54 
toplankton carbon, such as those given above, 
and the available data on rate of production of 
plant carbon. The basis for the estimates of 
standing crop is imperfect and most of the pro- 
duction data (Table 4) were obtained under 
nonnatural conditions of constant illumination. 
As a matter of interest, there are two noon sta- 
tions in the AB-11 and AB-8 series, numbers 
56 and 62 of Table 1, A, for which data are 
available for primary production rate measured 
by the more trustworthy in situ method, over 
the water column 0 -100 m. The values are 
respectively 134 and 290 mgC/m 2 /day (Black- 
burn et al., 1962, Appendix I); the correspond- 
ing estimated standing crops of phytoplankton 
for those stations (again using Strickland’s fac- 
tor 30 to multiply weights of chlorophyll a) 
are 1080 and 870 mgC/m 2 . 
The difference in range of sampling depth for 
the three standing crops has been noted in the 
preceding part of the discussion. As shown 
earlier, the depth was approximately 0-100 m 
for chlorophyll a, 0-300 m for zooplankton 
(including copepods), and 0-90 m for carni- 
vorous micronekton. Zooplankton hauls over 
0-300 m have long been standard in eastern 
tropical Pacific investigations, in order to mini- 
mize effects of diurnal vertical migration. Data 
in Appendix I show that the great bulk of zoo- 
plankton at 0-300 m is actually located at 0-140 
m in the eastern tropical Pacific. All standing 
crop measurements given in Tables 1 and 2 are, 
therefore, based wholly or mainly on material 
drawn from a water column or layer between 
0-90 m and 0-140 m, with a few exceptions 
noted elsewhere. 
Duration and Maintenance of 
Possible Steady State 
Assuming that the standing crops of the AB- 
11-Z and AB-8-H data series are in steady- 
state conditions, the question arises as to the 
minimum period of time over which these con- 
ditions prevailed. This would be about the aver- 
age time taken for phytoplankton material to be 
converted into tissue of small primary carnivores 
(ca. 1-10 cm), but the actual time is unknown. 
Blackburn (1963) assumed a period of three 
months in a neighboring area of the eastern 
PACIFIC SCIENCE, Vol. XX, January 1966 
tropical Pacific, and was thereby able to relate a 
series of seasonal changes in properties from 
wind velocity to abundance of tuna. This does 
not justify the three-month estimate, but in fact 
the estimate is not unreasonable; for the Gulf of 
Panama, Forsbergh ( 1963 ) estimated that about 
two weeks might suffice for herbivores to grow 
from eggs to adults, and Howard and Landa 
(1958) showed that a small pelagic fish grows 
to a length of about 5 cm in between two and 
three months. The supposed steady-state re- 
vealed by the standing crop data in the above- 
mentioned series of station-pairs may therefore 
be considered to have lasted at least from late 
February to late May, when the observations 
were made; it could have begun earlier, ended 
later, or prevailed all year. 
The data, then, are consistent with a steady 
balance between plants, herbivores, and primary 
carnivores, during the northern spring and pos- 
sibly longer, in most of the area east of 95 °W 
between 12° and 5°N (except the Costa Rica 
Dome). It might be asked how such a balance 
can be reconciled with the rather high standing 
crops and productivity observed, and other signs 
of biological richness such as the thick oxygen- 
poor layer that occurs at depth in the area 
( Wyrtki, 1962, and references cited there). 
The question arises because Cushing ( 1959^) 
thought such biological richness in tropical areas 
would be seasonal, as a result of upwelling, and 
therefore "unbalanced” as in higher latitudes. 
Seasonal upwelling does not seem to explain the 
observations put forward here; it does occur in 
the northern winter and spring on the conti- 
nental shelf in the Gulf of Panama (Schaefer 
et al., 1958), well to the north of A16 and A 17 
in Figure 2, but this paper does not deal with 
that area. 
Both Cushing (1959^, b) and Dunbar (I960) 
considered other ways by which a regular sup- 
ply of nutrients might be maintained in the 
tropical euphotic layer, such as regeneration 
through excretion by animals, but apparently 
did not consider them sufficient to maintain 
steady-state conditions in rich areas. This seemed 
to leave only upwelling, which Cushing thought 
would be seasonal, at least in its effects upon 
biota ( the standing crop of phytoplankton tem- 
porarily restricted by turbulence, then increasing 
