306
Chapter 10: The Indian Ocean
just started to appear on the shelf. When Ekman upwelling begins, surface nutrient levels
increase, the mixed layer becomes even thinner, and very low oxygen concentrations
(<05 ml O 2 liter
−1 ) characterize shelf waters below the thermocline.
The upwelling period of the Southwest Monsoon (August–October) is associated with
algal blooms reaching biomass levels of as much as 40 mg chl m
−3 , perhaps fueled not
only to the advection of nutrients in the upwelled water, but also by remineralization
of nutrients released from soft sediments. High chlorophyll concentrations, in the range
1–3 mg chl m
−3 , extend out beyond the continental shelf across the full width of the
Laccadive Sea, forming jets and spirals readily observed in satellite imagery (Lierheimer
and Banse, 2002).
During these near-surface algal blooms, water clarity is very low (chlorophyll maximum
at <5 m; water column chlorophyll <200 mg m
−2 ; Secchi disc, 2–3 m) and diatom cells
take high concentrations of <3×10
5 cells liter
−1 (Shah, 1973); these may be accompanied
by blooms of Trichodesmium, especially in boreal spring. Although I have located no
information on seasonal cycles of herbivore consumption in this region, Shah does suggest
that the relatively high ratio of phaeopigments to chlorophyll during upwelling periods
indicates strong coupling between herbivores and algal cells.
However, Madhupratap et al. (1994) have made another and very interesting suggestion
concerning the support of planktonic herbivores. Perhaps, they suggest, the massive
remobilization of green mud, heavy in POC/DOC from the inshore linear banks of this
substance, the well-known ‘chakara’ of this coast, together with the extremely strong
discharge of river water that occurs essentially the length of the western coast of India
(see later discussion), carrying heavy loads of decaying organic material, and hence of
bacteria, may directly fuel the zooplankton biomass via the microbial loop. In this model,
it is a land-based microbial loop that provides larval sardines with copepod food, so
that their recruitment success (see later discussion) would not be as closely correlated
with upwelling and diatom production as is indicated by other evidence. This is a novel
suggestion of great interest that is not contrary to the clear relationship that exists between
diatom blooms and their consumption by herbivorous fish. The concept of a land-based
coastal food chain that comprises DOC-bacteria-flagellates-microzooplankton-copepodfish may perhaps be of wider application?
Nevertheless, Menon and George (1977) show that the mesozooplankton biomass
over the Cochin shelf off southwest India (7–17
N) responds positively and rapidly
to the upwelling cycle that was, itself, indicated by the depth to the 10 ml O 2 liter
−1
isoline at standard stations across the shelf. Over a period of five annual upwelling
cycles (1971–1975), frequent observations over a grid of stations showed that there was
very little lag in zooplankton population growth in response to the start of upwelling.
Zooplankton biomass on this coast is routinely minimal (01 ml m
−3 ) from January to
April and maximal (07–08 ml m
−3 ) from July to September.
The composition of mesozooplankton here is entirely typical of tropical seas but we
should note how the distribution of species over the deeper shelf regions is constrained by
the presence of the zone of very low oxygen tension. Even in the Laccadive Sea, the normal
vertical distribution of the diel migrant copepods Pleuromamma spp. is strongly modified.
Excluded from their normal daytime residence depths, these species are anomalously
abundant throughout the 24 hours in the near-surface layers (Haq et al., 1973). This
occurs despite the fact that P. indica can tolerate oxygen tensions of < 01 ml liter
−1 ,
compared with the lower limit of around 05 ml liter
−1 tolerated by most other large
oceanic copepods. Although most studies of mesoplankton in the Indian coastal seas
have been taxonomic, enough is now known of regional ecology as to suggest that the
onshelf penetration of oceanic species is relatively limited, perhaps because of the rather
unusual conditions in the coastal zone. As on other tropical continental shelves, the
Chapter 10: The Indian Ocean
just started to appear on the shelf. When Ekman upwelling begins, surface nutrient levels
increase, the mixed layer becomes even thinner, and very low oxygen concentrations
(<05 ml O 2 liter
−1 ) characterize shelf waters below the thermocline.
The upwelling period of the Southwest Monsoon (August–October) is associated with
algal blooms reaching biomass levels of as much as 40 mg chl m
−3 , perhaps fueled not
only to the advection of nutrients in the upwelled water, but also by remineralization
of nutrients released from soft sediments. High chlorophyll concentrations, in the range
1–3 mg chl m
−3 , extend out beyond the continental shelf across the full width of the
Laccadive Sea, forming jets and spirals readily observed in satellite imagery (Lierheimer
and Banse, 2002).
During these near-surface algal blooms, water clarity is very low (chlorophyll maximum
at <5 m; water column chlorophyll <200 mg m
−2 ; Secchi disc, 2–3 m) and diatom cells
take high concentrations of <3×10
5 cells liter
−1 (Shah, 1973); these may be accompanied
by blooms of Trichodesmium, especially in boreal spring. Although I have located no
information on seasonal cycles of herbivore consumption in this region, Shah does suggest
that the relatively high ratio of phaeopigments to chlorophyll during upwelling periods
indicates strong coupling between herbivores and algal cells.
However, Madhupratap et al. (1994) have made another and very interesting suggestion
concerning the support of planktonic herbivores. Perhaps, they suggest, the massive
remobilization of green mud, heavy in POC/DOC from the inshore linear banks of this
substance, the well-known ‘chakara’ of this coast, together with the extremely strong
discharge of river water that occurs essentially the length of the western coast of India
(see later discussion), carrying heavy loads of decaying organic material, and hence of
bacteria, may directly fuel the zooplankton biomass via the microbial loop. In this model,
it is a land-based microbial loop that provides larval sardines with copepod food, so
that their recruitment success (see later discussion) would not be as closely correlated
with upwelling and diatom production as is indicated by other evidence. This is a novel
suggestion of great interest that is not contrary to the clear relationship that exists between
diatom blooms and their consumption by herbivorous fish. The concept of a land-based
coastal food chain that comprises DOC-bacteria-flagellates-microzooplankton-copepodfish may perhaps be of wider application?
Nevertheless, Menon and George (1977) show that the mesozooplankton biomass
over the Cochin shelf off southwest India (7–17
N) responds positively and rapidly
to the upwelling cycle that was, itself, indicated by the depth to the 10 ml O 2 liter
−1
isoline at standard stations across the shelf. Over a period of five annual upwelling
cycles (1971–1975), frequent observations over a grid of stations showed that there was
very little lag in zooplankton population growth in response to the start of upwelling.
Zooplankton biomass on this coast is routinely minimal (01 ml m
−3 ) from January to
April and maximal (07–08 ml m
−3 ) from July to September.
The composition of mesozooplankton here is entirely typical of tropical seas but we
should note how the distribution of species over the deeper shelf regions is constrained by
the presence of the zone of very low oxygen tension. Even in the Laccadive Sea, the normal
vertical distribution of the diel migrant copepods Pleuromamma spp. is strongly modified.
Excluded from their normal daytime residence depths, these species are anomalously
abundant throughout the 24 hours in the near-surface layers (Haq et al., 1973). This
occurs despite the fact that P. indica can tolerate oxygen tensions of < 01 ml liter
−1 ,
compared with the lower limit of around 05 ml liter
−1 tolerated by most other large
oceanic copepods. Although most studies of mesoplankton in the Indian coastal seas
have been taxonomic, enough is now known of regional ecology as to suggest that the
onshelf penetration of oceanic species is relatively limited, perhaps because of the rather
unusual conditions in the coastal zone. As on other tropical continental shelves, the