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Chapter 9: The Atlantic Ocean
available in the photic zone and a relatively weak late-winter bloom ensues, which is
followed by a long period in which the profile includes a DCM. The western basin
consistently supports a more active algal response to wind stress than the relatively oligotrophic eastern basin. The western winter–spring bloom is patchy, and differs in its
distribution rather significantly between years. For instance, in March 1999 there was a
strong accumulation of chlorophyll between southern France and Corsica that persisted
through the following month. Although the winter–spring bloom in each year tends to
accumulate more chlorophyll along the south coast of France than elsewhere, such a
strong offshore event did not occur in 1998, 2000, or 2001 and only weakly, to the west
of Corsica, in 2002.
It is only in winter months that positive net community production exceeds respiration
so that a winter production pulse (mid-January to mid-February) occurs off Southern
Spain (Rodriguez et al., 1987), both near-surface and subsurface chlorophyll maxima
being dominated by small autotrophic cells. At this season, 80–100% of the biomass
passed a 10-mm mesh and 20–60% passed even a 1-m Nuclepore filter. In the Adriatic
in summer, Revelante and Gilmartin (1994) found a twofold higher biomass of larger
cells in the DCM compared with the rest of the water column, even though picoplankton
formed 50% of the total biomass. By late spring, a DCM is established in both the eastern
and western Mediterranean basins; typical profiles show that this and the nutricline
occurs at the base of the thermocline at 75–80 m, with the topography of the density
surface following geostrophic flow. In anticyclonic features, the DCM generally coincides
with the nitracline rather than with density surfaces, suggesting that primary production
is limited primarily by nutrient supply and only secondarily by light and other factors.
At, or close to, the DCM is the expected layer of abundant zooplankton. All these features
deepen through midsummer, at a rate of about 15–20 m a month (Estrada et al., 1993).
Because deep and intermediate water masses are formed within the Mediterranean
basin by the modification of surface water, subpycnocline nutrient levels are significantly
lower than those in the open ocean, with maximum values in mid-depths of 95 M
at 250 m in the western basin (Coste et al., 1988). In fact, at the Straits of Gibraltar,
the balance between nutrients transported in the incoming and outgoing water masses
translates into a net gain of nutrients for the Mediterranean basin. The presence of
a discrete Atlantic water body in the Alboran Sea, separated across a density gradient
from the shoaler water mass, somewhat complicates observations. Rodriguez (in litt.)
suggests that the variable thickness of the Atlantic-Mediterranean water interface controls
the thickness of the DCM as well as its maximum chlorophyll concentration. In the
eastern basin, Yilmaz (1994) observed the control of DCM formation and maintenance
by variance in nutrient levels and irradiance. DCMs were shallower (50 m) and contained
more chlorophyll in late winter, and were deeper (100 m) and weaker in summer.
An accessory mechanism for vertical transport of nutrients has been described in the
western Mediterranean. Here, diel migrant herbivorous copepods, Centropages typicus,
are observed to feed continuously within the DCM by day but to occur near the surface
at night in food-poor conditions; this must force an active flux of organic nitrogen up
from the DCM into surface water. It has also been shown that individual weather systems
may now induce transient blooms by delivery of nitrate and other nutrients in rainfall:
rain collected in the smoggy northern Adriatic contains <80 M nitrate and <36 M
ammonium (Malej, 1997). The response of the phytoplankton to such anthropogenic
inputs is a dynamic that has been unduly neglected for—given the widespread turbidity
of the lower atmosphere in recent decades—it is surely not restricted to the Adriatic
Sea alone?
The majority of the available chlorophyll images, at all seasons, show significant
enhancement over the wide continental shelf on the eastern coast of Tunisia. This is
strongest just inshore of the small archipelago off Sfax, but in the oligotrophic season it
Chapter 9: The Atlantic Ocean
available in the photic zone and a relatively weak late-winter bloom ensues, which is
followed by a long period in which the profile includes a DCM. The western basin
consistently supports a more active algal response to wind stress than the relatively oligotrophic eastern basin. The western winter–spring bloom is patchy, and differs in its
distribution rather significantly between years. For instance, in March 1999 there was a
strong accumulation of chlorophyll between southern France and Corsica that persisted
through the following month. Although the winter–spring bloom in each year tends to
accumulate more chlorophyll along the south coast of France than elsewhere, such a
strong offshore event did not occur in 1998, 2000, or 2001 and only weakly, to the west
of Corsica, in 2002.
It is only in winter months that positive net community production exceeds respiration
so that a winter production pulse (mid-January to mid-February) occurs off Southern
Spain (Rodriguez et al., 1987), both near-surface and subsurface chlorophyll maxima
being dominated by small autotrophic cells. At this season, 80–100% of the biomass
passed a 10-mm mesh and 20–60% passed even a 1-m Nuclepore filter. In the Adriatic
in summer, Revelante and Gilmartin (1994) found a twofold higher biomass of larger
cells in the DCM compared with the rest of the water column, even though picoplankton
formed 50% of the total biomass. By late spring, a DCM is established in both the eastern
and western Mediterranean basins; typical profiles show that this and the nutricline
occurs at the base of the thermocline at 75–80 m, with the topography of the density
surface following geostrophic flow. In anticyclonic features, the DCM generally coincides
with the nitracline rather than with density surfaces, suggesting that primary production
is limited primarily by nutrient supply and only secondarily by light and other factors.
At, or close to, the DCM is the expected layer of abundant zooplankton. All these features
deepen through midsummer, at a rate of about 15–20 m a month (Estrada et al., 1993).
Because deep and intermediate water masses are formed within the Mediterranean
basin by the modification of surface water, subpycnocline nutrient levels are significantly
lower than those in the open ocean, with maximum values in mid-depths of 95 M
at 250 m in the western basin (Coste et al., 1988). In fact, at the Straits of Gibraltar,
the balance between nutrients transported in the incoming and outgoing water masses
translates into a net gain of nutrients for the Mediterranean basin. The presence of
a discrete Atlantic water body in the Alboran Sea, separated across a density gradient
from the shoaler water mass, somewhat complicates observations. Rodriguez (in litt.)
suggests that the variable thickness of the Atlantic-Mediterranean water interface controls
the thickness of the DCM as well as its maximum chlorophyll concentration. In the
eastern basin, Yilmaz (1994) observed the control of DCM formation and maintenance
by variance in nutrient levels and irradiance. DCMs were shallower (50 m) and contained
more chlorophyll in late winter, and were deeper (100 m) and weaker in summer.
An accessory mechanism for vertical transport of nutrients has been described in the
western Mediterranean. Here, diel migrant herbivorous copepods, Centropages typicus,
are observed to feed continuously within the DCM by day but to occur near the surface
at night in food-poor conditions; this must force an active flux of organic nitrogen up
from the DCM into surface water. It has also been shown that individual weather systems
may now induce transient blooms by delivery of nitrate and other nutrients in rainfall:
rain collected in the smoggy northern Adriatic contains <80 M nitrate and <36 M
ammonium (Malej, 1997). The response of the phytoplankton to such anthropogenic
inputs is a dynamic that has been unduly neglected for—given the widespread turbidity
of the lower atmosphere in recent decades—it is surely not restricted to the Adriatic
Sea alone?
The majority of the available chlorophyll images, at all seasons, show significant
enhancement over the wide continental shelf on the eastern coast of Tunisia. This is
strongest just inshore of the small archipelago off Sfax, but in the oligotrophic season it