Central Pacific Salpidae — Yount 
113 
chiefly on vertical mixing of the waters 
(Sverdrup, et al . , 1942: 785; Tait, 1952: 76; 
Harvey, 1955: 99) and additions of dissolved 
nutrients from the land (Kiinne, 1950: 57; 
Dakin, 1952: 7), which bring nutrients to the 
euphotic zone where they can be used. In 
temperate waters, vertical mixing and result- 
ant enrichment are useful for production prin- 
cipally in the spring and fall; in midwinter the 
standing crop of organisms is low, due pri- 
marily to insufficient light intensity, while in 
midsummer the low standing crop is appar- 
ently the result of depletion of nutrients in 
the euphotic zone (Russell and Yonge, 1936: 
246). In tropical waters, on the other hand, 
there is generally a low concentration of nu- 
trients in the euphotic zone with an attendant 
low standing crop throughout the year, ap- 
parently because upper tropical waters (to a 
depth of about 300 meters) are characterized 
by a three-layer system — an upper warm layer, 
a transition layer, and a lower cool one- 
present during a large portion of the year. 
Thus sinking of decomposing substances from 
the surface layer to a depth below the transi- 
tion layer results in a removal of nutrients 
from the euphotic zone until vertical mixing 
of the waters again brings the nutrients back. 
The enrichment in the zone between about 
2° N. and 5° S. in the area studied undoubt- 
edly originates principally from vertical water 
movements; i.e., it probably does not origi- 
nate in the Peru Current from the east. The 
phosphate and standing crops of the region 
studied probably also reflect, in small part, 
a horizontal movement of water from the east. 
No quantitative data on the westerly limits 
of the effects of the Peru Current are available, 
but considering the great distance involved, 
one can assume that the phosphate from up- 
welling in the Peru Current is utilized long 
before reaching the meridians studied. In 
other words, the water from the Peru Current 
has "aged” by the time it reaches these meri- 
dians, i.e., has moved far from the upwelling 
which initially enriched it (Steemann-Nielsen, 
1954). However, since these waters of the 
meridians studied are in a continuous state of 
upwelling they are "young,” that is, they 
have a high concentration of nutrient chemi- 
cals, being more or less continuously replen- 
ished from below. 
Another factor should be considered in 
regard to enrichment: phosphates probably 
affect salps only indirectly. As only plants are 
considered able to utilize dissolved nutrients 
such as phosphates for biological production, 
salps are probably affected by phosphates only 
by way of their food, which consists primarily 
of phytoplankters. Salps thus stand one troph- 
ic level away from the dissolved nutrients as 
shown by their trophic relationships: dis- 
solved nutrients — » phytoplankton (produc- 
ers) — > herbivorous zooplankton (primary 
consumers). Time, therefore, is required for 
the phosphates to be used by the phyto- 
plankton in order that the primary consumers 
can be affected by the increased production. 
For this reason, it is possible that the salps 
captured at the same time would show little 
correlation, station to station, with the phos- 
phates. However, the increased phosphate 
found in the region of upwelling is a zonal 
condition and thus not transient, and results 
in generally larger standing crops of plankton 
in this region (Cromwell, 1953; King and 
Demond, 1953). 
Considering the above factors, then, one 
would expect increased availability of nu- 
trients in the regions of upwelling and in 
other regions in which frequent vertical mix- 
ing occurs, such as in zones of shear at the 
edge of currents (the regions of "Ansamm- 
lungen” of Apstein, 1906), and in waters near 
island masses where vertical mixing and dif- 
fusion may occur as a result of eddies and 
where dissolved nutrients from the land may 
be added. Therefore, the a priori expectation 
for this study was that larger standing crops 
of salps would be related to those ecologic 
factors favorable to biological production. 
One further subject— that of the sources of 
error to which the results are liable — should 
be discussed before a description of the meri- 
