DISTRIBUTION OF PHOSPHATE IN THE SEA 



content is in most cases below the lower limit of the 

 transition zone, that is, it is found in the stratosphere. 



With respect to the relation of the phosphate transi- 

 tion zone to the density gradient, it is interesting to in- 

 spect figures C2 to C8, which show the close correlation 

 existing between the phosphate and density gradients at 

 seven Carnegie stations representing various localities 

 of the Atlantic and Pacific oceans. 



Figure C2 represents conditions at station 137 (lati- 

 tude 24° 02' north, longitude 145° 33' west). It has been 

 selected as typical of the Pacific for regions in which 

 there is a strong thermal stratification of the water and 

 in which there are no prominent dynamic features. The 

 density curve given in the figure shows a pronounced 

 difference between the lighter water near the surface 

 and the heavy, nearly uniform, water in the stratosphere 

 From the surface to a depth of 200 meters the water is 

 practically depleted of phosphate. In the phosphate tran- 

 sition zone, which extends from 200 to 675 meters, the 

 concentration of phosphate increases to 290 mg PO4 per 

 cubic meter. At 1000 meters the maximum is attained, 

 with about 300 mg PO4 per cubic meter. This occurs at 

 about the depth where the rapid increase in the density 

 of the water ceases. At lower levels the phosphate con- 

 tent diminishes more or less regularly, reaching 225 

 mg per cubic meter at 4000 meters. The phosphate 

 curve shows no modifications attributable to subsurface 

 currents. 



When other phosphate curves are referred to the 

 cvirve for station 137 as a base, the anomalies due to cir- 

 culation can readily be seen. Figure C3 represents the 

 distribution of phosphate at station 17 (latitude 11° 57' 

 south, longitude 78° 37' west), off the coast of Peru in a 

 region of upwelling. Here the upper part of the curve is 

 displaced toward the surface. The density c\irve shows 

 a similar upward displacement. A high concentration of 

 phosphate, as compared with station 137, occurs at the 

 surface, and the surface layer is limited to the upper 20 

 meters. The transition zone extends to about 80 meters 

 only. Another interesting feature at this station is the 

 nature of the phosphate gradient between 80 and 900 me- 

 ters, where the maximum might be expected (as shown 

 by the dashed line), had all the upper 1000 meters of the 

 curve been displaced. It is probable that the peculiar 

 shape of the curve between 80 and 900 meters is owing 

 to the Antarctic Intermediate Current which occurs here. 

 For reasons which will be discussed later, this water 

 may be expected to contain less phosphate than ordinarily 

 occurs as a maximum in this locality. This current also 

 affects the density at these levels, as shown by the curves. 



Farther south the effects of the Antarctic Current 

 are still more pronounced. At station 60 in latitude 40° 

 24' south, longitude 97° 33' west, (fig. 04), where there 

 is no upwelling, this current is found between 400 and 

 1500 meters and its phosphate content is about 50 mg 

 PO4 per cubic meter less than at station 71. Here again 

 there is a remarkable correspondence between the den- 

 sity and phosphate curves. 



Station 130 in latitude 37° 05' north, longitude 123° 



43' west (fig. C5), is in the region of the upwelling water 

 off the coast of California. Here the distribution of 

 phosphate corresponds to that in the similar region off 

 the coast of Peru except that above 1000 meters the 

 phosphate content shows no definite effect of an interme- 

 diate current. The maximum shows the same develop- 

 ment as that at station 137 but occurs at 700 meters in- 

 stead of at 1000 meters. 



Figure C6 shows the distribution of phosphate at one 

 of the most northern stations occupied in the Pacific, 

 namely, station 122 at latitude 46° 16' north, longitude 

 174° 03' east. The phosphate curve for this station re- 

 sembles that at station 137 except that the surface val- 

 ues at station 122 are much higher and the transition 

 zone is considerably nearer the surface than at station 

 137. The surface value is 130 mg PO4 per cubic meter 

 and the surface layer is only 20 meters thick. The tran- 

 sition zone extends to only 400 meters. Below this is 

 the usual decrease in phosphate with increasing depth. 

 The high surface values are undoubtedly attributable 

 partly to the inflow of water rich in phosphate from the 

 Bering Sea and partly to the fact that at this latitude the 

 density gradient is considerably reduced during the win- 

 ter, thus permitting a considerable amount of mixing 

 among the various water strata. It may be noted that 

 even for July, when this station was occupied, the density 

 gradient is not well developed. This, too, may be owing 

 in part to the relatively cold water entering from the 

 Bering Sea. 



In the Atlantic, observations to great depths were 

 obtained at a station located at a still higher latitude 

 than station 122. Station 10 (fig. C7) is southeast of 

 Greenland at latitude 50° 19' north, longitude 34° 15' 

 west. Although these observations were made in July, 

 both the phosphate and density gradients show the effect 

 of vertical mixing during the previous winter. Below 

 about 100 meters the distribution of phosphate is practi- 

 cally uniform, as is the density, but above this depth the 

 curves show a slight reduction in phosphate as well as 

 in density. The low phosphate content in the Atlantic 

 deep water as compared with that in the Pacific will be 

 discussed later under "Regional distribution," (pp. 16- 



17). 



At station 29 (fig. C8), located in the western trough 

 of the Atlantic at latitude 13° 16' north, longitude 52° 13' 

 west, the maximum quantity of phosphate which occurred 

 at about 800 meters is much higher than would be expect- 

 ed from the quantity at lower levels. Since the Antarctic 

 Intermediate Current centers at about 800 meters, it is 

 fairly certain that the water in this current originally 

 had a high phosphate content although the quantity may 

 have been augmented somewhat during its progress to- 

 ward the north. It is interesting to note that antarctic 

 water, when intruded at this level into the Atlantic, rep- 

 resents a layer of relatively high phosphate content, but 

 when extended into the Pacific, as at stations 70 and 71 

 (figs. C4 and C3), it represents water low in phosphate 

 relative to the adjacent layers. 



