Indian Ocean Coastal Biome
295
Of the larger algae, Trichodesmium and Anabaena form a major fraction of total
biomass, especially in areas where surface nitrate is undetectable, and it is reported that
a gradient exists in the diversity of phytoplankton taxa. Low diversity and high biomass
characterize the estuarized northern region of the Shatt-al-Arab, whereas in the southern
regions off Kuwait, diversity is much higher and biomass lower. It is also reported
that, over the long term, the diversity of dinoflagellates has increased—from 34 species
observed in 1934 to 211 in 1990. In these polluted waters, the probability of toxic blooms
is thought by regional biologists to be high and the valuable penaeid shrimp fisheries to
be at risk.
Characteristic of the Arabian Gulf is the combination of high temperatures and strong
evaporation that leads to the precipitation, as gypsum, of calcium sulfate, and this
may have consequences for our interpretation of chlorophyll images, as noted earlier.
Gypsum precipitation is, of course, the mechanism for the formation of the characteristic
coastal form here, represented by the “sabkha” concretions that form on the inner
shelf and gradually coalesce to enclose extensive coastal lagoons. The extensive shallow
shelf regions are dominated by sandy deposits within which have been recognized local
representatives of the globally distributed Amphiura and Venus isocommunities of benthic
invertebrates. The shallow water of the Gulf makes coupling especially strong between
pelagic production and benthic consumption and there is, in consequence, a diverse
and abundant population of detrital-feeding penaeid shrimps throughout the Gulf; seven
species occur, of which Penaeus semisulcatus is the dominant. Demersal fish are dominated
by sandy-ground forms, of which the lutjanids, lethrinids, and serranids formed 50%
of the pristine standing stock—exactly as occurs on other very low-latitude sandy shelf
regions (Longhurst and Pauly, 1987). The diatom-filtering oil sardine of the coastal
regions of the Arabian Sea appears not to enter these two basins in significant numbers.
Synopsis
Case 5—Large-amplitude response to monsoonlike reversal of trade winds—Mixed-layer seasonality is dominated by boreal winter mixing to just below photic depth from 10–20 m
April–October, to 35 m in February. Since Z eu is consistently between 30 and 40 m, the
thermocline is illuminated in all months except perhaps briefly in January–February.
Seasonal productivity resembles seasonal changes in ARAB, but the SW Monsoon P
peak rate occurs later (October, 09–10 gC m
−2 d
−1 ) and is sustained longer (Fig. 10.3).
Accumulation of chlorophyll occurs only during the onset of the SW Monsoon, but accumulated biomass is sustained longer than in ARAB (June–November). The relationship
between chlorophyll biomass and P rate is consistent with an overall close match between
consumption and production, and also with some increase in consumer biomass after
the onset of the SW Monsoon.
Northwest Arabian Sea Upwelling Province (ARAB)
Extent of the Province
The ARAB province includes the coastal areas from central Kenya (12
S) to Pakistan, and
also the northwest Arabian Sea. This arrangement permits the processes on either flank
of the Southwest Monsoon Jet to be discussed as a single system, especially off the coasts
of Somalia and Oman.
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The African and Arabian coasts of this province have very narrow (< 5–10 km) shelves
above steep continental slopes, relieved only by a small area of shallow banks southwest
295
Of the larger algae, Trichodesmium and Anabaena form a major fraction of total
biomass, especially in areas where surface nitrate is undetectable, and it is reported that
a gradient exists in the diversity of phytoplankton taxa. Low diversity and high biomass
characterize the estuarized northern region of the Shatt-al-Arab, whereas in the southern
regions off Kuwait, diversity is much higher and biomass lower. It is also reported
that, over the long term, the diversity of dinoflagellates has increased—from 34 species
observed in 1934 to 211 in 1990. In these polluted waters, the probability of toxic blooms
is thought by regional biologists to be high and the valuable penaeid shrimp fisheries to
be at risk.
Characteristic of the Arabian Gulf is the combination of high temperatures and strong
evaporation that leads to the precipitation, as gypsum, of calcium sulfate, and this
may have consequences for our interpretation of chlorophyll images, as noted earlier.
Gypsum precipitation is, of course, the mechanism for the formation of the characteristic
coastal form here, represented by the “sabkha” concretions that form on the inner
shelf and gradually coalesce to enclose extensive coastal lagoons. The extensive shallow
shelf regions are dominated by sandy deposits within which have been recognized local
representatives of the globally distributed Amphiura and Venus isocommunities of benthic
invertebrates. The shallow water of the Gulf makes coupling especially strong between
pelagic production and benthic consumption and there is, in consequence, a diverse
and abundant population of detrital-feeding penaeid shrimps throughout the Gulf; seven
species occur, of which Penaeus semisulcatus is the dominant. Demersal fish are dominated
by sandy-ground forms, of which the lutjanids, lethrinids, and serranids formed 50%
of the pristine standing stock—exactly as occurs on other very low-latitude sandy shelf
regions (Longhurst and Pauly, 1987). The diatom-filtering oil sardine of the coastal
regions of the Arabian Sea appears not to enter these two basins in significant numbers.
Synopsis
Case 5—Large-amplitude response to monsoonlike reversal of trade winds—Mixed-layer seasonality is dominated by boreal winter mixing to just below photic depth from 10–20 m
April–October, to 35 m in February. Since Z eu is consistently between 30 and 40 m, the
thermocline is illuminated in all months except perhaps briefly in January–February.
Seasonal productivity resembles seasonal changes in ARAB, but the SW Monsoon P
peak rate occurs later (October, 09–10 gC m
−2 d
−1 ) and is sustained longer (Fig. 10.3).
Accumulation of chlorophyll occurs only during the onset of the SW Monsoon, but accumulated biomass is sustained longer than in ARAB (June–November). The relationship
between chlorophyll biomass and P rate is consistent with an overall close match between
consumption and production, and also with some increase in consumer biomass after
the onset of the SW Monsoon.
Northwest Arabian Sea Upwelling Province (ARAB)
Extent of the Province
The ARAB province includes the coastal areas from central Kenya (12
S) to Pakistan, and
also the northwest Arabian Sea. This arrangement permits the processes on either flank
of the Southwest Monsoon Jet to be discussed as a single system, especially off the coasts
of Somalia and Oman.
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
The African and Arabian coasts of this province have very narrow (< 5–10 km) shelves
above steep continental slopes, relieved only by a small area of shallow banks southwest