426
Chapter 11: The Pacific Ocean
Province, ARCH); this is a matter of convenience since these features may be sufficiently
large as to occur over both a shelf and an adjacent deep basin. A prominent feature of
this kind is the front aligned with the long shelf edge lying zonally across the South China
Sea; flow approximately normal to this generates a meandering front with associated
uplift of the nutricline by as much as 90 m (Lim, 1975).
Two of the most prominent and permanent areas of high pigment are on the southern
coast of New Guinea: in the west-facing Aru Gulf where the Digoel River discharges and
the eastward-facing Gulf of Papua, with the Fly River. Of these, the former is by far the
more prominent, especially in austral winter, when the southeasterly monsoon winds
sweep across this gulf. The high-pigment plume from the Aru Gulf often turns southwest
and in some images appears to be traceable across the Gulf of Carpentaria.
What we know of these two features appears to be rather contradictory, and does not
resolve the obvious question: are we looking at pigmented CDOM from the river effluents,
or are we looking at a chlorophyll plume resulting from upwelling or riverine nutrient
input? The outer Aru Gulf was the site of the Snellius II investigations in 1985, whereas
the Gulf of Papua was investigated more recently by the TROPICS investigations that were
intended to provide a model for “Tropical River-Ocean Processes in Coastal Settings.”
The Snellius II investigations showed that in August, during the SE monsoon, upwelling
occurs extensively over the continental shelf of the western Arafura Sea (Wetsteyn et al.,
1990). The mechanism appears to be that water from 100 to 150 m in the western part
of the Aru Gulf spreads up over the shelf reaching several hundred kilometers distant the
shelf edge; this phenomenon carries cool, saline, high-nutrient slope water (>10 M NO 3 )
far up over the shelf both north and south of the Aru Islands. This transport is supported
by a slow offshore Ekman drift of coastal, low-salinity water presumably, at least in
part, forced by the easterly component of the monsoon winds. These processes are
insufficient to upwell the slope water to the surface, the buoyancy imparted by river
water maintaining stratification, but increased levels of primary production during boreal
summer (Ilahude et al., 1990) and higher cell counts of large phytoplankters (Adnan,
1990) suggest that upwelled nutrients, mixed into the buoyant layer, largely support
the bloom: riverborne nutrient inputs, though utilized, are less important. Investigations
during the NW monsoon in February showed, to the contrary, that productivity is
then dependent on riverborne nutrients (Ilahude et al., 1990). As we would expect,
neritic diatoms dominate the macro-phytoplankton in the August blooms and are more
abundant then by a factor of 3 than in February when nanoplankton account for 70–90%
of chlorophyll except in Trichodesmium blooms.
The dynamics of the Gulf of Papua, facing east, appear to be different from this model
(Robertson et al., 1998). Here, upwelling is not advanced as a factor in nutrient supply,
but rather this comes from the 30-m-deep, buoyant plume of warm river water that overlies cooler, saline water. The plume has a high sediment load and carries much floating
plant debris with it. Nutrient dynamics are complex and whereas silicate is conservative,
nitrate and phosphate are taken up locally by autotrophs and also released from particulates into the dissolved phase. Locally, phosphate dynamics corresponded with changes in
chlorophyll concentration. Production rates were low near the coast (<20 mg C m
−2 d
−1 )
but increased to 225–350 mg C m
−2 d
−1 in the plume farther offshore. Bacterial production was highly variable but significantly correlated with autotrophic production, as also
was zooplankton biomass, at rates very close to those for autotrophic production. All
this suggested to Robertson et al. that bacteria in the Gulf of Papua use riverine DOC
and POC and that Gulf waters are probably net heterotrophic with the consequence that
benthic production in the Gulf of Papua is dependent largely on organic material derived
from riverine sources rather than from local autotrophic production.
