FISHERY BULLETIN: VOL. 70, NO. 3 



about 60 to 140 m depth. The inversion is par- 

 ticularly pronounced for nitrate, where concen- 

 trations in the minimum reach nearly undetect- 

 able levels. 



The inversions are also apparent in vertical 

 sections. Section II-NO3-V2 from the EASTRO- 

 PAC Atlas (Thomas, 1971) is shown in Figure 

 2. This section was taken along long 119°W 

 in February 1967 and shows that the inversion 

 extends from about lat 8° to 15 °S. 



Similar sections occupied in February-March 

 1967 showed the inversions along long 126 °W 

 (from lat 8° to 15°S) and along long 112°W 

 (from lat 8° to 10°S) . The nitrate data show in- 

 versions much more clearly than phosphate or 

 silicate data. Inversions in nitrate concentration 

 are barely detectable in sections to the east at 

 these latitudes— along long 105°W, 98°W, or 

 92 °W — because concentrations at similar depths 

 are higher. The inversion phenomenon is most 

 pronounced in the westerly sections of the EAS- 

 TROPAC data. 



NO3-N. S1O4- Si (/jg tjt /liter) 



10.0 20.0 



PO4-P >.pg ot /liter) 

 1.0 2.0 



EASTROPAC 

 Station 11.140 

 Lot 8° 41' S 

 Long 119° 00' W 

 Februory lO, 1967 



Figure 1. — Distribution of nitrate-N, silicate-Si, and 

 phosphate-P concentrations with depth at EASTROPAC 

 station 11.140, February 10, 1967. 



The nitrate inversion was again detected in 

 the southern summer of 1968 (February-March) 

 during the third EASTROPAC survey (Thomas, 

 unpublished Atlas data) . At that time it was 

 apparent along long 119°W (from lat 9° to 

 15°S) , but not at more easterly longitudes. The 

 long 126°W line was not sampled at this time. 



It is not known whether inversions along these 

 longitudes are permanent, nonseasonal features, 

 since the second EASTROPAC survey during 

 the southern winter (August-September 1967) 

 and the EASTROPAC monitoring cruises 

 (which took place between the multiship sur- 

 veys) reached only lat 5°S. At this latitude 

 inversions were not found. 



DISCUSSION 



Such nutrient inversions were not commented 

 upon prior to EASTROPAC because this was 

 the first expedition to measure a relatively com- 

 plete suite of nutrients over a broad area of the 

 eastern tropical Pacific Ocean. However, there 

 are indications of such inversions in phosphate 

 at many of the EQUAPAC stations taken in Au- 

 gust-September 1956 during Cruise 35 of the 

 RV Hiigh M. Smith (Austin, 1957). The Smith 

 traversed long 135°W, 143°W, 151°W, and 

 160°W from about lat 3°N to 19°S, and many of 

 the stations taken from lat 5°S to 19 °S show 

 minimum phosphate values at about 100 to 160 m 

 depth. The inversions probably would have been 

 readily apparent if nitrate analyses had been 

 made during Smith Cruise 35. 



Such inversions are not found in stations taken 

 from lat 7° to 15°S and long 162°W and 175°W 

 in the nitrate, phosphate, or silicate data from 

 cruise STYX in June-July 1968 (Reid, unpub- 

 lished data) . However, they are found to a slight 

 extent in this same area when the nitrate data 

 from a Gascoyne cruise in 1961 are examined 

 (C.S.I.R.O., 1967). 



This water having a minimum nutrient con- 

 centration has a high salinity. Figure 3 (from 

 Tsuchiya, unpublished Atlas data) shows a sa- 

 linity section along long 119°W in February 

 1967 from the EASTROPAC observations. The 

 maximum in salinity clearly corresponds to the 

 nutrient minimum shown in Figure 2. 



930 



