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Chapter 9: The Atlantic Ocean
North Sea the spring and autumn peaks may be of about the same magnitude, except in
the mixed region off the Dutch coast where a single summer bloom is the rule. In the
northern North Sea and the western English Channel, the spring bloom is stronger, relative to the autumn bloom, than elsewhere. The Baltic Sea has a double-bloom cycle, and
the autumn bloom may reach even higher chlorophyll values than those during spring
(Kullenberg, 1983). In the Irish Sea, and probably elsewhere, the spring bloom starts in
shallow embayments (as in a recent study of Dundalk Bay) and occurs progressively later
offshore and farther to the north.
Seasonal succession has been followed particularly closely in the western Channel, where Holligan and Harbour (1977) distinguished a near-surface spring bloom
(<4 mg chl m
−3 , 0–15 m, April) from a summer subsurface bloom in the thermocline
(2–4 mg chl m
−3 , 20–25 m, May–September, fueled by regenerated NH 4 ). An autumnal near-surface bloom (<2 mg chl m
−3 , 0–15 m, late September to October) followed.
Diatoms initiated the spring bloom and were abundant until May, when they were
progressively replaced by dinoflagellates and flagellates; this process was completed by
midsummer. In the autumn bloom, diatoms again became important. The spring bloom
of diatoms develops faster than herbivores can increase their consumption rate by population buildup; consequently, much of the plant biomass sediments to the sea floor
to provide at least a part of the regenerated nitrogen utilized by the microalgae of the
summer phytoplankton. Such an imbalance of copepods and diatoms has been observed
in several locations in NECS.
The summer subsurface chlorophyll maximum may be concentrated within a depth
range of only a few meters, as in the Skagerrak, where it has been observed to dome
centrally, following the density contours very precisely around the gyral circulation. In
the Belt Sea, a study by Smetacek et al. (1984) revealed what is probably a typical seasonal
cycle in shoal water. The spring bloom utilized nitrate that had accumulated in winter,
during which large-scale sinking of plant cells occurs. This bloom was followed by an
early summer population maximum of herbivorous zooplankton and consequently very
little sedimentation of plant cells. By midsummer a complex food web had developed,
based largely on regenerated ammonium. Finally, during autumn and after the seasonal
increase in wind strength, a bloom developed, based on nitrate that had accumulated in
subthermocline water during the summer.
Given the diverse range of characteristic situations in large shelf regions such as this, it
is probably not entirely satisfactory to categorize the relative significance of autotrophic
cell fractions. Nevertheless, the relatively new generalization that autotrophic pico- and
nanoplankton are a vital and important component is also valid in these shelf waters.
Joint and Williams (1985) computed that 36% of primary production over the western
shelf is the work of the 0.2 to 10 m cell fraction and that 77% is produced by the 0.2
to 50 m fraction. Even more recently it was suggested that production by autotrophic
picoplankton accounts for 50% of production prior to the spring bloom, but thereafter
the absolute production rate of the fraction changes little, subsequent seasonal increases
in productivity being due almost entirely to cells >2 m.
Much attention has been given in recent years to progressive evolution of ecosystem
structure and functions here, as in the North Pacific: a recent review by Alongi (1999)
is a good introduction to these issues. Here, it is enough to note that it is suggested
that there has been a shift of dominance from diatoms toward dinoflagellates and also to
earlier spring blooms with very persistent summer blooms of dinoflagellates.
The summer profile on the shelf may be typified by that of the Celtic Sea, a two-layered
system in which the upper water is at summer temperatures and the lower is at about 8 or
9
C, or close to winter values. At the interface, in the thermocline, a DCM is associated
with maxima of microflagellates and ciliates. Bacterial biomass is uniformly high in the
upper layer and uniformly low in the lower layer. Mesozooplankton partition this vertical
Chapter 9: The Atlantic Ocean
North Sea the spring and autumn peaks may be of about the same magnitude, except in
the mixed region off the Dutch coast where a single summer bloom is the rule. In the
northern North Sea and the western English Channel, the spring bloom is stronger, relative to the autumn bloom, than elsewhere. The Baltic Sea has a double-bloom cycle, and
the autumn bloom may reach even higher chlorophyll values than those during spring
(Kullenberg, 1983). In the Irish Sea, and probably elsewhere, the spring bloom starts in
shallow embayments (as in a recent study of Dundalk Bay) and occurs progressively later
offshore and farther to the north.
Seasonal succession has been followed particularly closely in the western Channel, where Holligan and Harbour (1977) distinguished a near-surface spring bloom
(<4 mg chl m
−3 , 0–15 m, April) from a summer subsurface bloom in the thermocline
(2–4 mg chl m
−3 , 20–25 m, May–September, fueled by regenerated NH 4 ). An autumnal near-surface bloom (<2 mg chl m
−3 , 0–15 m, late September to October) followed.
Diatoms initiated the spring bloom and were abundant until May, when they were
progressively replaced by dinoflagellates and flagellates; this process was completed by
midsummer. In the autumn bloom, diatoms again became important. The spring bloom
of diatoms develops faster than herbivores can increase their consumption rate by population buildup; consequently, much of the plant biomass sediments to the sea floor
to provide at least a part of the regenerated nitrogen utilized by the microalgae of the
summer phytoplankton. Such an imbalance of copepods and diatoms has been observed
in several locations in NECS.
The summer subsurface chlorophyll maximum may be concentrated within a depth
range of only a few meters, as in the Skagerrak, where it has been observed to dome
centrally, following the density contours very precisely around the gyral circulation. In
the Belt Sea, a study by Smetacek et al. (1984) revealed what is probably a typical seasonal
cycle in shoal water. The spring bloom utilized nitrate that had accumulated in winter,
during which large-scale sinking of plant cells occurs. This bloom was followed by an
early summer population maximum of herbivorous zooplankton and consequently very
little sedimentation of plant cells. By midsummer a complex food web had developed,
based largely on regenerated ammonium. Finally, during autumn and after the seasonal
increase in wind strength, a bloom developed, based on nitrate that had accumulated in
subthermocline water during the summer.
Given the diverse range of characteristic situations in large shelf regions such as this, it
is probably not entirely satisfactory to categorize the relative significance of autotrophic
cell fractions. Nevertheless, the relatively new generalization that autotrophic pico- and
nanoplankton are a vital and important component is also valid in these shelf waters.
Joint and Williams (1985) computed that 36% of primary production over the western
shelf is the work of the 0.2 to 10 m cell fraction and that 77% is produced by the 0.2
to 50 m fraction. Even more recently it was suggested that production by autotrophic
picoplankton accounts for 50% of production prior to the spring bloom, but thereafter
the absolute production rate of the fraction changes little, subsequent seasonal increases
in productivity being due almost entirely to cells >2 m.
Much attention has been given in recent years to progressive evolution of ecosystem
structure and functions here, as in the North Pacific: a recent review by Alongi (1999)
is a good introduction to these issues. Here, it is enough to note that it is suggested
that there has been a shift of dominance from diatoms toward dinoflagellates and also to
earlier spring blooms with very persistent summer blooms of dinoflagellates.
The summer profile on the shelf may be typified by that of the Celtic Sea, a two-layered
system in which the upper water is at summer temperatures and the lower is at about 8 or
9
C, or close to winter values. At the interface, in the thermocline, a DCM is associated
with maxima of microflagellates and ciliates. Bacterial biomass is uniformly high in the
upper layer and uniformly low in the lower layer. Mesozooplankton partition this vertical