Indian Ocean Trade Wind Biome
283
Sections taken across the northern part of the Arabian Sea in September and November
show that the dominance of large cells (>18 m), mostly diatoms, does not extend beyond
the coastal upwelling regions. Across the open ocean, during monsoon and intermonsoon
periods, even when chlorophyll biomass was approximately equally distributed between
phytoplankton size fractions there was, in general, a maximum of production in the smallest (02–20 m) class (Savidge and Gilpin, 1999). Nevertheless, phytoplankton biomass
dominated by the prokaryote fraction, among which Synechococcus was particularly abundant, reaching >10
8 cells liter
−1 . During the intermonsoon period, prochlorophytes
increased in relative abundance, and in the center of the gyre, where conditions became
extremely oligotrophic, these cells dominated the autotrophic community. Prochlorophytes appear to be preferentially distributed within a secondary fluorescence maximum
at the oxic-anoxic interface at the bottom of the euphotic zone, and well below the depth
of the primary chlorophyll maximum (Johnson et al., 1999). Here, the population was
composed almost exclusively of Prochlorococccus spp. This population is strongly light
adapted, having instantaneous growth rates of order = 0001 d
−1 . Even shallower-living
populations of Prochlorococccus have doubling times of less than once daily, whereas
Synechococcus have maximum growth rates of >2 d
−1 that respond rapidly to nutrient
availability.
During this intermonsoon season, toward the center of the gyre, coccolithophores also
become more relatively abundant than elsewhere (Tarran et al., 1999). At this season,
also, the chlorophyll profile more frequently exhibits a pigment maximum at or close
to the nutricline at around 75 m, and deeper than the top of the pycnocline, rather
than within the mixed layer above. In the central gyre, producer and consumer biomass
covaries seasonally within each of the pico, nano, and micro fractions.
During the Southwest and Northeast Monsoons, photosynthetic prokaryotes were
responsible for ∼25% and ∼50% of productivity, respectively, and whereas the growth
of Prochlorococccus is in balance with its consumption by protests, that of Synechococcus
largely escapes consumption, >50% compounding daily during the Southwest Monsoon
(Brown et al., 2002).
Among the consumer groups, including herbivores, the nano- and microzooplankton
fractions form a relatively constant biomass while mesozooplankton biomass fluctuates
more strongly with the seasons and responds to the onset of upwelling (Stelfox et al.,
1999). Along two sonar transects across the entire Arabian Sea, Luo et al. (2000) found
a fivefold increase in sonic scattering from zooplankton and mesopelagic fish during the
Southwest Monsoon compared with the spring intermonsoon. Heterotrophic nanoflagellate biomass (<855 cells ml
−1 ) exceeded that of the microheterotroph biomass, dominated
by aloricate ciliates and dinoflagellates (<333 cells ml
−1 ). These organisms were an order
of magnitude more abundant than sarcodines and crustacean nauplii. During and after
the Southwest Monsoon of 1994, in the open gyre, consumption rates of microzooplankton grazing represented up to 20–30% of daily primary production that ranged from 1.0
to 15 gC m
−2 d
−1 .
