However, instead of the deep, homogeneous 

 surface layer that should result from increased 

 wind mixing and convective overturn, there was 

 virtually none (figs. 24-33 and 36) in this area, 

 indicating that some other mixing forces must 

 be involved. 



Comparison with the temperature, 

 salinity, and dynamic topography shows that 

 the large horizontal gradients in surface PO4 

 correspond to sharp changes in these fields. 

 Again using the 160 W. section as an example, 

 the phosphate increased from 0. 17 ug. at. /I. 

 at station 21 to 0.40 ug. at. /I. at station 20, 

 and from 0. 36 ug. at. /I. at station 19 to 0. 55 

 Ug. at. /I. at station 18, corresponding to ab- 

 rupt changes in surface temperature (figs. 

 24-34), distinct shifts in the T-S curves (fig. 

 68), and zones of relatively large geostrophic 

 currents (fig. 17). In addition, the temperature- 

 salinity relationships (figs. 68-71) show that 

 the surface waters of the northwest part of the 

 area contain less water of northern origin than 

 the water at the same temperatures farther 

 east. This indicates that mixing as a result 

 of turbulence in the areas of relatively large 

 horizontal current shear in the transition zone 

 between waters of slightly different type 

 (Sverdrup et al. 1942, p. 472) contributes to 

 the enrichment of the surface water and ac- 

 counts for the largest surface phosphate values 

 being in the northwestern part of the area. 



The isolated cell of greater than 

 0.2 ug. at. /I. centered at about 30 N. on 

 165 W. (fig. 84) is based on the observations 



from a single station. However, it is probably 

 real since, in addition to the relatively large 

 horizontal velocity gradients and divergence in 

 the surface currents indicated by figure 17, the 

 station was occupied during a period of high 

 winds (Beaufort force 7) (fig. 5). 



In the southeastern part of the area, 

 the 0. 1 ug. at. /I. contour is drawn as a dotted 

 line to show that there was little or no phosphate 

 in the surface waters. Further evidence of 

 barrenness in this area was the deep blue color 

 of the water (see table 1). 



The vertical distribution of dissolved 

 inorganic phosphate is shown in figures 85-93. 

 As expected, its pattern is almost the opposite 

 to that of the dissolved oxygen. At the surface, 

 except in the areas of large horizontal velocity 

 gradient mentioned above, there is a layer in 

 which the concentration is relatively low and uni- 

 form. Below this layer the phosphate increases 

 rapidly and, in general, uniformly with depth to 

 about the 2. 8 ug. at. /I. isopleth, and the trend 

 continues until a maximum of between 3. 00 and 

 3. 86 ug. at. /I. is reached at about the same 

 depth as the oxygen minimum. 



Water Transparency 



Whenever the sea and weather 

 conditions permitted, water color determinations 

 according to the Forel scale and transparency 

 observations by means of a Secchi disk (Sverdrup 

 et al. 1942, p. 82) were made immediately after 

 the hydrographic cast. Only 13 reliable 



Table 1. Transparency observations 



J_/ Time lowering was started. 



2/ Estimated. 



12 



