Pacific Trade Winds Biome
371
that bacteria respond relatively slowly to upwelling fields and that this causes persistent
uncoupling with autotrophic biomass.
The distribution of picophytoplankton along the 140
W EqPac transect supported the
reality of the ecological boundary between PEQD and PNEC; beyond 7
N the abundance
and vertical distribution of small cells matched subtropical gyre profiles. Here, Prochlorococcus and heterotrophic bacteria occupied the upper euphotic zone, while maximum
abundance of Synechococcus and small autotrophic eukaryotes occurred deeper. In the
PEQD region, this vertical distinction broke down, and all cell-types of the pico-fraction
were abundant throughout the euphotic zone (Landry et al., 1996). Another EqPac study
also provided support for the ecological reality of the boundaries suggested for this
province. Optical studies of microorganisms along the section at 140
W revealed three
“biohydrographic regimes” (Chung et al., 1996). Characteristics of beam attenuation (Cp)
due to particles, dominated by heterotrophic bacteria, prochlorophytes, cyanophytes, and
small eukaryotes (<30 mm), showed that the 7
N–7
S zone could readily be distinguished
from the more poleward parts of the section: this zone corresponds well enough with
the region here defined here as the PEQD province. It was only in the central biohydrographic regime, corresponding to our PEQD, that the particle population responded
significantly to the 1992 El Niño event; a 30% increase in beam Cp and depth-integrated
Cp were noted only here.
There is much evidence that the major fluxes in the pelagic ecosystem are—as we have
come to expect in recent years—dominated by flux through the small cells of the microbial
loop. The basic composition of the autotrophs is approximately as follows (expressed as
mg C liter
−1 ): Synechococcus, 2.0; prochlorophytes, 4.5; prymnesiophytes, 3.0; autotrophic
dinoflagellates, 5.0; pennate diatoms, 2.0; and Phaeocystis, 0.5. We have become used to the
apparent predominance of just two taxa of prokaryotes in the picophytoplankton, usually
referred to simply as Prochlorococcus and Synechococcus without specific connotation.
Here, during EqPac, analysis of chlorophyll and carotenins showed that Prochlorococcus
occurs as at least three ecotypes having different pigment ratios and perhaps carbonto-pigment ratios. Synechococcus may also possess similar heterogeneity (Mackey et al.,
2002a). However that may be, the ecology of these small cells is now becoming clearer and
EqPac studies revealed a very marked diel cycle in abundance, in cellular light scattering,
and in fluorescence of the prokaryotes in PEQD: cell division is synchronized and occurs
in late afternoon or at night, whereas cell size, and hence light scattering, represents the
balance between cell growth and division. Diel changes in cell fluorescence are due to
changes in pigment content and light-dependent quenching (Binder and DuRand, 2002).
With the small autotrophic cells occur the protistan grazers, also expressed as mg C
liter
−1 : heterotrophic and dinoflagellates, 7.0; and mixotrophic ciliates, 0.25 (Coale et al.,
1996b). This is a recipe for a tightly coupled production/consumption system in which
the population size of the grazers can respond as fast as that of the autotrophic organisms.
Rate estimates for microzooplankton grazing at the EqPac stations and instantaneous
autotrophic growth rates were highly variable, but quite similar. Bulk chlorophyll estimates for February–March 1991 suggested that 083 d
−1 (10–20 m), 034 d
−1 (40–50 m),
and 022 d
−1 (70–80 m) would be useful average values. Corresponding rates for consumption by microzooplankton were 0.72, 0.22, and 021 d
−1 , respectively (Landry et al.,
1995). Thus, grazing by these organisms imposed a daily mortality representing 83%
of autotrophic growth at this season, which was reduced to only 55% in the second
study period in August–September, when only the smallest autotrophs were controlled by
protists. An independent analysis suggested that grazing by microzooplankton (especially
microflagellates and dinoflagellates) in February–March removed the entire daily production of picophytoplankton. In this study, microzooplankton consumption only balanced
the daily production of prymnesiophytes and cyanobacteria, while at least 50% of diatom
loss was attributed to mesozooplankton grazing and to sinking (Verity et al., 1996).
