Indian Ocean Coastal Biome
293
Surface drift occurs in response to the strong wind events that sweep the Gulf, especially
northeast Shamal winds along the axis of the gulf that break down the density stratification
to at least 60 m and temporarily destroy the mean cyclonic circulation.
Regional Response of the Pelagic Ecosystem
These two basins are both, in their own ways, extreme habitats for the organisms of
the pelagic ecosystem. Nevertheless, they remain quite productive and, as Sheppard and
Dixon (1998) suggest, for this reason they are very useful laboratories for the study of
environmental stress on the marine organism.
Salinity >49‰ is lethal to planktonic organisms, and because the concentrations just
quoted are beginning to approach that level, it is not surprising that periods when there
was apparently no planktonic ecosystem should be recorded in the sediments of the
Gulf of Aqaba (Fenton et al., 2000). Further, given the extreme water clarity at some
seasons, it is not surprising that UVBR-induced DNA damage should occur in bacterioand phytoplankton; in the upper 15 m of the water column, DNA damage to such cells
accumulates during the daytime is not entirely repaired during the hours of darkness.
This suggests that photomortality is a potential loss parameter among the picofraction of
autotrophs (Boelen et al., 2002).
The single significant source of new dissolved nutrients in the Red Sea is the surface
water flowing in from the Gulf of Aden, though tidal mixing may transport regenerated
nutrients into the photic zone. In the two northern gulfs (Suez and Eilat-Aqaba) local
inputs of industrial effluents have induced local blooms. Over most of the Red Sea,
the strong pycnocline is an effective and permanent barrier to the vertical mixing of
nutrients from below. Consequently, the overall productivity of the mixed layer decreases
progressively northward.
Offshore Red Sea water has great clarity and primary production is generally extremely
low (about 100 mgC m
−2 day
−1 ), though locally, in coastal situations, rates may be higher.
It is only in the southern region that there appear to be significant offshore algal blooms;
the chlorophyll images from CZCS to MODIS all agree that the entire southern area
of wide shallow shelf (that is, south of about Massawa) has relatively high surface
chlorophyll during the Southwest Monsoon. Highest chlorophyll concentration occurs
over the shelves on both coastlines, but during the boreal winter over the southern
Red Sea surface chlorophyll is characteristically >10 mg m
−3 , although over the adjacent
shelves it is somewhat higher. In the summer, rather elevated chlorophyll extends farther
north and shows evidence of enhancement around major eddy features. Generally, the
coastal regions always show slightly higher chlorophyll compared with the central region
down the axis of the Red Sea, and a curious feature has been described at the coast
(Niemann et al., 2004): in the Gulf of Aqaba, the intense cooling that is induced by
offshore breeze in the winter cools surface water sufficiently that it sinks down submarine
gullies at the shelf edge, carrying with it the surface phytoplankton to depths of >300 m.
Such gravity current cascades may provide a previously overlooked pathway of nutrients
to the deeper zooplankton. It should also be noted, however, that the extensive coastal
coral reefs, especially on the southern shelves, support major consumers of phytoplankton
cells, the herbivorous soft corals; surface water masses that have passed across such reefs
are phytoplankton-deficient.
In the extreme north, in the Gulf of Aqaba, a strong spring bloom has been described
with a weaker autumn bloom (Labiosa et al., 2003). Here, a gradient of chlorophyll
concentration across the Gulf is attributable to upwelling on the eastern side that appears
to be a stronger source of nutrients than convective entrainment.
Though the Red Sea has an anomalous density profile, the profiles of algal biomass
and nutrient distribution closely resemble the typical tropical situation with a significant DCM (Weikert, 1987). Picoplankton (02–20 m) are the dominant size fraction of
293
Surface drift occurs in response to the strong wind events that sweep the Gulf, especially
northeast Shamal winds along the axis of the gulf that break down the density stratification
to at least 60 m and temporarily destroy the mean cyclonic circulation.
Regional Response of the Pelagic Ecosystem
These two basins are both, in their own ways, extreme habitats for the organisms of
the pelagic ecosystem. Nevertheless, they remain quite productive and, as Sheppard and
Dixon (1998) suggest, for this reason they are very useful laboratories for the study of
environmental stress on the marine organism.
Salinity >49‰ is lethal to planktonic organisms, and because the concentrations just
quoted are beginning to approach that level, it is not surprising that periods when there
was apparently no planktonic ecosystem should be recorded in the sediments of the
Gulf of Aqaba (Fenton et al., 2000). Further, given the extreme water clarity at some
seasons, it is not surprising that UVBR-induced DNA damage should occur in bacterioand phytoplankton; in the upper 15 m of the water column, DNA damage to such cells
accumulates during the daytime is not entirely repaired during the hours of darkness.
This suggests that photomortality is a potential loss parameter among the picofraction of
autotrophs (Boelen et al., 2002).
The single significant source of new dissolved nutrients in the Red Sea is the surface
water flowing in from the Gulf of Aden, though tidal mixing may transport regenerated
nutrients into the photic zone. In the two northern gulfs (Suez and Eilat-Aqaba) local
inputs of industrial effluents have induced local blooms. Over most of the Red Sea,
the strong pycnocline is an effective and permanent barrier to the vertical mixing of
nutrients from below. Consequently, the overall productivity of the mixed layer decreases
progressively northward.
Offshore Red Sea water has great clarity and primary production is generally extremely
low (about 100 mgC m
−2 day
−1 ), though locally, in coastal situations, rates may be higher.
It is only in the southern region that there appear to be significant offshore algal blooms;
the chlorophyll images from CZCS to MODIS all agree that the entire southern area
of wide shallow shelf (that is, south of about Massawa) has relatively high surface
chlorophyll during the Southwest Monsoon. Highest chlorophyll concentration occurs
over the shelves on both coastlines, but during the boreal winter over the southern
Red Sea surface chlorophyll is characteristically >10 mg m
−3 , although over the adjacent
shelves it is somewhat higher. In the summer, rather elevated chlorophyll extends farther
north and shows evidence of enhancement around major eddy features. Generally, the
coastal regions always show slightly higher chlorophyll compared with the central region
down the axis of the Red Sea, and a curious feature has been described at the coast
(Niemann et al., 2004): in the Gulf of Aqaba, the intense cooling that is induced by
offshore breeze in the winter cools surface water sufficiently that it sinks down submarine
gullies at the shelf edge, carrying with it the surface phytoplankton to depths of >300 m.
Such gravity current cascades may provide a previously overlooked pathway of nutrients
to the deeper zooplankton. It should also be noted, however, that the extensive coastal
coral reefs, especially on the southern shelves, support major consumers of phytoplankton
cells, the herbivorous soft corals; surface water masses that have passed across such reefs
are phytoplankton-deficient.
In the extreme north, in the Gulf of Aqaba, a strong spring bloom has been described
with a weaker autumn bloom (Labiosa et al., 2003). Here, a gradient of chlorophyll
concentration across the Gulf is attributable to upwelling on the eastern side that appears
to be a stronger source of nutrients than convective entrainment.
Though the Red Sea has an anomalous density profile, the profiles of algal biomass
and nutrient distribution closely resemble the typical tropical situation with a significant DCM (Weikert, 1987). Picoplankton (02–20 m) are the dominant size fraction of