Standing Crops and Trophic Levels — Blackburn 
37 
excesses of material at some trophic levels being 
causally associated with temporary deficiencies at 
other levels, and in so doing he summarized 
much observation and interpretation of standing 
crop changes from many investigations in tem- 
perate and cold seas. 
Recently a different concept, that of a vir- 
tually undisturbed balance between standing 
crops of biota, has been considered applicable 
to tropical ocean areas. For example, Nielsen 
(1958) said, "A direct relationship between phy- 
toplankton and zooplankton is normal in the sea 
. . . with stable hydrographic conditions such as 
occur mainly in tropical and subtropical regions, 
the standing crop of phytoplankton and its pro- 
duction is stable”; Holmes (1958) spoke of "a 
situation approaching a steady state” in regard to 
relationships between standing crops of chloro- 
phyll a and zooplankton in the eastern tropical 
Pacific; Cushing ( 1959^ b) distinguished two 
extreme types of productive cycle, one "unbal- 
anced” (with the features noted by Harvey) in 
temperate and cold seas, the other "fully bal- 
anced” or in a "steady state” (with no change 
in standing crops) in non-upwelling tropical 
regions; Menzel and Ryther ( 1961 ) found phy- 
toplankton production and zooplankton standing 
crop to be rather constant over most of the year 
and directly related over all of it, in the sub- 
tropical Sargasso Sea. None of these authors pre- 
sented data on primary carnivores. 
It is important to know where and when 
steady states occur because they are freely in- 
corporated in analytical models of food-chain 
relations concerning various parts of the ocean 
(Riley, 1963). At the practical level they could 
be of use in making forecasts. 
This paper deals with statistical analysis of 
measurements of standing crops of phytoplank- 
ton (as chlorophyll a ), herbivores (as total 
zooplankton and, in part, as copepods), and pri- 
mary carnivores (as micronektonic fish and 
cephalopods) which were all taken together at 
different times and places in tropical and sub- 
tropical parts of the eastern Pacific Ocean. The 
object of the study was to show for which (if 
any) of these time-space situations all stand- 
ing crop measurements were consistent with a 
steady-state productive cycle. It was realized that 
such data could not prove the existence of such 
a cycle. It was necessary to make many of the 
measurements as opportunities offered, in the 
course of cruises undertaken for other purposes. 
The results of this work may encourage the 
making of additional much-needed observations 
of the same kind, or of better ones for the same 
purpose. 
The work formed part of the research of the 
Scripps Tuna Oceanography Research Program 
of the Scripps Institution of Oceanography and 
Institute of Marine Resources, University of 
California. It was supported by the U. S. Bureau 
of Commercial Fisheries under several contracts. 
I am grateful to my colleagues R. W. Holmes, 
R. C. Griffiths, and K. Wyrtki for unpublished 
data and useful comments, and to E. W. Fager, 
M. B. Schaefer, and W. H. Thomas who also 
made valuable suggestions. 
MATERIAL: CRUISES, MEASUREMENTS, 
AND STATIONS 
Cruises 
Standing crops of chlorophyll a, zooplankton, 
and micronekton were measured frequently at 
oceanographic stations on eight cruises in trop- 
ical and subtropical parts of the eastern Pacific. 
On three of these cruises the observations were 
confined to a few small areas and do not serve 
the purpose of this study. The other cruises were 
those known as Scot (or TO- 5 8-1 ) and TO- 
60—1, both made in the Northern Hemisphere 
and northern spring; Step-I (or TO-60-2 ) , 
made in the Southern Hemisphere and southern 
spring; TO-59-1, in the Northern Hemisphere 
and northern winter; and TO-59-2, in the 
Northern Hemisphere and northern summer. 
Certain measurements made on these cruises 
were suitable for the present study in ways de- 
scribed below, and they are listed in Table 1. 
Similar measurements of chlorophyll a and 
zooplankton, but not micronekton, were made 
on three other cruises, and those suitable for the 
present study are listed in Table 2. The cruises, 
known as Eastropic, Scope, and Costa Rica 
Dome, were all made in the Northern Hemis- 
phere and northern autumn. Figure 1 shows the 
areas in which the measurements of Tables 1 
and 2 were made. 
