The Useful Results from 150 Years of Marine Biogeography
27
study, appropriately, was entitled “Ocean circulation and marine life,” words that might
have been used as a subtitle for this book.
The critical conclusion to be drawn from the now-classical studies of the Scripps
biogeographers is that the distribution envelopes of species of epizooplankton represent
neither a random pattern at the ocean surface nor a simple gradient from poles to equator, but fall into a small number of discrete classes that are related to distance from coasts
and to latitude. Since we must assume that the distribution of each species represents its
response to the living conditions, then we have direct confirmation from biogeography
that the oceanic habitat may reasonably be partitioned into ecologically consistent
regions rather then being treated as a continuum. That some species exist in several
adjacent regions does not nullify the argument: Brinton lists 6 species of epipelagic Pacific
euphausiids that occupy both his equatorial and central regions, compared with 11 and
7 species, respectively, that are restricted to these regions. Mesopelagic and bathypelagic
species are progressively more ubiquitous as, indeed, we should expect them to be.
The integration that best, to my mind, summarizes the accumulated information got
by almost 150 years of investigations of the pelagic fauna is the brief sketch of the problem
by Backus (1986). This is especially useful because he formally (for I think the first time)
relates the boundaries of biogeographic zones to major frontal systems in the global ocean
circulation. The thrust of his argument was that there was, by 1985, sufficient agreement
in the work of many authors that a synthesis was within reach, and he was clearly correct
in this. Indeed, his proposals for zonal patterns of organisms as divergent as oceanic
phytoplankton and epipelagic fishes converge very closely and are inapplicable only to
organisms whose distribution, like that of some siphonophores, cannot be understood
by reference only to the upper few hundred meters of the water column.
Backus argues for the retention of the classical nine-zone system: Arctic and Antarctic
zones, Sub-Arctic and Sub-Antarctic zones, North and South Temperate (or Transition)
zones, North and South Subtropical zones, and a Tropical zone. He suggests that the
Temperate zones are bounded equatorward by the Subtropical Convergence of each
hemisphere and that, at least in the south, the oceanic Polar front is the poleward limit of
the Sub-Antarctic zone. He allows that further subdivision is both inevitable and desirable,
to reflect the reality of species distributions. As examples, he suggests that the Gulf of
Mexico, the Mauritanian upwelling, the eastern and western Mediterranean basins, and
other regions of this dimension can profitably be treated as entities in any biogeographical
partition of the oceans. Though he did not expressly make the point, I note that the
examples he gives are peripheral to the major ocean basins, or else comprise semienclosed
marginal seas. This, as we shall discuss later, closely resembles the partition proposed here.
If we accept that this arrangement is a reasonable model for partitioning the biogeography of the open ocean, our attention is then directed toward discontinuity in the
ocean circulation, and so toward the convergent and divergent frontal systems associated
with discontinuity in the global wind field. Between the Subtropical and Subpolar gyres
at 40–60
latitude lie the convergent Polar fronts, convergent because their western root
is at the meeting place of the opposing western boundary currents of each gyre. Vertical Ekman transport of opposite sign in the two gyres results in the subduction and
equatorward transport of Intermediate Water masses into the interior of the ocean. At
yet higher latitudes, forced by the change in strength of the resultant Ekman transport
with increasing latitude, a Polar divergence occurs, bounding the characteristic density
stratification of polar oceans.
At the Subtropical Convergence (STC) fronts, usually near 30
latitude, Central water
masses are formed by subduction, passing down and equatorward within the deep permanent thermocline. Poleward of each STC, the seasonal thermocline is destroyed each
autumn by the increase of westerly wind stress so that the mixed layer (as defined by a
density criterion, sigma-t = 0125) is significantly deepened. Equatorward of each STC
27
study, appropriately, was entitled “Ocean circulation and marine life,” words that might
have been used as a subtitle for this book.
The critical conclusion to be drawn from the now-classical studies of the Scripps
biogeographers is that the distribution envelopes of species of epizooplankton represent
neither a random pattern at the ocean surface nor a simple gradient from poles to equator, but fall into a small number of discrete classes that are related to distance from coasts
and to latitude. Since we must assume that the distribution of each species represents its
response to the living conditions, then we have direct confirmation from biogeography
that the oceanic habitat may reasonably be partitioned into ecologically consistent
regions rather then being treated as a continuum. That some species exist in several
adjacent regions does not nullify the argument: Brinton lists 6 species of epipelagic Pacific
euphausiids that occupy both his equatorial and central regions, compared with 11 and
7 species, respectively, that are restricted to these regions. Mesopelagic and bathypelagic
species are progressively more ubiquitous as, indeed, we should expect them to be.
The integration that best, to my mind, summarizes the accumulated information got
by almost 150 years of investigations of the pelagic fauna is the brief sketch of the problem
by Backus (1986). This is especially useful because he formally (for I think the first time)
relates the boundaries of biogeographic zones to major frontal systems in the global ocean
circulation. The thrust of his argument was that there was, by 1985, sufficient agreement
in the work of many authors that a synthesis was within reach, and he was clearly correct
in this. Indeed, his proposals for zonal patterns of organisms as divergent as oceanic
phytoplankton and epipelagic fishes converge very closely and are inapplicable only to
organisms whose distribution, like that of some siphonophores, cannot be understood
by reference only to the upper few hundred meters of the water column.
Backus argues for the retention of the classical nine-zone system: Arctic and Antarctic
zones, Sub-Arctic and Sub-Antarctic zones, North and South Temperate (or Transition)
zones, North and South Subtropical zones, and a Tropical zone. He suggests that the
Temperate zones are bounded equatorward by the Subtropical Convergence of each
hemisphere and that, at least in the south, the oceanic Polar front is the poleward limit of
the Sub-Antarctic zone. He allows that further subdivision is both inevitable and desirable,
to reflect the reality of species distributions. As examples, he suggests that the Gulf of
Mexico, the Mauritanian upwelling, the eastern and western Mediterranean basins, and
other regions of this dimension can profitably be treated as entities in any biogeographical
partition of the oceans. Though he did not expressly make the point, I note that the
examples he gives are peripheral to the major ocean basins, or else comprise semienclosed
marginal seas. This, as we shall discuss later, closely resembles the partition proposed here.
If we accept that this arrangement is a reasonable model for partitioning the biogeography of the open ocean, our attention is then directed toward discontinuity in the
ocean circulation, and so toward the convergent and divergent frontal systems associated
with discontinuity in the global wind field. Between the Subtropical and Subpolar gyres
at 40–60
latitude lie the convergent Polar fronts, convergent because their western root
is at the meeting place of the opposing western boundary currents of each gyre. Vertical Ekman transport of opposite sign in the two gyres results in the subduction and
equatorward transport of Intermediate Water masses into the interior of the ocean. At
yet higher latitudes, forced by the change in strength of the resultant Ekman transport
with increasing latitude, a Polar divergence occurs, bounding the characteristic density
stratification of polar oceans.
At the Subtropical Convergence (STC) fronts, usually near 30
latitude, Central water
masses are formed by subduction, passing down and equatorward within the deep permanent thermocline. Poleward of each STC, the seasonal thermocline is destroyed each
autumn by the increase of westerly wind stress so that the mixed layer (as defined by a
density criterion, sigma-t = 0125) is significantly deepened. Equatorward of each STC