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1986) that is responsible for considerable
biological enhancement in the region.
The seasonal pattern of mixed-layer depth
varies from north to south with water
mass and with the large-scale circulation
(Longhurst 1998). The seasonal pattern
of mixed layer depth interacts with the
nutrient characteristics of the water masses,
freshwater water inflow, and light penetration
(mainly through the depth of the sunlit
surface layer) to determine the seasonal
patterns of primary production, nutrient
depletion, and ecosystem structure.
A comprehensive overview of the
distribution of plankton is available only
for phytoplankton as observed from space
through the time period 1997–2000 (Murphy
et al. 2001). STW to the north and in the Tasman
Sea has a classical cycle of spring and
autumn chlorophyll blooms consistent with
production being co-limited by nitrate and
light. Chlorophyll-a concentrations varied
annually between about 0.1 and 0.4mg m -3 to
the north and in the Tasman Sea, but east of
New Zealand the mean maximum was 0.8mg
m -3 . SAW has a low-amplitude annual cycle
of chlorophyll abundance that peaks in early
autumn, consistent with production being
limited predominantly by a combination of
iron and light. SAW chlorophyll-a concentrations
varied from 0.1 to 0.3mg m -3 and rarely
exceed 0.4mg m -3 . Chlorophyll-a is generally
greatest in the STF and has the greatest
variability with concentrations varying from
≤0.1mg m -3 to ≥1mg m -3 . Through winter,
elevated chlorophyll concentrations over the
Chatham Rise often occur in a narrow band
south of the Rise and tend not to extend as
far as the Chatham Islands. At other times of
the year there was a broader and more complex
region of increased chlorophyll stretching
across the whole length of the Rise and
beyond the Chatham Islands. Chlorophyll
concentrations were higher
in the STF east of New Zealand (mean
0.6mg m -3 ) with some evidence of a spring
and autumn peak in the seasonal cycle compared
with the STF to the west (mean of
0.4mg m -3 ) and without a consistent seasonal
pattern across the years analysed.
There is no similarly extensive picture of the
quantities of zooplankton present in the New
Zealand region. Bradford and Roberts (1978)
show that, using limited data for the whole
New Zealand region, there is a significant
positive correlation between zooplankton
biomass and surface chlorophyll-a.
Nevertheless, no such relationship is evident
for STW analysed separately. It is clear that
the zooplankton biomass picture is much
more complicated than can be interpreted
from the limited data that we have for the
New Zealand region. Any understanding
of the spatial and temporal distribution of
zooplankton in the New Zealand region will
have to take into account the dynamics of
the planktonic production system, ecosystem
structure, and trophic relationships among
plants and animals in the system.
Contributors:
Janet Bradford-Grieve and Philip Sutton.
The copepod, Neocalanus tonsus, a common zooplankton species in New Zealand’s offshore waters.
BIODIVERSITY – NEW ZEALAND’S MARINE ECOREGION ■ 17
M.D. Ohman, Pelagic Invertebrates Collection, Scripps Institution of Oceanography