DISTRIBUTION OF PHOSPHATE IN THE SEA 



15 



lacking, especially in the central part of the section 

 which includes the most northern stations occupied in 

 the Pacific. The transition zone was relatively thick ex- 

 cept for the stations off the Aleutian Islands, extending 

 only to 175 meters at station 123. It will be noted that 

 where the high surface values were obtained east of sta- 

 tion 117 and where the transition zone extended to the 

 surface, the layer of uniform phosphate content was en- 

 tirely lacking. The mean phosphate values in the tran- 

 sition zone ranged from 104 mg PO4 per cubic meter at 

 station 115 to 228 mg per cubic meter at station 120. In 

 this region the great quantities of phosphate in the upper 

 layers is the combined result of the partial elimination 

 of the temperature gradient and of the addition of water 

 from the Bering Sea. 



South of the above line of stations is the area in 

 which phosphate was almost entirely lacking at the sur- 

 face. We may consider two lines of stations within this 

 region: (1) the line extending from California to Hawaii, 

 then north and east to station 146, (2) the line starting 

 with station 101 at latitude 13° 23' north, longitude 177° 

 27' east, and extending west and north to Japan. Except 

 near the coast of California all these stations showed a 

 deficiency of phosphate in the surface layer which ex- 

 tended to considerable depths, usually to more than 100 

 meters and to as much as 200 meters in many places. 

 At these stations also the transition zone was relatively 

 thick, which means that below the surface layer the in- 

 crease in phosphate with depth was comparatively slow. 

 North of Hawaii at stations 139 to 142, and again in the 

 western Pacific between Guam and Japan at stations 108 

 to 113, the transition occurred at lower levels than in 

 any other regions investigated, extending to more than 

 800 meters at all these stations and reaching 1000 me- 

 ters at stations 142 and 109. The phosphate content of 

 the transition zone at these stations was variable, but 

 nowhere attained the high values observed north of lati- 

 tude 35° north. It varied from 80 mg PO4 per cubic me- 

 ter at station 105 to 188 mg per cubic meter at station 

 139. From station 134 to the coast of California there 

 was a gradual narrowing of the surface layer and to a 

 slight extent of the transition zone, together with an in- 

 crease in the phosphate content of both zones. At sta- 

 tion 130, where the most pronounced effect of upwelling 

 of subsurface water was shown, the surface layer was 

 only 25 meters thick with a mean phosphate content of 

 35 mg PO4 per cubic meter. The transition zone had its 

 lower limit at about 475 meters and contained an aver- 

 age of 207 mg PO4 per cubic meter. 



Proceeding southward we next take up two lines of 

 stations that cross the equatorial currents. The eastern 

 one extends from stations 149 to 161, and the western 

 one from stations 94 to 103. Station 153 in the eastern 

 series of stations, and station 99 in the western series, 

 are in the center of the Equatorial Counter current. On 

 either side of this current the North and South Equatori- 

 al currents flow in the opposite direction to the counter- 

 current. The relation of these currents to the distribu- 

 tion of phosphate is more fully discussed on pp. 11 and 12 

 under the description of sections V and VIII. The note- 

 worthy features of the surface and transition zones in 

 this area are the relative thinness of the layers, partic- 

 ularly in the vicinity of the counter current, the absence 

 of extremely low values in the surface layer, and the 

 variability of the concentration of phosphate in both lay- 

 ers. 



At station 152, at the northern border of the counter - 



current, the lower limit of the phosphate transition zone 

 was at a depth of 175 meters, and that of the surface 

 layer at a depth of only 10 meters. Over the whole area 

 between stations 150 and 155 the entire transition zone 

 was above 300 meters. The average phosphate content 

 of the transition zone in the eastern line of stations var- 

 ied between 79 and 179 mg PO4 per cubic meter. In the 

 surface layer the average phosphate content was above 

 10 mg PO4 per cubic meter except in the extreme north 

 and reached 60 mg per cubic meter at station 157. Sim- 

 ilar but not quite so extreme conditions were found in, 

 the western series of stations, with the center of the 

 thin zones at about station 99. 



In the absence of disturbances due to horizontal cur- 

 rents, the distribution of phosphate at the stations just 

 discussed would no doubt be quite different from that 

 actually found. These stations lie within the tropics 

 where there is intense insolation of the surface water 

 throughout the year and this normally would be expected 

 to result in thick surface, as well as transition, zones, 

 with consequent reduction of vertical mixing. This 



would lead to a low concentration of phosphate in the 

 upper layers similar to that found in many of the stations 

 in the North Pacific. 



It is evident that in the equatorial regions the ef- 

 fects of the circulation far outweigh the effects of the 

 insolation. According to Sverdrup (I-A, p. 105) there is a 

 strong upward movement of the water along the bounda- 

 ries of the Equatorial Countercurrent which probably 

 accounts for the proximity to the surface of the transi- 

 tion zone and for the high concentration of phosphate 

 near the surface in the vicinity of the countercurrent. 

 Similar conditions prevail to some degree, however, 

 over the whole equatorial region. The explanation is no 

 doubt to be found in the horizontal water movements, 

 which, according to Sverdrup, produce eddies and possi- 

 bly other forms of vertical circulation. This may ac- 

 count for the relatively high phosphate values in the sur- 

 face layer but probably does not contribute to the thin- 

 ness of the transition zone unless this northward-flowing 

 water has a decided upward component. 



The line of stations from Samoa to Peru also lies in 

 the tropics but farther south than the two series just 

 discussed. It includes all the stations presented in Sec- 

 tion XI (see p. 11). In general both the surface layer 

 and the transition zone deepen from the coast westward. 

 The lower boundary of the transition zone was 75 meters 

 at station 71 near the Peruvian coast, and 650 meters at 

 station 83 in latitude 17° 00' south, longitude 129° 45' 

 west. As was the case in other sections in the tropics, 

 the phosphate content in both layers was variable. The 

 western part of the line lies in the same longitude as the 

 most easterly of the two series of stations crossing the 

 equatorial currents. Here the transition zone extended 

 to somewhat greater depths than in the other two lines 

 of stations just mentioned. This is probably because the 

 more southern stations are in a region of less disturbed 

 hydrographic conditions than prevail farther north. The 

 depth of the surface layer at these stations was the same 

 or greater than that at the more northern tropical sta- 

 tions. 



The narrow transition zone in the eastern part of the 

 section is probably caused. In part at least, by the Ant- 

 arctic Intermediate Current. Nearer the coast the dis- 

 tribution of phosphate is further affected by upwelling 

 and by the Peruvian Current. In the line of stations run- 

 ning from station 60 in latitude 40° 24' south, longitude 



