40 
PACIFIC SCIENCE, VoL XX, January 1966 
Chlorophyll a 
The standing crop of chlorophyll a was esti- 
mated in mg under 1 m 2 of sea surface for a 
water column 100 m deep, except for Step-I 
where 55 m was used. This was done by taking 
water samples of 3-6 liters at six to eight depths 
in Van Dorn closing plastic samplers; filtering, 
extracting from the residue, and determining 
chlorophyll a concentration in the extract by 
optical density measurements for each sample 
(Richards with Thompson, 1952); using the 
sample data to draw a smooth profile of chloro- 
phyll a with depth, and integrating the profile 
with a planimeter ( Holmes and Blackburn, 
I960). This standing crop was considered to 
represent, somewhat imperfectly, the standing 
crop of plant material (living or dead) on which 
animals could feed. 
Some of the chlorophyll a data in Tables 1 
and 2 have been published previously (Holmes 
et al., 1957; Holmes et ah, 1958; Holmes and 
Blackburn, I960; Blackburn et ah, 1962; Scripps 
Institution of Oceanography, I960, 1961). Other 
observations, including revisions of some values 
listed by Holmes and Blackburn (I960), are 
from R. W. Holmes (unpublished) and Grif- 
fiths (MS). 
Zooplankton 
Zooplankton was collected by making an ob- 
lique haul with an open subconical net between 
the desired maximum depth and the sea surface. 
The net was of the type that has been used for 
the past 15 years in the eastern and central 
Pacific by the Scripps Institution of Oceanog- 
raphy, EJ. S. Bureau of Commercial Fisheries 
(La Jolla and Honolulu), and Inter- American 
Tropical Tuna Commission (King and Demond, 
1953); the mouth diameter is 1 m, the total 
length is about 5 m, and the mesh apertures 
are 0.65 mm wide in the front and middle sec- 
tions and 0.31 mm in the rear section. The 
volume of water strained was estimated from 
readings of a calibrated flowmeter in the mouth 
of the net. 
In making a haul, the ship steamed slowly to 
TABLE 2 
Measurements of Standing Crops of Chlorophyll a (mg/m 2 ) and Zooplankton (ml/10 3 m 3 ) at 
Oceanographic Stations (stns.) in the Eastern Tropical Pacific* 
EXPEDITION 
STN. 
CHLOR. 
ZOOP. 
EXPEDITION 
STN. 
CHLOR. 
ZOOP. 
EASTROPIC, 
33 
24 
193 
SCOPE, 
1 
24 
49 
November 
35 
22 
164 
November - 
2 
25 
37 
1955 1 
37 
25 
143 
December 
3 
43 
32 
1956 2 
4 
44 
54 
5 
51 
85 
6 
74 
314 
COSTA RICA 
7 
20 
84 
8 
60 
125 
DOME, 
12 
22 
250 
9 
24 
95 
November- 
18 
8.8 
120 
9C 
27 
250 
December 
22 
9.6 
43 
9D 
36 
135 
1959 
26 
12 
100 
10 
45 
166 
33 
10 
190 
11 
70 
104 
36 
9.4 
140 
16 
32 
95 
43 
20 
70 
17 
33 
139 
46 
8.7 
120 
18 
32 
114 
50-7 
11 
54 
19 
27 
96 
50-16 
14 
56 
22 
59 
233 
56 
12 
59 
23 
30 
33 
24 
33 
77 
25B 
49 
47 
* In water columns or layers to about 100 m and 300 m respectively; abbreviated as chlor. and zoop. respectively. For 
further explanation see text and footnotes. 
1 Spencer F. Baird stations; station 57 was omitted, being south of Equator and remote from others in this list; chlorophyll 
a data were extrapolated to 100 m for stations 33 and 37. 
2 Zooplankton value for station 25A was used for the adjacent station 25B. 
