Standing Crops and Trophic Levels— Blackburn 
55 
with the crop of herbivores lagging behind, 
etc. ) . There are other possibilities in the part of 
the eastern tropical Pacific under consideration, 
however. 
According to Cromwell ( 1938) and Wyrtki 
(MS, with more abundant data) the region men - 
tioned above is characterised by a very shoal 
thermocline at all times of year. The mean depth 
of the mixed layer is <30 m in all months, ex- 
cept in the southwest corner of the region (near 
All and A 14, see Fig. 2) where it is >30 m 
(occasionally >40 m <50 m) from about August 
through January, and in the northwest corner 
(west of A 13) where it is >30 m in January 
and February. It is <20 m in large areas of the 
region from about March through June, al- 
though only in small areas near the Costa Rica 
Dome from about October through December. 
It is sometimes <10 m, even in areas outside the 
Costa Rica Dome, and was so observed in the 
neighborhood of A 16 at the time this station- 
pair was occupied in May, 1958 (Blackburn and 
associates, 1962, Fig. 9); a similar situation was 
found in about the same area in March, 1941 
(Wooster, 1959, Fig. 15). 
Nutrient concentrations are high in and just 
below the thermocline in this area (Wooster 
and Cromwell, 1958,). As a result, nutrients are 
always available in the lower part of the eupho- 
ric layer, and can be supplied to the upper part 
of it by vertical mixing caused by wind. Such 
mixing probably would not carry phytoplank- 
ton below the compensation depth. Blackburn 
(1962) showed that gales could stir the upper 
part of such shoal thermoclines so as to enrich 
the mixed layer; the mixed layer was deepened 
from about 20 m to 30 m, but the thermocline 
was not destroyed. 
The area under consideration is not very 
windy. No part of it has more than 20% of sur- 
face wind above force 4 Beaufort (11-16 knots) 
in any month, or more than 10% above force 4 
in any month from March through July (Mete- 
orological Office, London, 1956). It will be ob- 
served that winds are weakest and the mixed 
layer thinnest from March through July, and a 
little stronger and thicker during the rest of the 
year, as might be expected. It seems reasonable 
to suppose that these waters are stirred to greater 
than the average depth of the mixed layer when 
winds of above-average velocity occur, and that 
this process, probably intermittent, is sufficiently 
regular to maintain moderate nutrient concen- 
trations in the mixed layer during most of the 
year. 
A steady process of upwelling might have a 
similar effect, and the following observations by 
Wyrtki (unpublished) indicate the possibility 
of such a process occurring in the region of 
interest. There Is an excess of precipitation over 
evaporation, and so an influx of high-salinity 
water must occur to maintain the surface salinity 
at the observed levels. Such water is present be- 
low the thermocline, and some of it, therefore, 
probably ascends. 
In any event, surface concentrations of 0.50 
<P0 4 -P <1.00 fig- atm/1. occur in a large part 
of the area of interest, and concentrations >0.25 
<0.50 occur in the remainder of it, and these are 
moderately high concentrations for tropical Pa- 
cific surface waters (Reid, 1962). It appears, 
therefore, at least qualitatively, that a supply of 
nutrients can be made available to the euphotic 
zone in sufficient amount and with sufficient 
regularity to maintain steady-state conditions 
among the fairly high standing crops of biota 
in the region studied. 
Nothing definite can be said about the pos- 
sible effects of horizontal circulation of water 
in maintaining or opposing steady-state condi- 
tions in the region. Information is available on 
mean monthly direction and velocity of surface 
currents (Cromwell and Bennett, 1959), and 
the annual range of variation in direction has 
been summarized in a useful chart (Schaefer, 
1962, Fig. 15). On the whole the current pat- 
tern is rather stable from month to month, ex- 
cept in an area between Costa Rica and Cocos 
Island (including A14 and A15 of Fig. 2), and 
it is quite possible that the effects of currents in 
transporting nutrients or biota are sufficiently 
constant over long periods to permit a steady 
state to exist. 
The foregoing observations on physical fea- 
tures indicate that steady-state conditions might 
last all year instead of merely in the northern 
spring. If this were so, standing crops would 
probably not change much from season to season 
at any one place in the region of interest, al- 
though small gradual changes might occur. Un- 
