438
Chapter 11: The Pacific Ocean
(1991) note, a simple statistical correlation cannot be found between physical and nutrient
variables at groups of individual stations and biological variables at the same stations;
moreover, Chang et al. (1992) suggest that strong winds and unusually constant deep
wind mixing around the New Zealand coasts may constrain both the accumulation of
phytoplankton cells and the complete utilization of nitrate. This is too dynamic a region
for simple relationships between nitrate and chlorophyll to be evident in survey data, and
the relationships that might exist with phytoplankton growth rate are, as Vincent et al.
point out, probably masked by the fact that phytoplankton biomass is a secondary variable
representing, as I have noted several times, merely the resultant between accumulation
and loss. Nevertheless, high F ratios (new/regenerated or nitrate/ammonium utilization
ratios) reaching 0.9 are often encountered inshore in the Westland upwelling region.
Nitrate is often in excess of limiting concentration except in the shelf areas adjoining
the TASM province, where a simple Sverdrup model is probably appropriate and where
nitrate reaches limiting levels in surface water in summer. To the south and east of New
Zealand, the surface water masses are distinguishable by their NO 3 levels: 4–7 M in
subtropical and 12 M in subantarctic water. Elsewhere in the coastal regions, values of
0.5 to about 30 M are normally encountered. Production processes have been studied
extensively in Tasman Bay in terms of the relation between salinity stratification early in
the year and thermal stratification later in the summer; in early summer, the photic zone is
nutrient-deplete and has very low chlorophyll, but the turbid, well-mixed subpycnocline
water has relatively high phytoflagellate biomass: this distribution has significance for
filter-feeding benthic organisms. When the salinity stratification breaks down, and prior
to thermal stratification, a diatom bloom ensues (MacKenzie and Adamson, 2004). More
typical of shelf areas is the East Cape region studied by Bradford et al. (1982), where
the phytoplankton response to the anticyclonicity of the eddying induced by the change
in direction of flow around the cape. Breakdown of stratification and consequent high
near-surface nutrient concentration caused by flow over Ranfurly Bank is associated with
regional maxima in phytoplankton biomass and productivity. This, in turn, is closely
reflected in the regional distribution of zooplankton biomass.
Off Otago, on the southeast coast, the Southland Current remains largely subsurface,
below buoyant plumes of coastal water that sustain larger biomass of algal cells than
does the Southland Current when it surfaces; the diatom-rich buoyant water is several
kilometers wide along the coast, suggesting that riverborne nutrients are very important
here. The same pattern has been observed on the west coast of South Island: diatom-rich
coastal water, and phytoflagellates offshore. However, in summer (February) dinoflagellates may reach bloom concentrations both inshore and off, whereas diatoms occur
only at the bottom of the photic zone (Bradford and Chang, 1982). In El Niño years,
unusually cold SST around New Zealand may be associated with outbreak blooms of the
coccolithophores Gephyrocapsa oceanica and Emiliania huxleyi.
Along the northwest coast of South Island, both upwelling from aphotic depths and
river water enrich the euphotic zone in nutrients (MacKenzie et al., 1983) with resulting
elevated biomass of both heterotrophic bacteria and autotrophic cells; a DCM occurs
near the nutricline here.
Here, off New Zealand, as in so many other sea areas in the past 10 years, we have
confirmation of the paramount importance of the picophytoplankton, both prokaryotes
and eukaryotes. In the upwelling region of the Westland coast, Hall and Vincent (1994)
have demonstrated the important role these organisms play in the whole phytoplankton
community. In summer the < 2 − m fraction accounted for 73% of phytoplankton
particulate nitrogen, whereas in winter 40–80% of particulate nitrogen, 55% of chlorophyll, and 45–70% of primary productivity are attributable to the pico fraction. At single
stations (e.g., at the edge of the upwelling plumes) prokaryotes contributed 99% of
all cells. These studies also reinforce the general observation of the high variability in
Chapter 11: The Pacific Ocean
(1991) note, a simple statistical correlation cannot be found between physical and nutrient
variables at groups of individual stations and biological variables at the same stations;
moreover, Chang et al. (1992) suggest that strong winds and unusually constant deep
wind mixing around the New Zealand coasts may constrain both the accumulation of
phytoplankton cells and the complete utilization of nitrate. This is too dynamic a region
for simple relationships between nitrate and chlorophyll to be evident in survey data, and
the relationships that might exist with phytoplankton growth rate are, as Vincent et al.
