THOMAS: TROPICAL PACIFIC NUTRIENT INVERSIONS 



LATITUDE 

 lOS 



173 171 169 167 165 163 161 159 i SB 156 154 152 



Yh8 146 144 142 140 ['38 136 134 i«2 130 128 124 120 118 l'l4 I'lO 106 



Figure 2.— Vertical distribution of nitrate-N (/xg-at/liter) along long 119°W from lat 1°14'S to 20°00'S, Feb- 

 ruary 7-14, 1967. 



It is not easy to explain these distributions. 

 There is a possibility that the subsurface water 

 has been depleted of nutrients in situ since this 

 water has a maximum chlorophyll concentration 

 (Owen and Zeitzschel, 1971). On the other 

 hand, one can envisage two water masses that 

 impinge on each other. One of these, the low- 

 salinity, high-nutrient, surface, and near-surface 

 water, may have drifted to the west from the 

 Peru Current via the South Equatorial Current. 

 The horizontal maps of nutrients at 10 m (Thom- 

 as, 1971, and unpublished Atlas data) show that 

 near-surface nutrients are high at these latitudes 

 from Peru at least west to long 126 °W, and 

 Wyrtki (1966, Figures 1 and 2) indicates such 

 westward surface currents. One can then en- 

 visage that the second water mass, containing 

 high-salinity water that is depleted in nutrients, 

 has its origin in the South Pacific Gyre and has 

 sunk below the first water mass and drifted in 

 from the southeast. Geostrophic flow of the 

 maximum salinity water is predominantly from 

 the east or southeast in the EASTROPAC area 

 (Tsuchiya, unpublished Atlas observations) and 

 similar flow from the east was shown by Tsu- 

 chiya (1968) in his Figure 6 for the 300 cl/t 8t 



surface which has a depth of 50-100 m in this 

 area. It should be mentioned that the data are 

 very sparse in the area to the south and south- 

 east of these inversions and their full expla- 

 nation may await further data collection and 

 analysis. 



The nitrate data show the inversion more 

 clearly than data for other nutrients, because 

 nitrogen is more limiting to phytoplankton than 

 phosphate or silicate in near-surface water to 

 the south of these inversions (Thomas, 1969) 

 and is possibly limiting in the water where the 

 nutrients were depleted. 



ACKNOWLEDGMENTS 



Many persons aided in gathering these nu- 

 trient data during the EASTROPAC Expedition. 

 Of these, Edward Renger and Don Seibert were 

 of considerable help in analysing the data and 

 preparing sections for drawing by the computer. 

 I also thank Dr. Mizuki Tsuchiya and Joseph L. 

 Reid for supplying data prior to its publication 

 and for helpful discussions of the inversion phe- 

 nomenon. 



931 



