334
Chapter 11: The Pacific Ocean
shallow water of the Anadyr Strait injects Slope Current nutrients into the photic zone
as a cross-shelf flow (Springer and McRoy, 1993). This may induce a plume of intense
growth of chain-forming diatoms (reaching 16 g C m
−2 day
−1 and <600 mg chl m
−2 in
the center of the straits) that are transported into the Chukchi Sea through the Bering
Straits. Springer and McRoy describe the Anadyr Current as a “north-flowing river of
oceanic water maintains a portion of these shelf waters in a eutrophic bloom summerlong,” and Sambrotto et al. (1984) stated, “This phytoplankton production system from
June through September is analogous to a laboratory continuous culture.” In short,
nitrate is continually supplied as slope water passes up onto the shelf, while cells are
continuously lost by sinking, later to fuel high benthic production.
This domain of the Bering Sea supports an expatriate subarctic zooplankton
assemblage, advected through the Aleutian chain and dominated by North Pacific
biota: Neocalanus plumchrus, Neocalanus cristatus, Eucalanus bungii, and Metridia pacifica
which comprise 70–90% of the copepod biomass, together with small copepods such as
Pseudocalanus spp., the euphausiids Thysanoessa spp., the chaetognath Sagitta elegans, and
others. These zooplankters support the immense flocks of diving planktivorous seabirds
(mostly auklets, Aethia spp.) most of which nest in the northern part of the Bering Sea
and whose numbers are evocative of a rich food supply. The larger copepods pass through
one (N. cristatus, E. bungii, and M. pacifica) or two (N. plumchrus) generations a year
(Heinrich, 1962b) and their abundance peaks during the summer.
Unlike the C. pacificus association of the eastern shelves, these oceanic zooplankton,
which pass through the Slope Current and into the Anadyr Strait—that is, they are
advected through the highly productive shelf-edge front and Anadyr Straits algal blooms—
have a food demand that is far lower than the algal production rate, so the blooms are
uncontrolled by herbivore grazing pressure. In fact, the advection of expatriate North
Pacific planktonic biota into the Anadyr Straits is itself a significant carbon flux, headed for
the deposits of the Chukchi Sea: Springer et al. (1989) compute this flux as 18 × 10
12 g C
during a single summer.
Deep Basins The deep regions of the Okhotsk Sea are less productive than shelf
regions, although both are characterized by spring blooms in which production rates
>5 g C m
−2 d
−1 are typical, and highest productivity occurs along the edges of the
Sakhalin and Kashevarov Banks (Sorokin and Sorokin, 1999, 2002). During the course
of the summer, episodic blooms are typical of the same regions and production remains
quite high (∼07 g C m
−2 d
−1 ) through July–August, associated with a typical DCM
structure; the euphotic depth over deep water is 30–50 m, compared with only 12–25 m
over the shelf. Diatoms form 50–80% of regional phytoplankton biomass, and bacterial
and ciliate numbers are about 2–3 times higher over the shelf than in the open ocean.
Although the coccolith bloom over the Bering Sea shelf appears to be a relatively novel
phenomenon (see earlier discussion), blooms of Coccolithus pelagicus appear to be a
feature of early spring blooms in the open Okhotsk Sea that occur very early in the
season, as soon as the ice cover breaks up (Broerse et al., 2000). These blooms support a
typical cold-water zooplankton and euphausiids, comprising essentially the same genera
and species as in the Bering Sea.
