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3.1 The Oceanography of the
New Zealand Marine Ecoregion
New Zealand is a relatively long, narrow
archipelago that lies athwart the West Wind
Drift and forms the western boundary to
the South Pacific Ocean south of 34ºS. The
position of New Zealand and the shape of
the bathymetric platform ensure that currents
are guided by the topography (Carter et al.
1998). This results in shelf edge currents,
uplift over shallow features, and oceanic
eddies that interact with coastal waters.
Oceanic water maintains an intimate contact
with coastal waters. Offshore currents are
particularly important in maintaining shelfbreak
fronts, e.g. the East Auckland Current
and the Southland Current/Subtropical Front.
The South Pacific western boundary current,
the East Australian Current, separates
from the coast of Australia and a flow of
water crosses the Tasman Sea forming the
Tasman Front. A portion of this warm, salty
Subtropical Water (STW) flows adjacent
to the northeast New Zealand landmass
to form the East Auckland Current, which
transports water southwards along the northeast
continental shelf (Stanton et al. 1997,
Stanton and Sutton 2003). Most of this flow
deflects south around East Cape as the East
Cape Current before forming the northern
side of the Subtropical Front (STF) (Tilburg
et al. 2001, Carter et al. 1998, Heath 1985).
Three permanent eddies lie offshore of this
boundary current: the North Cape, East
Cape, and Wairarapa Eddies (Roemich and
Sutton 1998). Currents probably uplift over
certain bathymetric features such as Mernoo
Gap, East Cape Ridge, Puysegur Bank, and
Three Kings Rise (e.g. Bradford and Roberts
1978, Bradford et al. 1991).
New Zealand intersects the circumpolar STF
which separates the subtropical gyres from
the Southern Ocean, i.e., warm, salty STW
from cold, fresh Subantarctic Water (SAW).
The STF passes south of Australia and
Tasmania and approaches New Zealand at
around 45ºS off Fiordland (Heath 1985). The
front deviates south, along the continental
margin, before following the shelf break
northwards along the east coast of the
South Island, where it is locally known as
the Southland Front and has an associated
current called the Southland Current. The
Southland Current advects mainly SAW with
peak surface speeds of 20–30cm per second
(Chiswell 1996, Sutton 2003). The STF turns
east along the crest of the Chatham Rise at
43.5ºS where it is constrained by the shallow
bathymetry to a limited depth of 300–350m
and a narrow width of approximately 100
km (Sutton 2001). Although the southern
limits of the South Island are at latitudes
normally associated with SAW, in fact the
entire coastal region is bathed in water of
STW origin, with the transition to SAW (i.e.
the STF) occurring at the continental shelf
break around the southern extreme of the
South Island.
New Zealand’s semidiurnal tides (M 2
and N 2
) have a complete 360º range of
phase around New Zealand (Walters et al.
2001). The semidiurnal tides have been
characterised as a coastally trapped Kelvin
wave travelling anticlockwise around the
shelf. Tidal elevations increase towards the
coast with a degenerate amphidrome situated
at the centre of New Zealand (Heath 1985).
A by-product of this geometry is that tides
are always 180º out of phase through Cook
Strait, resulting in very high tidal velocities
through the strait. High tidal velocities also
occur north of Cape Reinga and in Foveaux
Strait. These areas of strong tides are
associated with tidal mixing (Bowman et al.
1980).
New Zealand is located on the pole-ward
boundary of the South Pacific subtropical
gyre in the southwest Pacific Ocean. For
this reason shelf edge (at 200m) nitrate
concentrations are modest (about 5-15mmol
m -3 ) in comparison with many regions of
the world (Conkright et al. 2002) and are
similar in range to the northeast Atlantic
Ocean, although this statement is based
on very limited local data. The absolute
concentrations of dissolved inorganic
nutrients in oceanic water around New
Zealand depend on the water mass involved.
Coastal water is mainly of subtropical origin,
although SAW lies adjacent to the southeast
South Island slope. These two water masses
have different nutrient characteristics that
result in a north to south gradient in mean
nitrate at 200m at the shelf break (Ridgway
et al. 2002). The distribution of mean nitrate
ranges from 10mmol m -3 in the northwest
to 16mmol m -3 in the southeast. The
distribution on the west coast ranges from
10mmol m -3 in the northwest to 12mmol m -
3
in the southwest, with a minimum off the
central west coast of ≤6mmol m -3 .
STW has a typical mix of nutrients
(Tomczak and Godfrey 1994). Nitrate
(NO 3
) and dissolved reactive silica (DRSi)
are depleted more or less together (Zentara
BIODIVERSITY – NEW ZEALAND’S MARINE ECOREGION ■ 15