300
Chapter 10: The Indian Ocean
surface chlorophyll becomes prominent in satellite images and is clearly associated with
upwelling centers. Such images suggest that the region occupied by this algal bloom
is equivalent to the total area covered by the upwelling in all four eastern boundary
currents. Even before CZCS images were available, it was suggested that Southwest
Monsoon bloom of the Arabian Sea might be equivalent in productivity to all eastern
boundary currents combined (Smith, 1984). But, even if the Somali upwelling does
have such potential, it is not realised. This deficit has been attributed to the relatively
short residence time of upwelled water in the Somali system, before being dispersed out
over the Arabian Sea; surface velocities in the Great Whirl are of order 250 cm sec
−1
and chlorophyll concentrations reduce rapidly from 2–3 g liter
−1 to 05–15 g liter
−1
(Hitchcock et al., 2000).
The reversal of the monsoon winds dominates biological response to mixed-layer
dynamics in the upwelling province, and this coupling is so tight that between-year
variability in timing and strength of monsoon winds is directly reflected in the seasonal
ecology. The weak monsoon of 1982 generated a coastal phytoplankton bloom that had
maximum pigment values only about one-fourth of those in a strong monsoon. Surface
chlorophyll fields, both from ship-based data (e.g., Halim, 1984) and from satellite images
(e.g., Banse and McClain, 1986), match the upwelling pattern computed from wind stress.
During the Southwest Monsoon, many of the anticipated mesoscale circulation features
can usually be identified in the surface chlorophyll field: the persistent anticyclonic eddy
off northern Somalia, the broad field of upwelling off Arabia, and the lower chlorophyll
over the Murray Ridge and in the central Gulf of Oman (see Color plate 12).
An understanding of algal dynamics in this complex province requires integration
of the vertical relations of density, nutrients, and light. Chlorophyll profiles respond
predictably to deepening of the pycnocline and the level of ambient radiation remaining at
the nutricline, which usually lies in the upper pycnocline. Meridional chlorophyll sections
through the Arabian Sea in boreal summer show that to the right of the atmospheric jet
the pycnocline and nutricline are deep (approx 80–100 m) and are associated with the
chlorophyll maximum. To the left of the jet, maximum chlorophyll occurs in the mixed
layer, and the nutricline approaches the surface.
The distribution of subsurface light in relation to mixed-layer depth can be used to
model the seasonality of primary production in the Arabian Sea. During the relatively
windless but brief intermonsoon periods, the Arabian Sea becomes oligotrophic with a
deep chlorophyll maximum just above the nutricline at < 50 m. The 1% isolume lies
deeper than this because the mixed-layer water has low chlorophyll and is very transparent
(Brock et al., 1993). At the onset of both monsoon seasons, the 1% isolume rises into the
mixed layer, as algal growth reduces transparency. In the oligotrophic seasons, production
at the DCM exceeds production in the mixed layer.
The general pattern of the response of autotrophic organisms to monsoon forcing is
logical and well-known: larger cells, mostly diatoms, are more important during periods of
upwelling, while the pico and nano fractions dominate in the oligotrophic, intermonsoon
periods and comprise 40–80% of the standing stock of POC (Burkill et al., 1993b;
Veldhuis et al., 1997). Production and consumption are unbalanced during upwelling
periods, so that strong flux of cells to the sea floor occurs, but during the intermonsoon
periods, balance is struck; nevertheless, it is argued (e.g., Marra and Barber, 2005)
that physiological rate parameters and productivity measurements suggest that columnintegrated phytoplankton growth is strongly limited by neither irradiance nor nutrient
supply. This argument suggests that variance in biomass depends importantly on mixing
at all scales, including upwelling and the presence of mesoscale eddies.
