Pacific Coastal Biome
437
associated with the gyre of the Tasman Sea, flow to the north along the coast is induced
over the shelf as the Westland Current.
Within these flows there are several sites of coastal enrichment. At the northern tip of
the South Island and at the entrance to Cook Strait, bottom friction induces upwelling
that is enhanced by the curvature of the coastline and curvature of the coastline at Cape
Farewell (Shirtcliffe et al., 1990). The very strong winds of Cook Strait then force the
nutrient-rich, upwelled water through the straits, and the intensification of this flow also
induces a sea-surface slope that accelerates water over the Kaharungi Shoals, thus creating
a strong upwelling source near Kaharungi Point. South of Cape Providence (46
S) there
is some uplift of water from the deep salinity maximum over the continental slope of
the Puysegur Bank resulting in high nitrate concentrations inshore (Bradford, 1983). At
the East Cape of North Island, which is rounded by the East Auckland Current, this is
accompanied by the formation of a persistent warm-core eddy offshore and upwelling
nearshore over Ranfurly Bank (Bradford et al., 1982).
However, even more important is the persistent upwelling in the open bight between
Cape Cascade (44
S) and Cape Foulwind (42
S). This simple wind-induced coastal divergence is limited by the shelf-edge front of the upwelled water that bears a series of
wavelike, regularly spaced plumes of cooler water (about 13
C) in the warmer subtropical
water mass of the Tasman Sea. Seaward of Abut Head the plume may reach 70 km offshore and carry much of the discharge from the Buller River. Inshore, both here and on
the east coast, freshwater runoff from these rainy islands creates a sandwich of multiple,
overlying surface-water masses identifiable in the density profiles of the upper 25 m. More
generally, cross-shelf canyons on this coast may induce cross-shelf plumes of buoyant,
inshore water that resemble the offshore squirts observed on the Californian coast: they
stabilize the water column and have consequences for the pelagic production system on
the outer shelf.
In the seas west of New Zealand, wherever frontal processes are weak, the mixed-layer
depth can be modeled simply by local irradiance and wind stress. The seasonal range in
mixed-layer depths is large—about 20 m in summer and 140–200 m in winter. This cycle is
largely convective, being generated more by variance in irradiance (simply, sun angle and
cloudiness) than by variance in wind stress. To the southwest, along the Fjordland coast,
the exceedingly heavy rainfall on the Southern Alps creates a well-defined buoyant layer,
5–10 m deep, which is extremely opaque within the fjords because of a very heavy load
of dissolved organic humics or “yellow substance.” This is widely distributed, at different
dilutions, over the shelves of the South Island. The deeper layers are oxygen-deficient
because of very low rates of light-limited primary production.
Regional Response of the Pelagic Ecosystem
This is a very difficult region in which to generalize the ecological processes, though a
winter-spring bloom, forced by the seasonal cycle of irradiance and changes in mixedlayer depth, is the dominant signal in the chlorophyll field around the islands. However,
the local dominance of a brief seasonal bloom in the confluence region east of the
Banks Peninsula over the Chatham Rise must also be emphasized, even though as noted
earlier, it occurs mostly on or beyond the logical boundary of the province. The brevity
of this bloom is striking, as seen in satellite images and in sediment trap mass fluxes
(King and Howard, 2001). More generally, chlorophyll accumulation is strongest during
September–October over the entire shelf area of New Zealand with strongest biomass
along the west and north coasts as well as the confluence region on the east coast from
Cook Strait to Banks peninsula.
Also observable in the seasonal images are the consequences of the dynamic processes
discussed above, especially in austral spring and summer. However, as Vincent et al.
437
associated with the gyre of the Tasman Sea, flow to the north along the coast is induced
over the shelf as the Westland Current.
Within these flows there are several sites of coastal enrichment. At the northern tip of
the South Island and at the entrance to Cook Strait, bottom friction induces upwelling
that is enhanced by the curvature of the coastline and curvature of the coastline at Cape
Farewell (Shirtcliffe et al., 1990). The very strong winds of Cook Strait then force the
nutrient-rich, upwelled water through the straits, and the intensification of this flow also
induces a sea-surface slope that accelerates water over the Kaharungi Shoals, thus creating
a strong upwelling source near Kaharungi Point. South of Cape Providence (46
S) there
is some uplift of water from the deep salinity maximum over the continental slope of
the Puysegur Bank resulting in high nitrate concentrations inshore (Bradford, 1983). At
the East Cape of North Island, which is rounded by the East Auckland Current, this is
accompanied by the formation of a persistent warm-core eddy offshore and upwelling
nearshore over Ranfurly Bank (Bradford et al., 1982).
However, even more important is the persistent upwelling in the open bight between
Cape Cascade (44
S) and Cape Foulwind (42
S). This simple wind-induced coastal divergence is limited by the shelf-edge front of the upwelled water that bears a series of
wavelike, regularly spaced plumes of cooler water (about 13
C) in the warmer subtropical
water mass of the Tasman Sea. Seaward of Abut Head the plume may reach 70 km offshore and carry much of the discharge from the Buller River. Inshore, both here and on
the east coast, freshwater runoff from these rainy islands creates a sandwich of multiple,
overlying surface-water masses identifiable in the density profiles of the upper 25 m. More
generally, cross-shelf canyons on this coast may induce cross-shelf plumes of buoyant,
inshore water that resemble the offshore squirts observed on the Californian coast: they
stabilize the water column and have consequences for the pelagic production system on
the outer shelf.
In the seas west of New Zealand, wherever frontal processes are weak, the mixed-layer
depth can be modeled simply by local irradiance and wind stress. The seasonal range in
mixed-layer depths is large—about 20 m in summer and 140–200 m in winter. This cycle is
largely convective, being generated more by variance in irradiance (simply, sun angle and
cloudiness) than by variance in wind stress. To the southwest, along the Fjordland coast,
the exceedingly heavy rainfall on the Southern Alps creates a well-defined buoyant layer,
5–10 m deep, which is extremely opaque within the fjords because of a very heavy load
of dissolved organic humics or “yellow substance.” This is widely distributed, at different
dilutions, over the shelves of the South Island. The deeper layers are oxygen-deficient
because of very low rates of light-limited primary production.
Regional Response of the Pelagic Ecosystem
This is a very difficult region in which to generalize the ecological processes, though a
winter-spring bloom, forced by the seasonal cycle of irradiance and changes in mixedlayer depth, is the dominant signal in the chlorophyll field around the islands. However,
the local dominance of a brief seasonal bloom in the confluence region east of the
Banks Peninsula over the Chatham Rise must also be emphasized, even though as noted
earlier, it occurs mostly on or beyond the logical boundary of the province. The brevity
of this bloom is striking, as seen in satellite images and in sediment trap mass fluxes
(King and Howard, 2001). More generally, chlorophyll accumulation is strongest during
September–October over the entire shelf area of New Zealand with strongest biomass
along the west and north coasts as well as the confluence region on the east coast from
Cook Strait to Banks peninsula.
Also observable in the seasonal images are the consequences of the dynamic processes
discussed above, especially in austral spring and summer. However, as Vincent et al.