Pacific Trade Winds Biome
387
Biological Response and Regional Ecology
The generalized chlorophyll field clearly shows the consequences of the baroclinicity of
the nitrate isopleths. McClain et al. have identified (for each of the subtropical gyres,
let it be said) the location of minimum surface chlorophyll in each of the four seasons.
The lowest values consistently occur very close to the location where the mixed layer is
deepest, and hence where the nitracline lies furthest removed from wind-induced mixing
at the surface. In the South Pacific gyre, this location is remarkably stable at about
25
S 120
W, as noted earlier, despite the significant seasonal change here in mixed-layer
depth. Yet we must not suppose that the chlorophyll field of the subtropical gyre is
featureless: the eddy field associated with STCC discussed previously is often located in
the SeaWiFS and MODIS chlorophyll images, especially to the west of the date line, as
a field of arcuate meanders of chlorophyll significantly higher than background. This
is observed more commonly in austral winter and recalls the more elusive feature very
occasionally seen in a comparable location in the South Atlantic subtropical gyre (see
SANT). A more constant anomaly in the regional chlorophyll field is associated with
the Marquesas archipelago at 10
N 140
W; why this group of islands should routinely
produce a chlorophyll anomaly, while the Tuamotos not so far to the south do not, is
something for which I can suggest no explanation.
Both the surface SeaWiFS and MODIS data used in this study, and those for the
smaller box lying centrally in the gyre used by McLain et al., yield the same result: a welldefined winter maximum occurs in August–September and the rate of seasonal change
in chlorophyll values follows very closely the seasonal change in environmental forcing;
here, equilibrium between irradiance, mixing, nutrient flux, and growth appears to be
constantly maintained. The chlorophyll values attributed to the entire SPTG province
are somewhat higher (006–013 mg m
−3 ) than those for the central box of McLain et al.
(002–008 mg m
−3 ); this is to be expected because the peripheral regions of the province
are everywhere adjacent to areas of higher productivity.
We have some sea-truth data obtained from ships of opportunity on the Panama
to Auckland route to support the seasonal cycles inferred from SeaWiFS and similar
data; for a box in the central part of this province, Dandonneau et al. (2004) confirm
the permanent oligotrophy of SPSG (chlorophyll always < 015 mg m
−3 ) and report
a weak winter maximum accompanied by unusually high numbers of picoeukaryotes
(5000 cells ml
−1 , compared with a few hundreds normally). These data also confirm
the general dominance of cyanobacteria and the extensive occurrence of Trichodesmium
blooms, especially in the southwest of the province and especially in summer.
An interesting meridional phytoplankton section along 115
W has been reported
(Hardy et al., 1996), which shows that phytoplankton taxonomic composition was
remarkably invariant from about 11–12
S, a line coinciding with the southern edge of the
PEQD province, right down to 36
S at the edge of the Subantarctic Convergence. Beyond
these two boundaries, species composition differs while carbon biomass, chlorophyll, and
primary productivity all take higher values. Principal component analysis of the whole
section from 10
N to 60
S delivered five groupings of relative abundance of phytoplankton taxa in such a way that the meridional separation of these groups corresponds very
well with the five provinces PNEC, PEQD, SPSG, SSTC, and SANT.
Apart from the EASTROPAC voyages of the 1960s in the northeast corner of the
province, there are no comprehensive ecological studies of the open-ocean plankton
ecosystem and the best that can be done is a brief extrapolation from other regions. It is
probably safe to say that the observations discussed earlier for the CLIMAX and HOT site
in the North Pacific subtropical gyre will obtain here—only in a more extreme sense. It is
also safe to assume that a subsurface maximum chlorophyll layer lies across the province
just shallower than the nitracline and on the upper part of the density gradient and
that it acts to trap to utilize any nitrate mixed up across the upper pycnocline. Finally,
387
Biological Response and Regional Ecology
The generalized chlorophyll field clearly shows the consequences of the baroclinicity of
the nitrate isopleths. McClain et al. have identified (for each of the subtropical gyres,
let it be said) the location of minimum surface chlorophyll in each of the four seasons.
The lowest values consistently occur very close to the location where the mixed layer is
deepest, and hence where the nitracline lies furthest removed from wind-induced mixing
at the surface. In the South Pacific gyre, this location is remarkably stable at about
25
S 120
W, as noted earlier, despite the significant seasonal change here in mixed-layer
depth. Yet we must not suppose that the chlorophyll field of the subtropical gyre is
featureless: the eddy field associated with STCC discussed previously is often located in
the SeaWiFS and MODIS chlorophyll images, especially to the west of the date line, as
a field of arcuate meanders of chlorophyll significantly higher than background. This
is observed more commonly in austral winter and recalls the more elusive feature very
occasionally seen in a comparable location in the South Atlantic subtropical gyre (see
SANT). A more constant anomaly in the regional chlorophyll field is associated with
the Marquesas archipelago at 10
N 140
W; why this group of islands should routinely
produce a chlorophyll anomaly, while the Tuamotos not so far to the south do not, is
something for which I can suggest no explanation.
Both the surface SeaWiFS and MODIS data used in this study, and those for the
smaller box lying centrally in the gyre used by McLain et al., yield the same result: a welldefined winter maximum occurs in August–September and the rate of seasonal change
in chlorophyll values follows very closely the seasonal change in environmental forcing;
here, equilibrium between irradiance, mixing, nutrient flux, and growth appears to be
constantly maintained. The chlorophyll values attributed to the entire SPTG province
are somewhat higher (006–013 mg m
−3 ) than those for the central box of McLain et al.
(002–008 mg m
−3 ); this is to be expected because the peripheral regions of the province
are everywhere adjacent to areas of higher productivity.
We have some sea-truth data obtained from ships of opportunity on the Panama
to Auckland route to support the seasonal cycles inferred from SeaWiFS and similar
data; for a box in the central part of this province, Dandonneau et al. (2004) confirm
the permanent oligotrophy of SPSG (chlorophyll always < 015 mg m
−3 ) and report
a weak winter maximum accompanied by unusually high numbers of picoeukaryotes
(5000 cells ml
−1 , compared with a few hundreds normally). These data also confirm
the general dominance of cyanobacteria and the extensive occurrence of Trichodesmium
blooms, especially in the southwest of the province and especially in summer.
An interesting meridional phytoplankton section along 115
W has been reported
(Hardy et al., 1996), which shows that phytoplankton taxonomic composition was
remarkably invariant from about 11–12
S, a line coinciding with the southern edge of the
PEQD province, right down to 36
S at the edge of the Subantarctic Convergence. Beyond
these two boundaries, species composition differs while carbon biomass, chlorophyll, and
primary productivity all take higher values. Principal component analysis of the whole
section from 10
N to 60
S delivered five groupings of relative abundance of phytoplankton taxa in such a way that the meridional separation of these groups corresponds very
well with the five provinces PNEC, PEQD, SPSG, SSTC, and SANT.
Apart from the EASTROPAC voyages of the 1960s in the northeast corner of the
province, there are no comprehensive ecological studies of the open-ocean plankton
ecosystem and the best that can be done is a brief extrapolation from other regions. It is
probably safe to say that the observations discussed earlier for the CLIMAX and HOT site
in the North Pacific subtropical gyre will obtain here—only in a more extreme sense. It is
also safe to assume that a subsurface maximum chlorophyll layer lies across the province
just shallower than the nitracline and on the upper part of the density gradient and
that it acts to trap to utilize any nitrate mixed up across the upper pycnocline. Finally,