288
Chapter 10: The Indian Ocean
to maintain the adult stock and sufficiently abundant mesozooplankton and nekton to
support the growth of their larvae and young-of-the-year. Fontenau’s maps of tuna fishing
effort support this observation, although this fishery apparently failed after about 1970,
perhaps because of stock failure. Fontenau also shows that fishing effort for yellowfin and
bigeye tuna continues higher than background in a linear zone across the southern Indian
Ocean that corresponds well with the location of the SEC eddy field. The distribution of
these top pelagic predators may be sufficient evidence for us to be reasonably confident
that the bloom in the SEC supports—as we would expect it to do—a pelagic ecosystem
with biomass significantly higher than background.
Suda (1973) examined the records of tuna catches in the Indian Ocean in the relatively
early years of the fishery (1966–68), when we may assume that the relative distribution of
species had become little modified by the fishery. He shows a distribution of the longline
hooking rate of albacore (Thunnus alalunga) that includes a very striking concentration of
fish across the entire southern Indian Ocean at about 30–32
S, apparently corresponding
to the eddying flow around the southern limb of the gyral circulation. Concentration of
fish is highest at about 90–100
E, or at about the longitude of the major reflux into the
interior of the gyre (see earlier discussion). The linear concentration of fish across the
ocean is quite distinct from a weaker linear zone at about 45–50
S that corresponds very
well with the eddying flow in the SSTC province.
Eastern Eddy Field During the late austral winter, from July to September, the eddy
field west of Australia exhibits higher chlorophyll biomass than during the first half of
the year and becomes continuous with the eastern region of high chlorophyll in the
SEC. During the early phase of this bloom development, one may observe in the images
many long, undulating streamers of relatively high chlorophyll that are rooted in the
western Australian eddy field but extending sufficiently far as to join the SEC region
in midocean. It is here that the only serial observations of the major components of
the pelagic ecosystem were completed during the IIOE (Tranter, 1973). The Australian
meridional transect along 110
E, at the longitude of central Java and from 10–32
S, was
worked so to give 16 day and 16 night stations every 2 months. By today’s standards,
the observations were not sophisticated but this is the only organized, seasonal data
set we possess of nutrients, productivity, chlorophyll, zooplankton, and micronekton at
comparable time and spatial scales.
The seasonal cycle obtained by the Australian IIOE matches very well what we now
observe in the serial chlorophyll images: an austral winter bloom occurs from March–
April until September–October, the period when NO 3 values, integrated to 100 m, are
almost an order of magnitude higher than in the period December–February. Productivity,
integrated over the same depths, increases from < 20 to >60 mgC m
−2 hr
−1 , and this
permits >25 mg m
−2 of chlorophyll biomass to be accumulated. Seasonal variability of
night zooplankton is relatively weak with an increase of < 20% to ∼20 mg m
−3 , and
there appears to be a lag of several months in their response to the phytoplankton bloom
and decline. Only a few species (e.g., Eucalanus concinna, Rhincalanus nasutus) appear
even to double their biomass seasonally. Tranter (1973) used a correlative matrix to
determine if the changes observed represented an ecosystem response or whether they
were artifactual, caused by physical translocation of biota through each station position.
He concluded that nitrate, primary production rate, and zooplankton biomass all vary
seasonally by about 40–50% of the annual mean value, whereas phytoplankton biomass
(on the evidence of chlorophyll) varies only by about 20%. This conclusion agrees with
our general assumptions about pelagic ecology in warm seas. However, looking more
closely at evidence for trophic succession, Tranter found the following pattern: in tropical
water the timing of zooplankton biomass correlates well with productivity and nitrate,
but in subtropical water chlorophyll correlates positively with productivity but negatively
Chapter 10: The Indian Ocean
to maintain the adult stock and sufficiently abundant mesozooplankton and nekton to
support the growth of their larvae and young-of-the-year. Fontenau’s maps of tuna fishing
effort support this observation, although this fishery apparently failed after about 1970,
perhaps because of stock failure. Fontenau also shows that fishing effort for yellowfin and
bigeye tuna continues higher than background in a linear zone across the southern Indian
Ocean that corresponds well with the location of the SEC eddy field. The distribution of
these top pelagic predators may be sufficient evidence for us to be reasonably confident
that the bloom in the SEC supports—as we would expect it to do—a pelagic ecosystem
with biomass significantly higher than background.
Suda (1973) examined the records of tuna catches in the Indian Ocean in the relatively
early years of the fishery (1966–68), when we may assume that the relative distribution of
species had become little modified by the fishery. He shows a distribution of the longline
hooking rate of albacore (Thunnus alalunga) that includes a very striking concentration of
fish across the entire southern Indian Ocean at about 30–32
S, apparently corresponding
to the eddying flow around the southern limb of the gyral circulation. Concentration of
fish is highest at about 90–100
E, or at about the longitude of the major reflux into the
interior of the gyre (see earlier discussion). The linear concentration of fish across the
ocean is quite distinct from a weaker linear zone at about 45–50
S that corresponds very
well with the eddying flow in the SSTC province.
Eastern Eddy Field During the late austral winter, from July to September, the eddy
field west of Australia exhibits higher chlorophyll biomass than during the first half of
the year and becomes continuous with the eastern region of high chlorophyll in the
SEC. During the early phase of this bloom development, one may observe in the images
many long, undulating streamers of relatively high chlorophyll that are rooted in the
western Australian eddy field but extending sufficiently far as to join the SEC region
in midocean. It is here that the only serial observations of the major components of
the pelagic ecosystem were completed during the IIOE (Tranter, 1973). The Australian
meridional transect along 110
E, at the longitude of central Java and from 10–32
S, was
worked so to give 16 day and 16 night stations every 2 months. By today’s standards,
the observations were not sophisticated but this is the only organized, seasonal data
set we possess of nutrients, productivity, chlorophyll, zooplankton, and micronekton at
comparable time and spatial scales.
The seasonal cycle obtained by the Australian IIOE matches very well what we now
observe in the serial chlorophyll images: an austral winter bloom occurs from March–
April until September–October, the period when NO 3 values, integrated to 100 m, are
almost an order of magnitude higher than in the period December–February. Productivity,
integrated over the same depths, increases from < 20 to >60 mgC m
−2 hr
−1 , and this
permits >25 mg m
−2 of chlorophyll biomass to be accumulated. Seasonal variability of
night zooplankton is relatively weak with an increase of < 20% to ∼20 mg m
−3 , and
there appears to be a lag of several months in their response to the phytoplankton bloom
and decline. Only a few species (e.g., Eucalanus concinna, Rhincalanus nasutus) appear
even to double their biomass seasonally. Tranter (1973) used a correlative matrix to
determine if the changes observed represented an ecosystem response or whether they
were artifactual, caused by physical translocation of biota through each station position.
He concluded that nitrate, primary production rate, and zooplankton biomass all vary
seasonally by about 40–50% of the annual mean value, whereas phytoplankton biomass
(on the evidence of chlorophyll) varies only by about 20%. This conclusion agrees with
our general assumptions about pelagic ecology in warm seas. However, looking more
closely at evidence for trophic succession, Tranter found the following pattern: in tropical
water the timing of zooplankton biomass correlates well with productivity and nitrate,
but in subtropical water chlorophyll correlates positively with productivity but negatively