332
Chapter 11: The Pacific Ocean
again increased, although—at least until the end of the century—it remained significantly
smaller than before 1976 (Schumacher et al., 2003). These changes are attributed to
interaction between steplike changes in regional processes and the longer-term effects of
the secular global warming trend.
Regional Response of the Pelagic Ecosystem
In these two marginal seas, algal growth is both light- and nutrient-limited. Since the
winter water column is well mixed, when irradiance reaches levels sufficient for growth
of cells, nitrate concentration at the surface is high (10–25 M NO 3 liter
−1 ), but is
rapidly reduced to limiting concentrations after early summer algal growth (McRoy and
Goering, 1974).
The satellite images are unequivocal, and record a regional maximum of chlorophyll
accumulation (3–5 mg chl m
−3 ) at the surface, and as integrated over the euphotic zone
(70–100 mg chl m
−2 ) in April of each year with great regularity. A secondary peak of
chlorophyll accumulation (∼15 mg chl m
−3 ) occurs in August–September. The highest
primary production rate coincides with the spring phytoplankton bloom, though it does
not decrease so rapidly as biomass; there is no increase at the time of the late summer
biomass increase, which is therefore attributable to reduced herbivory. Although, in this
analysis, the shelf and oceanic regions cannot be separated formally, examination of serial
images suggests that the date of greatest chlorophyll accumulation in the central regions
of each sea occurs 30–60 days later than on the shelves.
Shelf Regions Epontic algal growth occurs below sea ice as early as reduction in
snow cover and increase in sun angle permit. Although high biomass is achieved locally
(∼7 mg chl m
−3 ), integrated biomass in the water column is very small (<05 mg chl m
−2 )
compared with open-water blooms later in the season. Nevertheless, the local effect is
important and the epontic algae support a wide range of other organisms.
Ice-edge blooms occur as soon as meltwater from the receding ice and increasing
irradiance induces stability in the water column, although these blooms may be brief and
are terminated by wind-mixing events. Such processes vary in location and timing from
year to year. In coastal water, enclosed by the inner front, a single spring bloom occurs
early in the season, probably fueled by nutrients remineralized during the preceding
winter. In the pre-1976 cold period, this bloom was initiated during May and continued
for about 1 month while its depth of maximum chlorophyll values deepened by about
1 m d
−1 ; it moved progressively seaward as stability was induced in deeper water across
the middle and outer shelf domains. Ephemeral blooms occur throughout the summer,
especially in the middle shelf, whenever wind events disrupt the two-layered density
structure and allow nutrient-rich bottom water to be mixed into the euphotic zone; even
during the bloom the rate of primary production is lower (< 20 g C m
−2 day
−1 ) than
over the slope.
Nevertheless, the contemporary satellite images show very clearly how, as the season
advances through May–June, the middle and outer shelf fronts of the southeastern
Bering Sea each supports a major linear zone of high chlorophyll biomass, apparently
>10 mg chl m
−3 (see Color plate 14). This phenomenon has been called, appropriately
enough, the “Bering Sea Green Belt.” The northern part of the shelf carries a rather
uniform bloom, initiated earlier in the year, while, in the west, only minor blooms occur
along the steep-to coast of Kamchatka. Nutrient pumping in the bottom water onto the
shelf is induced by the passage of low-pressure systems, which also enhance the cross-shelf
advection rates; despite such events, nitrate-fueled production is minimal over the shelf
and maximal along the slope.
In May–June, the shelves surrounding the Okhotsk Sea support high chlorophyll more
uniformly, although in some images, there is evidence of a shelf edge frontal bloom along
Chapter 11: The Pacific Ocean
again increased, although—at least until the end of the century—it remained significantly
smaller than before 1976 (Schumacher et al., 2003). These changes are attributed to
interaction between steplike changes in regional processes and the longer-term effects of
the secular global warming trend.
Regional Response of the Pelagic Ecosystem
In these two marginal seas, algal growth is both light- and nutrient-limited. Since the
winter water column is well mixed, when irradiance reaches levels sufficient for growth
of cells, nitrate concentration at the surface is high (10–25 M NO 3 liter
−1 ), but is
rapidly reduced to limiting concentrations after early summer algal growth (McRoy and
Goering, 1974).
The satellite images are unequivocal, and record a regional maximum of chlorophyll
accumulation (3–5 mg chl m
−3 ) at the surface, and as integrated over the euphotic zone
(70–100 mg chl m
−2 ) in April of each year with great regularity. A secondary peak of
chlorophyll accumulation (∼15 mg chl m
−3 ) occurs in August–September. The highest
primary production rate coincides with the spring phytoplankton bloom, though it does
not decrease so rapidly as biomass; there is no increase at the time of the late summer
biomass increase, which is therefore attributable to reduced herbivory. Although, in this
analysis, the shelf and oceanic regions cannot be separated formally, examination of serial
images suggests that the date of greatest chlorophyll accumulation in the central regions
of each sea occurs 30–60 days later than on the shelves.
Shelf Regions Epontic algal growth occurs below sea ice as early as reduction in
snow cover and increase in sun angle permit. Although high biomass is achieved locally
(∼7 mg chl m
−3 ), integrated biomass in the water column is very small (<05 mg chl m
−2 )
compared with open-water blooms later in the season. Nevertheless, the local effect is
important and the epontic algae support a wide range of other organisms.
Ice-edge blooms occur as soon as meltwater from the receding ice and increasing
irradiance induces stability in the water column, although these blooms may be brief and
are terminated by wind-mixing events. Such processes vary in location and timing from
year to year. In coastal water, enclosed by the inner front, a single spring bloom occurs
early in the season, probably fueled by nutrients remineralized during the preceding
winter. In the pre-1976 cold period, this bloom was initiated during May and continued
for about 1 month while its depth of maximum chlorophyll values deepened by about
1 m d
−1 ; it moved progressively seaward as stability was induced in deeper water across
the middle and outer shelf domains. Ephemeral blooms occur throughout the summer,
especially in the middle shelf, whenever wind events disrupt the two-layered density
structure and allow nutrient-rich bottom water to be mixed into the euphotic zone; even
during the bloom the rate of primary production is lower (< 20 g C m
−2 day
−1 ) than
over the slope.
Nevertheless, the contemporary satellite images show very clearly how, as the season
advances through May–June, the middle and outer shelf fronts of the southeastern
Bering Sea each supports a major linear zone of high chlorophyll biomass, apparently
>10 mg chl m
−3 (see Color plate 14). This phenomenon has been called, appropriately
enough, the “Bering Sea Green Belt.” The northern part of the shelf carries a rather
uniform bloom, initiated earlier in the year, while, in the west, only minor blooms occur
along the steep-to coast of Kamchatka. Nutrient pumping in the bottom water onto the
shelf is induced by the passage of low-pressure systems, which also enhance the cross-shelf
advection rates; despite such events, nitrate-fueled production is minimal over the shelf
and maximal along the slope.
In May–June, the shelves surrounding the Okhotsk Sea support high chlorophyll more
uniformly, although in some images, there is evidence of a shelf edge frontal bloom along