The ascending velocity in upwelling, Wh, can be estimated in a number of ways, but because it is low, 0.1-3.0 m/d (McEwen, 1929; Yoshida, 1955; Hidaka, 1954; Posner, 1957; Wyrtki, 1964) no direct measurements are available. Wooster and Reid (1963) show that the Ekman transport off northern Peru in winter is the greatest of all the upwelling areas. It might follow, therefore, that the production in northern Peru in winter was the greatest. However, the quantities of carbon and zooplankton are no greater there than in other upwelling areas (see Table I); indeed, the data show the quantity of production to be less in northern Peru than in other major upwelling areas.
The simplest view of the upwelling production cycle is to consider it as starting at the bottom of the photic layer and continuing as the water rises. As production starts from low quantities, the cycle in the upwelling area resembles the temperate cycle rather closely. Heinrich (1961) suggests a generation time for the zooplankton in the upwelling areas of about 40 days, so effective grazing should start about 20 days after the start of upwelling. So the peak of the production cycle should occur at or near the surface, at or near the line of upwelling. A more complex case occurs if the plant production becomes vulnerable by mixture to zooplankton arising from earlier production. Jones and Folkard (1968), in their sections of the Canary Current, show distributions of physical properties which might bring the vertically migrating zooplankton inshore towards the line of upwelling.
Let us consider production in the rising water, without grazing. From Steele and Menzel (1962),
where Io is the average radiation at the surface in g cal/cm²/d (Steele and Menzel's figure of 180 g cal/cm²/d for the Sargasso Sea has been used);
Z is depth in m;
k is the extinction coefficient (the value k = 0.1 has been used, ignoring its possible increase with increasing production);
a is a constant (=0.48);
p is the daily production in gc/m³/d.
Let Zp be the depth of the photic zone, hence
where Wh is the ascending velocity in m/d.
Between time t and t + d t,
then the total production, in gC/m²/d, as water upwells from the bottom of the photic layer (Z = Zp, t = 0) to the surface (Z = 0, t = Zp/Wh) is
I am grateful to my colleague, Mr. J.G.K. Harris, for the development of equations (2) to (5).
Figure 8 shows the relation between total production during ascent (for two depths of photic layer) and ascending velocity. Ignoring grazing the slower the ascending velocity the greater the production during the ascent, irrespective of the depth of the photic layer. Let grazing become effective after 20 days. Then with a photic layer 20 m deep, grazing starts at the surface with an upwelling of 1 m/d, and if it is 100 m deep it starts at the surface with an upwelling of 5 m/d. So the upwelling production cycle, well grazed, can reach a peak at the line of upwelling at the surface. It is obvious from Figure 8 that an upwelling velocity of <0.5 m/d will give the production generating its own grazing capacity at the surface. A greater upwelling velocity, irrespective of the depth of the photic layer, means that the production has not gone on for very long (less than half a generation time), when it reaches the surface and may become vulnerable to the greater grazing capacity of other animal populations. Perhaps the faster rate of upwelling off Peru modifies the production cycle in this way, leading to rather lower standing crops than expected.
Figure 8. Relation between total production, p, during ascent and ascending velocity, Wh, (in m) for a range of depths of the photic layer.
Figure 9 shows a vertical section of carbon production (Japan, Science Council. National Committee for IIOE, 1966) from Java to Western Australia during the period of upwelling off Java. It is a section of a dynamic process, but we may imagine the upwelling entraining northwards and slowly increasing its production as it rises. It is not confirmation of the process imagined above, but an illustration of how such a system might work. A series of sections (like that in Figure 9), closely spaced in time, and on a smaller horizontal scale could be used to analyze the production cycle in an upwelling area and even estimate the rate of upwelling.
Figure 9. A vertical section of carbon production in mg C/m³/d, off Java during the period of upwelling (Japan, Science Council. National Committee for IIOE, 1966)
