P 04 -P >jg al/L 
280 
FISHERY BULLETIN OF FISH AND WILDLIFE SERVICE 
Figure 25.—East-west variations in surface inorganic 
phosphate concentrations (gg.at./L.) between 2° N. and 
2° S. latitude and 0° N. and 11° N. latitude. 
An important bioloo-ical consideration is tlie 
fact that alon^ the nortliern boundary of the 
Countercurrent tlie thermocline in tlie eastern Pa¬ 
cific is sufficiently shallow to be within the euphot- 
ic zone. If we assume that in these tropical wa¬ 
ters seasonal variation in li^ht is not a limiting 
factor, Sverdrup’s concept of critical depth is ap¬ 
plicable (Sverdrup 1958). With the thermocline 
penetrating into the euphotic zone (shallower 
than the compensation depth, ca. 100 meters), the 
phytoplankton in the mixed layer is retained 
within the dejith range of active photosynthesis; 
i.e., not carried by vertical mixing to a depth dur¬ 
ing the day where respiration exceeds photosyn¬ 
thesis. To the north and south of the northern 
boundary of the Countercurrent, and west of 
about 150° W., the thermocline deepens and is 
normally deeper than the compensation depth. 
Biological data from this area are ])ertinent. 
King and Hida (1957), discussing the results of 
POFI’s zoo])lankton program in the central equa¬ 
torial Pacific, 1951 to 1954, show that zooplankton 
volumes in the Countercurrent are comparatively 
low, 180th meridian to 150° W. longitude (ca. 25 
cc./lOOO m.'^) but increase to nearly 45 cc./lOOO m.-"^ 
between 140° W. and 120° W., a volume approxi¬ 
mately expial to that of the region of the diver¬ 
gence near the Kipiator. They also demonstrate 
(fig. 11) that, between 8° N. and 11° N., the 
])lankton volumes increase west to east (170° W. 
to 140° W.) as the depth of tlie thermocline de¬ 
creases. Thus, the available empirical data sug¬ 
gest that any relative increase in the standing 
crops of the biota along the northern boundary of 
the (V)untercurrent is more dii*ectly I’elated to the 
jiresence of a sliallow thermocline than to a di¬ 
vergence and u])welling in the surface waters. 
SUMMARY 
A cooperative oceanographic survey of the cen¬ 
tral and eastern tropical Pacific (expedition 
Eastropic) was conducted during the period Sep- 
tember-December 1955, Avith five research vessels 
participating, representing five agencies: Scripps 
Institution of Oceanography, Inter-American 
Tropical Tuna Commission, California Depart¬ 
ment of Fish and (xame, Pacihc Oceanic Fishery 
Investigations (POFI) of the I . S. I ish and 
Wildlife Service, and the Peruvian Navy. 
As POFFs ])ar1 icipation in Eastropic, the 
Ihiglx M. SmJth (cruise 81) completed an 8r)-day, 
141)0()-mile cruise, obtaining information on east- 
west gradients in temperature, salinity, phosphate, 
zooplankton, and forage fish abundance along the 
northern boundary of the Equatorial Counter- 
current and along the Equator betAveen 110® W. 
and 15()° W. longitude. A survey of tuna bait- 
Hsh AAUis conducted in the Manpiesas Islands and, 
in collalioration Avith the UniAursity of IlaAvaii, 
carbon fixation and chlorophyll measurements 
Avere made on the AAUstbound leg of the cruise. 
Sea surfa(‘e temperatures along the Equator 
Avere from 2° to 4° F. cooler than normal; those 
in the Eipiatorial Countercurrent deviated little 
from normal. Temperature anomalies for the 
easteiai Pacific, Alaskan Avaters south to Peru, 
AAwe generally negative ( — 0.6° F. to —2.0° F.) 
for the latter half of 1955. 
Near the Equator, the observed east-Avest slope 
of the thermocline Avas considerably steeper than 
normah shalloAving from near 500 feet beneath 
the surface at 160° W. longitude to at or near 
the surface at 125°W., then deepening someAvhat 
to the eas^:. 
A pronounced oceanic temperature front, across 
which there was a temperature change of apjirox- 
imately 8° F., Avas observed near 4° N., 120° W. 
The same or a similar feature Avas observed (1 
month later) from the Scripps Institution of 
Oceanography vessel, the Horizon^ near 8° N., 
120° W. 
(hdculated current velocities in the Avarmer 
Avaters to the north of the front Avere Avesterly, 1.8 
to 2.8 knots, decreasing to 0.2-knot Avesterly Hoav 
in the (‘ooler AAaiter to the south of the front. In 
the eastern poilion of the area suiweyed from the 
(Eipiator to 4° N., 120° W.), velocities 
OCEANOGRAPHY OF EAST CENTRAL EQUATORIAL PACIFIC 
281 
measured from Scripps vessels Avere as high as 
2.0 knots (824° T., measured at 8°08' N. near 
120° W.). 
The calculated east-Avest slope of the sea surface 
(0/400-db. level) was greater than the mean; 0.66 
dvnamic meters at 120° W. (mean = 0.85) sloping 
upAvard to 0.90 dynamic meters near 140° W. 
(mean = 0.92) for a diffierence of 0.24 dynamic 
meter as compared Avith 0.07 for the mean. 
