Relationships between Standing Crops at Three Successive 
Trophic Levels in the Eastern Tropical Pacific 1 
Maurice Blackburn 2 
ABSTRACT : Measurements of the following standing crops were made at each of 
several pairs of stations on various cruises in the eastern tropical Pacific: (a) chloro- 
phyll a, mg / rn 2 , 0-100 m; (b) zooplankton (total, and, for some cruises, copepods 
separately), ml/10 3 m 3 , 0-300 m; (c) small fish and cephalopods, combined, from 
net-caught micronekton, ml/10 3 m 3 , 0-90 m. These were considered to represent 
plants, herbivores, and primary carnivores. It was estimated that most of the zoo- 
plankton was located at 0-140 m. The stations of each station-pair were separated 
by less than 120 miles and 36 hr. 
Relationships between the logarithms of crops were investigated by simple cor- 
relations, partial correlations, and structural two-variable linear regressions. In 
the 36 station-pairs available from cruises made in the northern spring, both zoo- 
plankton and carnivores had a significant positive regression on chlorophyll a\ the 
points for 1 1 of these pairs fell within or close to the 95 % confidence limits of each 
of the regressions. For these 11 "statistically selected” pairs all simple correlation 
coefficients were positive and significant, the partial correlation coefficient of chloro- 
phyll a and zooplankton was positive and significant, and the other two partial 
correlation coefficients were non-significant. These results were considered to be 
consistent with steady-state conditions between the three standing crops. A similar 
analysis using copepods instead of total zooplankton gave a generally similar result. 
Chlorophyll a and primary productivity (by the C? 4 method) were positively and 
significantly correlated at 19 stations where both measurements were taken. 
Most of the station-pairs for which these results were obtained were located in the 
area bounded by 5°N, 95 °W, 12 °N, and the American coast (excluding the Costa 
Rica Dome). This is a moderately eutrophic area, where a steady state might 
not have been expected; however, there are indications that the process of eutrophi- 
cation, which probably is vertical mixing of the upper part of the very shoal thermo- 
cline ( <30 m) by wind, is itself fairly steady throughout the year. No definite 
indications of a steady state were obtained from any other area at any season; how- 
ever, the possibility of obtaining them from more copious material is not denied. 
The regression (slope) coefficients showed that standing crop of herbivores varied 
as some power <1.0 of standing crop of chlorophyll a, suggesting increasingly in- 
efficient utilization of plants by herbivores with increase of plant standing crop. On 
the other hand the crop of carnivores varied in an approximately linear way with 
that of herbivores. The standing crop ratios, copepods/plants (by weight of carbon) 
and carnivores/zooplankton (by displacement volume), were both roughly esti- 
mated at 0.04 under steady-state conditions; for various reasons the corresponding 
food-chain efficiency ratios, for standing crops of all material at the appropriate 
trophic levels, would be higher. 
1 Contribution from the Scripps Institution of 
Oceanography, New Series. Manuscript received July 
20, 1964. 
2 Scripps Institution of Oceanography and Institute 
of Marine Resources, University of California, La 
Jolla, California. 
Speaking of the sea, Harvey (1955) said, 
"In nature an equilibrium between the standing 
crop of plants, herbivores and carnivores is con- 
tinually passing in and out of balance.” He 
clarified this statement by reference to temporary 
36 
