302
Chapter 10: The Indian Ocean
of consumption rates and nutrient limitation in determining biomass of autotrophic cells
is not yet resolved.
Mesozooplankton (>200 m) biomass is greatest in the coastal regions and during the monsoons, especially the summer Southwest Monsoon when biomass reaches
400 mg C m
−2 requiring a consumption of <150 mg C m
−2 d
−1 for maintenance (Roman
et al., 2000). Such ingestion rates are relatively high compared to measured rates in
eutrophic regimes elsewhere. This upwelling province is characterized by the abundant
presence of the ontogenetic migrant Calanoides carinatus, which has very little diel migration activity but has a seasonal ecological cycle compatible with observations made on
the same species off West Africa (see Chapter 9, GUIN) and elsewhere. C. carinatus,
together with two other large copepods (Eucalanus monachus and E. crassus), dominates
the epipelagic mesoplankton in water <25
C, during the southwest monsoon; at this season the abundance of these species is inversely related to water temperature (Smith, 1982;
Smith et al., 1998b). In the Somali upwelling centers at 4–6
and 10–12
N, C. carinatus
has been observed at concentrations of 6000–9000 ind m
−2 , but between these centers
<1000 ind m
−2 have been found. During the intermonsoons and the Northeast Monsoon,
these species are sparse at depths shoaler than 200 m.
Both females and progressively older copepodites of Calanoides occur together in
upwelled water in a spatial sequence in ageing upwelled water, which indicates that reproduction occurs there. Generation time is probably of the order of 25 days, and this suggests
that no more than a single generation will occur before the population is forced below the
surface by rising temperature as each parcel of upwelled water is advected horizontally.
Smith suggests that subsurface circulation is apt to retain resting, lipid-replete stage 5
copepodites (C5s) at>500 m in a trajectory that will position them to return to the surface
in a succeeding upwelling cell with reproductive products ready to mature. Calanoides
consumes small diatoms in the range 25–75 m (Rhizoselenia, Nitzschia, Eucampia, etc.),
and Smith (1984) computes the diatom consumption of recently upwelled Calanoides
as 25–45% of the lower primary production nearshore but only 1–15% of the higher
daily rate offshore. Such consumption represents an ingestion rate of about 50% of body
carbon daily, necessary to support the observed growth rate: ∼25% of body weight daily
during the juvenile instars that leads to generation times of ∼15 days from egg to adult.
Egg production rate is also very high, and as many as 70 eggs are produced daily by each
female. Lipid content is as high as 45% in stage 5 copepodites that are already several
months into their period of diapause: this copepod is magnificently adapted to life in
seasonal coastal upwelling blooms. Some species of Eucalanus may likewise be very well
adapted to such situations, being able to withstand oxygen stress for long periods: off
Somalia (and presumably also off Oman) these covary with C. carinatus. Maximal concentrations of Eucalanus monachus (90 ind m
−3 ) and E. crassus (75 ind m
−3 ) may indeed
rival the numerical dominance of C. carinatus (170 ind m
−3 ). Nevertheless, Calanoides
is more sensitive to increasing water temperature than Eucalanus, for which reason it
descends earlier into diapause.
The existence of blooms during both the Southwest and the Northeast Monsoons suggest that Calanoides will have a more complex life history here than in simpler upwelling
situations. It remains to be seen if this involves a complex pattern of subpopulations
specializing in each regional, seasonal bloom or whether (more likely) the deep subthermocline water of the Arabian Sea everywhere carries a sufficient population of resting C5s
to seed any upwelling parcel of water. The fact that blooms occur in both seasons perhaps
also explains a paradox of the Arabian Sea mesozooplankton: that there is relatively very
low variability in overall plankton biomass between seasons (Madhupratap et al., 1996).
Although I have found no reference to modern investigations of the ecology of
euphausiids in this province, they must be a major component of the pelagic ecosystem,
as indeed is suggested by Brinton’s IIOE maps of euphausiid distribution. The JGOFS
Chapter 10: The Indian Ocean
of consumption rates and nutrient limitation in determining biomass of autotrophic cells
is not yet resolved.
