NUTRIENT INVERSIONS IN THE SOUTHEASTERN TROPICAL 



PACIFIC OCEAN '•' 



William H. Thomas' 



ABSTRACT 



Nutrient inversions in the offshore southeastern tropical Pacific Ocean are described. 

 At a typical station nutrients such as nitrate and phosphate are high at the surface, 

 are at a minimum at about 100 m depth, and then increase at greater depths. Silicate 

 follows this distribution to a lesser degree. These inversions occur from lat 8 to 15° S and 

 are more pronounced in sections along long 126°W and 119°W than in sections farther 

 east. The nutrient minimum is associated wdth vi'ater having a salinity maximum. It 

 is suggested that such v^^ater may have acquired its characteristics in the mixed layer 

 in areas to the south or southeast vi^here the nutrients were depleted by phytoplankton, 

 and has then sunk below high-nutrient and relatively fresh water carried westward from 

 the Peru Current. However, the nutrients may also have been depleted in situ since 

 the low-nutrient water contains a maximum amount of chlorophyll. 



In water where nutrients are limiting phyto- 

 plankton growth, they are typically at minimum 

 concentrations in the surface and mixed layer, 

 increase at the thermocline, and reach a max- 

 imum below this depth. Such typical distribu- 

 tions are shown by Sverdrup, Johnson, and Flem- 

 ing (1942) and by Riley and Skirrow (1965). 

 The EASTROPAC data show such distributions 

 in all parts of the nutrient-poor water of the 

 northeastern tropical Pacific Ocean (Thomas, 

 1970, 1971, and unpublished data). 



The purpose of this paper is to describe a dif- 

 ferent distribution occurring in the south trop- 

 ical Pacific Ocean in which nutrient concentra- 

 tions are typically high at the surface, are at a 

 minimum at depths just above the permanent 

 thermocline, and then increase at greater depths. 



^ Contribution from the Scripps Institution of Ocean- 

 ography. 



" This work was part of the STOR (Scripps Tuna 

 Oceanography Research) Program and is also a result 

 of the EASTROPAC Expedition, a cooperative study of 

 the biological, chemical, and physical oceanography of 

 the eastern tropical Pacific Ocean. The work was sup- 

 ported by National Science Foundation Grant No. GA- 

 27320 and by contracts #14-17-0007-963 and #14-17-0007- 

 989 between the National Marine Fisheries Service and 

 the Institute of Marine Resources. 



' Institute of Marine Resources, Scripps Institution of 

 Oceanography, University of California, San Diego, La 

 Jolla, CA 92037. 



In this paper this unusual distribution is called 

 a "nutrient inversion." 



METHODS 



The EASTROPAC sampling program and ves- 

 sel track charts are given in the Introduction 

 to the EASTROPAC Atlas (Love, 1970). Nu- 

 trients (nitrate, nitrite, phosphate, and silicate) 

 from Nansen bottle samples were analyzed with 

 the Technicon® Autoanalyzer' using the methods 

 of Armstrong, Stearns, and Strickland (1967) or 

 manually, using methods outlined by Strickland 

 and Parsons (1968). Vertical sections along 

 given longitudes were plotted by computer 

 (Love, 1970). 



RESULTS 



During February-March 1967, nutrient inver- 

 sions were found at several stations near lat 10 °S 

 and along long 112°W, 119°W, and 126°W. A 

 typical inversion is shown in Figure 1 for EAS- 

 TROPAC Station 11.140; three nutrients— ni- 

 trate, phosphate, and silicate — show minima at 



Manuscript accepted December 1971. 



FISHERY BULLETIN: VOL. 70, NO. 3, 1972. 



* Reference to trade names in this publication does 

 not imply endorsement of commercial products by the 

 National Marine Fisheries Service. 



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