Seasonal Studies of a Pelagic Ecosystem (Meridian 110° E)
515
1. Components of Instability
a) Fluctuations in Energy Input
The seasonal amplitude (coefficient of variation) in primary productivity is of the
order of 50% of the annual mean in both the tropical and subtropical regions of the
eastern Indian Ocean (Table 3), a fluctuation that is not so small compared with other
regions of the world ocean. For example, it is nearly as great as that at station "P" in the
Canadian Subarctic and it is much more than in the eastern tropical Pacific (Table 4).
Such instabilities in energy input are a latent source of instability throughout the food
web (RYTHER and MENZEL, 1965) and, no doubt, a large component of the seasonal
amplitude in zooplankton biomass (48%) has its origin here. On the other hand, the
seasonal amplitude in micronekton (dry weight) is only 20% of the annual mean, a level
that is much lower than that of the primary energy source, and much lower than that in the
eastern tropical Pacific (Table 4). An explanation for this anomaly will later be sought.
b) Time Lags
BLACKBURN et al. (1970) found the seasonal cycles of phytoplankton and zooplankton in
the eastern tropical Pacific to be low in amplitude and close in phase. Our results for the
tropical part of the eastern Indian Ocean are somewhat similar. However, the seasonal
cycle of micronekton showed such close coincidence with the plankton cycles at times that
there is doubt whether they belong to the same trophic sequence. This represents a second
anomaly.
The best known study of seasonal changes in a subtropical pelagic ecosystem is that
of MENZEL and RYTHER (1960) in the Sargasso Sea. Their system was typically in a
steady state, plankton values being fairly low, except in the spring, and the cycles close in
phase. Our results for the subtropical eastern Indian Ocean show similar low amplitudes
but, in contrast with the Sargasso Sea, cycles of phytoplankton and zooplankton are
distinctly out of phase for a large part of the year. This represents a third anomaly which
requires an explanation.
In other respects, these eastern Indian Ocean pelagic ecosystems, like those in the
eastern tropical Pacific and subtropical Sargasso Sea, follow the general pattern of
seasonal variation so well described by CUSHING (1958).
2. Horizontal Translation
Each of the anomalies noted above may be explained in terms of horizontal movement
of waters and the consequent translation of those populations that are unable, by their
own powers of locomotion, to move independently. The changes observed on our section
line take on more meaning if they are regarded as a combination of ecological succession
and translation. For example, horizontal translation introduces a time lag to the subtropical ecosystem where, previously, there may have been a steady state as in the
Sargasso Sea. Towards the origin of the food web, translation introduces instability; but
towards the periphery of the food web, translation introduces an element of stability, as
evidenced by the relatively low seasonal amplitude of micronekton and the phase
coincidence of the micronekton and plankton cycles.
It is of considerable importance to examine in closer detail the nature of this horizontal
translation, first from a biological, then from a physical point of view.
515
1. Components of Instability
a) Fluctuations in Energy Input
The seasonal amplitude (coefficient of variation) in primary productivity is of the
order of 50% of the annual mean in both the tropical and subtropical regions of the
eastern Indian Ocean (Table 3), a fluctuation that is not so small compared with other
regions of the world ocean. For example, it is nearly as great as that at station "P" in the
Canadian Subarctic and it is much more than in the eastern tropical Pacific (Table 4).
Such instabilities in energy input are a latent source of instability throughout the food
web (RYTHER and MENZEL, 1965) and, no doubt, a large component of the seasonal
amplitude in zooplankton biomass (48%) has its origin here. On the other hand, the
seasonal amplitude in micronekton (dry weight) is only 20% of the annual mean, a level
that is much lower than that of the primary energy source, and much lower than that in the
eastern tropical Pacific (Table 4). An explanation for this anomaly will later be sought.
b) Time Lags
BLACKBURN et al. (1970) found the seasonal cycles of phytoplankton and zooplankton in
the eastern tropical Pacific to be low in amplitude and close in phase. Our results for the
tropical part of the eastern Indian Ocean are somewhat similar. However, the seasonal
cycle of micronekton showed such close coincidence with the plankton cycles at times that
there is doubt whether they belong to the same trophic sequence. This represents a second
anomaly.
The best known study of seasonal changes in a subtropical pelagic ecosystem is that
of MENZEL and RYTHER (1960) in the Sargasso Sea. Their system was typically in a
steady state, plankton values being fairly low, except in the spring, and the cycles close in
phase. Our results for the subtropical eastern Indian Ocean show similar low amplitudes
but, in contrast with the Sargasso Sea, cycles of phytoplankton and zooplankton are
distinctly out of phase for a large part of the year. This represents a third anomaly which
requires an explanation.
In other respects, these eastern Indian Ocean pelagic ecosystems, like those in the
eastern tropical Pacific and subtropical Sargasso Sea, follow the general pattern of
seasonal variation so well described by CUSHING (1958).
2. Horizontal Translation
Each of the anomalies noted above may be explained in terms of horizontal movement
of waters and the consequent translation of those populations that are unable, by their
own powers of locomotion, to move independently. The changes observed on our section
line take on more meaning if they are regarded as a combination of ecological succession
and translation. For example, horizontal translation introduces a time lag to the subtropical ecosystem where, previously, there may have been a steady state as in the
Sargasso Sea. Towards the origin of the food web, translation introduces instability; but
towards the periphery of the food web, translation introduces an element of stability, as
evidenced by the relatively low seasonal amplitude of micronekton and the phase
coincidence of the micronekton and plankton cycles.
It is of considerable importance to examine in closer detail the nature of this horizontal
translation, first from a biological, then from a physical point of view.