Indian Ocean Trade Wind Biome
289
with zooplankton. He infers that consumption response is instantaneous in low latitudes
but is lagged by one cruise interval (several weeks) farther poleward.
The Madagascar Basin An enigmatic seasonal dendritic bloom in the southwest
Indian Ocean escaped observation until revealed by satellite images. These show that
chlorophyll biomass may reach 1–3 mg chl m
−3 against the oligotrophic oceanic background of order 005 mg chl m
−3 and that this bloom may, at its maximum extent, cover
as much as 3000 km in zonal extent, from 45
E to 80
E—that is to say, halfway to
Australia, as occurred in 1999 (Longhurst, 2001). It takes the form of a series of arcuate
patches lying north and south on either side of a zonal axis along about 25
S (see Color
plate 11).
The dendritic bloom originates in February near the retroflection of the East
Madagascar Current and very rapidly extends eastward zonally across the ocean to reach
its furthest extent in April, at which time the chlorophyll signal near the origin has attenuated significantly. The individual features appear faintly, and subsequently strengthen as
the bloom progresses. The evolution of juvenile to mature features is characteristic: rapid
spreading occurs of arcuate wisps of chlorophyll enhancement to form broader regions
of high chlorophyll that eventually overlie the entire feature around which they were first
developed. The rapid extension of the bloom toward the east led Srokosz et al. (2004)
to propose that the entire feature represents a “plankton wave,” caused by “the swirling
motions of the eddy field” that “diffuse the plankton against both the mean flow and the
eddy and Rossby wave propagation direction.” Further analysis of this suggestion was
not forthcoming.
The bloom is highly variable between years: it was strong in 1997 (Polder), 1999,
2000, 2002, and 2004 (SeaWiFS) and absent or very weak in 1998, 2001, 2003, and 2005
(SeaWiFS, MODIS). I can now find no relationship between this variability and any
readily available climatic index; earlier, I suggested a relationship with the SOI, but this
seems untenable now with better access to data.
There appear to be two probable explanations for the initiation of the bloom, although
others have been suggested; both probable explanations depend on a change in the
depth of the regional mixed layer. If this shoaled, then an existing subsurface bloom at
the chlorophyll maximum might become visible to the satellite sensors rather rapidly.
However, this is unlikely because a period of heat loss from the surface begins after austral
midsummer so that a seasonal deepening of the mixed layer is initiated in February
that takes the 40-m mixed layer of austral summer down to about 80 m by May and
even deeper prior to the next austral spring. Such conditions, to the east of Australia,
are known to initiate entrainment blooms there (Dandonneau and Gohin, 1984). I have
noted in Chapter 9 the probable existence of a homologous—but much fainter and less
frequent—bloom at exactly the homologous location in the South Atlantic. Further, we
should note that the deepening of the mixed layer progresses eastward, as it must given
the overall bowl form of the thermocline in the subtropical gyre. Srokosz et al. present
their “plankton wave” concept largely to explain the eastward movement: it seems to me
that this suggestion is unnecessary.
The deepening of the mixed layer provides us with a possible explanation for this
bloom by the simple entrainment of nutrients into the euphotic zone caused by downward
erosion of the nutricline. Nor should the rapidity of onset of this entrainment bloom
surprise us if, as seems probable, it is dominated by picoautotrophs. In this highly eddying
region, drifting buoys (and observations of the swirls) suggest a generally westward flow
(Lutjeharms et al., 1981); there is therefore no question that nutrient transport from the
East Madagascar Current might be involved as has been suggested. Given the probable
mechanism, one would anticipate a negative relationship between chlorophyll biomass
and the SLA in cms and, indeed, this is what we observe. There is, as has already been
289
with zooplankton. He infers that consumption response is instantaneous in low latitudes
but is lagged by one cruise interval (several weeks) farther poleward.
The Madagascar Basin An enigmatic seasonal dendritic bloom in the southwest
Indian Ocean escaped observation until revealed by satellite images. These show that
chlorophyll biomass may reach 1–3 mg chl m
−3 against the oligotrophic oceanic background of order 005 mg chl m
−3 and that this bloom may, at its maximum extent, cover
as much as 3000 km in zonal extent, from 45
E to 80
E—that is to say, halfway to
Australia, as occurred in 1999 (Longhurst, 2001). It takes the form of a series of arcuate
patches lying north and south on either side of a zonal axis along about 25
S (see Color
plate 11).
The dendritic bloom originates in February near the retroflection of the East
Madagascar Current and very rapidly extends eastward zonally across the ocean to reach
its furthest extent in April, at which time the chlorophyll signal near the origin has attenuated significantly. The individual features appear faintly, and subsequently strengthen as
the bloom progresses. The evolution of juvenile to mature features is characteristic: rapid
spreading occurs of arcuate wisps of chlorophyll enhancement to form broader regions
of high chlorophyll that eventually overlie the entire feature around which they were first
developed. The rapid extension of the bloom toward the east led Srokosz et al. (2004)
to propose that the entire feature represents a “plankton wave,” caused by “the swirling
motions of the eddy field” that “diffuse the plankton against both the mean flow and the
eddy and Rossby wave propagation direction.” Further analysis of this suggestion was
not forthcoming.
The bloom is highly variable between years: it was strong in 1997 (Polder), 1999,
2000, 2002, and 2004 (SeaWiFS) and absent or very weak in 1998, 2001, 2003, and 2005
(SeaWiFS, MODIS). I can now find no relationship between this variability and any
readily available climatic index; earlier, I suggested a relationship with the SOI, but this
seems untenable now with better access to data.
There appear to be two probable explanations for the initiation of the bloom, although
others have been suggested; both probable explanations depend on a change in the
depth of the regional mixed layer. If this shoaled, then an existing subsurface bloom at
the chlorophyll maximum might become visible to the satellite sensors rather rapidly.
However, this is unlikely because a period of heat loss from the surface begins after austral
midsummer so that a seasonal deepening of the mixed layer is initiated in February
that takes the 40-m mixed layer of austral summer down to about 80 m by May and
even deeper prior to the next austral spring. Such conditions, to the east of Australia,
are known to initiate entrainment blooms there (Dandonneau and Gohin, 1984). I have
noted in Chapter 9 the probable existence of a homologous—but much fainter and less
frequent—bloom at exactly the homologous location in the South Atlantic. Further, we
should note that the deepening of the mixed layer progresses eastward, as it must given
the overall bowl form of the thermocline in the subtropical gyre. Srokosz et al. present
their “plankton wave” concept largely to explain the eastward movement: it seems to me
that this suggestion is unnecessary.
The deepening of the mixed layer provides us with a possible explanation for this
bloom by the simple entrainment of nutrients into the euphotic zone caused by downward
erosion of the nutricline. Nor should the rapidity of onset of this entrainment bloom
surprise us if, as seems probable, it is dominated by picoautotrophs. In this highly eddying
region, drifting buoys (and observations of the swirls) suggest a generally westward flow
(Lutjeharms et al., 1981); there is therefore no question that nutrient transport from the
East Madagascar Current might be involved as has been suggested. Given the probable
mechanism, one would anticipate a negative relationship between chlorophyll biomass
and the SLA in cms and, indeed, this is what we observe. There is, as has already been