371
that bacteria respond relatively slowly to upwelling fields and that this causes persistent
uncoupling with autotrophic biomass.
The distribution of picophytoplankton along the 140
W EqPac transect supported the
reality of the ecological boundary between PEQD and PNEC; beyond 7
N the abundance
and vertical distribution of small cells matched subtropical gyre profiles. Here, Prochlorococcus and heterotrophic bacteria occupied the upper euphotic zone, while maximum
abundance of Synechococcus and small autotrophic eukaryotes occurred deeper. In the
PEQD region, this vertical distinction broke down, and all cell-types of the pico-fraction
were abundant throughout the euphotic zone (Landry et al., 1996). Another EqPac study
also provided support for the ecological reality of the boundaries suggested for this
province. Optical studies of microorganisms along the section at 140
W revealed three
“biohydrographic regimes” (Chung et al., 1996). Characteristics of beam attenuation (Cp)
due to particles, dominated by heterotrophic bacteria, prochlorophytes, cyanophytes, and
small eukaryotes (<30 mm), showed that the 7
N–7
S zone could readily be distinguished
from the more poleward parts of the section: this zone corresponds well enough with
the region here defined here as the PEQD province. It was only in the central biohydrographic regime, corresponding to our PEQD, that the particle population responded
significantly to the 1992 El Niño event; a 30% increase in beam Cp and depth-integrated
Cp were noted only here.
There is much evidence that the major fluxes in the pelagic ecosystem are—as we have
come to expect in recent years—dominated by flux through the small cells of the microbial
loop. The basic composition of the autotrophs is approximately as follows (expressed as
mg C liter
−1 ): Synechococcus, 2.0; prochlorophytes, 4.5; prymnesiophytes, 3.0; autotrophic
dinoflagellates, 5.0; pennate diatoms, 2.0; and Phaeocystis, 0.5. We have become used to the
apparent predominance of just two taxa of prokaryotes in the picophytoplankton, usually
referred to simply as Prochlorococcus and Synechococcus without specific connotation.
Here, during EqPac, analysis of chlorophyll and carotenins showed that Prochlorococcus
occurs as at least three ecotypes having different pigment ratios and perhaps carbonto-pigment ratios. Synechococcus may also possess similar heterogeneity (Mackey et al.,
2002a). However that may be, the ecology of these small cells is now becoming clearer and
EqPac studies revealed a very marked diel cycle in abundance, in cellular light scattering,
and in fluorescence of the prokaryotes in PEQD: cell division is synchronized and occurs
in late afternoon or at night, whereas cell size, and hence light scattering, represents the
balance between cell growth and division. Diel changes in cell fluorescence are due to
changes in pigment content and light-dependent quenching (Binder and DuRand, 2002).
With the small autotrophic cells occur the protistan grazers, also expressed as mg C
liter
−1 : heterotrophic and dinoflagellates, 7.0; and mixotrophic ciliates, 0.25 (Coale et al.,
1996b). This is a recipe for a tightly coupled production/consumption system in which
the population size of the grazers can respond as fast as that of the autotrophic organisms.
Rate estimates for microzooplankton grazing at the EqPac stations and instantaneous
autotrophic growth rates were highly variable, but quite similar. Bulk chlorophyll estimates for February–March 1991 suggested that 083 d
−1 (10–20 m), 034 d
−1 (40–50 m),
and 022 d
−1 (70–80 m) would be useful average values. Corresponding rates for consumption by microzooplankton were 0.72, 0.22, and 021 d
−1 , respectively (Landry et al.,
1995). Thus, grazing by these organisms imposed a daily mortality representing 83%
of autotrophic growth at this season, which was reduced to only 55% in the second
study period in August–September, when only the smallest autotrophs were controlled by
protists. An independent analysis suggested that grazing by microzooplankton (especially
microflagellates and dinoflagellates) in February–March removed the entire daily production of picophytoplankton. In this study, microzooplankton consumption only balanced
the daily production of prymnesiophytes and cyanobacteria, while at least 50% of diatom
loss was attributed to mesozooplankton grazing and to sinking (Verity et al., 1996).