388
Chapter 11: The Pacific Ocean
I also assume that maximum primary production rate will occur much shallower; that
nitrogen-fixing organisms will be prominent; that the microbial loop, or regeneration
of ammonium as a substrate for algal growth, will be very active and mediated by
bacterioplankton, pico-fraction cyanobacteria, prochlorophytes, and microflagellates; and
that most consumption will be by a complex protist community. Such mesozooplankton
as occur will be small, except for diel migrant metridiids, which will rise at night, some
to the DCM and some to graze on protists in the mixed layer.
For the EASTROPAC area, which extended only to 20
S at 90–125
W, some of these
suggestions are generally confirmed. A DCM slopes downward in the upper thermocline,
weakening to the south so that by 20
S it lies at about 120 m in February and March when
the depth of maximum primary production rate is at about 20 m (Longhurst, 1976). The
same pattern is repeated in August data, though the DCM is a little deeper. Whereas
mesozooplankton profiles in PNEC and PEQD to the north are strongly structured with
near-surface and near-DCM layers of maximum abundance, in SPSG their layering is
relatively weak with a very broad zone of relative abundance progressively weakening
downward to the midpycnocline.
Although coral reefs are not part of our discussion, a passing reference to their place in
this ocean microcosm is in order because the myriad islands inhabiting the province owe
their existence to the activities of reef-forming biota. If the growth of many of these reefforming corals and mollusks is supported by photosynthesis of symbiotic algae within their
tissues, a source of nitrogen still has to be found: this comes from three sources, the balance
among which is complex to compute. In part, it is provided by the activity of nitrogen-fixing
blue-green algae, in part by capture of the sparse oceanic plankton drifting over the reef,
and in part—perhaps in some cases most important—by endo-upwelling within the fractured carbonate rock of the reef platform. Very slight geothermal heat flux from the earth’s
crust is sufficient to maintain a slow upward movement of water within fractured rock,
drawing nitrate-replete water in at subpycnocline depths and releasing it at the surface. For
this system to function another requisite is perfect clarity of the surface water to obtain
maximum solar energy within the symbiotic tissues. These facts answer Darwin’s paradox:
why do we find the most exuberant growth of corals in the clearest water?
Therefore, where reef-forming organisms occur, we can be sure that seasonal blooms of
planktonic algae and upwelling processes are not significant. This suggestion is confirmed
by the observations of Rougerie and Rancher (1994), who found that in the immediate
vicinity of numerous atolls the vertical structure of the oligotrophic profile is preserved
intact. Right up to the coral front the nitracline (150–200 m), the pycnocline (100–175 m),
and the DCM (125 m) remain undisturbed by the proximity of the reef platform. Weak,
irregular flows past the atolls produce little turbulence or chlorophyll-enhanced wakes
downstream of the atolls.
It is interesting to note that once again, McGowan’s biogeography and Brinton’s
euphausiid maps support at least some of our conclusions. The distribution envelopes
of 10 central water biota (Stylocheiron suhmii, Euphausia brevis, Euphausia mutica,
Sagitta serratodentata, etc.) are a mirror image of the Northern Hemisphere pattern (see
NPSG) but with the important difference that it is more diffuse in the eastern part
of the ocean. This is exactly what we would expect from the hemispheric differences
between the circulation patterns. However, the distribution of large pelagic fish, exemplified by tuna, is somewhat contrary, highest abundance being concentrated marginally
along the northern and southern reaches of the province, adjacent to more productive
regions.
Synopsis
Case 3—Winter-spring production with nutrient limitation—Pycnocline depth undergoes a
weak austral winter excursion (30–40 m February–March, 80 m September) while photic
Chapter 11: The Pacific Ocean
I also assume that maximum primary production rate will occur much shallower; that
nitrogen-fixing organisms will be prominent; that the microbial loop, or regeneration
of ammonium as a substrate for algal growth, will be very active and mediated by
bacterioplankton, pico-fraction cyanobacteria, prochlorophytes, and microflagellates; and
that most consumption will be by a complex protist community. Such mesozooplankton
as occur will be small, except for diel migrant metridiids, which will rise at night, some
to the DCM and some to graze on protists in the mixed layer.
For the EASTROPAC area, which extended only to 20
S at 90–125
W, some of these
suggestions are generally confirmed. A DCM slopes downward in the upper thermocline,
weakening to the south so that by 20
S it lies at about 120 m in February and March when
the depth of maximum primary production rate is at about 20 m (Longhurst, 1976). The
same pattern is repeated in August data, though the DCM is a little deeper. Whereas
mesozooplankton profiles in PNEC and PEQD to the north are strongly structured with
near-surface and near-DCM layers of maximum abundance, in SPSG their layering is
relatively weak with a very broad zone of relative abundance progressively weakening
downward to the midpycnocline.
Although coral reefs are not part of our discussion, a passing reference to their place in
this ocean microcosm is in order because the myriad islands inhabiting the province owe
their existence to the activities of reef-forming biota. If the growth of many of these reefforming corals and mollusks is supported by photosynthesis of symbiotic algae within their
tissues, a source of nitrogen still has to be found: this comes from three sources, the balance
among which is complex to compute. In part, it is provided by the activity of nitrogen-fixing
blue-green algae, in part by capture of the sparse oceanic plankton drifting over the reef,
and in part—perhaps in some cases most important—by endo-upwelling within the fractured carbonate rock of the reef platform. Very slight geothermal heat flux from the earth’s
crust is sufficient to maintain a slow upward movement of water within fractured rock,
drawing nitrate-replete water in at subpycnocline depths and releasing it at the surface. For
this system to function another requisite is perfect clarity of the surface water to obtain
maximum solar energy within the symbiotic tissues. These facts answer Darwin’s paradox:
why do we find the most exuberant growth of corals in the clearest water?
Therefore, where reef-forming organisms occur, we can be sure that seasonal blooms of
planktonic algae and upwelling processes are not significant. This suggestion is confirmed
by the observations of Rougerie and Rancher (1994), who found that in the immediate
vicinity of numerous atolls the vertical structure of the oligotrophic profile is preserved
intact. Right up to the coral front the nitracline (150–200 m), the pycnocline (100–175 m),
and the DCM (125 m) remain undisturbed by the proximity of the reef platform. Weak,
irregular flows past the atolls produce little turbulence or chlorophyll-enhanced wakes
downstream of the atolls.
It is interesting to note that once again, McGowan’s biogeography and Brinton’s
euphausiid maps support at least some of our conclusions. The distribution envelopes
of 10 central water biota (Stylocheiron suhmii, Euphausia brevis, Euphausia mutica,
Sagitta serratodentata, etc.) are a mirror image of the Northern Hemisphere pattern (see
NPSG) but with the important difference that it is more diffuse in the eastern part
of the ocean. This is exactly what we would expect from the hemispheric differences
between the circulation patterns. However, the distribution of large pelagic fish, exemplified by tuna, is somewhat contrary, highest abundance being concentrated marginally
along the northern and southern reaches of the province, adjacent to more productive
regions.
Synopsis
Case 3—Winter-spring production with nutrient limitation—Pycnocline depth undergoes a
weak austral winter excursion (30–40 m February–March, 80 m September) while photic