Seasonal Studies of a Pelagic Ecosystem (Meridian 110° E)
519
their rations in the form of living algal biomass are inadequate: from VINOGRADOV
(1968) we learn that the total zooplankton content of the water column is, on the average,
2.5 times as much as in the upper 200 m in tropical eutrophic areas of the Indian Ocean,
and it is 4.3 times as much in subtropical oligotrophic areas. Applying these ratios to our
data, we find that the zooplankton content of the total water column is 137% of the
phytoplankton in the tropics, and 185% of the phytoplankton in the subtropics. If
the zooplankton require daily rations equivalent to 1/ 20 their body carbon (MENZEL and
RYTHER, 1961; MULLIN and BROOKS, 1970), then they would use each day in the tropics
about 46% of the primary algal production and, in the subtropics, about 71 %. Since, at
anyone time, barely 30% or so of algal production is present as living algae, the dead
component, viz. detritus, must serve to supplement the rations of the zoo plankton.
It is clear, that zooplankton recycle part of the daily algal production before it is
remineralized by decomposition. These avenues of re-use accelerate the flow of energy
through the ecosystem and add to its stability. There may be many other mechanisms of
recycling, particularly in warm waters.
4. Total Dynamics of the Ecosystem
The results indicate that elements of both translation and ecological succession are
responsible for the changes observed in the study area, the main interacting forces being:
a) Enrichment of tropical surface waters in winter by dynamic uplift associated with
the South Equatorial Current.
b) Impoverishment of subtropical surface waters in late summer by thermal stratification .
c) Meridional transport of tropical waters towards the south in autumn and winter,
and subtropical waters towards the north in spring and summer.
d) Heavy algal grazing and rapid nutrient recycling.
These changes constitute an annual cycle which may conveniently be considered to
begin in late summer (March ) when chlorophyll is at a minimum along the entire section
(Fig. 7; Table 1). The stratification and impoverishment of subtropical waters which drift
towards the north (ROCHfORD, 1969) combine with heavy grazing by both zooplankton
(Figs. 8, 9; Table 7) and macroplankton (Table 6) to establish a seasonal catastrophe
throughout the area. The zooplankton starve and are preyed upon by micronekton ; by
June, as predation continues (Fig. 10), they are much reduced in abundance (Figs. 8, 9).
In winter (June - August) the subtropical thermocline breaks down and regenerated
nutrients lying between the seasonal thermocline and the "permanent thermal
discontinuity" (ROCHFORD, 1969) become available for plant production. These are
used immediately, leaving little evidence of enrichment (ROCHFORD, 1969). Meanwhile
in the tropics the South Equatorial Stream gains strength, and the dynamic uplift associated with the current (WYRTKI, 1962; ROCHFORD, 1969) enriches the euphotic zone with
nutrients which here also, are immediately utilized in algal production (Figs. 6, 7). There
is a rapid response by tropical herbivores to this increased food supply, and a slower
response by the subtropical population (Figs. 8, 9) which has been exposed to greater
starvation (Fig. 7) and predation (Fig. 10). By September these populations have reached
a maximum throughout the section and are beginning to overgraze (Fig. 7). In the tropics,
their excretion leads to rapid remineralization of nutrients, the remineralization of
phosphate being more rapid than that of nitrate. In the subtropics, there is greater
assimilation, and as a consequence remineralization is less immediate.
519
their rations in the form of living algal biomass are inadequate: from VINOGRADOV
(1968) we learn that the total zooplankton content of the water column is, on the average,
2.5 times as much as in the upper 200 m in tropical eutrophic areas of the Indian Ocean,
and it is 4.3 times as much in subtropical oligotrophic areas. Applying these ratios to our
data, we find that the zooplankton content of the total water column is 137% of the
phytoplankton in the tropics, and 185% of the phytoplankton in the subtropics. If
the zooplankton require daily rations equivalent to 1/ 20 their body carbon (MENZEL and
RYTHER, 1961; MULLIN and BROOKS, 1970), then they would use each day in the tropics
about 46% of the primary algal production and, in the subtropics, about 71 %. Since, at
anyone time, barely 30% or so of algal production is present as living algae, the dead
component, viz. detritus, must serve to supplement the rations of the zoo plankton.
It is clear, that zooplankton recycle part of the daily algal production before it is
remineralized by decomposition. These avenues of re-use accelerate the flow of energy
through the ecosystem and add to its stability. There may be many other mechanisms of
recycling, particularly in warm waters.
4. Total Dynamics of the Ecosystem
The results indicate that elements of both translation and ecological succession are
responsible for the changes observed in the study area, the main interacting forces being:
a) Enrichment of tropical surface waters in winter by dynamic uplift associated with
the South Equatorial Current.
b) Impoverishment of subtropical surface waters in late summer by thermal stratification .
c) Meridional transport of tropical waters towards the south in autumn and winter,
and subtropical waters towards the north in spring and summer.
d) Heavy algal grazing and rapid nutrient recycling.
These changes constitute an annual cycle which may conveniently be considered to
begin in late summer (March ) when chlorophyll is at a minimum along the entire section
(Fig. 7; Table 1). The stratification and impoverishment of subtropical waters which drift
towards the north (ROCHfORD, 1969) combine with heavy grazing by both zooplankton
(Figs. 8, 9; Table 7) and macroplankton (Table 6) to establish a seasonal catastrophe
throughout the area. The zooplankton starve and are preyed upon by micronekton ; by
June, as predation continues (Fig. 10), they are much reduced in abundance (Figs. 8, 9).
In winter (June - August) the subtropical thermocline breaks down and regenerated
nutrients lying between the seasonal thermocline and the "permanent thermal
discontinuity" (ROCHFORD, 1969) become available for plant production. These are
used immediately, leaving little evidence of enrichment (ROCHFORD, 1969). Meanwhile
in the tropics the South Equatorial Stream gains strength, and the dynamic uplift associated with the current (WYRTKI, 1962; ROCHFORD, 1969) enriches the euphotic zone with
nutrients which here also, are immediately utilized in algal production (Figs. 6, 7). There
is a rapid response by tropical herbivores to this increased food supply, and a slower
response by the subtropical population (Figs. 8, 9) which has been exposed to greater
starvation (Fig. 7) and predation (Fig. 10). By September these populations have reached
a maximum throughout the section and are beginning to overgraze (Fig. 7). In the tropics,
their excretion leads to rapid remineralization of nutrients, the remineralization of
phosphate being more rapid than that of nitrate. In the subtropics, there is greater
assimilation, and as a consequence remineralization is less immediate.