160
Chapter 9: The Atlantic Ocean
the coastal boundary biome, such as over the Rockall Channel. Strass and Woods
point out that the southward slope of the deep chlorophyll maximum is coincident
with the southward deepening of the nitracline but is disjunct from Z m , sloping
downward and southward at a much slighter angle. New production continues
at the deep chlorophyll maximum during the oligotrophic summer period after
the termination of the spring bloom in the mixed layer. Some enhancement of
surface chlorophyll extends into early autumn, producing a shoulder on the regional,
seasonal graph of satellite-derived chlorophyll.
Because this province lies centrally in the area affected by the North Atlantic spring
bloom, it has generally been believed to be the ideal case of the diatom-copepod food
chain. However, we now know, thanks to the results of the 1989 North Atlantic Bloom
Study (NABE; Ducklow and Harris, 1993) that nano- and picoplankton contribute significantly to total primary production in the Atlantic spring bloom and that diatoms are
not consistently the dominant large cells. Silicate was reduced to limiting values before
nitrate limitation occurred at the NABE time series stations at 18
and 40
W, representing the zonal extent of the province at about 45
N, so this unexpected result must
be typical. Before silicate limitation, the dominant large cells were diatoms (mostly Rhizosolenia, Fragillariopsis, Thalassema, Thalassiosira, and Nitzschia) at 18
W, but dinoflagellates (perhaps facultative heterotrophs) dominated in optically counted samples at
45
W. Dominance shifted rapidly to an abundance (10
4 cells ml
−1 ) of small (2–5 m)
flagellates as soon as silicate limitation occurred, without reduction of overall chlorophyll biomass. The unconsumed, silicon-depleted diatoms sank out, leaving behind an
abundant mucopolysaccharide residue in the photic zone. In repeated zonal transects
between April and October, Li and Harrison (2001) observed that the carbon biomass
of the picoautotrophic fraction represented only 6% of the total autotrophs in NADR,
heterotrophic bacteria representing 16% of autotrophs.
During the spring bloom studies at the western JGOFS-NABE sites (Harrison et al.,
1993) in NADR, microheterotrophs (ciliates and other protists) represented 11% of total
living organic carbon compared with 4% for zooplankton. Protists were the principal
consumers of primary production, taking 25% of the standing stock of plant cells per day
and 90% of primary production based on phytoplankton growth rates of about 0.4–0.7
doublings d
−1 , whereas grazing by zooplankton herbivores took less than 10% of daily
production. Of the copepods, small forms (e.g., Oithona) took as much chlorophyll as
the medium (Metridia) and large-size classes (Calanus and Pleuromamma) combined.
Biomass and grazing impact of three size classes of zooplankters (0.2–05, 0.5–1.0, and
1.0–2.0 mm) was investigated at the eastern NABE stations at 47
N 20
W during May
1989 (Dam et al., 1993). Group biomass was inversely related to body size, the smallest
class forming >50% of the total biomass that itself increased by a factor of 3 during the
month, even as individual size increased sixfold. Accordingly, 66% of all consumption of
autotrophic biomass was performed by the small fraction at the start of the period, and
44% by the medium fraction at the end. The active flux of DON out of the photic zone
during diel migration was about 25% of the passive PON flux.
During the postbloom period, interaction between biota rather than physical forcing
may—at least in some years—dominate the seasonal development of spatial and temporal
variability of the pelagic ecosystem. Popova et al. (2002) discussed such a situation in
April–May of 1997 some weeks after the peak of the seasonal bloom to the west of
the British Isles (48
N 17
W). Here, deep wind-induced mixing during spring after
deeper-than-usual winter mixing created a situation that approached the so-called HNLC
condition, which—as the authors of the study remark—is unusual here. The application
of a coupled 3D physical-biological model to this situation suggested that what was
observed was a phytoplankton that was grazer-controlled rather than limited by nitrate
availability—this did not drop below 10 M m
−3 during the observations, and biomass
Chapter 9: The Atlantic Ocean
the coastal boundary biome, such as over the Rockall Channel. Strass and Woods
point out that the southward slope of the deep chlorophyll maximum is coincident
with the southward deepening of the nitracline but is disjunct from Z m , sloping
downward and southward at a much slighter angle. New production continues
at the deep chlorophyll maximum during the oligotrophic summer period after
the termination of the spring bloom in the mixed layer. Some enhancement of
surface chlorophyll extends into early autumn, producing a shoulder on the regional,
seasonal graph of satellite-derived chlorophyll.