point out, probably masked by the fact that phytoplankton biomass is a secondary variable
representing, as I have noted several times, merely the resultant between accumulation
and loss. Nevertheless, high F ratios (new/regenerated or nitrate/ammonium utilization
ratios) reaching 0.9 are often encountered inshore in the Westland upwelling region.
Nitrate is often in excess of limiting concentration except in the shelf areas adjoining
the TASM province, where a simple Sverdrup model is probably appropriate and where
nitrate reaches limiting levels in surface water in summer. To the south and east of New
Zealand, the surface water masses are distinguishable by their NO 3 levels: 4–7 M in
subtropical and 12 M in subantarctic water. Elsewhere in the coastal regions, values of
0.5 to about 30 M are normally encountered. Production processes have been studied
extensively in Tasman Bay in terms of the relation between salinity stratification early in
the year and thermal stratification later in the summer; in early summer, the photic zone is
nutrient-deplete and has very low chlorophyll, but the turbid, well-mixed subpycnocline
water has relatively high phytoflagellate biomass: this distribution has significance for
filter-feeding benthic organisms. When the salinity stratification breaks down, and prior
to thermal stratification, a diatom bloom ensues (MacKenzie and Adamson, 2004). More
typical of shelf areas is the East Cape region studied by Bradford et al. (1982), where
the phytoplankton response to the anticyclonicity of the eddying induced by the change
in direction of flow around the cape. Breakdown of stratification and consequent high
near-surface nutrient concentration caused by flow over Ranfurly Bank is associated with
regional maxima in phytoplankton biomass and productivity. This, in turn, is closely
reflected in the regional distribution of zooplankton biomass.
Off Otago, on the southeast coast, the Southland Current remains largely subsurface,
below buoyant plumes of coastal water that sustain larger biomass of algal cells than
does the Southland Current when it surfaces; the diatom-rich buoyant water is several
kilometers wide along the coast, suggesting that riverborne nutrients are very important
here. The same pattern has been observed on the west coast of South Island: diatom-rich
coastal water, and phytoflagellates offshore. However, in summer (February) dinoflagellates may reach bloom concentrations both inshore and off, whereas diatoms occur
only at the bottom of the photic zone (Bradford and Chang, 1982). In El Niño years,
unusually cold SST around New Zealand may be associated with outbreak blooms of the
coccolithophores Gephyrocapsa oceanica and Emiliania huxleyi.
Along the northwest coast of South Island, both upwelling from aphotic depths and
river water enrich the euphotic zone in nutrients (MacKenzie et al., 1983) with resulting
elevated biomass of both heterotrophic bacteria and autotrophic cells; a DCM occurs
near the nutricline here.
Here, off New Zealand, as in so many other sea areas in the past 10 years, we have
confirmation of the paramount importance of the picophytoplankton, both prokaryotes
and eukaryotes. In the upwelling region of the Westland coast, Hall and Vincent (1994)
have demonstrated the important role these organisms play in the whole phytoplankton
community. In summer the < 2 − m fraction accounted for 73% of phytoplankton
particulate nitrogen, whereas in winter 40–80% of particulate nitrogen, 55% of chlorophyll, and 45–70% of primary productivity are attributable to the pico fraction. At single
stations (e.g., at the edge of the upwelling plumes) prokaryotes contributed 99% of
all cells. These studies also reinforce the general observation of the high variability in