282
Chapter 10: The Indian Ocean
occurs to levels sustained during the period of the Northeast Monsoon. At the onset of
the spring intermonsoon period, chlorophyll biomass is rapidly decreased, so that from
April to June the entire offshore Arabian Sea is extremely oligotrophic (<01 mg chl m
−3 ).
Only a meridional line of higher chlorophyll at the Maldive Islands along 73
E forms
a recurrent feature in surface chlorophyll images at this season: westward flow across
this line creates chlorophyll enhancement downstream to the west, a situation that is
reversed in boreal summer, when flow is in the opposite direction. At the same season,
the regional deep chlorophyll maximum of the anticyclonic Bay of Bengal gyre deepens
progressively toward the center of the gyre.
The onset of the winter monsoon coincides with a minor buildup of phytoplankton
biomass around the periphery of the Bay of Bengal, including around the southern limb
of the cyclonic gyre that spins up at that season; this region of relatively high chlorophyll
is rooted at Sri Lanka. The equatorial region generally shows as a low-chlorophyll zone,
adjacent to higher chlorophyll lying around the north of the south subtropical gyre,
beyond the 10
S hydrochemical front (see earlier discussion). The eastward equatorial
jet of intermonsoon periods does, in some years, appear to be associated with high
chlorophyll biomass that is aligned with the equator, but this is more diffuse than
the typical narrow, meandering equatorial divergence signal seen in the other oceans.
Thus, this feature was especially strongly developed in November–December 1997 from
midocean across to Sumatra exhibiting, particularly in the latter month, evidence of
meandering at the Rossby scale.
You may have wondered about the observation that I discussed briefly earlier, concerning a rather high concentration of NO 3 in the mixed layer of the central Arabian Sea
during the Northeast Monsoon; this might seem, at first sight, anomalous in view of the
continued relatively high chlorophyll biomass there—high, that is, for a tropical openocean situation—at this season. Both SeaSoar observations and models of this process
give the same result: during the Northeast Monsoon, diurnal shallow stratification within
the mixed layer permits accumulation of phytoplankton biomass that is mixed down
each night, thus reducing the number of cells in the near-surface zone. Ammonium is
regenerated each night within the “microbial loop” as cells are consumed and digested,
and is available each morning within the mixed layer in sufficient concentration as to
inhibit the uptake of NO 3 (Wiggert et al., 2000).
Thus, phytoplankton response (productivity of order 10–15 gC m
−2 d
−1 ) to deep convective mixing is instantaneous while, at the same time, the NO 3 that is entrained into
the deepening mixed layer across the eroding nutricline is not fully utilized and accumulates there. Prior to the bloom, during the oligotrophic period, NO 3 utilization (at
about 015 M d
−1 ) is balanced with the entrainment of this molecule into the euphotic
zone. This mechanism supports the finding of Watts and Owens (1999) of the suppression of nitrate assimilation here and more generally the observation of Harrison et al.
(1996) concerning nitrate and ammonium interaction at nanomolar concentrations in
the oligotrophic ocean.
Because, in occasional years, chlorophyll biomass remains below 10 mg m
−3 , some
have suggested that this is yet another high-S region (see Chapter 5). There is no reason,
however, to postulate either Fe or SiO 3 limitation here because, for one thing, if Fe is
delivered by dust in nonlimiting quantities anywhere at the oceans surface, it must be
here: Sharon Smith has described this region as “Mother Nature’s Iron Experiment.”
Nor is it necessary to suggest that it is grazing control that restrains phytoplankton
accumulation. The daily stratification cycle appears to be a sufficient explanation because,
once shallow stratification is established permanently after monsoon wind stress at the
surface is reduced, then phytoplankton biomass accumulates, the NH 4 pool is consumed,
and plant growth begins to take up the previously entrained NO 3 and to reduce its
concentration.
Chapter 10: The Indian Ocean
occurs to levels sustained during the period of the Northeast Monsoon. At the onset of
the spring intermonsoon period, chlorophyll biomass is rapidly decreased, so that from
April to June the entire offshore Arabian Sea is extremely oligotrophic (<01 mg chl m
−3 ).
Only a meridional line of higher chlorophyll at the Maldive Islands along 73
E forms
a recurrent feature in surface chlorophyll images at this season: westward flow across
this line creates chlorophyll enhancement downstream to the west, a situation that is
reversed in boreal summer, when flow is in the opposite direction. At the same season,
the regional deep chlorophyll maximum of the anticyclonic Bay of Bengal gyre deepens
progressively toward the center of the gyre.
The onset of the winter monsoon coincides with a minor buildup of phytoplankton
biomass around the periphery of the Bay of Bengal, including around the southern limb
of the cyclonic gyre that spins up at that season; this region of relatively high chlorophyll
is rooted at Sri Lanka. The equatorial region generally shows as a low-chlorophyll zone,
adjacent to higher chlorophyll lying around the north of the south subtropical gyre,
beyond the 10
S hydrochemical front (see earlier discussion). The eastward equatorial
jet of intermonsoon periods does, in some years, appear to be associated with high
chlorophyll biomass that is aligned with the equator, but this is more diffuse than
the typical narrow, meandering equatorial divergence signal seen in the other oceans.
Thus, this feature was especially strongly developed in November–December 1997 from
midocean across to Sumatra exhibiting, particularly in the latter month, evidence of
meandering at the Rossby scale.
You may have wondered about the observation that I discussed briefly earlier, concerning a rather high concentration of NO 3 in the mixed layer of the central Arabian Sea
during the Northeast Monsoon; this might seem, at first sight, anomalous in view of the
continued relatively high chlorophyll biomass there—high, that is, for a tropical openocean situation—at this season. Both SeaSoar observations and models of this process
give the same result: during the Northeast Monsoon, diurnal shallow stratification within
the mixed layer permits accumulation of phytoplankton biomass that is mixed down
each night, thus reducing the number of cells in the near-surface zone. Ammonium is
regenerated each night within the “microbial loop” as cells are consumed and digested,
and is available each morning within the mixed layer in sufficient concentration as to
inhibit the uptake of NO 3 (Wiggert et al., 2000).
Thus, phytoplankton response (productivity of order 10–15 gC m
−2 d
−1 ) to deep convective mixing is instantaneous while, at the same time, the NO 3 that is entrained into
the deepening mixed layer across the eroding nutricline is not fully utilized and accumulates there. Prior to the bloom, during the oligotrophic period, NO 3 utilization (at
about 015 M d
−1 ) is balanced with the entrainment of this molecule into the euphotic
zone. This mechanism supports the finding of Watts and Owens (1999) of the suppression of nitrate assimilation here and more generally the observation of Harrison et al.
(1996) concerning nitrate and ammonium interaction at nanomolar concentrations in
the oligotrophic ocean.
Because, in occasional years, chlorophyll biomass remains below 10 mg m
−3 , some
have suggested that this is yet another high-S region (see Chapter 5). There is no reason,
however, to postulate either Fe or SiO 3 limitation here because, for one thing, if Fe is
delivered by dust in nonlimiting quantities anywhere at the oceans surface, it must be
here: Sharon Smith has described this region as “Mother Nature’s Iron Experiment.”
Nor is it necessary to suggest that it is grazing control that restrains phytoplankton
accumulation. The daily stratification cycle appears to be a sufficient explanation because,
once shallow stratification is established permanently after monsoon wind stress at the
surface is reduced, then phytoplankton biomass accumulates, the NH 4 pool is consumed,
and plant growth begins to take up the previously entrained NO 3 and to reduce its
concentration.