376
Chapter 11: The Pacific Ocean
is some evidence for midday photoinhibition of primary production in the extremely clear
water. Serial observations demonstrate very close coincidence of the time-varying depths
of DCM, of nutricline and of maximum stability (Brunt-Väisälä frequency >002 sec
−1 )
at around 100 m over periods of several days.
During Niño events, the principal change is the progressive shoaling of both thermocline and nutricline with a consequent shift in the depth of greatest productivity,
although this remains close to the depth of 10% surface irradiance; vertical nutrient flux
is somewhat enhanced by the stress at the sea surface imposed by westerly wind bursts.
However, during the 1986–87 El Niño, when the thermocline shallowed significantly,
surface enrichment and a doubling of primary production rates was quite ephemeral
(Radenac and Rodier, 1996). The Australian transects of 1992–92, done at the end of
another Niño event, likewise found a relatively shallow thermocline, a relatively thin
barrier layer, and a DCM that was everywhere deeper than the top of the thermocline.
Surface water was undersaturated with CO 2 during the Niño event, although under what
Radenac and Rodier (1996) call “reference” conditions, or in the absence of El Niño,
pCO 2 is in equilibrium with the atmosphere.
The serial satellite images show that near-surface chlorophyll values are somewhat
higher a few degrees on either side of the equator, this effect being clearer in the eastern
and western parts of the province but weaker centrally. During those ENSO events that
are sufficiently strong to bring the nutricline up to lie at the bottom of the wind-mixed
layer, we may reasonably assume that this is associated with the 25–50% increase in
average primary production that occurs in the WARM province in these years (Barber
and Chavez, 1991).
Under reference conditions, relative productivity may also depend on the thickness
of the barrier layer itself (Mackey et al., 1997), which is very nonuniform; in 1990 at
155
E, it was 88 m deep at 2
S, compared with only 6 m at 1
N. Associated with this
variance in barrier layer thickness are changes in chlorophyll biomass and productivity:
chlorophyll in the DCM was 03 g/liter
−1 at 2
S and almost 05 g/liter
−1 at 1
N. The
regions of thinner barrier layer have been modeled to have higher column productivity
and higher numbers of cyanobacteria in the upper layer, both thought to reflect higher
rates of intermittent mixing.
Because mixed-layer chlorophyll concentrations are so low, the deep euphotic zone
generally comprises two ecologically distinct depth strata: an upper nitrate-limited,
cyanobacteria-dominated zone and a deeper light-limited zone dominated by eukaryotic microalgae (Le Boutiller et al., 1992). As in the PEQD, the picophytoplankton is
dominated by Prochlorococcus and Synechococcus and, when transient nutrient flux induces
increased productivity, it is the latter that shows a small increase in abundance along
with diatoms and chlorophytes (Mackey et al., 2002a). A recent novelty, Bolidomonas,
comprises <4% of pico-fraction chlorophyll.
There is a strong zonal discontinuity in mesozooplankton biomass at around 170
W,
or just to the east of the date line, chosen as the statutory boundary of the WARM
province (Le Borgne and Rodier, 1997). Biomass to the west of this discontinuity, in
the WARM province, is about one-third of that in PEQD to the east and its vertical
distribution is different: in WARM, overall zooplankton biomass lies deeper, and there is
a greater distinction in distribution between microzooplankton (here, principally shallow
and associated with the cyanobacterial populations) and the deeper mesozooplankton.
Biomass is concentrated in a broad zone from the DCM to the surface but is biased
toward the depth of the DCM, especially at night. Under both El Niño and reference
conditions, zooplankton biomass from 20
S to 6
N takes low values in the range 0.5–1.0 g
dry weight m
−2 ; at the equator, when westward trade wind stress is strong and upwelling
occurs, zooplankton biomass rises to 2.5 g dry weight m
−2 .
Chapter 11: The Pacific Ocean
is some evidence for midday photoinhibition of primary production in the extremely clear
water. Serial observations demonstrate very close coincidence of the time-varying depths
of DCM, of nutricline and of maximum stability (Brunt-Väisälä frequency >002 sec
−1 )
at around 100 m over periods of several days.
During Niño events, the principal change is the progressive shoaling of both thermocline and nutricline with a consequent shift in the depth of greatest productivity,
although this remains close to the depth of 10% surface irradiance; vertical nutrient flux
is somewhat enhanced by the stress at the sea surface imposed by westerly wind bursts.
However, during the 1986–87 El Niño, when the thermocline shallowed significantly,
surface enrichment and a doubling of primary production rates was quite ephemeral
(Radenac and Rodier, 1996). The Australian transects of 1992–92, done at the end of
another Niño event, likewise found a relatively shallow thermocline, a relatively thin
barrier layer, and a DCM that was everywhere deeper than the top of the thermocline.
Surface water was undersaturated with CO 2 during the Niño event, although under what
Radenac and Rodier (1996) call “reference” conditions, or in the absence of El Niño,
pCO 2 is in equilibrium with the atmosphere.
The serial satellite images show that near-surface chlorophyll values are somewhat
higher a few degrees on either side of the equator, this effect being clearer in the eastern
and western parts of the province but weaker centrally. During those ENSO events that
are sufficiently strong to bring the nutricline up to lie at the bottom of the wind-mixed
layer, we may reasonably assume that this is associated with the 25–50% increase in
average primary production that occurs in the WARM province in these years (Barber
and Chavez, 1991).
Under reference conditions, relative productivity may also depend on the thickness
of the barrier layer itself (Mackey et al., 1997), which is very nonuniform; in 1990 at
155
E, it was 88 m deep at 2
S, compared with only 6 m at 1
N. Associated with this
variance in barrier layer thickness are changes in chlorophyll biomass and productivity:
chlorophyll in the DCM was 03 g/liter
−1 at 2
S and almost 05 g/liter
−1 at 1
N. The
regions of thinner barrier layer have been modeled to have higher column productivity
and higher numbers of cyanobacteria in the upper layer, both thought to reflect higher
rates of intermittent mixing.
Because mixed-layer chlorophyll concentrations are so low, the deep euphotic zone
generally comprises two ecologically distinct depth strata: an upper nitrate-limited,
cyanobacteria-dominated zone and a deeper light-limited zone dominated by eukaryotic microalgae (Le Boutiller et al., 1992). As in the PEQD, the picophytoplankton is
dominated by Prochlorococcus and Synechococcus and, when transient nutrient flux induces
increased productivity, it is the latter that shows a small increase in abundance along
with diatoms and chlorophytes (Mackey et al., 2002a). A recent novelty, Bolidomonas,
comprises <4% of pico-fraction chlorophyll.
There is a strong zonal discontinuity in mesozooplankton biomass at around 170
W,
or just to the east of the date line, chosen as the statutory boundary of the WARM
province (Le Borgne and Rodier, 1997). Biomass to the west of this discontinuity, in
the WARM province, is about one-third of that in PEQD to the east and its vertical
distribution is different: in WARM, overall zooplankton biomass lies deeper, and there is
a greater distinction in distribution between microzooplankton (here, principally shallow
and associated with the cyanobacterial populations) and the deeper mesozooplankton.
Biomass is concentrated in a broad zone from the DCM to the surface but is biased
toward the depth of the DCM, especially at night. Under both El Niño and reference
conditions, zooplankton biomass from 20
S to 6
N takes low values in the range 0.5–1.0 g
dry weight m
−2 ; at the equator, when westward trade wind stress is strong and upwelling
occurs, zooplankton biomass rises to 2.5 g dry weight m
−2 .