Pacific Coastal Biome
433
so-called Bass Strait Cascade where cold, dense straits water slips below lighter Tasman
Sea water, producing a linear zone over the upper slope of enhanced chlorophyll that
extends to the coast of Victoria where the bloom may continue later than to the east of
the Strait itself. It is suggested by Gibbs et al. (1991) that phytoplankton growth continues
actively in the plume in the northward flow along the shelf break after it has ceased at
the entrance to Bass Strait. Eventually, the constituent cells will be carried to depth when
the plume enters the cascade area itself off the coast of Victoria.
The 4-year study in South Bay on the Tasmanian shelf, referred to previously, demonstrated the ecosystem consequences of the proximity of the strongly meandering South
Subtropical Convergence Zone: pelagic ecosystem structure, the timing of the spring
bloom, and the recruitment success of fish stocks may all be correlated with incursions of
water from the Convergence Zone at the coast (Harris et al., 1991). The pelagic ecosystem
responded strongly to conditions obtaining during a relatively warm, calm year after
several cooler windy years, associated with changes in mixed-layer depth and hence in
nutrient supply at the end of winter: cooler SST, deeper MLD, and more 10-m nutrients
in windy years. The phytoplankton particle-size spectrum significantly changes between
such periods, being less steep in windy years and dominated by large cells. Small cells
were more dominant in years of greater stability. Hence, the flux of organic matter to
the shelf sediments is much greater in windy, more productive years. However, nitrate
is drawn down to limiting levels for several months every summer, but phosphate only
briefly.
Net-caught plankton volumes were greater by an order of magnitude in the windy
year 1986–87 than in other years, a result of larger fractions both of diatoms and of salps.
The dominance of larger organisms in net-taken samples changed strikingly between
years: Nyctiphanes dominated in 1985–86, salps in 1986–87, and salps and Nyctiphanes
in 1987–88. In 1988–89, only small volumes of small copepods were taken. These last are
the “background” plankton, on which are imposed Nyctiphanes and salps in years when
production is dominated by external sources of nutrients, rather than on regeneration.
Regional Benthic and Demersal Ecology
If only because the coral reefs along the Queensland coast are the largest example of this
habitat anywhere, it will be appropriate to note some of the critical characteristics of this
benthic system, so far ignored in this work. In what follows I lean heavily on the classical
review of John Lewis (1981) and on Alongi (1998).
The first questions that come to mind when examining this region concern the
relationship between the regional ocean circulation discussed earlier and the extraordinary
accumulation of benthic biomass represented by the Great Barrier Reef. To what extent
are the fluxes of slope water through the channels in the reef onto the inner shelf,
carrying inorganic nutrients, particulate organic matter, and oceanic plankton, involved
in maintaining the great biomass of the reef? The observation that the reef biota are
most strongly developed where the shelf edge is steepest led Orr (1933) to suggest that
the richness of the Great Barrier Reef might be a response to upwelling in the western
boundary current.
So it may be, but in that case the mechanism is very complex and obscure: as Lewis
emphasizes, there is generally little reduction in the nitrate content of water that flows
across reefs, whereas near-bottom phosphate concentration in water over a reef appear
to be higher than background. And although oceanic zooplankton are consumed during
their passage across a reef, it is calculated that this can provide only a small part of what
is required for growth of the reef ensemble.
Rather than looking for external sources of particulate and dissolved nitrogen, we
should perhaps remember that fixation rates of < 50–250 g N m
−2 y
−1 are typical for
433
so-called Bass Strait Cascade where cold, dense straits water slips below lighter Tasman
Sea water, producing a linear zone over the upper slope of enhanced chlorophyll that
extends to the coast of Victoria where the bloom may continue later than to the east of
the Strait itself. It is suggested by Gibbs et al. (1991) that phytoplankton growth continues
actively in the plume in the northward flow along the shelf break after it has ceased at
the entrance to Bass Strait. Eventually, the constituent cells will be carried to depth when
the plume enters the cascade area itself off the coast of Victoria.
The 4-year study in South Bay on the Tasmanian shelf, referred to previously, demonstrated the ecosystem consequences of the proximity of the strongly meandering South
Subtropical Convergence Zone: pelagic ecosystem structure, the timing of the spring
bloom, and the recruitment success of fish stocks may all be correlated with incursions of
water from the Convergence Zone at the coast (Harris et al., 1991). The pelagic ecosystem
responded strongly to conditions obtaining during a relatively warm, calm year after
several cooler windy years, associated with changes in mixed-layer depth and hence in
nutrient supply at the end of winter: cooler SST, deeper MLD, and more 10-m nutrients
in windy years. The phytoplankton particle-size spectrum significantly changes between
such periods, being less steep in windy years and dominated by large cells. Small cells
were more dominant in years of greater stability. Hence, the flux of organic matter to
the shelf sediments is much greater in windy, more productive years. However, nitrate
is drawn down to limiting levels for several months every summer, but phosphate only
briefly.
Net-caught plankton volumes were greater by an order of magnitude in the windy
year 1986–87 than in other years, a result of larger fractions both of diatoms and of salps.
The dominance of larger organisms in net-taken samples changed strikingly between
years: Nyctiphanes dominated in 1985–86, salps in 1986–87, and salps and Nyctiphanes
in 1987–88. In 1988–89, only small volumes of small copepods were taken. These last are
the “background” plankton, on which are imposed Nyctiphanes and salps in years when
production is dominated by external sources of nutrients, rather than on regeneration.
Regional Benthic and Demersal Ecology
If only because the coral reefs along the Queensland coast are the largest example of this
habitat anywhere, it will be appropriate to note some of the critical characteristics of this
benthic system, so far ignored in this work. In what follows I lean heavily on the classical
review of John Lewis (1981) and on Alongi (1998).
The first questions that come to mind when examining this region concern the
relationship between the regional ocean circulation discussed earlier and the extraordinary
accumulation of benthic biomass represented by the Great Barrier Reef. To what extent
are the fluxes of slope water through the channels in the reef onto the inner shelf,
carrying inorganic nutrients, particulate organic matter, and oceanic plankton, involved
in maintaining the great biomass of the reef? The observation that the reef biota are
most strongly developed where the shelf edge is steepest led Orr (1933) to suggest that
the richness of the Great Barrier Reef might be a response to upwelling in the western
boundary current.
So it may be, but in that case the mechanism is very complex and obscure: as Lewis
emphasizes, there is generally little reduction in the nitrate content of water that flows
across reefs, whereas near-bottom phosphate concentration in water over a reef appear
to be higher than background. And although oceanic zooplankton are consumed during
their passage across a reef, it is calculated that this can provide only a small part of what
is required for growth of the reef ensemble.
Rather than looking for external sources of particulate and dissolved nitrogen, we
should perhaps remember that fixation rates of < 50–250 g N m
−2 y
−1 are typical for