Atlantic Coastal Biome
229
new production there. But Barber and Smith (1981) remind us that we should think not
only about new production fueled by upwelled nitrate in eastern boundary currents but
also about nitrogen regenerated there as NH 4 . Because of the episodic nature of upwelling
pulses, and hence of blooms, much of the pelagic organic matter that is produced sinks
unconsumed to the sediments. Here, in the Canary Current, the relatively high wind
stress and strong equatorward and cross-shelf currents prevent accumulation and ensure
that benthic utilization and remineralization rates are high. A further consequence of
the relatively high wind stress and deep wind mixing is that only weak near-surface
stratification develops during the immediate post-upwelling production phase of each
episode. In the other eastern boundary currents, accumulating phytoplankton cells are
more frequently mixed down, even below the euphotic zone, and their concentration
diluted. One of the findings of the CUEA project, in which the Oregon, Peruvian, and
northwestern African regions were compared, was that because of high wind stress,
primary production rates are relatively lower in this province than off the western coasts
of America (Barber and Smith, 1981).
The giant filament south of Cape Blanc, reaching as much as 400 km offshore, may
appear as a persistent chlorophyll feature or it may be seen principally in the temperature
field. To explain the appearance of a chlorophyll feature so far offshore, Gabric et al.
(1993) proposed a mechanism for the conservation of nutrients in the upwelled water as
it advected to the west. For this to occur, the algal cells seeding the upwelling must be
entrained from below the photic layer when upwelling occurs over the shelf break: in this
event, a delay occurs before a bloom can be initiated, as the cells undergo physiological
conditioning to a photic environment. Probably more typical, however, is the sequence
observed by Joint et al. (2001b) during two 5-day Lagrangian drift studies in a filament
at the shelf break off northern Portugal; during the first 5-day period, within the body
of the filament as it drifted along the shelf edge, nutrients were progressively reduced;
this occurred first at the surface, then progressively deeper as a DCM was established
and as a shift occurred from small flagellates and dinoflagellates to a diatom-dominated
community; during this period the f-ratio declined from 0.7 to 0.5. The second 5-day
drift followed the head of the filament as it moved out over the deep ocean, when it
comprised extremely oligotrophic water, in which NH 4 concentration exceeded NO 3 ,
and which was dominated by autotrophic picoplankton that were responsible for 65% of
total production, diatoms being almost absent. Bacterial biomass remained little changed
during both drifts, but growth rates were significantly higher in the first than in the
second experiment.
Because the Canary Current, to the north of Cape Blanc, is not compensated by a
countercurrent over the shelf break, offshore advection in the resulting single upwelling
cell is sufficiently rapid (a residence time of about 10 days) that ecological succession is
significantly lagged. Consequently, the processes of production and consumption are not
tightly linked and herbivore development lags the growth of phytoplankton biomass by
several weeks. Thus, the rate of remineralization lags the rate of new production of algal
biomass, and diatom biomass is therefore continually lost by sedimentation. Further, the
rate of excretion of N and P by herbivores is insufficiently matched by excretion rate
of Si, so that diatom growth is limited by the upwelling supply of this latter element.
South of Cape Blanc, during the winter upwelling season, the poleward flow of the
NECC performs as a shelf-break countercurrent beyond the southward, upwelling flow
of Canary Current water. Two upwelling cells are thus formed of which the inner, over
the shelf, is partially closed so that it retains the elements of the pelagic ecosystem and
reduces offshore advection. In this way, primary and secondary production within the
shelf break are coupled more effectively than in any regions to the north of Cape Blanc.
In a classical series of papers, Margalef suggested that in upwelling regions the phytoplankton species associations are arranged sequentially (and successionally) around each
229
new production there. But Barber and Smith (1981) remind us that we should think not
only about new production fueled by upwelled nitrate in eastern boundary currents but
also about nitrogen regenerated there as NH 4 . Because of the episodic nature of upwelling
pulses, and hence of blooms, much of the pelagic organic matter that is produced sinks
unconsumed to the sediments. Here, in the Canary Current, the relatively high wind
stress and strong equatorward and cross-shelf currents prevent accumulation and ensure
that benthic utilization and remineralization rates are high. A further consequence of
the relatively high wind stress and deep wind mixing is that only weak near-surface
stratification develops during the immediate post-upwelling production phase of each
episode. In the other eastern boundary currents, accumulating phytoplankton cells are
more frequently mixed down, even below the euphotic zone, and their concentration
diluted. One of the findings of the CUEA project, in which the Oregon, Peruvian, and
northwestern African regions were compared, was that because of high wind stress,
primary production rates are relatively lower in this province than off the western coasts
of America (Barber and Smith, 1981).
The giant filament south of Cape Blanc, reaching as much as 400 km offshore, may
appear as a persistent chlorophyll feature or it may be seen principally in the temperature
field. To explain the appearance of a chlorophyll feature so far offshore, Gabric et al.
(1993) proposed a mechanism for the conservation of nutrients in the upwelled water as
it advected to the west. For this to occur, the algal cells seeding the upwelling must be
entrained from below the photic layer when upwelling occurs over the shelf break: in this
event, a delay occurs before a bloom can be initiated, as the cells undergo physiological
conditioning to a photic environment. Probably more typical, however, is the sequence
observed by Joint et al. (2001b) during two 5-day Lagrangian drift studies in a filament
at the shelf break off northern Portugal; during the first 5-day period, within the body
of the filament as it drifted along the shelf edge, nutrients were progressively reduced;
this occurred first at the surface, then progressively deeper as a DCM was established
and as a shift occurred from small flagellates and dinoflagellates to a diatom-dominated
community; during this period the f-ratio declined from 0.7 to 0.5. The second 5-day
drift followed the head of the filament as it moved out over the deep ocean, when it
comprised extremely oligotrophic water, in which NH 4 concentration exceeded NO 3 ,
and which was dominated by autotrophic picoplankton that were responsible for 65% of
total production, diatoms being almost absent. Bacterial biomass remained little changed
during both drifts, but growth rates were significantly higher in the first than in the
second experiment.
Because the Canary Current, to the north of Cape Blanc, is not compensated by a
countercurrent over the shelf break, offshore advection in the resulting single upwelling
cell is sufficiently rapid (a residence time of about 10 days) that ecological succession is
significantly lagged. Consequently, the processes of production and consumption are not
tightly linked and herbivore development lags the growth of phytoplankton biomass by
several weeks. Thus, the rate of remineralization lags the rate of new production of algal
biomass, and diatom biomass is therefore continually lost by sedimentation. Further, the
rate of excretion of N and P by herbivores is insufficiently matched by excretion rate
of Si, so that diatom growth is limited by the upwelling supply of this latter element.
South of Cape Blanc, during the winter upwelling season, the poleward flow of the
NECC performs as a shelf-break countercurrent beyond the southward, upwelling flow
of Canary Current water. Two upwelling cells are thus formed of which the inner, over
the shelf, is partially closed so that it retains the elements of the pelagic ecosystem and
reduces offshore advection. In this way, primary and secondary production within the
shelf break are coupled more effectively than in any regions to the north of Cape Blanc.
In a classical series of papers, Margalef suggested that in upwelling regions the phytoplankton species associations are arranged sequentially (and successionally) around each