Because this province lies centrally in the area affected by the North Atlantic spring
bloom, it has generally been believed to be the ideal case of the diatom-copepod food
chain. However, we now know, thanks to the results of the 1989 North Atlantic Bloom
Study (NABE; Ducklow and Harris, 1993) that nano- and picoplankton contribute significantly to total primary production in the Atlantic spring bloom and that diatoms are
not consistently the dominant large cells. Silicate was reduced to limiting values before
nitrate limitation occurred at the NABE time series stations at 18
and 40
W, representing the zonal extent of the province at about 45
N, so this unexpected result must
be typical. Before silicate limitation, the dominant large cells were diatoms (mostly Rhizosolenia, Fragillariopsis, Thalassema, Thalassiosira, and Nitzschia) at 18
W, but dinoflagellates (perhaps facultative heterotrophs) dominated in optically counted samples at
45
W. Dominance shifted rapidly to an abundance (10
4 cells ml
−1 ) of small (2–5 m)
flagellates as soon as silicate limitation occurred, without reduction of overall chlorophyll biomass. The unconsumed, silicon-depleted diatoms sank out, leaving behind an
abundant mucopolysaccharide residue in the photic zone. In repeated zonal transects
between April and October, Li and Harrison (2001) observed that the carbon biomass
of the picoautotrophic fraction represented only 6% of the total autotrophs in NADR,
heterotrophic bacteria representing 16% of autotrophs.
During the spring bloom studies at the western JGOFS-NABE sites (Harrison et al.,
1993) in NADR, microheterotrophs (ciliates and other protists) represented 11% of total
living organic carbon compared with 4% for zooplankton. Protists were the principal
consumers of primary production, taking 25% of the standing stock of plant cells per day
and 90% of primary production based on phytoplankton growth rates of about 0.4–0.7
doublings d
−1 , whereas grazing by zooplankton herbivores took less than 10% of daily
production. Of the copepods, small forms (e.g., Oithona) took as much chlorophyll as
the medium (Metridia) and large-size classes (Calanus and Pleuromamma) combined.
Biomass and grazing impact of three size classes of zooplankters (0.2–05, 0.5–1.0, and
1.0–2.0 mm) was investigated at the eastern NABE stations at 47
N 20
W during May
1989 (Dam et al., 1993). Group biomass was inversely related to body size, the smallest
class forming >50% of the total biomass that itself increased by a factor of 3 during the
month, even as individual size increased sixfold. Accordingly, 66% of all consumption of
autotrophic biomass was performed by the small fraction at the start of the period, and
44% by the medium fraction at the end. The active flux of DON out of the photic zone
during diel migration was about 25% of the passive PON flux.
During the postbloom period, interaction between biota rather than physical forcing
may—at least in some years—dominate the seasonal development of spatial and temporal
variability of the pelagic ecosystem. Popova et al. (2002) discussed such a situation in
April–May of 1997 some weeks after the peak of the seasonal bloom to the west of
the British Isles (48
N 17
W). Here, deep wind-induced mixing during spring after
deeper-than-usual winter mixing created a situation that approached the so-called HNLC
condition, which—as the authors of the study remark—is unusual here. The application
of a coupled 3D physical-biological model to this situation suggested that what was
observed was a phytoplankton that was grazer-controlled rather than limited by nitrate
availability—this did not drop below 10 M m
−3 during the observations, and biomass