TTpAvelling and Avind mixing, coupled Avith a 
shalloAV thermocline, resulted in considerable en¬ 
richment of the surface Avaters along the Equator. 
Concentrations of inorganic phosphate at the sur¬ 
face Avere between 0.8 and 1.2 /xg. at./L. in the 
region of the shalloAV thermocline (near the 
Equator and 125° W. and near 4° S. and 115° 
W.). Phosphate concentrations in the surface 
Avaters of the Countercurrent and along the north¬ 
ern edge of the Countercurrent Avere uniformly 
loAv, ^0.4 jug. at./E. 
Cooler than normal surface temperatures, com¬ 
paratively steep east-Avest slopes in the sea surface 
and the thermocline, comparatively high calcu¬ 
lated and measured current velocities, and high 
inorganic phosphate concentrations in the surface 
layer, all suggest the horizontal and vertical cir¬ 
culation features, 5° N. to 5° S. along the Equator, 
Avere more dynamic during Eastropic than nor¬ 
mally observed. 
LITERATURE CITED 
Austin, Thomas S. 
1954a. Mid-Pacific oceanography. Part III. Trans- 
eqiiatorial w^aters, Augiist-October 1951. U.S. 
Fish and AVildlife Service, Spec. Sci. Kept.—Fish. 
No. 181, 50 pp. 
1954h. Mid-Pacific oceanography. Part V. Trans- 
eciuatorial Avaters, May-.Tune 1952, August 1952. 
tl.S. Fish and Wildlife Service, Siiec. Sci. Rei)t.— 
Fish. No. <S() pp. 
1957. Suniinary, (x-eanographic and fishery data, 
MaiMpiesas Islands area, August-Septeinher, 1956 
(E(piapac). U.S. Fish and Wildlife Service, Spec. 
S('i. Kept.—Fish. No. 217, 1<S6 pp. 
195S. Vailations Avith depth of oceanographic prop¬ 
erties along the E(iiiator in the Pacific*. Trans. 
Ani(‘r. Geoj)hys. Union 89((>) ; 105.5-1068. 
Ai s'iT\. Thomas S.. and IMai hu’e O. Kinkkl. 
In press. Variatii. is in upAvelling in the ecpiatorial 
Pacific. Proc. 9th Pacific Science Uongress. 
British Air AIinistry. 
1950. ^Monthly meteorological charts of the eastern 
Pacific Ocean. Prepared in the Marine Branch 
of the AIeteorologi(‘al Office. London: His Majest.A’S 
Stationery Office, M.O. 51S, 122 pp. 
Bruneau, L., N. G. Jerlov, and F. F. Kocza'. 
1958. Physical and chemical methods. Rei)orts of 
the SAvedish Deep-Sea Expedition 8(4) ; 101-125. 
CROMAVELL, TowNSENl). 
1951. Mid-Pacific* oc*eanography, January-March 
1950. U.S. Fish and Wildlife SerAic*e, Spc'c. Sci. 
Rept.—Fish. No. .54, 76 pp. 
19.54. IMid-Pacific* oc*cNUiography. Part II. Trans- 
ecpiatorial Avatcws, .Tune-August 19.50, .lanuar.A'- 
Marc*h 1951. U.S. Fish and Wildlife Servic*e, Spec. 
Sc*i. Rept.—Fish. No. 181. 179 pp. 
1956. Eastropic expedition: Avhat it seeks, and 
Avhere it fits into prac*tic*al industrial fishing. 
Pacific Pish. .54(8) : 25, 27, 29. 
Cromavell, Toavnseni), R. B. Montgomera', and E. I). 
Stroup. 
1954. Ecpiatorial undercurrent in Pacific Ocean re- 
A'ealed by neA\^ methods. Science 119(8097) : 648- 
649. 
Cromwell, Townsend, and J. L. Reid, .Tr. 
1956. A study of oceanic fronts. Tellus 8(1) ; 
95-101. 
Fleming, J. A., H. U. Sverdrup, C. C. Ennis, S. L. Seaton, 
and W. 0. Hendrix. 
1945. Scientific results of cruise YII of the Carnegie 
during 1928-1929 under command of Captain J. P. 
Ault. Observations and results in physical ocea¬ 
nography—I—B. Carnegie Inst. AVasli., Publ. No. 
545, 815 pp. 
Fofonoff, N. P., and R. B. Montgomery. 
19.55. The ecpiatorial undercurrent in the light of 
the AT>rticity equation. Tellus 7(4) : 518-.521. 
Graham, H. AV. 
1941. Plankton prc)duc*tic)n in relation to character 
of Avater in the open Pac*ific*. .Tour. Alar. Res. 
4(8) ; 189-197. 
Jerlov, N. G. 
1956. The ecpiatorial currents in the Pacific Ocean. 
Report of the SAvedish Deei>-Sea Expedition 8(6) : 
12t)-154. 
Ketchum, B. H. 
1989. The development and restoration of deficien- 
c*ies in the phosphorus and nitrogen composition of 
unicellular plants. Jour. Cell. (5)mp. Physiol., vol. 
18 
87.8-381. 
King, Joseph E., Thomas S. Austin, 
Doty. 
and AIaxavell S. 
Preliminary report on cxpc'clition Eastropic*. 
Fish and AATldlife Serviccx Spec. Sci. Rept.— 
Fish. No. 201,155 ])p. 
