278 
FISHERY BULLETIN OF FISH AND WILDLIFE SERVICE 
From consideration of the standing crops of the 
biota in the eiiphotic zone near the Equator, oxy¬ 
gen values in the waters beneath the thermocline at 
the Equator should essentially be depleted and 
lower than to the north and south unless there v as 
active replenishment. An inference to the stand¬ 
ing crop of the phytoplankton may be made fiom 
the results of the use of the carbon isotope during 
expedition Eastropic (hg- 24). The highest 
rates of photosynthesis were at or very near the 
Equator where there is enrichment by upwelling 
(fig. 18). It is reasonable to assume that these 
higher rates of carbon fixation were associated 
with the larger standing crops of the phytoplank¬ 
ton. The different rates of photosynthesis among 
various species in, and the ages of, the populations 
(whether vigorously growing or senescent) are 
quantitatively unknown variables. King and 
Hida (1957, figs. 8 and 10) have shown that the 
standing crop of zooplankton reaches a maximum 
between approximately 1.5° K. and 1.5° S. latitude. 
These facts and our present knowledge of the 
pattern of flow near the Equator lead to the con¬ 
clusion that there is advection of waters with 
higher oxygen content from the west by the 
Equatorial Undercurrent. This subsurface, east¬ 
erly directed flow was first reported by Cromwell, 
Montgomery, and Stroup (1954). They observed 
that the Undercurrent was both in the lower part 
of the surface layer and in the upper part of the 
thermocline. Its total depth range, however, was 
not determined. 
More recently, Knauss and King (1958), pre¬ 
senting preliminary results of a detailed survey of 
the Undercurrent made at 440° W. longitude, re¬ 
port that the vertical extent of the Undercurrent 
is between about 30 and 300 meters with the high¬ 
est easterly velocities (2.0 to 3.5 knots) recorded at 
a depth of 100 meters. The Undercurrent was 
symmetrical about the Equator. At 2° K. and 2° 
S. latitude, the average thickness had decreased to 
30 meters and the average maximum velocity to 
0.6 knot. Of particular interest to the discussion 
of the results from expedition Eastropic, Knauss 
and King report that during the period of their 
cruise (March-June 1958), the Undercurrent 
showed no diminution in velocity between 140° W. 
and 92° W. and that the depth of its core rose 
from 100 meters at 140° W. to 42 meters at 98° W. 
Farther to the east, at 95° W. and 92° AT., it once 
again deepened. At 89° W. the Undercurrent 
was missing. 
In the section on vertical distribution of tem¬ 
perature (fig- 6), we discussed the configuration of 
the isotherms, particularly in the thermocline, as 
related to the Undercurrent. Between 2° N. and 
2° S. there Avas a spreading of the isotherms, re- 
sidting in a shalloAV ridge and a deeper trough 
Figukk 24. —Rate of carbon fixation 
(nig.(t/lir./in.'’) by photosynthesis as measured i)y uptake of the isotope carbon 14. 
(Data from King et ah, 1957.) 
OCEANOGRAPHY OF EAST CENTRAL EQUATORIAL PACIFIC 
279 
about the Equator. This Avas interpreted as re¬ 
sulting from adA^ection from the Avest of the some- 
A\4iat Avarmer Avaters of the Ihidercurrent. Near 
the Equator, the thermocline shalloAved betAveen 
160° AV. and 120°-T25° AA^. (fig. 5), then deepened 
once again. Considering these data and the re¬ 
sults reported by Knauss and King (1958), it is 
assumed that, during Eastropic, the core of the 
Undercurrent exhibited a Avest-east Auiriation in 
depth similar to that of the thermocline. 
Oxygen and inorganic phosphate are both non¬ 
conservative (biologically affected) properties. 
