356
Chapter 11: The Pacific Ocean
off New South Wales, developed highest biomass only in November–December (Tranter
et al., 1980); the DCM was near the summer thermocline at about 50 m. Investigations
of other warm-core eddies confirmed that their productivity was high relative to the
surrounding water mass and was fuelled by nutrients delivered by deep winter mixing
within the core of each eddy.
Warm-core eddies containing water originating in the Coral Sea have a more complex
horizontal distribution of surface chlorophyll, in which maximal values often occur near
their western edges. One such eddy, tracked over a 19-month period, persistently had
integrated chlorophyll of 60–80 mg chl m
−2 , of which 60–70% was contributed by very
small cells of the pico- and nanoplankton; this contribution reached 90% in some of
the more oligotrophic stations worked within the eddy. Edge enrichment may occur also
where warm-core eddies entrain crescents of cooler, nitrate-replete water (about 50 M)
from the coastal regions, especially where the edge of the eddy is comparatively straight.
Here, populations of the upwelling copepod Calanoides carinatus may occur and the
general biological enhancement (NO 3 50–100 M; chlorophyll, 15 mg liter
−1 ; copepods,
50–70 m
−3 ) may attract schools of the southern bluefin tuna (Tranter et al., 1983). The
eddies associated with the retroflection area, and those shed from the Tasman Front
as it passes toward North Cape of New Zealand, may entrain expatriated subtropical
organisms from the Coral Sea; this has been investigated for the case of several tropical
hyperiid amphipods entrained in these features.
That this should be a very dynamic and changeable part of the ocean could be
predicted from its boundary conditions, lying between the Southern Ocean and the
Coral Sea and adjacent to the great landmass of Australia. This is confirmed by the
observations of variability not only within seasons but also between seasons (Harris
et al., 1988, 1991). Though most relevant observations have been made on the east
Tasmanian coast (see AUSE), they are generally applicable to the entire province. The
observations compare the effects of different wind stress during two 2-year periods.
The summers of 1986 and 1987 were cool and windy, whereas in 1988 and 1989 they
were warmer and quieter. The cool years more closely resembled the subantarctic (deep
winter mixing and high nitrate) and the warm years subtropical conditions (shallow
winter mixing and low nitrate). Particle size of all planktonic components indicated a
trend toward subtropical oligotrophic conditions in 1988 and 1989. Primary production
was reduced, small copepods dominated, and all large zooplankters (especially salps
and Nyctiphanes australis) were eliminated: these organisms appear to be dependent
on large-cell new production rather than on small-cell regenerated production. In cool
years, large populations of euphausiids occur, whose swarms support a rich Trachurus
fishery, which collapsed during the two warm years. Harris et al. suggest that this entire
chain of events was a regional manifestation of the Pacific-wide ENSO warm event
of 1988.
Synopsis
Case 3—Winter-spring production with nutrient limitation—The pycnocline undergoes
a strong seasonal excursion from 25 m in austral summer to 150 m in August; Z eu
varies only marginally, so that the pycnocline is illuminated only in austral summer
(November–April). From a summer minimum (02 mg C m
−2 d
−1 ) in April–May, P
shows a progressive fourfold rate increase through austral fall and winter (Fig. 11.7),
to reach an annual maximum in October–November (early austral summer). Thereafter
a progressive decline sets in, leading to the annual minimum in April. Chlorophyll
accumulation follows the rate of P during the spring bloom but not the winter depression:
at that season, a minor secondary accumulation occurs.
Chapter 11: The Pacific Ocean
off New South Wales, developed highest biomass only in November–December (Tranter
et al., 1980); the DCM was near the summer thermocline at about 50 m. Investigations
of other warm-core eddies confirmed that their productivity was high relative to the
surrounding water mass and was fuelled by nutrients delivered by deep winter mixing
within the core of each eddy.
Warm-core eddies containing water originating in the Coral Sea have a more complex
horizontal distribution of surface chlorophyll, in which maximal values often occur near
their western edges. One such eddy, tracked over a 19-month period, persistently had
integrated chlorophyll of 60–80 mg chl m
−2 , of which 60–70% was contributed by very
small cells of the pico- and nanoplankton; this contribution reached 90% in some of
the more oligotrophic stations worked within the eddy. Edge enrichment may occur also
where warm-core eddies entrain crescents of cooler, nitrate-replete water (about 50 M)
from the coastal regions, especially where the edge of the eddy is comparatively straight.
Here, populations of the upwelling copepod Calanoides carinatus may occur and the
general biological enhancement (NO 3 50–100 M; chlorophyll, 15 mg liter
−1 ; copepods,
50–70 m
−3 ) may attract schools of the southern bluefin tuna (Tranter et al., 1983). The
eddies associated with the retroflection area, and those shed from the Tasman Front
as it passes toward North Cape of New Zealand, may entrain expatriated subtropical
organisms from the Coral Sea; this has been investigated for the case of several tropical
hyperiid amphipods entrained in these features.
That this should be a very dynamic and changeable part of the ocean could be
predicted from its boundary conditions, lying between the Southern Ocean and the
Coral Sea and adjacent to the great landmass of Australia. This is confirmed by the
observations of variability not only within seasons but also between seasons (Harris
et al., 1988, 1991). Though most relevant observations have been made on the east
Tasmanian coast (see AUSE), they are generally applicable to the entire province. The
observations compare the effects of different wind stress during two 2-year periods.
The summers of 1986 and 1987 were cool and windy, whereas in 1988 and 1989 they
were warmer and quieter. The cool years more closely resembled the subantarctic (deep
winter mixing and high nitrate) and the warm years subtropical conditions (shallow
winter mixing and low nitrate). Particle size of all planktonic components indicated a
trend toward subtropical oligotrophic conditions in 1988 and 1989. Primary production
was reduced, small copepods dominated, and all large zooplankters (especially salps
and Nyctiphanes australis) were eliminated: these organisms appear to be dependent
on large-cell new production rather than on small-cell regenerated production. In cool
years, large populations of euphausiids occur, whose swarms support a rich Trachurus
fishery, which collapsed during the two warm years. Harris et al. suggest that this entire
chain of events was a regional manifestation of the Pacific-wide ENSO warm event
of 1988.
Synopsis
Case 3—Winter-spring production with nutrient limitation—The pycnocline undergoes
a strong seasonal excursion from 25 m in austral summer to 150 m in August; Z eu
varies only marginally, so that the pycnocline is illuminated only in austral summer
(November–April). From a summer minimum (02 mg C m
−2 d
−1 ) in April–May, P
shows a progressive fourfold rate increase through austral fall and winter (Fig. 11.7),
to reach an annual maximum in October–November (early austral summer). Thereafter
a progressive decline sets in, leading to the annual minimum in April. Chlorophyll
accumulation follows the rate of P during the spring bloom but not the winter depression:
at that season, a minor secondary accumulation occurs.