342
Chapter 11: The Pacific Ocean
Turning now to the western gyre, PSAG(W), we have less information than for the
Alaskan gyre although, as discussed earlier, the SeaWiFS and MODIS images show clearly
that a spring bloom does occur here with all that that implies for ecosystem functioning.
As already noted, we also now have information from the KNOT station investigations,
although this station is somewhat anomalous because the influence of the Oyashio influx
into the gyral circulation. There is also useful serial nutrient data, routinely obtained by
container ships on the Vancouver-Japan route that passes directly across both subgyres
of the Subarctic Province.
These data (Wong et al., 2002) show that NO 3 is drawn down to lower levels in
summer in the western gyre than at OWS P, especially in the southern part near Station
KNOT (Kyoto North Pacific Ocean Time Series, 44
N 155
E), where June values of
2 M liter
−1 are not unusual. Moreover, the SiO 3 /NO 3 ratios in these data indicate
that much of the nutrient utilization supports a diatom bloom, as captured in the
serial observations at KNOT (Liu, 2002a; Mochizuki et al., 2002). Here, centric diatom
(Thalassiosira, Coscinodiscus) numbers dominate the large cell fraction year-round with
seasonal species succession, whereas pennate diatoms (Fragilariopsis, Neodenticulata) are
important only in spring. Biomass is dominated by the smaller cell fraction year-round
against a seasonally varying abundance of diatoms that form >30% of autotrophic biomass
in May, but <10% at other times.
Bacterial abundance at KNOT, like the picophytoplankton, shows greater seasonal
change (014–13 × 10
−6 cells liter
−1 ) than in the Alaskan gyre, the seasonal maximum
being lagged several months behind the seasonal chlorophyll maximum. Seasonal changes
occur in the composition of the pico fraction of the autotrophic cells, so that the
spring bloom diatoms are followed first by picoeukaryotes, that are then succeeded by
Synechococcus in late summer.
Here, it has been proposed that the spring diatom bloom is induced by Fe that is
delivered in dust from the deserts of China; would it be facetious to suggest that fashion
currently demands that wherever you need it, Fe is found to be right at hand? It is
not hard to think of other reasons why diatoms here, as elsewhere, should build their
population size in the spring. For one, the influence of Oyashio conditions in the region
of KNOT would appear to be a sufficient explanation: here, after winter recharge of
surface nutrient levels, silicate limitation apparently terminates a diatom bloom that is
initiated by stratification and enhanced irradiance (e.g., Limsakul et al., 2002). Here,
unlike at OWS P, mixed-layers depths exceed 100 m in winter and are <20 m in summer.
Current thinking among KNOT researchers also favors significant constraints on diatom
biomass by herbivorous mesozooplankton (e.g., Fujii et al., 2002).
The generally more productive ecosystem of the western gyre supports larger populations of higher trophic levels than the Alaskan gyre. The migrations of Southern Ocean
shearwaters (Puffinus spp.) take them into the Pacific subarctic during boreal summer
and they are (as reported by Springer et al., 1999) an order of magnitude more abundant
in the western gyre than in the east. Several species of seabirds that nest in the eastern
region resort later to the western gyre to feed. These observations all suggest that the
prey of these birds (euphausiids, saury, and Atka mackerel) is more relatively abundant
in the western gyre. Baleen whales and plankton-feeding seabirds do not follow the same
pattern but are more evenly distributed. Interannual variability in the abundance of the
components of the oceanic ecosystem is critical for the breeding success of many seabirds
around the coasts of the region.
Synopsis
Case 2—Nutrient-limited spring production peak—Pycnocline undergoes a boreal winter
excursion that is deeper in PASG (W) than in the east, but shoaling does not begin
Chapter 11: The Pacific Ocean
Turning now to the western gyre, PSAG(W), we have less information than for the
Alaskan gyre although, as discussed earlier, the SeaWiFS and MODIS images show clearly
that a spring bloom does occur here with all that that implies for ecosystem functioning.
As already noted, we also now have information from the KNOT station investigations,
although this station is somewhat anomalous because the influence of the Oyashio influx
into the gyral circulation. There is also useful serial nutrient data, routinely obtained by
container ships on the Vancouver-Japan route that passes directly across both subgyres
of the Subarctic Province.
These data (Wong et al., 2002) show that NO 3 is drawn down to lower levels in
summer in the western gyre than at OWS P, especially in the southern part near Station
KNOT (Kyoto North Pacific Ocean Time Series, 44
N 155
E), where June values of
2 M liter
−1 are not unusual. Moreover, the SiO 3 /NO 3 ratios in these data indicate
that much of the nutrient utilization supports a diatom bloom, as captured in the
serial observations at KNOT (Liu, 2002a; Mochizuki et al., 2002). Here, centric diatom
(Thalassiosira, Coscinodiscus) numbers dominate the large cell fraction year-round with
seasonal species succession, whereas pennate diatoms (Fragilariopsis, Neodenticulata) are
important only in spring. Biomass is dominated by the smaller cell fraction year-round
against a seasonally varying abundance of diatoms that form >30% of autotrophic biomass
in May, but <10% at other times.
Bacterial abundance at KNOT, like the picophytoplankton, shows greater seasonal
change (014–13 × 10
−6 cells liter
−1 ) than in the Alaskan gyre, the seasonal maximum
being lagged several months behind the seasonal chlorophyll maximum. Seasonal changes
occur in the composition of the pico fraction of the autotrophic cells, so that the
spring bloom diatoms are followed first by picoeukaryotes, that are then succeeded by
Synechococcus in late summer.
Here, it has been proposed that the spring diatom bloom is induced by Fe that is
delivered in dust from the deserts of China; would it be facetious to suggest that fashion
currently demands that wherever you need it, Fe is found to be right at hand? It is
not hard to think of other reasons why diatoms here, as elsewhere, should build their
population size in the spring. For one, the influence of Oyashio conditions in the region
of KNOT would appear to be a sufficient explanation: here, after winter recharge of
surface nutrient levels, silicate limitation apparently terminates a diatom bloom that is
initiated by stratification and enhanced irradiance (e.g., Limsakul et al., 2002). Here,
unlike at OWS P, mixed-layers depths exceed 100 m in winter and are <20 m in summer.
Current thinking among KNOT researchers also favors significant constraints on diatom
biomass by herbivorous mesozooplankton (e.g., Fujii et al., 2002).
The generally more productive ecosystem of the western gyre supports larger populations of higher trophic levels than the Alaskan gyre. The migrations of Southern Ocean
shearwaters (Puffinus spp.) take them into the Pacific subarctic during boreal summer
and they are (as reported by Springer et al., 1999) an order of magnitude more abundant
in the western gyre than in the east. Several species of seabirds that nest in the eastern
region resort later to the western gyre to feed. These observations all suggest that the
prey of these birds (euphausiids, saury, and Atka mackerel) is more relatively abundant
in the western gyre. Baleen whales and plankton-feeding seabirds do not follow the same
pattern but are more evenly distributed. Interannual variability in the abundance of the
components of the oceanic ecosystem is critical for the breeding success of many seabirds
around the coasts of the region.
Synopsis
Case 2—Nutrient-limited spring production peak—Pycnocline undergoes a boreal winter
excursion that is deeper in PASG (W) than in the east, but shoaling does not begin