28
Chapter 2: Biogeographic Partition of the Ocean
the relative lack of seasonality in trade wind stress and irradiance induces the formation
of a relatively shallow permanent tropical thermocline; here, seasonal changes occur in
mixed-layer depth principally due to geostrophic response to the regional wind field. In
an ideal equatorial current system, as in the eastern Pacific, the South Equatorial Current
lies above the equator; here, because Ekman transport takes opposite sign in the two
hemispheres, transport is poleward on each side of the equator. Evidently, within the
upper 200 m or so, this must create an equatorial divergence zone readily observable both
at sea and in satellite images. This may have an extraordinarily sharp boundary between
cool upwelled water and warmer surface water: it has been described as “a line in the
sea” (Yoder et al., 1993).
We shall find that the evidence that these oceanographic frontal zones act as biogeographic boundaries is equivocal. For some authors, for some organisms and especially
those confined to the epipelagic zone shoaler than the seasonal or tropical thermoclines,
they clearly separate different faunas. For other authors, for other organisms and especially
those with representatives in mid-depths, there is no such evidence.
A final problem presents itself in the deep expatriation of species at convergent fronts.
The surface layers (in which we are interested) lie above deeper water masses that may
have been subducted from the surface at convergent fronts even thousands of kilometers
distant, together with some of the planktonic organisms that inhabit that water mass. In
this way, the surface organisms of a subtropical water mass may lie only a few hundred
meters above organisms that were expatriated from a subarctic environment. Oblique
plankton net samples may deliver organisms from two entirely separate environments.
Species characteristic of both may appear with great regularity in the same sample, and one
such example puzzled me a long time ago now. About 500 m below the tropical copepods
in the EASTROPAC zooplankton profiles, I often found quite large numbers of Eucalanus
bungii, a copepod of the Subarctic North Pacific that had apparently been subducted at
the convergent Polar Front; the specimens were translucent, sluggish, nonreproductive,
and existed—one might think—only to confuse the biogeographer.
Geographic Component of Benthic–Pelagic Coupling
To what extent can the logic of an open ocean partition be relevant also to the biogeography of continental shelf organisms? This problem is not so divorced from the ecology
of the pelagic realm as it might appear to be, because the nutrient regime experienced by
phytoplankton over the continental shelves is modified both by the biotic regeneration
of dissolved nutrients from sedimented pelagic organic material and by the consumption of planktonic biota by filter-feeding benthic invertebrates. It may be useful at this
point, then, very briefly to review the biogeography of shelf benthos communities. We
shall find that some of the early generalizations concerning the composition and differential distribution of communities of benthic invertebrates are highly relevant to the
present study.
Ekman (1953) partitioned the biogeography of the continental shelf benthos simply by
water temperature, thus: warm-water, temperate, boreal, and polar faunas. He suggested
that the principal geographic distinction was between the warm- and cool-water faunas,
and he placed the discontinuity between these near the latitudes of the oceanic subtropical
divergences. The separation, for instance, of his North Atlantic boreal from temperate
benthic faunas lies near Cape Cod in the west, and off the western English Channel in the
east. This arrangement reflects the penetration of Labrador Current water south to Nova
Scotia, and the bifurcation of North Atlantic Drift flow off Western Europe. Ekman’s
next level of partition derives from coastal topography rather than from any features of
ocean circulation.
Chapter 2: Biogeographic Partition of the Ocean
the relative lack of seasonality in trade wind stress and irradiance induces the formation
of a relatively shallow permanent tropical thermocline; here, seasonal changes occur in
mixed-layer depth principally due to geostrophic response to the regional wind field. In
an ideal equatorial current system, as in the eastern Pacific, the South Equatorial Current
lies above the equator; here, because Ekman transport takes opposite sign in the two
hemispheres, transport is poleward on each side of the equator. Evidently, within the
upper 200 m or so, this must create an equatorial divergence zone readily observable both
at sea and in satellite images. This may have an extraordinarily sharp boundary between
cool upwelled water and warmer surface water: it has been described as “a line in the
sea” (Yoder et al., 1993).
We shall find that the evidence that these oceanographic frontal zones act as biogeographic boundaries is equivocal. For some authors, for some organisms and especially
those confined to the epipelagic zone shoaler than the seasonal or tropical thermoclines,
they clearly separate different faunas. For other authors, for other organisms and especially
those with representatives in mid-depths, there is no such evidence.
A final problem presents itself in the deep expatriation of species at convergent fronts.
The surface layers (in which we are interested) lie above deeper water masses that may
have been subducted from the surface at convergent fronts even thousands of kilometers
distant, together with some of the planktonic organisms that inhabit that water mass. In
this way, the surface organisms of a subtropical water mass may lie only a few hundred
meters above organisms that were expatriated from a subarctic environment. Oblique
plankton net samples may deliver organisms from two entirely separate environments.
Species characteristic of both may appear with great regularity in the same sample, and one
such example puzzled me a long time ago now. About 500 m below the tropical copepods
in the EASTROPAC zooplankton profiles, I often found quite large numbers of Eucalanus
bungii, a copepod of the Subarctic North Pacific that had apparently been subducted at
the convergent Polar Front; the specimens were translucent, sluggish, nonreproductive,
and existed—one might think—only to confuse the biogeographer.
Geographic Component of Benthic–Pelagic Coupling
To what extent can the logic of an open ocean partition be relevant also to the biogeography of continental shelf organisms? This problem is not so divorced from the ecology
of the pelagic realm as it might appear to be, because the nutrient regime experienced by
phytoplankton over the continental shelves is modified both by the biotic regeneration
of dissolved nutrients from sedimented pelagic organic material and by the consumption of planktonic biota by filter-feeding benthic invertebrates. It may be useful at this
point, then, very briefly to review the biogeography of shelf benthos communities. We
shall find that some of the early generalizations concerning the composition and differential distribution of communities of benthic invertebrates are highly relevant to the
present study.
Ekman (1953) partitioned the biogeography of the continental shelf benthos simply by
water temperature, thus: warm-water, temperate, boreal, and polar faunas. He suggested
that the principal geographic distinction was between the warm- and cool-water faunas,
and he placed the discontinuity between these near the latitudes of the oceanic subtropical
divergences. The separation, for instance, of his North Atlantic boreal from temperate
benthic faunas lies near Cape Cod in the west, and off the western English Channel in the
east. This arrangement reflects the penetration of Labrador Current water south to Nova
Scotia, and the bifurcation of North Atlantic Drift flow off Western Europe. Ekman’s
next level of partition derives from coastal topography rather than from any features of
ocean circulation.