Pacific Westerly Winds Biome
339
provinces ALSK and CALC, and the polar province BERS. The oceanic region of the
Pacific Subarctic is their center of distribution from which some individuals are lost in
surface flow and by subduction. Of these, E. bungii, occurs just below the thermocline
in the equatorial Pacific (see Chapter 2), whereas N. cristatus is transported equatorward
at 600–800 m with the submergence of Oyashio water below the Kuroshio off Japan: but
who knows how far some expatriate individuals of these and other species may not be
transported?
PSAG(E) is the site of two of the great enigmas that puzzled biological oceanographers
for many decades: (i) why should a spring bloom (in the sense of an accumulation of
chlorophyll) not occur here as it does in the North Atlantic? and (ii) why does primary
production during summer not utilize all the available mixed-layer nitrate? I shall return
to these questions later, but readers will recall that the explanation that for many years
seemed satisfactory was that the phytoplankton at OWS P was dominated by small
(< 20 m) cells, consumed by copepods whose populations are sufficiently large at the
end of winter as to suppress the accumulation of chlorophyll when productivity increases
in spring. This model, of course, has been replaced by others that all involve some aspect
of Fe limitation—it will be recalled that this region was the first high-S region to be
recognized and the site of the first Fe-limitation experiments—but I suggest that the real
system is far more complex than suggested by simple Fe-limitation models, such as that
of Banse and English (1999). This was based on seasonal CZCS images in which neither
spring nor summer blooms could be observed satisfactorily anywhere in the Subarctic
Pacific, although some episodic autumn blooms were seen. These, it was suggested, were
responses to iron in episodic falls of volcanic dust. Unfortunately for this model, the
inferred lack of a chlorophyll accumulation cycle was artifactual, due to the very poor
coverage of the region by the CZCS sensors.
That a seasonal cycle of production and chlorophyll accumulation does occur here is
now reasonably well known, although our accounts of it are heavily weighted by observations at OWS P: almost all accounts of the Pacific Subarctic regime continue to emphasize
the “lack of a spring bloom” or rather the lack of seasonal accumulation of chlorophyll.
But, as the SeaWiFS and MODIS images make clear, there are significant differences
between seasonal cycles in the two subgyres: in PSAG(E), as expected from OWS P data,
chlorophyll accumulation and production rate both remain relatively high in summer,
with a seasonal range of 025–06 chl m
−3 . This corresponds well with observations at
OWS P of seasonal changes in chlorophyll biomass of 02–04 mg chl m
−3 . Peripherally
around the gyre, however, this range increases and a spring maximum is observed in the
SeaWiFS data; at 200 km beyond the shelf, this occurs in May, and the annual range is
05–14 chl m
−3 (Brickley and Thomas, 2004). In the data representing the entire Western
gyre, a sharper peak in both chlorophyll biomass and production rate may occurs during
May, as it did in four years (1998–2001) in the SeaWiFS observations and again in 2004
in MODIS data; this spring bloom appears to be a typical seasonal response. Further, the
seasonal range of chlorophyll biomass in PSAG(W) is greater than in PSAG(E), increasing
from winter values of 025 chl m
−3 to 13 chl m
−3 in spring.
The JGOFS section along the P line, and at OWS P, in 1992–1997 showed that small
(< 5 m) cells dominated the phytoplankton everywhere, with some evolution offshore
along the line toward a relative increase in the importance of very small cells. Further,
even in the presence of a DCM, there was very little change in size composition down the
profiles (Boyd and Harrison, 1999). These cells are predominantly autotrophic flagellates.
Episodic contributions of larger cells (>20 m) to total biomass and productivity were
noted, as at the inshore end of P line in March 1993, and in March 1996 and September
1995 at the outermost stations. In such episodes, small diatoms are dominant, and these
cells are more important every year during the period of higher chlorophyll biomass.
339
provinces ALSK and CALC, and the polar province BERS. The oceanic region of the
Pacific Subarctic is their center of distribution from which some individuals are lost in
surface flow and by subduction. Of these, E. bungii, occurs just below the thermocline
in the equatorial Pacific (see Chapter 2), whereas N. cristatus is transported equatorward
at 600–800 m with the submergence of Oyashio water below the Kuroshio off Japan: but
who knows how far some expatriate individuals of these and other species may not be
transported?
PSAG(E) is the site of two of the great enigmas that puzzled biological oceanographers
for many decades: (i) why should a spring bloom (in the sense of an accumulation of
chlorophyll) not occur here as it does in the North Atlantic? and (ii) why does primary
production during summer not utilize all the available mixed-layer nitrate? I shall return
to these questions later, but readers will recall that the explanation that for many years
seemed satisfactory was that the phytoplankton at OWS P was dominated by small
(< 20 m) cells, consumed by copepods whose populations are sufficiently large at the
end of winter as to suppress the accumulation of chlorophyll when productivity increases
in spring. This model, of course, has been replaced by others that all involve some aspect
of Fe limitation—it will be recalled that this region was the first high-S region to be
recognized and the site of the first Fe-limitation experiments—but I suggest that the real
system is far more complex than suggested by simple Fe-limitation models, such as that
of Banse and English (1999). This was based on seasonal CZCS images in which neither
spring nor summer blooms could be observed satisfactorily anywhere in the Subarctic
Pacific, although some episodic autumn blooms were seen. These, it was suggested, were
responses to iron in episodic falls of volcanic dust. Unfortunately for this model, the
inferred lack of a chlorophyll accumulation cycle was artifactual, due to the very poor
coverage of the region by the CZCS sensors.
That a seasonal cycle of production and chlorophyll accumulation does occur here is
now reasonably well known, although our accounts of it are heavily weighted by observations at OWS P: almost all accounts of the Pacific Subarctic regime continue to emphasize
the “lack of a spring bloom” or rather the lack of seasonal accumulation of chlorophyll.
But, as the SeaWiFS and MODIS images make clear, there are significant differences
between seasonal cycles in the two subgyres: in PSAG(E), as expected from OWS P data,
chlorophyll accumulation and production rate both remain relatively high in summer,
with a seasonal range of 025–06 chl m
−3 . This corresponds well with observations at
OWS P of seasonal changes in chlorophyll biomass of 02–04 mg chl m
−3 . Peripherally
around the gyre, however, this range increases and a spring maximum is observed in the
SeaWiFS data; at 200 km beyond the shelf, this occurs in May, and the annual range is
05–14 chl m
−3 (Brickley and Thomas, 2004). In the data representing the entire Western
gyre, a sharper peak in both chlorophyll biomass and production rate may occurs during
May, as it did in four years (1998–2001) in the SeaWiFS observations and again in 2004
in MODIS data; this spring bloom appears to be a typical seasonal response. Further, the
seasonal range of chlorophyll biomass in PSAG(W) is greater than in PSAG(E), increasing
from winter values of 025 chl m
−3 to 13 chl m
−3 in spring.
The JGOFS section along the P line, and at OWS P, in 1992–1997 showed that small
(< 5 m) cells dominated the phytoplankton everywhere, with some evolution offshore
along the line toward a relative increase in the importance of very small cells. Further,
even in the presence of a DCM, there was very little change in size composition down the
profiles (Boyd and Harrison, 1999). These cells are predominantly autotrophic flagellates.
Episodic contributions of larger cells (>20 m) to total biomass and productivity were
noted, as at the inshore end of P line in March 1993, and in March 1996 and September
1995 at the outermost stations. In such episodes, small diatoms are dominant, and these
cells are more important every year during the period of higher chlorophyll biomass.