Atlantic Westerly Winds Biome
159
in many monthly images. Similarly, the “blue holes” in the chlorophyll field to the west of
the British Isles seen in the May–July 1999 images are repeated in many summer months
in other years; this appears to be a very “quiet” region with little mesoscale eddy activity.
A region of high chlorophyll is repeatedly associated with Rockall Bank, lying across the
boundary between NADR and SARC to the northwest of Scotland. Here, it is known that
upwelling occurs onto the bank. It is clear, from examination of these images, that the
assumption often made that major biological gradients in the North Atlantic do not exist
in the zonal sense, but only in the meridional sense, is incorrect.
The between-year variability now revealed is significant so that the overall patterns
remain, but their sequence differs strongly: the null hypothesis must be variability in
meteorological forcing. Observations made at the BIOTRANS station (47
N 20
W) have
revealed very clearly the instability of the early spring bloom (Koeve et al., 2002). In
April, 1992, when the doubling time of mixed-layer phytoplankton biomass was of order
7 days, the development of a stable, near-surface mixed layer was far from progressive,
but rather was interrupted by episodes of deep mixing during the passage of storms. Heat
flux into the mixed layer was reversed during these episodes, and the loss of cells into the
interior of the ocean represented as much as 60% of total spring bloom production; thus,
the sign of the heat flux across the sea surface was constantly changing during March and
April, and became permanently positive only in May. Reevaluation of the NABE data for
1989 suggest that convective loss followed the same pattern and was equivalent to about
40% of total new production. Model studies of the same phenomenon suggest that where
the Z cr /Z m ratio is relatively low, because of deep winter mixing, the early bloom is very
sensitive to irruptive mixing events and hence to meteorological modulation.
The University of Kiel “Sea Rover” meridional transects along 30
W that were obtained
during spring and summer of 1984–1986 (Strass and Woods, 1991) provide us with
synoptic sections of chlorophyll biomass to 150 m, and very usefully serve support the
satellite data in the recognition of three phytoplankton seasons:
The development of a spring bloom. The first indications (April 18–24) of a bloom
occurred where meandering flow was strongest, across 44–47
N, with a small
southerly outlying patch at about 40
N to the south of this province, in NAST.
Elsewhere, the bloom had not started. By April 24–29, consistent with rapid onset of
density stratification, a near-surface bloom with values >125 mg chl m
−3 extended
clear from 39
to 50
N, a distance of 1500 km. This seems to confirm, then, that
initial stratification (and the first patches of surface bloom) is related to the effect of
increasing irradiance on cells brought near the surface in mesoscale eddy dynamics,
rather than on density stratification, which occurs almost simultaneously across a
rather wide zonal swath of ocean.
Transition to summer oligotrophy. By the end of June, the transition to oligotrophic
conditions had occurred, though patchily, poleward to 46
N. There was residual
nitrate in the photic zone only in small areas having exceptionally shoal pycnoclines,
suggesting an ephemeral event. Maximum chlorophyll values occurred near the OPF
and from there south to 46
N the deep chlorophyll maximum lay at, or shallower
than, the mixed-layer depth. Further to the south yet, nitrate was depleted and
the chlorophyll maximum was significantly deeper than the bottom of the mixed
layer. Oligotrophy propagates poleward during the summer at about 3
of latitude
a month, whereas Z m deepens at about 10 m month
−1 .
Late summer regrowth. By the end of August, everywhere south of the Oceanic Polar
Front, which lay at 52–54
N, a chlorophyll maximum occurred within the seasonal
pycnocline, deepening toward the south at about 3.5 m across each degree of latitude.
Only to the north of the OPF was the chlorophyll maximum within the mixed layer,
and here it took maximal values. Late summer blooms of coccolithophores occur
consistently in the northern part of NADR and are especially frequent bordering
159
in many monthly images. Similarly, the “blue holes” in the chlorophyll field to the west of
the British Isles seen in the May–July 1999 images are repeated in many summer months
in other years; this appears to be a very “quiet” region with little mesoscale eddy activity.
A region of high chlorophyll is repeatedly associated with Rockall Bank, lying across the
boundary between NADR and SARC to the northwest of Scotland. Here, it is known that
upwelling occurs onto the bank. It is clear, from examination of these images, that the
assumption often made that major biological gradients in the North Atlantic do not exist
in the zonal sense, but only in the meridional sense, is incorrect.
The between-year variability now revealed is significant so that the overall patterns
remain, but their sequence differs strongly: the null hypothesis must be variability in
meteorological forcing. Observations made at the BIOTRANS station (47
N 20
W) have
revealed very clearly the instability of the early spring bloom (Koeve et al., 2002). In
April, 1992, when the doubling time of mixed-layer phytoplankton biomass was of order
7 days, the development of a stable, near-surface mixed layer was far from progressive,
but rather was interrupted by episodes of deep mixing during the passage of storms. Heat
flux into the mixed layer was reversed during these episodes, and the loss of cells into the
interior of the ocean represented as much as 60% of total spring bloom production; thus,
the sign of the heat flux across the sea surface was constantly changing during March and
April, and became permanently positive only in May. Reevaluation of the NABE data for
1989 suggest that convective loss followed the same pattern and was equivalent to about
40% of total new production. Model studies of the same phenomenon suggest that where
the Z cr /Z m ratio is relatively low, because of deep winter mixing, the early bloom is very
sensitive to irruptive mixing events and hence to meteorological modulation.
The University of Kiel “Sea Rover” meridional transects along 30
W that were obtained
during spring and summer of 1984–1986 (Strass and Woods, 1991) provide us with
synoptic sections of chlorophyll biomass to 150 m, and very usefully serve support the
satellite data in the recognition of three phytoplankton seasons:
The development of a spring bloom. The first indications (April 18–24) of a bloom
occurred where meandering flow was strongest, across 44–47
N, with a small
southerly outlying patch at about 40
N to the south of this province, in NAST.
Elsewhere, the bloom had not started. By April 24–29, consistent with rapid onset of
density stratification, a near-surface bloom with values >125 mg chl m
−3 extended
clear from 39
to 50
N, a distance of 1500 km. This seems to confirm, then, that
initial stratification (and the first patches of surface bloom) is related to the effect of
increasing irradiance on cells brought near the surface in mesoscale eddy dynamics,
rather than on density stratification, which occurs almost simultaneously across a
rather wide zonal swath of ocean.
Transition to summer oligotrophy. By the end of June, the transition to oligotrophic
conditions had occurred, though patchily, poleward to 46
N. There was residual
nitrate in the photic zone only in small areas having exceptionally shoal pycnoclines,
suggesting an ephemeral event. Maximum chlorophyll values occurred near the OPF
and from there south to 46
N the deep chlorophyll maximum lay at, or shallower
than, the mixed-layer depth. Further to the south yet, nitrate was depleted and
the chlorophyll maximum was significantly deeper than the bottom of the mixed
layer. Oligotrophy propagates poleward during the summer at about 3
of latitude
a month, whereas Z m deepens at about 10 m month
−1 .
Late summer regrowth. By the end of August, everywhere south of the Oceanic Polar
Front, which lay at 52–54
N, a chlorophyll maximum occurred within the seasonal
pycnocline, deepening toward the south at about 3.5 m across each degree of latitude.
Only to the north of the OPF was the chlorophyll maximum within the mixed layer,
and here it took maximal values. Late summer blooms of coccolithophores occur
consistently in the northern part of NADR and are especially frequent bordering