DISTRIBUTION OF PHOSPHATE IN THE SEA 



13 



The chart also indicates the months during which the 

 observations were obtained. It will be noted that, ex- 

 cept in the tropics where there is little range in tem- 

 perature, all the investigations in both hemispheres 

 were carried out in the summer. 



Atlantic Ocean . --In the Atlantic, north of latitude 

 36" north, north of station 16, no surface values less 

 than 10 mg PO4 per cubic meter were observed. 

 Throughout this part of the Atlantic there is an active 

 circulation and the thermal stratification is less pro- 

 noimced than farther south. It is interesting to note that 

 at station 2 in May the surface water contained 58 mg 

 PO4 per cubic meter whereas in August of the same year 

 at station 15, which is in the same vicinity, only 11 mg 

 PO4 per cubic meter were found. This corresponds to 

 the seasonal variation observed by Atkins (1930) in Brit- 

 ish waters. He found that a concentration of phosphate 

 of about 60 mg PO4 per cubic meter in the winter was 

 regularly reduced to practically zero by July to Septem- 

 ber. The low values in summer were attributed to the 

 utilization of phosphate by phytoplankton and to the lack 

 of a renewal from lower levels owing to the stratifica- 

 tion of the water during that season. 



South of latitude 36° north all the Carnegie stations 

 showed a low phosphate content at the surface and to 

 considerable depths. In the Sargasso Sea, at stations 18 

 and 19, the water contained less than 10 mg PO4 per cu- 

 bic meter to a depth of 225 meters. In the tropics this 

 dearth of phosphate did not extend to quite such great 

 depths, but occurred consistently. The distribution of 

 phosphate observed in the Sargasso Sea may be attributed 

 to the fact that in this area there is sinking of surface 

 water poor in phosphate (see Wust, 1928). The low phos- 

 phate content in the tropics must be attributed to the well- 

 developed thermocline and the absence of any complex 

 currents to disturb the thermal stratification of the water. 



Stations 24 to 13, Section I, were occupied by the 

 Carnegie in August and it is interesting to note that Sei- 

 well and Seiwell (1934) report equally low surface values 

 for February and March at six stations in the western 

 North Atlantic and west of Carnegie Section I. At these 

 stations concentrations of less than 10 mg PO4 per cu- 

 bic meter were observed to even greater depths than at 

 the Carnegie stations. At their station 1222 (latitude 33° 

 15' north, longitude 67° 35' west), such low values were 

 observed to a depth of 470 meters. 



Pacific Ocean . --For the Pacific the distribution of 

 phosphate at the surface has been summarized in a pre- 

 vious paper by Moberg, Seiwell, Graham, and Paul (1930). 

 The distribution is intimately associated with the current 

 systems of that ocean. At all the stations lying north of 

 latitude 40" north, the phosphate content was markedly 

 high, ranging from about 25 to about 150 mgP04 per cu- 

 bic meter with the exception of station 117, east of Ja- 

 pan. Between this station and the next station (118) 

 toward the northeast, there was an extremely abrupt in- 

 crease in the phosphate content, surface values at sta- 

 tions 117 and 118 being 3 and 90 mg PO4 per cubic me- 

 ter respectively. From between these two stations to 

 near the Aleutian Islands there is a southerly flowing 

 current which apparently originates in the Bering Sea. 

 Mixing of water to great depths probably takes place dur- 

 ing a considerable part of the year and an abundance of 

 phosphate in the surface water consequently is to be ex- 

 pected. Apparently this phosphate-rich water flows 

 southward until diverted eastward by the North Pacific 

 West TOnd Drift (commonly called the Japan Current). 



This drift carries thephosphate-rich water eastward 

 and then southward by the California Current, which is 

 formed in the area where the West Wind Drift reaches 

 the Aleutian Islands. The highest phosphate content any- 

 where obtained at the surface, 142 mg PO4 per cubic 

 meter, was found at station 119, in latitude 45° 24' north, 

 longitude 159° 36' east. From this station the concen- 

 tration steadily diminishes eastward, to 25 mg per cubic 

 meter at station 129. As regards the high phosphate 

 content in this region, the fact must not be overlooked 

 that in latitudes above 40° north there is considerable 

 convective mixing of the water during the winter months, 

 and this, as has been pointed out previously, will in- 

 crease the phosphate content in the surface layer. The 

 Carnegie observations were made in July when one would 

 expect that much of the phosphate brought to the surface 

 in this manner would be consumed were there no other 

 source of supply. 



Between latitudes 10° and 40° north, with the excep- 

 tion of a few stations near the coast of California, the 

 phosphate content was extremely low, never exceeding 

 10 mg per cubic meter and often less than half that 

 amount. These low values in this area extended to con- 

 siderable depths, being found in the upper 100 meters or 

 more at practically all stations with the exception of a 

 few northeast of Japan. The maximum depth to which 

 they extended was 300 meters at station 140. 



The small quantities of phosphate observed in the 

 North Pacific between latitudes 10° and 40° north, ap- 

 pear to be associated with two hydrographic features of 

 the region. First, there is a steep density gradient re- 

 sulting in a minimum amount of mixing of deep water 

 with the surface water. Second, in this part of the Pa- 

 cific there are no well-developed current systems ex- 

 cept near the coasts of California and Japan. It is prob- 

 able that very little water enters or leaves the area 

 except to a relatively small extent in the two localities 

 mentioned. Thus there appears to be no agency that can 

 materially augment the phosphate content in the surface 

 layer. 



The relatively high phosphate values obtained off 

 the coast of California in July and September may be at- 

 tributed to the upwelling of subsurface water from inter- 

 mediate depths and possibly also to the transport of 

 phosphate-rich water from the north by the California 

 Current. 



Matsudaira (1932) reports the results of phosphate 

 determinations made on surface samples collected by 

 the Japanese training ship, Sintbku Maru, on a cruise 

 from Kobe to San Diego, from San Diego to Hilo, and 

 from Hilo to Kobe, June to September, 1930. Apparently 

 the samples were not analyzed until the vessel's arrival 

 in Japan. The phosphate values obtained are consider- 

 ably higher than those obtained by the Carnegie in the 

 open Pacific and by the Scripps Institution of Oceanog- 

 raphy off San Diego. In these two cases the samples 

 were analyzed soon after collection and it seems neces- 

 sary to conclude that the values obtained by Matsudaira 

 may be in error owing to prolonged storage of the sam- 

 ples. 



South of latitude 10° north, the phosphate contentwas 

 in general intermediate between that found in the other 

 two parts of the Pacific discussed. In the whole region 

 extending well over the southeastern and tropical Pacific, 

 extremely low concentrations of phosphate occurred at 

 only three stations south of latitude 7° north and these 

 were south of Easter Island. The highest values observed 



