Indian Ocean Coastal Biome
307
relatively high abundance of cladocerans (Evadne, Penilia) would surprise a cold-water
planktologist.
As along other upwelling coastlines, the seasonal diatom blooms support a pelagic
food chain that includes shoaling, planktivorous fish—here, they are the oil sardine (Sardinella longiceps), the Indian mackerel (Rastrelliger kanagurta), and anchovies (Stolephorus
spp.). But because of the reversing monsoon winds, and some special characteristics of
terrestrial runoff, the pelagic ecosystem off western India is unique. Here, also, the direct
link between diatom production and adult fish growth is perhaps better demonstrated
than anywhere else so that—at short time-scales—the growth rates of both sardine and
mackerel are strongly related to the availability of their planktonic food. Like some
other coasts in warm seas where seasonal diatom blooms are exceptionally strong, this
province supports a pelagic clupeid fish that depends almost entirely on diatoms for
food, obtained by active filtration on gill-rakers: this is Sardinella longiceps, the oil sardine
of the Indian Ocean. Species having comparable ecology occur in the western Atlantic
(Brevoortia) and in the Gulf of Guinea (Ethmalosa). Note that here, as in the Gulf of
Guinea, high concentrations of diatoms are associated with relatively higher numbers
of large cells than occurs at lower concentrations, as was discussed by Banse et al.
(1996). The larger cells presumably escape ingestion by small herbivores (e.g., small
copepods) and it is hypothesized that population size of the larger copepods requires a
few weeks to respond; oil sardine populations may respond more rapidly, being available
to consume diatoms whenever upwelling occurs—and the larger the diatoms are, the
better!
The oil sardine is extremely abundant in this province and supports a fishery that
has been sustained since the early 1900s, and whose long-term variability was discussed
in Chapter 8. The arrival of sardines at the coast coincides with the seasonal bloom
of the diatom Fragillaria (= Nitzschia) oceanica and also of Coscinodiscus, Pleurosigma,
and Biddulphia. The first indications of a bloom, at the start of the monsoon season, coincides with the arrival of prespawning adults, and the peak of the bloom, in
September or October, coincides with the main fishery for juvenile sardines, these being
the fish-of-the-year. Dinoflagellates and copepods, together with soft organic-rich material resuspended by tidal streams from the coastal banks of almost liquid mud, appear
to sustain the sardines from October to January. The abundance of this rapidly growing
fish is therefore closely linked to the productivity of the ecosystem; having long records
of sardine abundance, we can hindcast the relative performance of the plankton over the
same period. This was discussed in Chapter 8, in which statistical series of physical time
series representing an upwelling index were compared with sardine landings.
The long-term data on landings of mackerel (Rastrelliger) and anchovies (Stylephorus) suggest that the stock biomass of these species is not so intimately dependent on
upwelling—and hence on diatom blooms—as is that of the oil sardine. I tentatively
venture to suggest that this may be some support for the significance of a land-based
microbial web in fueling the mesozooplankton of this coastline.
Regional Benthic and Demersal Ecology
The inner shelf off the alluvial coast of Kerala (8–12
S) is a mobile mud regime, the inshore
mud banks forming during the boreal winter and remobilizing during the summer, when
longshore currents and wind mixing are maximal. The ecology of the benthic fauna here
can be presumed to resemble that in the deposition-resuspension mud regime off the
mouth of the Amazon (see GUIA).
The continental shelf of western India is an example of a region that currently exhibits
extreme hypoxia in the bottom waters and eutrophication in surface waters due, in large
part, to increasing runoff of nutrients, especially nitrate, from terrestrial sources (Naqui
307
relatively high abundance of cladocerans (Evadne, Penilia) would surprise a cold-water
planktologist.
As along other upwelling coastlines, the seasonal diatom blooms support a pelagic
food chain that includes shoaling, planktivorous fish—here, they are the oil sardine (Sardinella longiceps), the Indian mackerel (Rastrelliger kanagurta), and anchovies (Stolephorus
spp.). But because of the reversing monsoon winds, and some special characteristics of
terrestrial runoff, the pelagic ecosystem off western India is unique. Here, also, the direct
link between diatom production and adult fish growth is perhaps better demonstrated
than anywhere else so that—at short time-scales—the growth rates of both sardine and
mackerel are strongly related to the availability of their planktonic food. Like some
other coasts in warm seas where seasonal diatom blooms are exceptionally strong, this
province supports a pelagic clupeid fish that depends almost entirely on diatoms for
food, obtained by active filtration on gill-rakers: this is Sardinella longiceps, the oil sardine
of the Indian Ocean. Species having comparable ecology occur in the western Atlantic
(Brevoortia) and in the Gulf of Guinea (Ethmalosa). Note that here, as in the Gulf of
Guinea, high concentrations of diatoms are associated with relatively higher numbers
of large cells than occurs at lower concentrations, as was discussed by Banse et al.
(1996). The larger cells presumably escape ingestion by small herbivores (e.g., small
copepods) and it is hypothesized that population size of the larger copepods requires a
few weeks to respond; oil sardine populations may respond more rapidly, being available
to consume diatoms whenever upwelling occurs—and the larger the diatoms are, the
better!
The oil sardine is extremely abundant in this province and supports a fishery that
has been sustained since the early 1900s, and whose long-term variability was discussed
in Chapter 8. The arrival of sardines at the coast coincides with the seasonal bloom
of the diatom Fragillaria (= Nitzschia) oceanica and also of Coscinodiscus, Pleurosigma,
and Biddulphia. The first indications of a bloom, at the start of the monsoon season, coincides with the arrival of prespawning adults, and the peak of the bloom, in
September or October, coincides with the main fishery for juvenile sardines, these being
the fish-of-the-year. Dinoflagellates and copepods, together with soft organic-rich material resuspended by tidal streams from the coastal banks of almost liquid mud, appear
to sustain the sardines from October to January. The abundance of this rapidly growing
fish is therefore closely linked to the productivity of the ecosystem; having long records
of sardine abundance, we can hindcast the relative performance of the plankton over the
same period. This was discussed in Chapter 8, in which statistical series of physical time
series representing an upwelling index were compared with sardine landings.
The long-term data on landings of mackerel (Rastrelliger) and anchovies (Stylephorus) suggest that the stock biomass of these species is not so intimately dependent on
upwelling—and hence on diatom blooms—as is that of the oil sardine. I tentatively
venture to suggest that this may be some support for the significance of a land-based
microbial web in fueling the mesozooplankton of this coastline.
Regional Benthic and Demersal Ecology
The inner shelf off the alluvial coast of Kerala (8–12
S) is a mobile mud regime, the inshore
mud banks forming during the boreal winter and remobilizing during the summer, when
longshore currents and wind mixing are maximal. The ecology of the benthic fauna here
can be presumed to resemble that in the deposition-resuspension mud regime off the
mouth of the Amazon (see GUIA).
The continental shelf of western India is an example of a region that currently exhibits
extreme hypoxia in the bottom waters and eutrophication in surface waters due, in large
part, to increasing runoff of nutrients, especially nitrate, from terrestrial sources (Naqui