290
Chapter 10: The Indian Ocean
suggested, the required coincidence between elements of the chlorophyll field (lenses,
whirls) and the SLA field (anticyclonic elevation, cyclonic depression).
Shallow Topography of the Central Regions Finally, the serial chlorophyll images
clearly reveal discrete seasonal blooms that are associated with the shallow banks
(Nazareth, Hawkins, and Saya de Malha banks on the Mauritius-Seychelles Ridge, 5–20
S)
and down the Chagos archipelago, especially during austral winter. This appears to confirm previous suggestions that the Mauritius-Seychelles Ridge causes divergence in the
SEC during this season, leading to nutrient enrichment of surface water on its western
side; similar effects occur at the Seychelles themselves, which lie athwart the thermal ridge
at 10
S (Ragoonaden et al., 1987; Vethamony et al., 1987).
However, these banks are sandy flats that have depths of only 15–60 m and lie atop the
topography that rises from abyssal depths. The flats are dotted with coral heads but not
occupied by massive reef-coral development as elsewhere, and in their shallower regions
they support very extensive sea-grass meadows. Examination of 4-km MODIS images
confirms that the regions of enhanced chlorophyll associated with these midocean banks
have a permanent “core” that represents the shape of the banks themselves, surrounded
by a variable “aura” of apparently lower chlorophyll biomass. This is especially evident on
the Chagos banks where central, permanent patches of apparently rather high chlorophyll
biomass are observed.
We have very little organized knowledge of the ecology of the Indian Ocean banks,
beyond surveys done for fish resource assessment prior to exploitation: these are reviewed
by Longhurst and Pauly (1987). Obviously, the fish fauna is dominated by hard-substrate
genera: Serranidae, Lutjanidae, Lethrinidae, and Labridae.
Synopsis
Case 3—Winter-spring production with nutrient limitation—Z m undergoes an austral winter excursion (20–25 m December–January, 75 m September) and since Z eu is always at
50–70 m, the thermocline is illuminated in all except 3 winter months, July–September.
P responds to the period (June–August) of nutrient input due to winter deepening of
the mixed layer with a sustained rate increase, though over a small dynamic range to
an annual maximum in austral spring (August–October). The period of decreasing P
rate (November–August) to the summer minimum coincides with period when Z eu lies
within thermocline. Chlorophyll biomass takes a very low dynamic range, with only a
brief (June–July) period of accumulation to slightly higher (020 mgC m
−3 ) biomass from
summer low biomass of 005 mgC m
−3 (Fig. 10.2). Failure to sustain chlorophyll accumulation after the initial increase in productivity is consistent with vertical migration or
other response of herbivores.
Indian Ocean Coastal Biome
Red Sea, Arabian Gulf Province (REDS)
Extent of the Province
This province comprises the Red Sea north of the Straits of Bab-el-Mandeb and the
Arabian Gulf within the Straits of Hormuz. Note that in older literature, what is now
generally called the Arabian Gulf may be given as the Persian Gulf. The Gulfs of Aden
and Oman are considered to be a part of the adjacent coastal boundary province (ARAB).
Chapter 10: The Indian Ocean
suggested, the required coincidence between elements of the chlorophyll field (lenses,
whirls) and the SLA field (anticyclonic elevation, cyclonic depression).
Shallow Topography of the Central Regions Finally, the serial chlorophyll images
clearly reveal discrete seasonal blooms that are associated with the shallow banks
(Nazareth, Hawkins, and Saya de Malha banks on the Mauritius-Seychelles Ridge, 5–20
S)
and down the Chagos archipelago, especially during austral winter. This appears to confirm previous suggestions that the Mauritius-Seychelles Ridge causes divergence in the
SEC during this season, leading to nutrient enrichment of surface water on its western
side; similar effects occur at the Seychelles themselves, which lie athwart the thermal ridge
at 10
S (Ragoonaden et al., 1987; Vethamony et al., 1987).
However, these banks are sandy flats that have depths of only 15–60 m and lie atop the
topography that rises from abyssal depths. The flats are dotted with coral heads but not
occupied by massive reef-coral development as elsewhere, and in their shallower regions
they support very extensive sea-grass meadows. Examination of 4-km MODIS images
confirms that the regions of enhanced chlorophyll associated with these midocean banks
have a permanent “core” that represents the shape of the banks themselves, surrounded
by a variable “aura” of apparently lower chlorophyll biomass. This is especially evident on
the Chagos banks where central, permanent patches of apparently rather high chlorophyll
biomass are observed.
We have very little organized knowledge of the ecology of the Indian Ocean banks,
beyond surveys done for fish resource assessment prior to exploitation: these are reviewed
by Longhurst and Pauly (1987). Obviously, the fish fauna is dominated by hard-substrate
genera: Serranidae, Lutjanidae, Lethrinidae, and Labridae.
Synopsis
Case 3—Winter-spring production with nutrient limitation—Z m undergoes an austral winter excursion (20–25 m December–January, 75 m September) and since Z eu is always at
50–70 m, the thermocline is illuminated in all except 3 winter months, July–September.
P responds to the period (June–August) of nutrient input due to winter deepening of
the mixed layer with a sustained rate increase, though over a small dynamic range to
an annual maximum in austral spring (August–October). The period of decreasing P
rate (November–August) to the summer minimum coincides with period when Z eu lies
within thermocline. Chlorophyll biomass takes a very low dynamic range, with only a
brief (June–July) period of accumulation to slightly higher (020 mgC m
−3 ) biomass from
summer low biomass of 005 mgC m
−3 (Fig. 10.2). Failure to sustain chlorophyll accumulation after the initial increase in productivity is consistent with vertical migration or
other response of herbivores.
Indian Ocean Coastal Biome
Red Sea, Arabian Gulf Province (REDS)
Extent of the Province
This province comprises the Red Sea north of the Straits of Bab-el-Mandeb and the
Arabian Gulf within the Straits of Hormuz. Note that in older literature, what is now
generally called the Arabian Gulf may be given as the Persian Gulf. The Gulfs of Aden
and Oman are considered to be a part of the adjacent coastal boundary province (ARAB).