WWF Shining a spotlight on the biodiversity of ... - MarineNZ.org.nz

The dinoflagellate, Karenia brevisulcata, a
New Zealand phytoplankton species that
produces a potent toxin.
and Kamykowski
1981). SAW, as
well as being low
in iron and copper
(Sedwick et al.
1997, Croot and
Hunter 1998),
has an excess of
NO 3
relative to
DRSi (Zentara and
Kamykowski 1981).
The interaction of
SAW and STW at
the STF, especially
over the Chatham
Rise, results in
this region being
highly productive
(Bradford-Grieve
et al. 1999).
In addition,
atmospheric
transport of iron
from arid and
semi-arid parts of
Australia may be
a source of iron to surface seawater in this
region (Kieber et al. 2001; Boyd et al. 2004).
It is thought that freshwater is generally
not an important source of nutrients on the
open coast, given the degree of dilution
(e.g. Hawke and Hunter 1992). Deep winter
mixing and upwelling of deep waters are
more likely to be dominant in enhancing
nutrients in surface waters.
The nutrient content of the oceans has
implications for ecosystem structure and
function. For example, the Southern Plateau
behaves like a system that is low in nutrients
(despite high NO 3
) and primary production,
with phytoplankton dominated by very
small cells. It is a low total biomass, low
productivity system with high transfer
efficiency (Bradford-Grieve et al. 2003).
In this region there is very little organic
matter arriving at the sea floor and most
of the production occurs in the water
column. This system is apparently tightly
coupled. On the other hand, productivity is
much greater in the STF over the Chatham
Rise, phytoplankton cells are much larger,
and there is much more sedimentation of
organic matter to the sea floor (Nodder and
Northcote 2001).
It is not only the water mass that has
significance for nutrient supply to
phytoplankton. Mesoscale processes that
influence the supply of nutrients to surface
waters and seasonal patterns of heating
and cooling (as reflected in the depth of
the surface mixed layer) are significant
contributors to the patterns of phytoplankton
distribution as seen from space as sea colour
(Murphy et al. 2001).
The dynamical signature of the warmcore
eddies down the east coast of the
North Island results in deep, surface winter
mixing (greater than would occur in the
adjacent water from which the eddies
formed (Bradford et al. 1982, Bradford and
Chapman 1988). These eddies contribute
to enhanced winter nutrient renewal, lower
winter phytoplankton biomass, and are
probably responsible for the generally
extensive spring phytoplankton bloom in
the region (Murphy et al. 2001). Not only
do these eddies change the seasonal pattern
of nutrient renewal but they can also retain
larvae of coastal organisms (Chiswell and
Booth 1999) or entrain coastal water taking
it offshore (Bradford and Chapman 1988).
Winds are important in driving upwellings
that bring nutrient-rich water to the surface.
In the Southern Hemisphere winds blowing
parallel to a coast cause surface waters
to move to the left of the wind. Thus a
northwesterly wind blowing along the coast
north of Auckland (Sharples and Greig 1998)
and a southwesterly wind blowing off the
west coast of the South Island (Stanton and
Moore 1992) will move surface water away
from the coast and bring nutrient-rich deeper
water to the surface inshore. A conspicuous
summer feature of western Cook Strait is a
large upwelling plume that originates from
Kahurangi Point (Bradford-Grieve et al.
©NIWA/Hoe Chang
16 ■ BIODIVERSITY – NEW ZEALAND’S MARINE ECOREGION