During the intermonsoons, daily consumption represents 30–50% of a rather smaller
productivity (03–08 gC m
−2 d
−1 ). Mesozooplankton herbivory was judged by Stelfox
et al. to represent around 25–30% of that of microzooplankton. The grazing dynamics of
mesoplankton were addressed independently by Roman et al. (2000), who found, overall,
that the copepod mesozooplankton (dominated by ∼50 species of copepods) represented
a biomass in the range of 10–50 mgC m
−2 that ingested 10–40 mgC m
−2 of food daily,
representing 10–60% of daily depth-integrated primary production. Lowest ratios of
ingestion/primary production were found during the Northeast Monsoon. The seasonal
response of copepod biomass to autotrophic productivity was rapid, so that in the central
gyre during the spring intermonsoon this was of order 20–50 mgC m
−2 , whereas during
283
Sections taken across the northern part of the Arabian Sea in September and November
show that the dominance of large cells (>18 m), mostly diatoms, does not extend beyond
the coastal upwelling regions. Across the open ocean, during monsoon and intermonsoon
periods, even when chlorophyll biomass was approximately equally distributed between
phytoplankton size fractions there was, in general, a maximum of production in the smallest (02–20 m) class (Savidge and Gilpin, 1999). Nevertheless, phytoplankton biomass
dominated by the prokaryote fraction, among which Synechococcus was particularly abundant, reaching >10
8 cells liter
−1 . During the intermonsoon period, prochlorophytes
increased in relative abundance, and in the center of the gyre, where conditions became
extremely oligotrophic, these cells dominated the autotrophic community. Prochlorophytes appear to be preferentially distributed within a secondary fluorescence maximum
at the oxic-anoxic interface at the bottom of the euphotic zone, and well below the depth
of the primary chlorophyll maximum (Johnson et al., 1999). Here, the population was
composed almost exclusively of Prochlorococccus spp. This population is strongly light
adapted, having instantaneous growth rates of order = 0001 d
−1 . Even shallower-living
populations of Prochlorococccus have doubling times of less than once daily, whereas
Synechococcus have maximum growth rates of >2 d
−1 that respond rapidly to nutrient
availability.
During this intermonsoon season, toward the center of the gyre, coccolithophores also
become more relatively abundant than elsewhere (Tarran et al., 1999). At this season,
also, the chlorophyll profile more frequently exhibits a pigment maximum at or close
to the nutricline at around 75 m, and deeper than the top of the pycnocline, rather
than within the mixed layer above. In the central gyre, producer and consumer biomass
covaries seasonally within each of the pico, nano, and micro fractions.
During the Southwest and Northeast Monsoons, photosynthetic prokaryotes were
responsible for ∼25% and ∼50% of productivity, respectively, and whereas the growth
of Prochlorococccus is in balance with its consumption by protests, that of Synechococcus
largely escapes consumption, >50% compounding daily during the Southwest Monsoon
(Brown et al., 2002).
Among the consumer groups, including herbivores, the nano- and microzooplankton
fractions form a relatively constant biomass while mesozooplankton biomass fluctuates
more strongly with the seasons and responds to the onset of upwelling (Stelfox et al.,
1999). Along two sonar transects across the entire Arabian Sea, Luo et al. (2000) found
a fivefold increase in sonic scattering from zooplankton and mesopelagic fish during the
Southwest Monsoon compared with the spring intermonsoon. Heterotrophic nanoflagellate biomass (<855 cells ml
−1 ) exceeded that of the microheterotroph biomass, dominated
by aloricate ciliates and dinoflagellates (<333 cells ml
−1 ). These organisms were an order
of magnitude more abundant than sarcodines and crustacean nauplii. During and after
the Southwest Monsoon of 1994, in the open gyre, consumption rates of microzooplankton grazing represented up to 20–30% of daily primary production that ranged from 1.0
to 15 gC m
−2 d
−1 .
During the intermonsoons, daily consumption represents 30–50% of a rather smaller
productivity (03–08 gC m
−2 d
−1 ). Mesozooplankton herbivory was judged by Stelfox
et al. to represent around 25–30% of that of microzooplankton. The grazing dynamics of
mesoplankton were addressed independently by Roman et al. (2000), who found, overall,
that the copepod mesozooplankton (dominated by ∼50 species of copepods) represented
a biomass in the range of 10–50 mgC m
−2 that ingested 10–40 mgC m
−2 of food daily,
representing 10–60% of daily depth-integrated primary production. Lowest ratios of
ingestion/primary production were found during the Northeast Monsoon. The seasonal
response of copepod biomass to autotrophic productivity was rapid, so that in the central
gyre during the spring intermonsoon this was of order 20–50 mgC m
−2 , whereas during