312
Chapter 10: The Indian Ocean
as during the Northeast Monsoon of January–February, primary production rates are
significantly higher, especially where blooms of Thallasiosira subtilis occur.
The mesozooplankton of the shelf from 13
N to 20
N (Madras-Calcutta) was surveyed
during the Southwest Monsoon with IIOE protocols in 1978 by Nair et al. (1981),
who found significantly higher dry weight biomass (24 g 100 m
−3 ) south of 17
N than
to the north of that latitude (077 g 100 m
−3 ). As was found also by the more recent
investigations, biomass over the shelf was significantly higher than in the oceanic regions.
Indeed, comment Nair et al., the biomass in the first of these two zones was comparable
with biomass taken off Arabia during other IIOE investigations. Later in the season, other
contemporary studies showed that the high-biomass region extends to lower latitudes
than 13
N, and it has been suggested that this is associated with upwelling on the coast
between Madras and the Palk Strait. Other studies have suggested a significant decoupling
between zooplankton biomass, the latter peaking significantly later than the local maximal
algal biomass.
The IIOE samples (0.3 mm mesh) were dominated by copepods (∼80% numerically) of
40 species in 31 genera, and the JGOFS transect sampled a mesozooplankton dominated,
as expected, by copepods (Para- and Clausoclanus, Eucalanus) and cyclopoids. Some of the
JGOFS samples were dominated by massive blooms of the mucous/filter feeding Pyrosoma
that is otherwise more relatively abundant in the oceanic region of the Bay of Bengal.
The numbers observed suggested that this species was perhaps capable of significantly
lowering the ambient chlorophyll biomass. Other observations on the northern shelves
off the deltaic region suggest that mesozooplankton biomass is maximal during, and
just after, the Northeast Monsoon with relatively very low biomass during the Southeast
Monsoon.
Regional Benthic and Demersal Ecology
There is some information that suggests a general coupling between benthic biomass on
the eastern Indian shelves and the productivity of the water column above. Parekular et al.
(1982) suggest that macrobenthos biomass maxima (averaging >50 g m
−2 ) occur in three
regions and that here the biomass of this fraction exceeds that of meiobenthos, otherwise
relatively more important on the slope. These regions are adjacent to Palk Strait, in the
central upwelling region (15–17
N) and on the shelf to the west of the BrahmaputraGanges delta. On loose, sandy soil, maximum biomass reached 150 g m
−2 but is much
lower in the highly organic estuarine muds that are transported so generously onto the
shelf in this province. Yet once again here, most of the ubiquitous genera of the infauna
would be familiar to a temperate-zone benthic ecologist: Eunice, Nephthys, Glycera, Neries,
Cardium, Mactra, Ampelisca, Ophiothrix—and so on (Harkantra et al., 1982). It is among
the vagile epifauna (e.g., Penaeus) and the demersal fish (e.g., Sciaenidae, Polynemidae)
that specifically tropical genera dominate.
The demersal fish faunas of the shelves of this province are typical of those on other
similar shelves. On muddy, estuarized shelves, especially inshore, we should expect sciaenids, polynemids, ariids, and various pleuronectiformes; on sandy deposits and usually
deeper, sparids, lutjanids, serranids, and lethrinids are characteristic. This arrangement
has been discussed in some detail for all tropical shelves by myself and Pauly (1987); the
division between shallow and deep fish faunas occurs unusually deep at the head of the
Bay of Bengal – at around 100 m. This is probably the consequence of the widespread distribution of soft deposits off the very large delta of the Brahmaputra-Ganges river system
(Garces et al., 2006) A significant study of the effects of several decades of trawl-fishing
on these faunas suggests serious modification to the pristine food web on the eastern
coast of India (Vivekanandan et al., 2005). On the southeastern sector, the dominant
Chapter 10: The Indian Ocean
as during the Northeast Monsoon of January–February, primary production rates are
significantly higher, especially where blooms of Thallasiosira subtilis occur.
The mesozooplankton of the shelf from 13
N to 20
N (Madras-Calcutta) was surveyed
during the Southwest Monsoon with IIOE protocols in 1978 by Nair et al. (1981),
who found significantly higher dry weight biomass (24 g 100 m
−3 ) south of 17
N than
to the north of that latitude (077 g 100 m
−3 ). As was found also by the more recent
investigations, biomass over the shelf was significantly higher than in the oceanic regions.
Indeed, comment Nair et al., the biomass in the first of these two zones was comparable
with biomass taken off Arabia during other IIOE investigations. Later in the season, other
contemporary studies showed that the high-biomass region extends to lower latitudes
than 13
N, and it has been suggested that this is associated with upwelling on the coast
between Madras and the Palk Strait. Other studies have suggested a significant decoupling
between zooplankton biomass, the latter peaking significantly later than the local maximal
algal biomass.
The IIOE samples (0.3 mm mesh) were dominated by copepods (∼80% numerically) of
40 species in 31 genera, and the JGOFS transect sampled a mesozooplankton dominated,
as expected, by copepods (Para- and Clausoclanus, Eucalanus) and cyclopoids. Some of the
JGOFS samples were dominated by massive blooms of the mucous/filter feeding Pyrosoma
that is otherwise more relatively abundant in the oceanic region of the Bay of Bengal.
The numbers observed suggested that this species was perhaps capable of significantly
lowering the ambient chlorophyll biomass. Other observations on the northern shelves
off the deltaic region suggest that mesozooplankton biomass is maximal during, and
just after, the Northeast Monsoon with relatively very low biomass during the Southeast
Monsoon.
Regional Benthic and Demersal Ecology
There is some information that suggests a general coupling between benthic biomass on
the eastern Indian shelves and the productivity of the water column above. Parekular et al.
(1982) suggest that macrobenthos biomass maxima (averaging >50 g m
−2 ) occur in three
regions and that here the biomass of this fraction exceeds that of meiobenthos, otherwise
relatively more important on the slope. These regions are adjacent to Palk Strait, in the
central upwelling region (15–17
N) and on the shelf to the west of the BrahmaputraGanges delta. On loose, sandy soil, maximum biomass reached 150 g m
−2 but is much
lower in the highly organic estuarine muds that are transported so generously onto the
shelf in this province. Yet once again here, most of the ubiquitous genera of the infauna
would be familiar to a temperate-zone benthic ecologist: Eunice, Nephthys, Glycera, Neries,
Cardium, Mactra, Ampelisca, Ophiothrix—and so on (Harkantra et al., 1982). It is among
the vagile epifauna (e.g., Penaeus) and the demersal fish (e.g., Sciaenidae, Polynemidae)
that specifically tropical genera dominate.
The demersal fish faunas of the shelves of this province are typical of those on other
similar shelves. On muddy, estuarized shelves, especially inshore, we should expect sciaenids, polynemids, ariids, and various pleuronectiformes; on sandy deposits and usually
deeper, sparids, lutjanids, serranids, and lethrinids are characteristic. This arrangement
has been discussed in some detail for all tropical shelves by myself and Pauly (1987); the
division between shallow and deep fish faunas occurs unusually deep at the head of the
Bay of Bengal – at around 100 m. This is probably the consequence of the widespread distribution of soft deposits off the very large delta of the Brahmaputra-Ganges river system
(Garces et al., 2006) A significant study of the effects of several decades of trawl-fishing
on these faunas suggests serious modification to the pristine food web on the eastern
coast of India (Vivekanandan et al., 2005). On the southeastern sector, the dominant