Pacific Trade Winds Biome
359
Large-scale gradients are lacking, as indicated by the depth and kind of the DCM,
compared with the NPST province north of the Subtropical Convergence; zonal transPacific sections of chlorophyll and primary production along 24
N (Venrick, 1989, 1991)
have much greater uniformity than zonal sections along 47
N. The ridge-trough topography of the pycnocline associated with the zonal equatorial current system dominates
the stability and vertical eddy diffusivity of the regional water column and, hence, the
long-term supply of nutrients to the photic zone. The vertical ecological structure is
modified by the development of a seasonal shallow thermocline (Hayward et al., 1983),
following the general model for a subtropical ocean with moderate winter mixing, as in
the Sargasso Sea (see NATR).
The deep, nitrate-depleted euphotic zone of this province in summer, reaching even to
100 m, lies above a permanent nutricline. Year-round, and each year, and right across the
subtropical gyre, a DCM occurs at 80–120 m, lying just above the nitracline and the permanent thermocline, and close to the 1% isolume (Venrick, 1991). In meridional sections
(Venrick et al., 1973), the DCM is seen to conform to the general northward upsloping of
the thermocline across the province. Some seasonal change occurs in the depth of this feature from <100 m in winter to >100 m in summer. The DCM and nitracline remain deep
when the weak summer thermocline is established at 35–45 m, well above them. The summer DCM contains maximum values of around 02–04 mg chl m
−3 , whereas the winter
DCM is a weaker feature, usually about 01 mg chl m
−3 . Primary production rate is maximal rather shallower in the mixed layer, at 30–60 m. The profiles studied at ALOHA, and
first reported by Letelier et al. (1996), integrate very well into Venrick’s transpacific section
and show that although primary production rates increase somewhat in summer, the
interannual differences are at least as great as seasonal differences: the seasonal/interannual
range is between 200 and 700 mg C m
−2 d
−1 . The effect of seasonal changes in surface
irradiance is very marked: there is no regular seasonality in production rate in the 0–100 m
layer, but at 100–200 m the rate (about 20% of that in the shallower zone) shows very
clear midsummer maxima in 4 of the 5 years reported by Karl et al., (1996).
The vertical diffusive nutrient flux apparently required by these observations can only be
partially satisfied by observed nitrate uptake, and the mechanism by which calculated new
production is sustained above the summer thermocline remains unexplained. In the presence
of a stable vertical ecological structure, as is observed, we would expect any upward physically
driven nitrate flux across the nitracline to be taken up in the DCM (Hayward, 1987).
Karl et al. (2001) have integrated the observations at ALOHA with the data from
CLIMAX. The very long series he obtained in this way runs from 1968 to 1997 and
demonstrates that what these authors term a “domain shift” in the pelagic production
system occurred during this period. The CLIMAX observations already showed that from
1968 to 1985 the rate of primary production by autotrophic cells had almost doubled
during the summer (May–October). It remains uncertain if this was a continuous process
or whether a step-function increase occurred between 1973 and 1980, partly because this
was a period of great change in the methods of biological oceanography. Nevertheless,
the data have been very carefully intercompared, and the results are convincing. Timeintegrated profiles of chlorophyll and autotrophic production show that the significant
increase in production occurred mostly within the 0–50 m layer, while the DCM did
not respond to whatever forced the increase and remained close to 100 m throughout
the series. Karl et al. (2001) suggested that these changes occurred in response to an
unprecedented period of sustained values of the SOI (see Chapter 8) favorable to the
development of El Niño conditions.
These changes were accompanied by a reduction in dissolved silicate and phosphate
and a shift in the structure of the phytoplankton community that was convincingly
indicated by the relative increase in chlb that occurred during this period. Such a shift
toward an ecosystem dominated increasingly by prokaryotes carries major implications
359
Large-scale gradients are lacking, as indicated by the depth and kind of the DCM,
compared with the NPST province north of the Subtropical Convergence; zonal transPacific sections of chlorophyll and primary production along 24
N (Venrick, 1989, 1991)
have much greater uniformity than zonal sections along 47
N. The ridge-trough topography of the pycnocline associated with the zonal equatorial current system dominates
the stability and vertical eddy diffusivity of the regional water column and, hence, the
long-term supply of nutrients to the photic zone. The vertical ecological structure is
modified by the development of a seasonal shallow thermocline (Hayward et al., 1983),
following the general model for a subtropical ocean with moderate winter mixing, as in
the Sargasso Sea (see NATR).
The deep, nitrate-depleted euphotic zone of this province in summer, reaching even to
100 m, lies above a permanent nutricline. Year-round, and each year, and right across the
subtropical gyre, a DCM occurs at 80–120 m, lying just above the nitracline and the permanent thermocline, and close to the 1% isolume (Venrick, 1991). In meridional sections
(Venrick et al., 1973), the DCM is seen to conform to the general northward upsloping of
the thermocline across the province. Some seasonal change occurs in the depth of this feature from <100 m in winter to >100 m in summer. The DCM and nitracline remain deep
when the weak summer thermocline is established at 35–45 m, well above them. The summer DCM contains maximum values of around 02–04 mg chl m
−3 , whereas the winter
DCM is a weaker feature, usually about 01 mg chl m
−3 . Primary production rate is maximal rather shallower in the mixed layer, at 30–60 m. The profiles studied at ALOHA, and
first reported by Letelier et al. (1996), integrate very well into Venrick’s transpacific section
and show that although primary production rates increase somewhat in summer, the
interannual differences are at least as great as seasonal differences: the seasonal/interannual
range is between 200 and 700 mg C m
−2 d
−1 . The effect of seasonal changes in surface
irradiance is very marked: there is no regular seasonality in production rate in the 0–100 m
layer, but at 100–200 m the rate (about 20% of that in the shallower zone) shows very
clear midsummer maxima in 4 of the 5 years reported by Karl et al., (1996).
The vertical diffusive nutrient flux apparently required by these observations can only be
partially satisfied by observed nitrate uptake, and the mechanism by which calculated new
production is sustained above the summer thermocline remains unexplained. In the presence
of a stable vertical ecological structure, as is observed, we would expect any upward physically
driven nitrate flux across the nitracline to be taken up in the DCM (Hayward, 1987).
Karl et al. (2001) have integrated the observations at ALOHA with the data from
CLIMAX. The very long series he obtained in this way runs from 1968 to 1997 and
demonstrates that what these authors term a “domain shift” in the pelagic production
system occurred during this period. The CLIMAX observations already showed that from
1968 to 1985 the rate of primary production by autotrophic cells had almost doubled
during the summer (May–October). It remains uncertain if this was a continuous process
or whether a step-function increase occurred between 1973 and 1980, partly because this
was a period of great change in the methods of biological oceanography. Nevertheless,
the data have been very carefully intercompared, and the results are convincing. Timeintegrated profiles of chlorophyll and autotrophic production show that the significant
increase in production occurred mostly within the 0–50 m layer, while the DCM did
not respond to whatever forced the increase and remained close to 100 m throughout
the series. Karl et al. (2001) suggested that these changes occurred in response to an
unprecedented period of sustained values of the SOI (see Chapter 8) favorable to the
development of El Niño conditions.
These changes were accompanied by a reduction in dissolved silicate and phosphate
and a shift in the structure of the phytoplankton community that was convincingly
indicated by the relative increase in chlb that occurred during this period. Such a shift
toward an ecosystem dominated increasingly by prokaryotes carries major implications