372
Chapter 11: The Pacific Ocean
Turning now to mesozooplankton, Roman et al. (2002) used the EqPac transects
to specify the relationship between biomass of the >200-m fraction and the location
of maximal autotrophic production in the upwelling zone. In accordance with theory,
they found mesozooplankton biomass to be shifted “downstream” because of the delay
involved in the response of mesozooplankton life cycles to enhanced food availability.
Mesozooplankton biomass was generally ∼25% of that of autotrophs in equivalent carbon
units and ∼30% of bacterial biomass. Mesozooplankton grazing was estimated in this
study to be <5% daily of the standing stock of autotrophs, a figure that implies that mesozooplankton largely consume microzooplankton and detritus to support their growth
(058 d
−1 in the 64–200 m fraction, 008 d
−1 in the 1–2 mm fraction). Independent estimates, also made during EqPac, put mesozooplankton grazing rates at <9% of chlorophyll
and <12% of primary production daily (Dam et al., 1996). This study suggested that
>80% of the carbon ingested by mesozooplankton is not phytoplankton, yet may represent removal of >27% of the biomass of large (>2 m) diatoms daily. But, as was noted
earlier (see PNEC), although generalizations such as these concerning mesozooplankton
feeding have become very familiar to us recently, they do tend to obscure the highly
structured and highly differentiated pattern of consumption characteristic of individual
species, and individual growth stages of species and of meso- and microzooplankton.
The mesozooplankton species and stages are arranged in relation to the stratification
of the water column in a reasonably simple and well-known arrangement (Roman et al.,
1995). As the mixed layer deepens toward the west across the ocean and poleward from
the equator, it carries with it the permanent DCM and associated vertical layering of
zooplankton, of which the depth of greatest abundance lies near the depth of maximal
primary production rate in the upper mixed layer and therefore significantly shoaler
than the DCM. At 155
W the core of the DCM (0.2–03 mg chl m
−3 ) deepens from 50 m
under the equator to 100 m at 15
S, whereas the rate of normalized primary production is
maximal in the upper 10 m at about 3 or 4 g C g chl
−1 h
−1 . Diel migrants (euphausiids
and metridiid copepods) are at 400–500 m by day and join the epiplankton in the upper
50 m at night.
Although, as I suggested earlier and in Chapter 5, the “enigma” of Fe limitation in
PEQD may be resolved quite simply and without recourse to subaerial fluxes, it may nevertheless be useful to comment very briefly here on the IronEx I and II experiments that
were done in PEQD. IronEx I, in 1993, led by John Martin, was the first such experiment
in a now long series, and was only a partial success: Fe was released into the ship’s wake
and response of the autotrophic biota was instantaneous, with photosynthetic efficiency
peaking after 2 or 3 days, when chlorophyll concentration and primary production rate
had doubled. Unfortunately, after 5 days the patch was subducted below fast-moving
low-salinity surface water and lost.
IronEx II, in 1995, was a resounding success (Coale et al., 1996a,b). Repeated injections
of Fe into the ship’s wake during about 1 week produced a 70-km
2 patch with an Fe
concentration of 20 M. During this time, nitrate was drawn down from an initial
>10 M to <50 M, chlorophyll increased from <02 mg m
−3 to >30 mg m
−3 in the
patch center, and pCO 2 decreased as carbon was taken up for photosynthesis. When
injection ceased, the patch began to weaken both through diffusion and by the loss of
Fe. In control patches, there was no response to injection of biologically inert molecules.
The autotrophic biota showed differential responses that make perfect sense. The cells
that responded most rapidly and most completely were diatoms, which increased in
abundance by a factor of 85, whereas the picoautotrophs responded only by doubling
their numbers. The micrograzers responded in step with the picoautotrophs, but mesozooplankton responded very little—as indeed their long generation time would ensure.
The resulting imbalance between copepods and diatoms allowed a bloom to occur that
was analogous to the imbalance during a high-latitude spring bloom.