Their concentrations normally exhibit reciprocal 
variations in the sea. This Avas the case in the 
Avaters beneath the Equator during Eastropic, at 
least along the 110° W. section (hg. 13), the only 
one for Avhich adequate phosphate data are aA^ail- 
able. BetAA^een approximately 2° K. and 2° S., the 
meridional extent of the Undercurrent, the higher 
oxygen Auilues preAhously discussed Avere accom¬ 
panied by loAver inorganic phosphate A^alues, 1.6 
/xg.at./L. or less as compared Avith 2.0 /xg.at./L. or 
greater at comparative depths to the north and 
south. Thus, the distribution of the tAvo noncon- 
serAuitive properties and of temperature beneath 
the thermocline at the Equator is largely governed 
by the easterly floAving Undercurrent, Avhile their 
distribution in the shalloAver portion of the ther¬ 
mocline and the mixed layer is largely related to 
upAvelling. 
There are various references to the oceano¬ 
graphic conditions along the northern boundary 
of the Countercurrent Avhich use the Avords “di- 
A^ergence” or ‘hipAA-elling.” Relative measure¬ 
ments of the standing crops of the marine biota, 
especially zooplankton, have been used to support 
the hypothesis that divergence of the surface 
Avaters along this boundary has resulted in enrich¬ 
ment Avithin the euphotic zone. Sverdrup et ah 
(1942, p. 711), suggest that a transverse circula¬ 
tion is superimposed on the floAvs to the east (the 
Countercurrent) and to the Avest (the Korth and 
South Equatorial Currents). Such a transverse 
circulation Avould require a divergence at the 
northern boundary of the Countercurrent and a 
convergence at the southern boundary. Referring 
to the relative volumes of the plankton samples 
taken aboard the Carnegie as reported by Graham 
(4941), they suggest that the relatively high 
Amlume at Carnegie station 151 (13° X.) Avas asso¬ 
ciated Avith a diA^ergence centered near 10° X. 
(Sverdrup et ah, 1942, fig. 219). This biological 
evidence may be someAvhat speculative as Graham 
(1941, p. 193) states that this sample (station 151) 
Avas ‘‘not quite comparable as it contained a large 
colony of salps." The sample (expressed as dry 
Aveight and not as volume) Avas not used by 
Graham in his analyses of plankton abundance 
along the Carnegie/s transequatorial section (his 
fig. 41). 
Jerlov (1956, p. 150), discussing the results of 
the AJhatross expedition in the central equatorial 
Pacific, assumes from consideration of the relative 
distribution of particles that “there is ascending 
Avater moA^ement along o-t-surfaces aaOiIcIi enriches 
the upper layer Avith nutrients.” His figure 34 
shoAvs a maximum concentration of particles at 
the northern edge of the Countercurrent. Jerlov 
suggests that the distribution of these particles 
“largely represents phytoplankton population and 
plankton remnants, as the supply of terrigenous 
components in this area must be Ioav.” 
Austin (1954a) does not recognize the presence 
of upAvelling at the northern boundary of the 
Equatorial Countercurrent in the central Pacific, 
but does acknoAvledge the fact that the shalloAv 
thermocline in this area (see fig. 15, this report), 
coupled Avith Avind mixing, may result in an in¬ 
crease in the biota. A question of semantics may 
be involved. Austin and Rinkel (in press) liaA^e 
defined upAvelling as a local, Avind-induced diA^er- 
gence of the surface Avaters resulting in a mixing 
of the deeper, cooler, nutrient-rich Avaters Avith 
those at the surface. Witlnn the scope of tliis defi¬ 
nition, there is no evidence of upAvelling along the 
northern boundaiy of the Countercurrent in the 
Eastropic data. Although there Avas a marked 
ridge in the tliermocline, there Avas no eAudent 
cooling of the surface Avaters (see fig. 14). At the 
northern edge of the Countercurrent any upAvel- 
linc’ Avould result in a band of more-saline Avaters 
and AATiters AAuth higher ])hosphate concentrations. 
Salinities Avere comparatively Ioav, increasing to 
either side of the northern edge of the Countercur¬ 
rent (fig. 17). Phosphate concentrations Avere 
loAV, averaging 0.4 /xg. at./E. or less, these to be 
compared to 0.8 to 1.0 /xg. at./E. near the Equatoi 
(fig. 25). 
