382
Chapter 11: The Pacific Ocean
Despite such difficulties, it is clear that the pelagic ecosystem here responds to the
strong seasonality of the reversing monsoon winds with winter phytoplankton blooms in
each hemisphere: surface chlorophyll is maximum in the Coral Sea and adjacent basins
in austral winter (July) and in the South China Sea in boreal winter (January). Although
in the smaller archipelagic seas the seasonal signal is not so clear, the Coral Sea seasonal
bloom does extend westward through the Banda and Flores Seas, while the Sulu and
Celebes Seas follow—rather generally—the same pattern as the South China Sea. This
pattern appears to be a simple response to the seasonal deepening of the mixed layer in
the two hemispheres and the consequent flux of nutrients into the euphotic zone.
It is in this region that some of the earliest direct observations were made using satellite
data of phytoplankton response to mesoscale physical processes, at least partly because
here such responses are as direct and strong as anywhere in the oceans. Island wakes
up to 300 km long were observed as high-contrast features in early CZCS images of the
Bismarck Sea, representing the effects of turbulent plumes downwind of the mountains of
New Britain and high islands such as Sakar to the east of the Vitiaz Straits (Wolanski et al.,
1986). These plumes represent cooler water surfacing in regions where the surface water is
highly turbid, either with phytoplankton or suspended sediments; fields of large internal
waves seen in the southern Bismarck Sea probably represent deeper, clearer water being
brought to the surface. Some of these features have been examined at sea; thus, in the
Coral Sea, temporary zonal ridges in the variable circulation have been observed at 12 and
17
S, with consequences for mixed-layer nutrient levels (Rougerie and Henin, 1977). We
can anticipate that many such features will be revealed throughout the province now that
examination of detailed images has become routine; we can also expect that their orientation and location will be found to depend on seasonal and shorter changes in wind direction and strength. Examination of almost any high-resolution chlorophyll or SST image
of this province (including those shown here) will yield further examples, perhaps especially of plumes of high chlorophyll in the eddying flow through passes in the island arcs
surrounding the Coral Sea. The consequences of eddying flow behind coral atolls has been
investigated in the Coral Sea (Rissik et al., 1997): uplift of the nutricline in the island wake
induces enhanced production of phytoplankton and microzooplankton, although with little alteration of specific composition compared with the free stream. Ecosystem response
occurs, principally in the organisms associated with the DCM, up to at least the micronekton level, with myctophid fish being observed to feed more successfully in the wake.
In the Solomon Sea, there are observations of a shade-adapted DCM (06 mg chl m
−3
and 015 mg C mg chl hr
−1 ) at about 75 m (near the 1.2% isolume), containing almost
half of the total 0–200 m chlorophyll (Satoh et al., 1992). Phytoplankton profiles in the
Coral Sea have a somewhat deeper DCM, of order 100 m, where light-dependent maximum assimilation numbers of cyanobacteria are twice those for near surface populations,
whereas for larger phytoplankton the relevant value is almost five times greater (Furnas
and Mitchell, 1996b); this is a typical result for very oligotrophic open oceans. The southern part of the Solomon Sea, and the internal basin of the Louisiade Archipelago, which
forms the southern margin of this body of water, has relatively high productivity. As in
other oligotrophic regions, there is evidence in the stratified region that the cells at the
DCM are more dependent on nitrate than those closer to the surface, and that the pico
fraction generally dominates the assemblage of autotrophic cells. Most of the primary production occurs shoaler than the midday 20% isolume, and very little is lost to sinking, so
we can assume that production and consumption of small cells are held closely in balance.