Chapter 11: The Pacific Ocean
Turning now to mesozooplankton, Roman et al. (2002) used the EqPac transects
to specify the relationship between biomass of the >200-m fraction and the location
of maximal autotrophic production in the upwelling zone. In accordance with theory,
they found mesozooplankton biomass to be shifted “downstream” because of the delay
involved in the response of mesozooplankton life cycles to enhanced food availability.
Mesozooplankton biomass was generally ∼25% of that of autotrophs in equivalent carbon
units and ∼30% of bacterial biomass. Mesozooplankton grazing was estimated in this
study to be <5% daily of the standing stock of autotrophs, a figure that implies that mesozooplankton largely consume microzooplankton and detritus to support their growth
(058 d
−1 in the 64–200 m fraction, 008 d
−1 in the 1–2 mm fraction). Independent estimates, also made during EqPac, put mesozooplankton grazing rates at <9% of chlorophyll
and <12% of primary production daily (Dam et al., 1996). This study suggested that
>80% of the carbon ingested by mesozooplankton is not phytoplankton, yet may represent removal of >27% of the biomass of large (>2 m) diatoms daily. But, as was noted
earlier (see PNEC), although generalizations such as these concerning mesozooplankton
feeding have become very familiar to us recently, they do tend to obscure the highly
structured and highly differentiated pattern of consumption characteristic of individual
species, and individual growth stages of species and of meso- and microzooplankton.
The mesozooplankton species and stages are arranged in relation to the stratification
of the water column in a reasonably simple and well-known arrangement (Roman et al.,
1995). As the mixed layer deepens toward the west across the ocean and poleward from
the equator, it carries with it the permanent DCM and associated vertical layering of
zooplankton, of which the depth of greatest abundance lies near the depth of maximal
primary production rate in the upper mixed layer and therefore significantly shoaler
than the DCM. At 155
W the core of the DCM (0.2–03 mg chl m
−3 ) deepens from 50 m
under the equator to 100 m at 15
S, whereas the rate of normalized primary production is
maximal in the upper 10 m at about 3 or 4 g C g chl
−1 h
−1 . Diel migrants (euphausiids
and metridiid copepods) are at 400–500 m by day and join the epiplankton in the upper
50 m at night.
Although, as I suggested earlier and in Chapter 5, the “enigma” of Fe limitation in
PEQD may be resolved quite simply and without recourse to subaerial fluxes, it may nevertheless be useful to comment very briefly here on the IronEx I and II experiments that
were done in PEQD. IronEx I, in 1993, led by John Martin, was the first such experiment
in a now long series, and was only a partial success: Fe was released into the ship’s wake
and response of the autotrophic biota was instantaneous, with photosynthetic efficiency
peaking after 2 or 3 days, when chlorophyll concentration and primary production rate
had doubled. Unfortunately, after 5 days the patch was subducted below fast-moving
low-salinity surface water and lost.
IronEx II, in 1995, was a resounding success (Coale et al., 1996a,b). Repeated injections
of Fe into the ship’s wake during about 1 week produced a 70-km
2 patch with an Fe
concentration of 20 M. During this time, nitrate was drawn down from an initial
>10 M to <50 M, chlorophyll increased from <02 mg m
−3 to >30 mg m
−3 in the
patch center, and pCO 2 decreased as carbon was taken up for photosynthesis. When
injection ceased, the patch began to weaken both through diffusion and by the loss of
Fe. In control patches, there was no response to injection of biologically inert molecules.
The autotrophic biota showed differential responses that make perfect sense. The cells
that responded most rapidly and most completely were diatoms, which increased in
abundance by a factor of 85, whereas the picoautotrophs responded only by doubling
their numbers. The micrograzers responded in step with the picoautotrophs, but mesozooplankton responded very little—as indeed their long generation time would ensure.
The resulting imbalance between copepods and diatoms allowed a bloom to occur that
was analogous to the imbalance during a high-latitude spring bloom.