128
Chapter 8: Longer Term Responses: From Seasons to Centuries
for in order to explain any massive natural increase in sardine abundance. In fact, the
explanation is quite unexpected and is surely explicable in terms of the “larval retention
area/adult stock size” hypothesis of Sinclair (1988), as argued convincingly by Binet
(1997). The argument runs that the Atlantic Ocean experienced anomalously strong flow
of eastward zonal currents during this period. Downstream of both Cape Palmas (Ivory
Coast) and Cape Three Points (Ghana), these unusual flows supported larger than usual
turbulent gyres. These gyres serve as the retention areas for the larvae of Sardinella aurita,
the sardine of this upwelling region, so that their enlargement is likely sufficient reason
for the observed increase in stock size.
These case studies in different oceans are directed at two sardines having quite different
ecologies. S. longiceps is one of the few species of clupeids to graze directly on blooms
of large diatoms, often Coscinodiscus spp., whereas S. aurita mainly eats copepods and
depends principally on a population of Calanoides carinatus for its nutrition. Both fish
provide evidence, in their time-dependent variability, of the extent to which the whole
pelagic ecosystem undergoes long-term variability in the tropical seas.
Conclusion: Stable Partitions
in a Varying Ocean?
You may have wondered at my emphasis in this chapter on the impermanence of
marine ecosystems (or at least of ecological conditions) after earlier having proposed
that it is feasible, and would be useful, to partition the ocean into regions, each having
characteristic conditions. You may have concluded that the constant change in planetary
weather systems likely induce so much (and such constant) change in circulation patterns,
and hence in the response of ecosystems, that any attempt to partition the ocean must be
fallacious. I suggest, on the contrary, that some degree of formalism and partition may,
in fact, assist us in comprehending changes in such a vastly complex, interacting whole.
It may, at least, help us to keep our thoughts in order.
We shall have to be alert to the fact that at least some of the key features of ocean
circulation that we use to define our partition may themselves change location with
changes in sign of the SOI, the NAO, or another index. We shall have to be alert also to
the fact that within many of our compartments the ecosystem may exist in more than a
single state. Finally, we shall have to accept that in some cases, it is possible that partitions
between adjacent provinces can under some conditions no longer be identified.
An instructive case is the observation that the circulation of the NE Pacific exists in
two states, Type A when winter atmospheric sea-level pressure is anomalously high over
the Gulf of Alaska, and Type B when it is relatively low (Francis et al., 1998). Under Type
A, the flux of the West Wind Drift, lying across the ocean at 45–50
N, splits strongly
to the south and weakly to the north on encountering the west coast of North America.
During Type B conditions, the reverse obtains, so that flow down the California Current
is weak, and flow north around the Alaska coastal current is strong. But note that the
general oceanographic features characteristic of each region remain approximately static.
Conditions within regions change, but boundaries between them do not.
It is perhaps only in the Trade Wind biome and perhaps the equatorward parts of the
Westerlies biome that the boundaries of provinces risk being modified significantly during,
for example, an El Niño event. It is, of course, the provinces of the Indo-Pacific Trade
Wind biome that have the potential to show the greatest modification, and especially the
western Pacific Warm Pool Province (WARM), whose eastern boundary might become
difficult to define. In extremely strong events, the conditions across the whole Pacific
basin, from the Indo-Pacific archipelago to the coastal boundary of the Americas, might
Chapter 8: Longer Term Responses: From Seasons to Centuries
for in order to explain any massive natural increase in sardine abundance. In fact, the
explanation is quite unexpected and is surely explicable in terms of the “larval retention
area/adult stock size” hypothesis of Sinclair (1988), as argued convincingly by Binet
(1997). The argument runs that the Atlantic Ocean experienced anomalously strong flow
of eastward zonal currents during this period. Downstream of both Cape Palmas (Ivory
Coast) and Cape Three Points (Ghana), these unusual flows supported larger than usual
turbulent gyres. These gyres serve as the retention areas for the larvae of Sardinella aurita,
the sardine of this upwelling region, so that their enlargement is likely sufficient reason
for the observed increase in stock size.
These case studies in different oceans are directed at two sardines having quite different
ecologies. S. longiceps is one of the few species of clupeids to graze directly on blooms
of large diatoms, often Coscinodiscus spp., whereas S. aurita mainly eats copepods and
depends principally on a population of Calanoides carinatus for its nutrition. Both fish
provide evidence, in their time-dependent variability, of the extent to which the whole
pelagic ecosystem undergoes long-term variability in the tropical seas.
Conclusion: Stable Partitions
in a Varying Ocean?
You may have wondered at my emphasis in this chapter on the impermanence of
marine ecosystems (or at least of ecological conditions) after earlier having proposed
that it is feasible, and would be useful, to partition the ocean into regions, each having
characteristic conditions. You may have concluded that the constant change in planetary
weather systems likely induce so much (and such constant) change in circulation patterns,
and hence in the response of ecosystems, that any attempt to partition the ocean must be
fallacious. I suggest, on the contrary, that some degree of formalism and partition may,
in fact, assist us in comprehending changes in such a vastly complex, interacting whole.
It may, at least, help us to keep our thoughts in order.
We shall have to be alert to the fact that at least some of the key features of ocean
circulation that we use to define our partition may themselves change location with
changes in sign of the SOI, the NAO, or another index. We shall have to be alert also to
the fact that within many of our compartments the ecosystem may exist in more than a
single state. Finally, we shall have to accept that in some cases, it is possible that partitions
between adjacent provinces can under some conditions no longer be identified.
An instructive case is the observation that the circulation of the NE Pacific exists in
two states, Type A when winter atmospheric sea-level pressure is anomalously high over
the Gulf of Alaska, and Type B when it is relatively low (Francis et al., 1998). Under Type
A, the flux of the West Wind Drift, lying across the ocean at 45–50
N, splits strongly
to the south and weakly to the north on encountering the west coast of North America.
During Type B conditions, the reverse obtains, so that flow down the California Current
is weak, and flow north around the Alaska coastal current is strong. But note that the
general oceanographic features characteristic of each region remain approximately static.
Conditions within regions change, but boundaries between them do not.
It is perhaps only in the Trade Wind biome and perhaps the equatorward parts of the
Westerlies biome that the boundaries of provinces risk being modified significantly during,
for example, an El Niño event. It is, of course, the provinces of the Indo-Pacific Trade
Wind biome that have the potential to show the greatest modification, and especially the
western Pacific Warm Pool Province (WARM), whose eastern boundary might become
difficult to define. In extremely strong events, the conditions across the whole Pacific
basin, from the Indo-Pacific archipelago to the coastal boundary of the Americas, might