Under such conditions, it is small wonder that the Coral Sea and the other marginal
seas share other ecological characteristics with the oligotrophic South Pacific, including
the persistent occurrence of blooms of nitrogen-fixing tufts of cyanobacteria. Very large
(9×10
4 km
2 ) blooms of these organisms, identified as Trichodesmium by its chromatic signature, have been observed at 10
S 165
E New Caledonia in the surface chlorophyll field
Chapter 11: The Pacific Ocean
Despite such difficulties, it is clear that the pelagic ecosystem here responds to the
strong seasonality of the reversing monsoon winds with winter phytoplankton blooms in
each hemisphere: surface chlorophyll is maximum in the Coral Sea and adjacent basins
in austral winter (July) and in the South China Sea in boreal winter (January). Although
in the smaller archipelagic seas the seasonal signal is not so clear, the Coral Sea seasonal
bloom does extend westward through the Banda and Flores Seas, while the Sulu and
Celebes Seas follow—rather generally—the same pattern as the South China Sea. This
pattern appears to be a simple response to the seasonal deepening of the mixed layer in
the two hemispheres and the consequent flux of nutrients into the euphotic zone.
It is in this region that some of the earliest direct observations were made using satellite
data of phytoplankton response to mesoscale physical processes, at least partly because
here such responses are as direct and strong as anywhere in the oceans. Island wakes
up to 300 km long were observed as high-contrast features in early CZCS images of the
Bismarck Sea, representing the effects of turbulent plumes downwind of the mountains of
New Britain and high islands such as Sakar to the east of the Vitiaz Straits (Wolanski et al.,
1986). These plumes represent cooler water surfacing in regions where the surface water is
highly turbid, either with phytoplankton or suspended sediments; fields of large internal
waves seen in the southern Bismarck Sea probably represent deeper, clearer water being
brought to the surface. Some of these features have been examined at sea; thus, in the
Coral Sea, temporary zonal ridges in the variable circulation have been observed at 12 and
17
S, with consequences for mixed-layer nutrient levels (Rougerie and Henin, 1977). We
can anticipate that many such features will be revealed throughout the province now that
examination of detailed images has become routine; we can also expect that their orientation and location will be found to depend on seasonal and shorter changes in wind direction and strength. Examination of almost any high-resolution chlorophyll or SST image
of this province (including those shown here) will yield further examples, perhaps especially of plumes of high chlorophyll in the eddying flow through passes in the island arcs
surrounding the Coral Sea. The consequences of eddying flow behind coral atolls has been
investigated in the Coral Sea (Rissik et al., 1997): uplift of the nutricline in the island wake
induces enhanced production of phytoplankton and microzooplankton, although with little alteration of specific composition compared with the free stream. Ecosystem response
occurs, principally in the organisms associated with the DCM, up to at least the micronekton level, with myctophid fish being observed to feed more successfully in the wake.
In the Solomon Sea, there are observations of a shade-adapted DCM (06 mg chl m
−3
and 015 mg C mg chl hr
−1 ) at about 75 m (near the 1.2% isolume), containing almost
half of the total 0–200 m chlorophyll (Satoh et al., 1992). Phytoplankton profiles in the
Coral Sea have a somewhat deeper DCM, of order 100 m, where light-dependent maximum assimilation numbers of cyanobacteria are twice those for near surface populations,
whereas for larger phytoplankton the relevant value is almost five times greater (Furnas
and Mitchell, 1996b); this is a typical result for very oligotrophic open oceans. The southern part of the Solomon Sea, and the internal basin of the Louisiade Archipelago, which
forms the southern margin of this body of water, has relatively high productivity. As in
other oligotrophic regions, there is evidence in the stratified region that the cells at the
DCM are more dependent on nitrate than those closer to the surface, and that the pico
fraction generally dominates the assemblage of autotrophic cells. Most of the primary production occurs shoaler than the midday 20% isolume, and very little is lost to sinking, so
we can assume that production and consumption of small cells are held closely in balance.
Under such conditions, it is small wonder that the Coral Sea and the other marginal
seas share other ecological characteristics with the oligotrophic South Pacific, including
the persistent occurrence of blooms of nitrogen-fixing tufts of cyanobacteria. Very large
(9×10
4 km
2 ) blooms of these organisms, identified as Trichodesmium by its chromatic signature, have been observed at 10
S 165
E New Caledonia in the surface chlorophyll field