in the strait connecting Lake Maracaibo with the sea. During the rainy 

 season this strait is swept out by water escaping from the lake. The net move- 

 ment is seaward at all depths. Although the deeper layers have lower velocities 

 than the surface the strait is too short to permit any notable change in the 

 concentrations of oxygen to develop along its length. With the advent of the 

 dry season the outflow slackens and water of higher salinity crosses the shallow 

 sill separating the strait from the sea, and flows lake ward beneath the outflow 

 of surface water. Within a few months the oxygen content of the deep counter 

 current becomes greatly reduced in the direction of its movement (see Fig. 12). 



o 



20 

 40 

 60 



20 



50 



Fig. 12. Distribution of oxygen in section along Strait of Maracaibo during wet season, 

 May, and dry season, March. Lake Maracaibo lies to the right. Arrows indicate the 

 direction of non-tidal currents. Ordinate, depth in feet; abscissa, scale of miles; 

 contour interval, 1 ml oxygen/m 3 . (After Redfield, 1955.) 



Extreme examples of the accumulation of nonconservative elements are 

 provided by fjords. These estuaries are separated from the outer sea by sills 

 which limit the active exchange of water to a relatively shallow layer and thus 

 permit the deep water to stagnate. They thus represent the limiting case of 

 differential advection in which the ratio of the velocities of the upper and lower 

 layers is very large. The sinking of organic matter into the deep basins leads 

 to reduction in oxygen content, the production of hydrogen sulfide and the 

 accumulation of unusual concentrations of phosphate. In a large series of fjords 

 examined by Strom (1936) great variations in the development of these effects 

 are observed, varying from those in which complete anoxia does not occur to 

 others in which as much as 40 ml/1. H2S is present. In the extreme cases as 

 much as 10 mg atoms/m 3 of phosphate phosphorus may accumulate in the deep 

 water. 



In fjords with large quantities of fresh water in the upper layers, the deep 



