392
Chapter 11: The Pacific Ocean
There is, of course, an opposite flux of nutrients in the large anticyclonic eddies
discussed earlier because, as Whitney et al. (2005) point out, much of the transported
shelf water mass both has a near-coastal origin and lies deeper than the euphotic layer.
The consequent transport of macronutrients to the subthermocline layer of the open
subarctic gyre may not be significant, but the transport of micronutrients may well be.
Whitney et al. suggest that iron transported in this manner, originating in the coastal
discharges, may be a significant source of this element for the gyre as a whole. Anticyclonic
eddies that propagate across the Gulf of Alaska form a major transport system from
coastal waters to the open ocean, providing a steady flux from their iron-rich core water
(Johnson et al., 2005).
Circulation, buoyancy, and relative temperatures are clearly highly sensitive to short
and long-term periodic changes in atmospheric forcing, as well as to the aperiodic changes
that occur each year in the effects attributable to individual storm systems. As I shall
discuss later, it is also a place of great fisheries, and concern for the rational management of
these has created an unusual accumulation of observations on the interdecadal changes in
the marine environment. The dominant determinant of long-term changes in conditions
is the Pacific Decadal Oscillation (see Chapter 8) associated with the alternation of ENSO
conditions. In recent decades, the warm conditions of the very strong 1997–98 Niño
were followed by cold conditions in 1999, and the subsequent return to warmer inshore
temperatures continued at least until 2003, leading to comments about “regime changes.”
Again, freshwater discharge has been rather higher than the 50-year average since the
mid-1980s and a progressive shoaling of MLD in the open Gulf of Alaska in the recent
decades must be reflected appropriately in these coastal regions.
Regional Response of the Pelagic Ecosystem
Until recently, there has been very little information on the biological consequences
of circulation and buoyancy within this province (Anderson et al., 1977), save that
production in shelf waters is thought to occur at about twice the open-ocean rate
(200–300 compared to 70–100 g C m
−2 y
−1 ) and that this is concentrated in a spring
bloom that occurs progressively northward. Here, of course, the required stability for a
bloom to occur is given by the salinity profile, rather than by progressive warming of a
surface layer. We now have more comprehensive information, based mostly on analysis
of SeaWiFS data by Brickley and Thomas (2004), who present seasonal data for several
compartments of this province showing a spring bloom that is sustained well into summer
with higher chlorophyll concentrations on the shelf than in a 200-km strip beyond the
shelf. Their global estimates of seasonal changes in surface chlorophyll for the shelf region
correspond quite closely to the analysis presented later as a climatology. Relatively weak
phytoplankton growth in spring 1998 is coincident with positive SST anomalies associated
with the 1997–98 Niño. Brickley and Thomas suggest a general relationship between
strong winter downwelling conditions and a subsequent weak spring bloom.
Early ecological studies in the Strait of Georgia, at 48
N between Vancouver Island
and the Canadian mainland, have long served as a model for the other major sounds
(which were given regal names by their navigating discoverers: Queen Charlotte, Prince
of Wales, Alexander, and Prince William) up to 60
N in Alaska (Harrison et al., 1983).
A strong spring bloom is initiated in March (about 15 mg chl m
−3 ) and phytoplankton
biomass then progressively declines throughout the whole summer to reach very low
values (<1 mg chl m
−3 ) in November. A succession of diatom species occupies the seasons: Thalassiosira and Skeletonema dominate the spring bloom, followed by Chaetoceros,
Ditylum, Nitzschia, and Leptocylindricus in summer and Coscinodiscus in autumn. During
the spring bloom, these large cells exhibit growth rates from 0.5 to >15 doublings per
Chapter 11: The Pacific Ocean
There is, of course, an opposite flux of nutrients in the large anticyclonic eddies
discussed earlier because, as Whitney et al. (2005) point out, much of the transported
shelf water mass both has a near-coastal origin and lies deeper than the euphotic layer.
The consequent transport of macronutrients to the subthermocline layer of the open
subarctic gyre may not be significant, but the transport of micronutrients may well be.
Whitney et al. suggest that iron transported in this manner, originating in the coastal
discharges, may be a significant source of this element for the gyre as a whole. Anticyclonic
eddies that propagate across the Gulf of Alaska form a major transport system from
coastal waters to the open ocean, providing a steady flux from their iron-rich core water
(Johnson et al., 2005).
Circulation, buoyancy, and relative temperatures are clearly highly sensitive to short
and long-term periodic changes in atmospheric forcing, as well as to the aperiodic changes
that occur each year in the effects attributable to individual storm systems. As I shall
discuss later, it is also a place of great fisheries, and concern for the rational management of
these has created an unusual accumulation of observations on the interdecadal changes in
the marine environment. The dominant determinant of long-term changes in conditions
is the Pacific Decadal Oscillation (see Chapter 8) associated with the alternation of ENSO
conditions. In recent decades, the warm conditions of the very strong 1997–98 Niño
were followed by cold conditions in 1999, and the subsequent return to warmer inshore
temperatures continued at least until 2003, leading to comments about “regime changes.”
Again, freshwater discharge has been rather higher than the 50-year average since the
mid-1980s and a progressive shoaling of MLD in the open Gulf of Alaska in the recent
decades must be reflected appropriately in these coastal regions.
Regional Response of the Pelagic Ecosystem
Until recently, there has been very little information on the biological consequences
of circulation and buoyancy within this province (Anderson et al., 1977), save that
production in shelf waters is thought to occur at about twice the open-ocean rate
(200–300 compared to 70–100 g C m
−2 y
−1 ) and that this is concentrated in a spring
bloom that occurs progressively northward. Here, of course, the required stability for a
bloom to occur is given by the salinity profile, rather than by progressive warming of a
surface layer. We now have more comprehensive information, based mostly on analysis
of SeaWiFS data by Brickley and Thomas (2004), who present seasonal data for several
compartments of this province showing a spring bloom that is sustained well into summer
with higher chlorophyll concentrations on the shelf than in a 200-km strip beyond the
shelf. Their global estimates of seasonal changes in surface chlorophyll for the shelf region
correspond quite closely to the analysis presented later as a climatology. Relatively weak
phytoplankton growth in spring 1998 is coincident with positive SST anomalies associated
with the 1997–98 Niño. Brickley and Thomas suggest a general relationship between
strong winter downwelling conditions and a subsequent weak spring bloom.
Early ecological studies in the Strait of Georgia, at 48
N between Vancouver Island
and the Canadian mainland, have long served as a model for the other major sounds
(which were given regal names by their navigating discoverers: Queen Charlotte, Prince
of Wales, Alexander, and Prince William) up to 60
N in Alaska (Harrison et al., 1983).
A strong spring bloom is initiated in March (about 15 mg chl m
−3 ) and phytoplankton
biomass then progressively declines throughout the whole summer to reach very low
values (<1 mg chl m
−3 ) in November. A succession of diatom species occupies the seasons: Thalassiosira and Skeletonema dominate the spring bloom, followed by Chaetoceros,
Ditylum, Nitzschia, and Leptocylindricus in summer and Coscinodiscus in autumn. During
the spring bloom, these large cells exhibit growth rates from 0.5 to >15 doublings per