Indian Ocean Coastal Biome
311
of stations worked recently during the Southwest Monsoon (Madhupratap et al., 2003).
I can only agree with the authors of this study when they lament that reliable information
on ecological characteristics of this region is, as they write, “elusive.” If we did not have
recourse to satellite images, it would be very difficult to say anything useful about the
ecology of this province.
Some early coastal shelf studies, such as that of Subba Rao (1976) off Waltair, suggested
that there was a bimodal phytoplankton bloom, increased growth occurring during both
monsoons, although all concur that the maximal accumulation of biomass occurs during
the Southwest Monsoon. There is also general concurrence concerning the effects of
estuarization of the northern part of the eastern India coast and of upwelling along the
central part of the coast. What I have not found is any recognition of the fact that the
entire shelf region of the Gulf of Manar and the Palk Straits supports relatively high
chlorophyll biomass throughout the year, filaments of which are carried offshore and
to the northeast. That this occurs is evident from satellite images, which show it to be
especially strong during the Southwest Monsoon. The seasonal images also make it clear
that the two shelf regions facing the Brahmaputra-Ganges and the Irrawaddy deltas each
support apparently high chlorophyll biomass at all seasons; we must, of course, assume
that at least a part of this signal is due to presently unquantifiable amounts of CDOM, as
off the Amazon. At all seasons, on the other hand, the eastern coast of Burma between
these regions lacks any significant bloom.
Although the east coast of India is aligned parallel with that of Somalia and Arabia and
shares a generally similar seasonal wind regime, the ecological consequences of upwelling
during the Southwest Monsoon off India are in no way comparable in the two regions.
This is evident from a scan of the relevant seasonal and monthly chlorophyll images that
indicate that this is a far weaker bloom than in the western Arabian Sea, although they
confirm that boreal summer upwelling does result in enhanced near-surface chlorophyll
along the shelf from Madras to about 20
N. Even during this season, the water column
is sufficiently stabilized that a DCM occurs in all sections of this coast, at about 60–80 m
at the edge of the western boundary current and sloping up (and weakening) toward the
coast. This DCM has maximum chlorophyll values of about 075–10 mg chl m
−3 and is
aligned with the nitracline and the ammonium maximum, suggesting that nutrient depletion occurs in the mixed layer above—as, indeed, we would expect (Sarma and Aswanikumar, 1991). The single modern coastwise transect revealed a mixed layer that decreased
in depth progressively toward the north (30 m at 12
N, 5 m at 19
N); the chlorophyll
maximum was everywhere deeper than this (40–80 m) with increasing biomass toward
the north (015–04 mg m
−3 ). Productivity maxima (<15 mgC m
−3 d
−1 ) were either coincident with the DCM or close under the surface. Phytoplankton of cell size >5 m was
dominated by diatoms (Biddulphia, Chaetoceros, Thallasiothrix, Thallasionema, and others) with some dinoflagellates (Peridinium, Ceratium), whereas autotrophic picoplankton
counts were much lower than in the central Bay of Bengal except only at 19
N, where a
maximum of 94 × 10
−7 liter
−1 was reached in an essentially unstratified water column
below the freshwater plume. Phytoplankton diversity here is higher than in comparable
situations in the Arabian Sea.
All this, as Madhupratap et al. (2003) point out, is consistent with the observed
physical regime—relatively low ambient macronutrients and a very shallow (∼5 to 10 m)
euphotic zone on the shelf because of reduced irradiance and heavy sediment discharges
during the Southwest Monsoon period. Other recent studies (Gomez et al., 2000) have
also found that below the heavy cloud cover of the Southwest Monsoon, even where
high chlorophyll biomass has built up (∼90 mg m
−3 ), production rates are very low
(03 mgC m
−2 d
−1 ) and do not significantly increase until near the end of the monsoon
period. Earlier in the season and before the heavy cloud cover is established, as well
311
of stations worked recently during the Southwest Monsoon (Madhupratap et al., 2003).
I can only agree with the authors of this study when they lament that reliable information
on ecological characteristics of this region is, as they write, “elusive.” If we did not have
recourse to satellite images, it would be very difficult to say anything useful about the
ecology of this province.
Some early coastal shelf studies, such as that of Subba Rao (1976) off Waltair, suggested
that there was a bimodal phytoplankton bloom, increased growth occurring during both
monsoons, although all concur that the maximal accumulation of biomass occurs during
the Southwest Monsoon. There is also general concurrence concerning the effects of
estuarization of the northern part of the eastern India coast and of upwelling along the
central part of the coast. What I have not found is any recognition of the fact that the
entire shelf region of the Gulf of Manar and the Palk Straits supports relatively high
chlorophyll biomass throughout the year, filaments of which are carried offshore and
to the northeast. That this occurs is evident from satellite images, which show it to be
especially strong during the Southwest Monsoon. The seasonal images also make it clear
that the two shelf regions facing the Brahmaputra-Ganges and the Irrawaddy deltas each
support apparently high chlorophyll biomass at all seasons; we must, of course, assume
that at least a part of this signal is due to presently unquantifiable amounts of CDOM, as
off the Amazon. At all seasons, on the other hand, the eastern coast of Burma between
these regions lacks any significant bloom.
Although the east coast of India is aligned parallel with that of Somalia and Arabia and
shares a generally similar seasonal wind regime, the ecological consequences of upwelling
during the Southwest Monsoon off India are in no way comparable in the two regions.
This is evident from a scan of the relevant seasonal and monthly chlorophyll images that
indicate that this is a far weaker bloom than in the western Arabian Sea, although they
confirm that boreal summer upwelling does result in enhanced near-surface chlorophyll
along the shelf from Madras to about 20
N. Even during this season, the water column
is sufficiently stabilized that a DCM occurs in all sections of this coast, at about 60–80 m
at the edge of the western boundary current and sloping up (and weakening) toward the
coast. This DCM has maximum chlorophyll values of about 075–10 mg chl m
−3 and is
aligned with the nitracline and the ammonium maximum, suggesting that nutrient depletion occurs in the mixed layer above—as, indeed, we would expect (Sarma and Aswanikumar, 1991). The single modern coastwise transect revealed a mixed layer that decreased
in depth progressively toward the north (30 m at 12
N, 5 m at 19
N); the chlorophyll
maximum was everywhere deeper than this (40–80 m) with increasing biomass toward
the north (015–04 mg m
−3 ). Productivity maxima (<15 mgC m
−3 d
−1 ) were either coincident with the DCM or close under the surface. Phytoplankton of cell size >5 m was
dominated by diatoms (Biddulphia, Chaetoceros, Thallasiothrix, Thallasionema, and others) with some dinoflagellates (Peridinium, Ceratium), whereas autotrophic picoplankton
counts were much lower than in the central Bay of Bengal except only at 19
N, where a
maximum of 94 × 10
−7 liter
−1 was reached in an essentially unstratified water column
below the freshwater plume. Phytoplankton diversity here is higher than in comparable
situations in the Arabian Sea.
All this, as Madhupratap et al. (2003) point out, is consistent with the observed
physical regime—relatively low ambient macronutrients and a very shallow (∼5 to 10 m)
euphotic zone on the shelf because of reduced irradiance and heavy sediment discharges
during the Southwest Monsoon period. Other recent studies (Gomez et al., 2000) have
also found that below the heavy cloud cover of the Southwest Monsoon, even where
high chlorophyll biomass has built up (∼90 mg m
−3 ), production rates are very low
(03 mgC m
−2 d
−1 ) and do not significantly increase until near the end of the monsoon
period. Earlier in the season and before the heavy cloud cover is established, as well