During oligotrophic periods, even close to the coast, diel variation in mixed-layer
depth is small and nitrate is almost totally depleted; analysis of critical depths suggests
that active growth continues at the deep chlorophyll maximum, which lies ∼50 m and
Chapter 10: The Indian Ocean
surface chlorophyll becomes prominent in satellite images and is clearly associated with
upwelling centers. Such images suggest that the region occupied by this algal bloom
is equivalent to the total area covered by the upwelling in all four eastern boundary
currents. Even before CZCS images were available, it was suggested that Southwest
Monsoon bloom of the Arabian Sea might be equivalent in productivity to all eastern
boundary currents combined (Smith, 1984). But, even if the Somali upwelling does
have such potential, it is not realised. This deficit has been attributed to the relatively
short residence time of upwelled water in the Somali system, before being dispersed out
over the Arabian Sea; surface velocities in the Great Whirl are of order 250 cm sec
−1
and chlorophyll concentrations reduce rapidly from 2–3 g liter
−1 to 05–15 g liter
−1
(Hitchcock et al., 2000).
The reversal of the monsoon winds dominates biological response to mixed-layer
dynamics in the upwelling province, and this coupling is so tight that between-year
variability in timing and strength of monsoon winds is directly reflected in the seasonal
ecology. The weak monsoon of 1982 generated a coastal phytoplankton bloom that had
maximum pigment values only about one-fourth of those in a strong monsoon. Surface
chlorophyll fields, both from ship-based data (e.g., Halim, 1984) and from satellite images
(e.g., Banse and McClain, 1986), match the upwelling pattern computed from wind stress.
During the Southwest Monsoon, many of the anticipated mesoscale circulation features
can usually be identified in the surface chlorophyll field: the persistent anticyclonic eddy
off northern Somalia, the broad field of upwelling off Arabia, and the lower chlorophyll
over the Murray Ridge and in the central Gulf of Oman (see Color plate 12).
An understanding of algal dynamics in this complex province requires integration
of the vertical relations of density, nutrients, and light. Chlorophyll profiles respond
predictably to deepening of the pycnocline and the level of ambient radiation remaining at
the nutricline, which usually lies in the upper pycnocline. Meridional chlorophyll sections
through the Arabian Sea in boreal summer show that to the right of the atmospheric jet
the pycnocline and nutricline are deep (approx 80–100 m) and are associated with the
chlorophyll maximum. To the left of the jet, maximum chlorophyll occurs in the mixed
layer, and the nutricline approaches the surface.
The distribution of subsurface light in relation to mixed-layer depth can be used to
model the seasonality of primary production in the Arabian Sea. During the relatively
windless but brief intermonsoon periods, the Arabian Sea becomes oligotrophic with a
deep chlorophyll maximum just above the nutricline at < 50 m. The 1% isolume lies
deeper than this because the mixed-layer water has low chlorophyll and is very transparent
(Brock et al., 1993). At the onset of both monsoon seasons, the 1% isolume rises into the
mixed layer, as algal growth reduces transparency. In the oligotrophic seasons, production
at the DCM exceeds production in the mixed layer.
The general pattern of the response of autotrophic organisms to monsoon forcing is
logical and well-known: larger cells, mostly diatoms, are more important during periods of
upwelling, while the pico and nano fractions dominate in the oligotrophic, intermonsoon
periods and comprise 40–80% of the standing stock of POC (Burkill et al., 1993b;
Veldhuis et al., 1997). Production and consumption are unbalanced during upwelling
periods, so that strong flux of cells to the sea floor occurs, but during the intermonsoon
periods, balance is struck; nevertheless, it is argued (e.g., Marra and Barber, 2005)
that physiological rate parameters and productivity measurements suggest that columnintegrated phytoplankton growth is strongly limited by neither irradiance nor nutrient
supply. This argument suggests that variance in biomass depends importantly on mixing
at all scales, including upwelling and the presence of mesoscale eddies.
During oligotrophic periods, even close to the coast, diel variation in mixed-layer
depth is small and nitrate is almost totally depleted; analysis of critical depths suggests
that active growth continues at the deep chlorophyll maximum, which lies ∼50 m and