Seasonal Studies of a Pelagic Ecosystem (Meridian 110
0
E)
517
1971) concerning this tropical-subtropical interaction. Centred at a depth of about 100 m
there is a weakly developed "west Australian gyre", bounded by the South Equatorial Current in the north, meridian 100
0
E. in the west, the West Wind Drift in the South, and the coast
of western Australia (between Cape Leeuwin and Northwest Cape) in the east. This
regional gyre separates from the southern flank of the South Equatorial Current in early
autumn and turns south and east across the study section (110
0
E.) carrying tropical water
well into the subtropical zone. The southward drift continues into late autumn and early
winter, and is gradually replaced in spring and summer by drift from the south central
Indian Ocean towards the east and north across our study section.
This pattern of oceanic drift provides a continuous corridor, over the space of one
year, for organisms to drift in a full anti-clockwise circle. Whether or not they do so
depends on whether, when, and for how long they join the roundabout. For example,
early phyllosoma might move directly out to sea in summer because they are confined to
the immediate surface stratum (CHITILEBOROUGH and THOMAS, 1969).
I believe that the West Australian gyre explains the otherwise anomalous subtropical
concentrations of micronekton (Fig. 10) near which sperm whales and southern bluefin
tuna aggregate on their northward migrations (TOWNSEND, 1935; MIMURA and
WARASHINA, 1962). I suggest that these concentrations are derived principally from
plankton production of tropical origin, perhaps as a result of nutrient enrichment at the
Java Dome (CUSHING, 1969). I would also propose that the gyre is responsible in part for
the observed instabilities and anomalies earlier in the food web; these are the result, in
part, of the seasonal impact of monsoon generated water movements upon a subtropical
system which might otherwise have maintained a steady state.
3. Components of Stability
The variety of ways by which the primary source of energy can filter through the food
web is a measure of the stability of the system (MACARTHUR, 1955). Species diversity is a
measure of the variety of energy users; recycling mechanisms provide avenues of re-use.
a) Species Diversity
Little can be said at present about species diversity as a whole because the available
data include certain groups of copepods only; later studies will deal with diversity in a
wider context. However, both the patterns of diversity that I obtained (Figs. 15 a and b)
and those of GUEREDRAT (1971) show that diversity is generally high throughout the study
area, particularly where tropical and subtropical waters mix.
No doubt this increase is due, in part, to the union of two slightly different faunas.
However, more than this would seem to be involved. It is curious that species diversity in
the copepods that I examined is a function of zooplankton biomass in the subtropics,
but not in the tropics (Table 10). If the subtropics are as poor as nutrient concentrations
would suggest (Fig. 2, and WYRTKI, 1973 Fig. 2 this volume), then the subtropical
habitat may have reached the species saturation limit, there being a limit to the similarity
of species with limited resources (MACARTHUR, 1965). It would follow that species
diversity would increase where subtropical waters are enriched by tropical intrusions.
I would think that such a response would lead to composite stability at least equal to
that of either interacting system. If so, then instabilities resulting from the interaction of 2
trophic sequences that are out of phase would gradually dissipate, rather than gain
0
E)
517
1971) concerning this tropical-subtropical interaction. Centred at a depth of about 100 m
there is a weakly developed "west Australian gyre", bounded by the South Equatorial Current in the north, meridian 100
0
E. in the west, the West Wind Drift in the South, and the coast
of western Australia (between Cape Leeuwin and Northwest Cape) in the east. This
regional gyre separates from the southern flank of the South Equatorial Current in early
autumn and turns south and east across the study section (110
0
E.) carrying tropical water
well into the subtropical zone. The southward drift continues into late autumn and early
winter, and is gradually replaced in spring and summer by drift from the south central
Indian Ocean towards the east and north across our study section.
This pattern of oceanic drift provides a continuous corridor, over the space of one
year, for organisms to drift in a full anti-clockwise circle. Whether or not they do so
depends on whether, when, and for how long they join the roundabout. For example,
early phyllosoma might move directly out to sea in summer because they are confined to
the immediate surface stratum (CHITILEBOROUGH and THOMAS, 1969).
I believe that the West Australian gyre explains the otherwise anomalous subtropical
concentrations of micronekton (Fig. 10) near which sperm whales and southern bluefin
tuna aggregate on their northward migrations (TOWNSEND, 1935; MIMURA and
WARASHINA, 1962). I suggest that these concentrations are derived principally from
plankton production of tropical origin, perhaps as a result of nutrient enrichment at the
Java Dome (CUSHING, 1969). I would also propose that the gyre is responsible in part for
the observed instabilities and anomalies earlier in the food web; these are the result, in
part, of the seasonal impact of monsoon generated water movements upon a subtropical
system which might otherwise have maintained a steady state.
3. Components of Stability
The variety of ways by which the primary source of energy can filter through the food
web is a measure of the stability of the system (MACARTHUR, 1955). Species diversity is a
measure of the variety of energy users; recycling mechanisms provide avenues of re-use.
a) Species Diversity
Little can be said at present about species diversity as a whole because the available
data include certain groups of copepods only; later studies will deal with diversity in a
wider context. However, both the patterns of diversity that I obtained (Figs. 15 a and b)
and those of GUEREDRAT (1971) show that diversity is generally high throughout the study
area, particularly where tropical and subtropical waters mix.
No doubt this increase is due, in part, to the union of two slightly different faunas.
However, more than this would seem to be involved. It is curious that species diversity in
the copepods that I examined is a function of zooplankton biomass in the subtropics,
but not in the tropics (Table 10). If the subtropics are as poor as nutrient concentrations
would suggest (Fig. 2, and WYRTKI, 1973 Fig. 2 this volume), then the subtropical
habitat may have reached the species saturation limit, there being a limit to the similarity
of species with limited resources (MACARTHUR, 1965). It would follow that species
diversity would increase where subtropical waters are enriched by tropical intrusions.
I would think that such a response would lead to composite stability at least equal to
that of either interacting system. If so, then instabilities resulting from the interaction of 2
trophic sequences that are out of phase would gradually dissipate, rather than gain