SeaWiFS images clearly confirm the spatial context of these observations in both gulfs,
but probably overestimate the contribution of chlorophyll everywhere in this region. In
Chapter 11: The Pacific Ocean
Province, ARCH); this is a matter of convenience since these features may be sufficiently
large as to occur over both a shelf and an adjacent deep basin. A prominent feature of
this kind is the front aligned with the long shelf edge lying zonally across the South China
Sea; flow approximately normal to this generates a meandering front with associated
uplift of the nutricline by as much as 90 m (Lim, 1975).
Two of the most prominent and permanent areas of high pigment are on the southern
coast of New Guinea: in the west-facing Aru Gulf where the Digoel River discharges and
the eastward-facing Gulf of Papua, with the Fly River. Of these, the former is by far the
more prominent, especially in austral winter, when the southeasterly monsoon winds
sweep across this gulf. The high-pigment plume from the Aru Gulf often turns southwest
and in some images appears to be traceable across the Gulf of Carpentaria.
What we know of these two features appears to be rather contradictory, and does not
resolve the obvious question: are we looking at pigmented CDOM from the river effluents,
or are we looking at a chlorophyll plume resulting from upwelling or riverine nutrient
input? The outer Aru Gulf was the site of the Snellius II investigations in 1985, whereas
the Gulf of Papua was investigated more recently by the TROPICS investigations that were
intended to provide a model for “Tropical River-Ocean Processes in Coastal Settings.”
The Snellius II investigations showed that in August, during the SE monsoon, upwelling
occurs extensively over the continental shelf of the western Arafura Sea (Wetsteyn et al.,
1990). The mechanism appears to be that water from 100 to 150 m in the western part
of the Aru Gulf spreads up over the shelf reaching several hundred kilometers distant the
shelf edge; this phenomenon carries cool, saline, high-nutrient slope water (>10 M NO 3 )
far up over the shelf both north and south of the Aru Islands. This transport is supported
by a slow offshore Ekman drift of coastal, low-salinity water presumably, at least in
part, forced by the easterly component of the monsoon winds. These processes are
insufficient to upwell the slope water to the surface, the buoyancy imparted by river
water maintaining stratification, but increased levels of primary production during boreal
summer (Ilahude et al., 1990) and higher cell counts of large phytoplankters (Adnan,
1990) suggest that upwelled nutrients, mixed into the buoyant layer, largely support
the bloom: riverborne nutrient inputs, though utilized, are less important. Investigations
during the NW monsoon in February showed, to the contrary, that productivity is
then dependent on riverborne nutrients (Ilahude et al., 1990). As we would expect,
neritic diatoms dominate the macro-phytoplankton in the August blooms and are more
abundant then by a factor of 3 than in February when nanoplankton account for 70–90%
of chlorophyll except in Trichodesmium blooms.
The dynamics of the Gulf of Papua, facing east, appear to be different from this model
(Robertson et al., 1998). Here, upwelling is not advanced as a factor in nutrient supply,
but rather this comes from the 30-m-deep, buoyant plume of warm river water that overlies cooler, saline water. The plume has a high sediment load and carries much floating
plant debris with it. Nutrient dynamics are complex and whereas silicate is conservative,
nitrate and phosphate are taken up locally by autotrophs and also released from particulates into the dissolved phase. Locally, phosphate dynamics corresponded with changes in
chlorophyll concentration. Production rates were low near the coast (<20 mg C m
−2 d
−1 )
but increased to 225–350 mg C m
−2 d
−1 in the plume farther offshore. Bacterial production was highly variable but significantly correlated with autotrophic production, as also
was zooplankton biomass, at rates very close to those for autotrophic production. All
this suggested to Robertson et al. that bacteria in the Gulf of Papua use riverine DOC
and POC and that Gulf waters are probably net heterotrophic with the consequence that
benthic production in the Gulf of Papua is dependent largely on organic material derived
from riverine sources rather than from local autotrophic production.
SeaWiFS images clearly confirm the spatial context of these observations in both gulfs,
but probably overestimate the contribution of chlorophyll everywhere in this region. In