24' 



N. 



22' 



2 0°- 



MEXICO 



MAZATLAN 



CAPE 

 CORRIENTES 



SAN BENEDICTO I 



II4»W. 

 Figure 36.— 



112° 



110° 



108° 



106° 



Horizontal distribution of the dynamic height anomaly at 125 m. over the 500- andl.OOO-decibar surfaces, as 

 in figure 35. The contour interval Is 0.025 dynamic meter. 



is somewhat below 100 percent saturation 

 and probably was upwelled recently. Extend- 

 ing out of the Gulf and turning westward, is 

 a tongue of generally undersaturated water 

 (80 - 100 percent). Agreement seems tobe good 

 between this tongue and one of high surface 

 zooplankton volumes, but I do not think the data 

 allow us to assert a causal relationship. 



Immediately below the surface, off Lower 

 California, the oxygen distribution shows the 

 upwelling very strongly, especially at Cape 

 Falso. Off Cape Corrientes, too, is a strong 

 gradient to low dissolved oxygen values near 

 the coast, extending northward beyond 

 Mazatlan. The low-oxygen tongue extending 

 from the Gulf becomes a high-oxygen tongue 



at 20 m.; it was suggested earlier that this 

 change was due to local phytoplankton activity 

 near the thermocline. 



At 50 m. (fig. 39), upwelling off Lower 

 California and between Cape Corrientes and 

 Mazatlan is evident. At this depth more than 

 at others, there is some sort of frontal sys- 

 tem like the one shown by the temperature 

 and salinity distributions. 



At 100 m. most of the southeastern part 

 of the area is occupied by water withan oxygen 

 content of about 0.5 ml./l., from just above 

 the subtropical minimum. At greater depths 

 the oxygen minimum covers the Gulf entrance 

 as well as the area off western Lower Cali- 

 fornia. The strong gradient running roughly 



41 