Mesozooplankton (>200 m) biomass is greatest in the coastal regions and during the monsoons, especially the summer Southwest Monsoon when biomass reaches
400 mg C m
−2 requiring a consumption of <150 mg C m
−2 d
−1 for maintenance (Roman
et al., 2000). Such ingestion rates are relatively high compared to measured rates in
eutrophic regimes elsewhere. This upwelling province is characterized by the abundant
presence of the ontogenetic migrant Calanoides carinatus, which has very little diel migration activity but has a seasonal ecological cycle compatible with observations made on
the same species off West Africa (see Chapter 9, GUIN) and elsewhere. C. carinatus,
together with two other large copepods (Eucalanus monachus and E. crassus), dominates
the epipelagic mesoplankton in water <25
C, during the southwest monsoon; at this season the abundance of these species is inversely related to water temperature (Smith, 1982;
Smith et al., 1998b). In the Somali upwelling centers at 4–6
and 10–12
N, C. carinatus
has been observed at concentrations of 6000–9000 ind m
−2 , but between these centers
<1000 ind m
−2 have been found. During the intermonsoons and the Northeast Monsoon,
these species are sparse at depths shoaler than 200 m.
Both females and progressively older copepodites of Calanoides occur together in
upwelled water in a spatial sequence in ageing upwelled water, which indicates that reproduction occurs there. Generation time is probably of the order of 25 days, and this suggests
that no more than a single generation will occur before the population is forced below the
surface by rising temperature as each parcel of upwelled water is advected horizontally.
Smith suggests that subsurface circulation is apt to retain resting, lipid-replete stage 5
copepodites (C5s) at>500 m in a trajectory that will position them to return to the surface
in a succeeding upwelling cell with reproductive products ready to mature. Calanoides
consumes small diatoms in the range 25–75 m (Rhizoselenia, Nitzschia, Eucampia, etc.),
and Smith (1984) computes the diatom consumption of recently upwelled Calanoides
as 25–45% of the lower primary production nearshore but only 1–15% of the higher
daily rate offshore. Such consumption represents an ingestion rate of about 50% of body
carbon daily, necessary to support the observed growth rate: ∼25% of body weight daily
during the juvenile instars that leads to generation times of ∼15 days from egg to adult.
Egg production rate is also very high, and as many as 70 eggs are produced daily by each
female. Lipid content is as high as 45% in stage 5 copepodites that are already several
months into their period of diapause: this copepod is magnificently adapted to life in
seasonal coastal upwelling blooms. Some species of Eucalanus may likewise be very well
adapted to such situations, being able to withstand oxygen stress for long periods: off
Somalia (and presumably also off Oman) these covary with C. carinatus. Maximal concentrations of Eucalanus monachus (90 ind m
−3 ) and E. crassus (75 ind m
−3 ) may indeed
rival the numerical dominance of C. carinatus (170 ind m
−3 ). Nevertheless, Calanoides
is more sensitive to increasing water temperature than Eucalanus, for which reason it
descends earlier into diapause.
The existence of blooms during both the Southwest and the Northeast Monsoons suggest that Calanoides will have a more complex life history here than in simpler upwelling
situations. It remains to be seen if this involves a complex pattern of subpopulations
specializing in each regional, seasonal bloom or whether (more likely) the deep subthermocline water of the Arabian Sea everywhere carries a sufficient population of resting C5s
to seed any upwelling parcel of water. The fact that blooms occur in both seasons perhaps
also explains a paradox of the Arabian Sea mesozooplankton: that there is relatively very
low variability in overall plankton biomass between seasons (Madhupratap et al., 1996).
Although I have found no reference to modern investigations of the ecology of
euphausiids in this province, they must be a major component of the pelagic ecosystem,
as indeed is suggested by Brinton’s IIOE maps of euphausiid distribution. The JGOFS