308
Chapter 10: The Indian Ocean
et al., 2000). A zone of anoxia develops in late summer and autumn: here, adjacent to
the shelf, the highest accumulations of H 2 S and N 2 O yet recorded have been observed
in recent years. The water upwelled onto the shelf (see earlier discussion) is already
hypoxic and is capped by a shallow layer of low-salinity water formed by admixture with
river effluents. Strong stratification ensues, imposing very reduced ventilation of upwelled
water that is in contact with shelf sediments. This rapidly becomes anoxic, by oxidation
of phytoplankton and other organic material sinking from the strong seasonal bloom
(again, see the earlier discussion). This shallow anoxic layer was determined by Naqui
et al. to be distinct from the deeper, perennial hypoxic zone of the Arabian Sea with
which it joins when the latter is entrained onto the shelf as described previously. The
formation of excessive N 2 O is perhaps the consequence here, as in lakes, of nitrification
occurring in the near absence of oxygen.
The consequence of these various processes is that the benthic fauna and demersal fish populations along the entire continental shelf of western India and Pakistan
may be essentially absent deeper than about 50 m. I have been able to locate several
benthic surveys of this shelf, and each reports a preponderance of polychaetes—these
comprised almost 60% of macrobenthic biomass on the inner shelf off Goa (Harkantra
and Parukelar, 1981). Off Bombay, another survey showed that 61% of the meiofauna
was composed of polychaetes, and that the meiofaunal biomass was dominated (>90%)
by foraminiferans. Of the remainder of the meiofauna, almost 20% were mollusks,
amongst which bivalves of the infauna were dominant: Mactra, Arca, Tellina, and
so on. Epifauna comprised natant crustacea (e.g., Metapenaeus, Mysis) and echinoderms (e.g., Ophiothrix). These characteristics, and genera, are very similar to the
Amphioplus isocommunity of very soft, muddy deposits in the Gulf of Guinea
(Longhurst, 1958).
The remobilization of the predominantly soft deposits of the shallower parts of the
shelf not only interrupts the continuity of the inshore chakara but also requires that
the benthic fauna become re-established annually (Seshappa, 1953). Of course, infaunal
polychaetes dominate the recruiting organisms.
All along the west coast of India, the demersal fish fauna is typical of tropical soft
deposits and resembles, even if it is more diverse, the fauna of the muddy parts of the
Gulf of Guinea. Sciaenids form 40–60% of the trawlable biomass and are dominated by
species of Pseudosciaena and Otolithoides; these occur together with polynemids, ariids,
and rays. This fauna, as in the eastern Atlantic, extends down to about 60–75 m across
the shelf. Where deposits are sandier, toward the southern end of the coastline and
into the Gulf of Oman, sciaenids are scarce and various breams and groupers take their
place.
Synopsis
Case 5—Large-amplitude response to monsoonlike reversal of trade winds—Mixed-layer
seasonality is dominated by boreal winter mixing to 40–45 m in January–February; SW
Monsoon upwelling slightly deepens the summer mixed layer. Z eu shoals rapidly to
30 m from 50 m at the onset of the SW Monsoon, but the thermocline is permanently
illuminated except briefly in boreal winter. The seasonal maximum rate of P occurs in
September (12–14 gC m
−2 d
−1 ) with a rapid decline to a postmonsoon minimum in
December (Fig. 10.6). Chlorophyll accumulation occurs only during the onset of the
SW Monsoon, declining from an August peak biomass of around 15 mg m
−3 to an
intermonsoon value of about 025 mgC m
−3 , after buildup of herbivore population causes
a change of sign. This cycle is consistent with a close match between consumption and
production rates, and perhaps some increase in consumer biomass after the onset of the
SW Monsoon.
Chapter 10: The Indian Ocean
et al., 2000). A zone of anoxia develops in late summer and autumn: here, adjacent to
the shelf, the highest accumulations of H 2 S and N 2 O yet recorded have been observed
in recent years. The water upwelled onto the shelf (see earlier discussion) is already
hypoxic and is capped by a shallow layer of low-salinity water formed by admixture with
river effluents. Strong stratification ensues, imposing very reduced ventilation of upwelled
water that is in contact with shelf sediments. This rapidly becomes anoxic, by oxidation
of phytoplankton and other organic material sinking from the strong seasonal bloom
(again, see the earlier discussion). This shallow anoxic layer was determined by Naqui
et al. to be distinct from the deeper, perennial hypoxic zone of the Arabian Sea with
which it joins when the latter is entrained onto the shelf as described previously. The
formation of excessive N 2 O is perhaps the consequence here, as in lakes, of nitrification
occurring in the near absence of oxygen.
The consequence of these various processes is that the benthic fauna and demersal fish populations along the entire continental shelf of western India and Pakistan
may be essentially absent deeper than about 50 m. I have been able to locate several
benthic surveys of this shelf, and each reports a preponderance of polychaetes—these
comprised almost 60% of macrobenthic biomass on the inner shelf off Goa (Harkantra
and Parukelar, 1981). Off Bombay, another survey showed that 61% of the meiofauna
was composed of polychaetes, and that the meiofaunal biomass was dominated (>90%)
by foraminiferans. Of the remainder of the meiofauna, almost 20% were mollusks,
amongst which bivalves of the infauna were dominant: Mactra, Arca, Tellina, and
so on. Epifauna comprised natant crustacea (e.g., Metapenaeus, Mysis) and echinoderms (e.g., Ophiothrix). These characteristics, and genera, are very similar to the
Amphioplus isocommunity of very soft, muddy deposits in the Gulf of Guinea
(Longhurst, 1958).
The remobilization of the predominantly soft deposits of the shallower parts of the
shelf not only interrupts the continuity of the inshore chakara but also requires that
the benthic fauna become re-established annually (Seshappa, 1953). Of course, infaunal
polychaetes dominate the recruiting organisms.
All along the west coast of India, the demersal fish fauna is typical of tropical soft
deposits and resembles, even if it is more diverse, the fauna of the muddy parts of the
Gulf of Guinea. Sciaenids form 40–60% of the trawlable biomass and are dominated by
species of Pseudosciaena and Otolithoides; these occur together with polynemids, ariids,
and rays. This fauna, as in the eastern Atlantic, extends down to about 60–75 m across
the shelf. Where deposits are sandier, toward the southern end of the coastline and
into the Gulf of Oman, sciaenids are scarce and various breams and groupers take their
place.
Synopsis
Case 5—Large-amplitude response to monsoonlike reversal of trade winds—Mixed-layer
seasonality is dominated by boreal winter mixing to 40–45 m in January–February; SW
Monsoon upwelling slightly deepens the summer mixed layer. Z eu shoals rapidly to
30 m from 50 m at the onset of the SW Monsoon, but the thermocline is permanently
illuminated except briefly in boreal winter. The seasonal maximum rate of P occurs in
September (12–14 gC m
−2 d
−1 ) with a rapid decline to a postmonsoon minimum in
December (Fig. 10.6). Chlorophyll accumulation occurs only during the onset of the
SW Monsoon, declining from an August peak biomass of around 15 mg m
−3 to an
intermonsoon value of about 025 mgC m
−3 , after buildup of herbivore population causes
a change of sign. This cycle is consistent with a close match between consumption and
production rates, and perhaps some increase in consumer biomass after the onset of the
SW Monsoon.