I can locate very little direct information on the production ecology of the oceanic part
of the Bering Sea, perhaps because the shelf fisheries have almost exclusively occupied
the attention of funding agencies. Thus, little modern research seems to have addressed
the central cyclonic gyre since the review of Motoda and Minoda (1974), who gathered
together scattered, earlier observations. It is clear from their study that (as could be
expected) the populations of large oceanic copepods in the open gyral region resemble
those advected into the Slope Current (see earlier discussion) with biomass (20–40 g wet
weight m
−2 ) not different from values over the shelf; only in the Slope Current itself
Chapter 11: The Pacific Ocean
shallow water of the Anadyr Strait injects Slope Current nutrients into the photic zone
as a cross-shelf flow (Springer and McRoy, 1993). This may induce a plume of intense
growth of chain-forming diatoms (reaching 16 g C m
−2 day
−1 and <600 mg chl m
−2 in
the center of the straits) that are transported into the Chukchi Sea through the Bering
Straits. Springer and McRoy describe the Anadyr Current as a “north-flowing river of
oceanic water maintains a portion of these shelf waters in a eutrophic bloom summerlong,” and Sambrotto et al. (1984) stated, “This phytoplankton production system from
June through September is analogous to a laboratory continuous culture.” In short,
nitrate is continually supplied as slope water passes up onto the shelf, while cells are
continuously lost by sinking, later to fuel high benthic production.
This domain of the Bering Sea supports an expatriate subarctic zooplankton
assemblage, advected through the Aleutian chain and dominated by North Pacific
biota: Neocalanus plumchrus, Neocalanus cristatus, Eucalanus bungii, and Metridia pacifica
which comprise 70–90% of the copepod biomass, together with small copepods such as
Pseudocalanus spp., the euphausiids Thysanoessa spp., the chaetognath Sagitta elegans, and
others. These zooplankters support the immense flocks of diving planktivorous seabirds
(mostly auklets, Aethia spp.) most of which nest in the northern part of the Bering Sea
and whose numbers are evocative of a rich food supply. The larger copepods pass through
one (N. cristatus, E. bungii, and M. pacifica) or two (N. plumchrus) generations a year
(Heinrich, 1962b) and their abundance peaks during the summer.
Unlike the C. pacificus association of the eastern shelves, these oceanic zooplankton,
which pass through the Slope Current and into the Anadyr Strait—that is, they are
advected through the highly productive shelf-edge front and Anadyr Straits algal blooms—
have a food demand that is far lower than the algal production rate, so the blooms are
uncontrolled by herbivore grazing pressure. In fact, the advection of expatriate North
Pacific planktonic biota into the Anadyr Straits is itself a significant carbon flux, headed for
the deposits of the Chukchi Sea: Springer et al. (1989) compute this flux as 18 × 10
12 g C
during a single summer.
Deep Basins The deep regions of the Okhotsk Sea are less productive than shelf
regions, although both are characterized by spring blooms in which production rates
>5 g C m
−2 d
−1 are typical, and highest productivity occurs along the edges of the
Sakhalin and Kashevarov Banks (Sorokin and Sorokin, 1999, 2002). During the course
of the summer, episodic blooms are typical of the same regions and production remains
quite high (∼07 g C m
−2 d
−1 ) through July–August, associated with a typical DCM
structure; the euphotic depth over deep water is 30–50 m, compared with only 12–25 m
over the shelf. Diatoms form 50–80% of regional phytoplankton biomass, and bacterial
and ciliate numbers are about 2–3 times higher over the shelf than in the open ocean.
Although the coccolith bloom over the Bering Sea shelf appears to be a relatively novel
phenomenon (see earlier discussion), blooms of Coccolithus pelagicus appear to be a
feature of early spring blooms in the open Okhotsk Sea that occur very early in the
season, as soon as the ice cover breaks up (Broerse et al., 2000). These blooms support a
typical cold-water zooplankton and euphausiids, comprising essentially the same genera
and species as in the Bering Sea.
I can locate very little direct information on the production ecology of the oceanic part
of the Bering Sea, perhaps because the shelf fisheries have almost exclusively occupied
the attention of funding agencies. Thus, little modern research seems to have addressed
the central cyclonic gyre since the review of Motoda and Minoda (1974), who gathered
together scattered, earlier observations. It is clear from their study that (as could be
expected) the populations of large oceanic copepods in the open gyral region resemble
those advected into the Slope Current (see earlier discussion) with biomass (20–40 g wet
weight m
−2 ) not different from values over the shelf; only in the Slope Current itself