352
Chapter 11: The Pacific Ocean
Although these provinces have not been investigated comprehensively, we may take
some comfort from biogeographical data because they do support a small but very
characteristic assemblage of species for which this appears to be the unique distribution
area. Some of the clearest demonstrations of this are the ZETES winter 1966 and the
URSA MAJOR summer 1964 sections along 155
W (Venrick, 1974). Group analysis of 66
diatom taxa identified several recurrent groups of species whose distributions matched
subarctic and subtropical gyres, and also a group of three species that are endemic to
a “transitional domain” (our NPST Province at about 35–42
N) in winter. Two other
groups of species comprise 99% of the diatom biomass in this zone in summer. Because
the two meandering boundary fronts must be leaky, it is no surprise to find that the
species groups endemic to the adjacent domains to the north and south overlap into this
province. However, it is the relative fidelity of the “transition species” to their zone that
is significant.
We have even better evidence from zooplankton. An early review of the biogeography
of the Pacific Ocean (Reid, 1962) revealed a remarkable zonal strip of high biomass of
mesozooplankton across the ocean at about 38–45
N that was very clearly separated by
low biomass occupying most of the subarctic gyre from another zone of high values
in the Bering Sea. Even more striking are the envelopes for the distribution of 12
“transition zone” species assembled by McGowan (1971); these lie in a tight group across
the ocean with average meridional extents from 35
N to 45
N. They represent a wide
range of taxa (euphausiids, copepods, mollusks, and foraminifera) and some, such as
Nematoscelis difficilis, have a most remarkable fidelity to this province. These species go
with the flow into the coastal currents off western North America (see CALC). How
these species maintain their populations in this zonal river in the ocean is an unanswered
question.
To all this early work, we can now add recent evidence, some obtained by satellitemonitored tags attached to large animals (Polovina et al., 2001). Thus, loggerhead
(Caretta) and Ridley (Lepidochelys) turtles are found to be associated preferentially with
the Transition Zone, migrating seasonally with the temperature front, but displaying
specific preferences within the zone. Ridleys are more southerly, in warmer water, and
loggerheads more northerly. Distribution of the turtles responds to between-year differences in the location of the Transition Zone. It is also now known that albacore tuna
travel along the frontal zones in the seasonal trans-Pacific migrations. A new but perhaps already overfished jig fishery by both U.S. and Japanese boats for the flying squid
(Ommastrephes) is concentrated in the Transition Zone.
A remarkable study by Hyrenbach (2002) of satellite-tracked albatrosses (Phoebastria
spp.) that nest on the central tropical Pacific islands show that both species fly fast
and directly between there and their feeding grounds, where flight becomes slower and
constantly changes direction. Feeding grounds for both Black-footed and Laysan albatross
are in the Transition Domain (sic) and Subarctic Frontal Zone during the brooding
period. Precise definition of conditions (temperature and chlorophyll) selected by the
birds as they foraged along the transition zone was achieved by reference to contemporary
satellite imagery. When both species are present, Laysan albatrosses exploit the colder
(10–15
C) waters of the SAFZ (and are associated with the so-called Transition Zone
Chlorophyll Front) while Black-footed albatrosses feed south of the STFZ in warmer
water (>20
C).
Synopsis
Case 3—Winter-spring production with nutrient limitation—The interaction between the
boreal winter excursion of the pycnocline and the depth of the irradiance-driven photic
Chapter 11: The Pacific Ocean
Although these provinces have not been investigated comprehensively, we may take
some comfort from biogeographical data because they do support a small but very
characteristic assemblage of species for which this appears to be the unique distribution
area. Some of the clearest demonstrations of this are the ZETES winter 1966 and the
URSA MAJOR summer 1964 sections along 155
W (Venrick, 1974). Group analysis of 66
diatom taxa identified several recurrent groups of species whose distributions matched
subarctic and subtropical gyres, and also a group of three species that are endemic to
a “transitional domain” (our NPST Province at about 35–42
N) in winter. Two other
groups of species comprise 99% of the diatom biomass in this zone in summer. Because
the two meandering boundary fronts must be leaky, it is no surprise to find that the
species groups endemic to the adjacent domains to the north and south overlap into this
province. However, it is the relative fidelity of the “transition species” to their zone that
is significant.
We have even better evidence from zooplankton. An early review of the biogeography
of the Pacific Ocean (Reid, 1962) revealed a remarkable zonal strip of high biomass of
mesozooplankton across the ocean at about 38–45
N that was very clearly separated by
low biomass occupying most of the subarctic gyre from another zone of high values
in the Bering Sea. Even more striking are the envelopes for the distribution of 12
“transition zone” species assembled by McGowan (1971); these lie in a tight group across
the ocean with average meridional extents from 35
N to 45
N. They represent a wide
range of taxa (euphausiids, copepods, mollusks, and foraminifera) and some, such as
Nematoscelis difficilis, have a most remarkable fidelity to this province. These species go
with the flow into the coastal currents off western North America (see CALC). How
these species maintain their populations in this zonal river in the ocean is an unanswered
question.
To all this early work, we can now add recent evidence, some obtained by satellitemonitored tags attached to large animals (Polovina et al., 2001). Thus, loggerhead
(Caretta) and Ridley (Lepidochelys) turtles are found to be associated preferentially with
the Transition Zone, migrating seasonally with the temperature front, but displaying
specific preferences within the zone. Ridleys are more southerly, in warmer water, and
loggerheads more northerly. Distribution of the turtles responds to between-year differences in the location of the Transition Zone. It is also now known that albacore tuna
travel along the frontal zones in the seasonal trans-Pacific migrations. A new but perhaps already overfished jig fishery by both U.S. and Japanese boats for the flying squid
(Ommastrephes) is concentrated in the Transition Zone.
A remarkable study by Hyrenbach (2002) of satellite-tracked albatrosses (Phoebastria
spp.) that nest on the central tropical Pacific islands show that both species fly fast
and directly between there and their feeding grounds, where flight becomes slower and
constantly changes direction. Feeding grounds for both Black-footed and Laysan albatross
are in the Transition Domain (sic) and Subarctic Frontal Zone during the brooding
period. Precise definition of conditions (temperature and chlorophyll) selected by the
birds as they foraged along the transition zone was achieved by reference to contemporary
satellite imagery. When both species are present, Laysan albatrosses exploit the colder
(10–15
C) waters of the SAFZ (and are associated with the so-called Transition Zone
Chlorophyll Front) while Black-footed albatrosses feed south of the STFZ in warmer
water (>20
C).
Synopsis
Case 3—Winter-spring production with nutrient limitation—The interaction between the
boreal winter excursion of the pycnocline and the depth of the irradiance-driven photic