142
Chapter 9: The Atlantic Ocean
range of depths, from cavities in sea ice down to at least 300 m. It is entirely nektonic for
its first year and subsists throughout life very largely on planktonic crustaceans.
Benthos-feeding walrus (Odobenus rosmarus) and gray whales (Eschrichtius robustus)
are restricted to open water, the former occurring in polynyas in winter, the latter
migrating south to the California coast. It is the abundance of bivalve mollusks, which they
dig from the sediments with their downwardly directed tusks, in the benthic communities
described earlier that enable walrus to maintain populations at such high latitudes as that
of the Chukchi Sea. Walrus must have access to dry land for hauling out, which they do
in very large herds.
Gray whales are, of course, now long extinct in the Atlantic, but a population of 15,000
or so still migrates between the Chukchi Sea and California (to the great profit of the tourboat operators there). These curious whales occupy the whole shelf habitat of the Chukchi
Sea in summer and fatten by feeding on the benthic invertebrate infauna. These they
dredge—it is the only word to use—from the bottom, and then filter the organisms from
the soft deposits with their baleen plates. Nevertheless, their principal item of food is not
so different from that of the baleen whales that feed on krill in the Southern Ocean, for
example. Recall that one of the main benthic species associations on the Chukchi shelf
is that of the Ampelisca community, which is characterized by crowded masses of small
tubicolous, filter-feeding amphipods, and it is these that form the principal diet of the
whales. Feeding heavily but moving little, they are able to fuel themselves not only for
reproduction, but also for the long southern migration to their wintering grounds. They
take little food there, or on the journeys to and fro.
Synopsis
Case 1—Polar irradiance-mediated production peak. Because some parts of the province
are south of the Arctic Circle, the mean seasonal cycle does not include winter darkness;
midwinter data that are incorporated into the seasonal cycle represent regions where
sunlight occurs year-round. Nevertheless, mean regional photic depth is strongly affected
by ice cover and the winter darkness that does occur over most of this province. Z m
is shallow at all seasons in open water, and Z eu is usually deeper (at least in the sunlit
regions), so that the response of productivity to the seasonal light field is very rapid
(Fig. 9.2) and the seasonal evolution of P rate is symmetrical about midsummer peak
rate. Chlorophyll tracks the linear increase of P rate to a midsummer peak, then declines
(with P) but subsequently accumulates again to reach the annual chlorophyll maximum
in October. Thus, consumption and sinking do not constrain accumulation during the
vernal increase in P and the late summer renewal of plant biomass accumulation is
consistent with relaxation of the loss term as the large herbivorous copepods descend to
overwintering depths in July–September.
Atlantic Arctic Province (ARCT)
Extent of the Province
This is an entirely oceanic province that lies between the edge of the Greenland coastal
currents and the Oceanic Polar or Subarctic Front (Dietrich, 1964) that crosses the
ocean diagonally from Flemish Cap to Spitzbergen. The province thus excludes the East
Greenland Coastal Current but includes the central part of the Labrador Sea and a broad
zone between eastern Greenland and Spitzbergen including the Iceland and Greenland
Seas. The limits of the province are rather variable, because set by oceanic and shelf-edge
fronts, and its southern boundary probably cannot often be traced in satellite imagery.
Chapter 9: The Atlantic Ocean
range of depths, from cavities in sea ice down to at least 300 m. It is entirely nektonic for
its first year and subsists throughout life very largely on planktonic crustaceans.
Benthos-feeding walrus (Odobenus rosmarus) and gray whales (Eschrichtius robustus)
are restricted to open water, the former occurring in polynyas in winter, the latter
migrating south to the California coast. It is the abundance of bivalve mollusks, which they
dig from the sediments with their downwardly directed tusks, in the benthic communities
described earlier that enable walrus to maintain populations at such high latitudes as that
of the Chukchi Sea. Walrus must have access to dry land for hauling out, which they do
in very large herds.
Gray whales are, of course, now long extinct in the Atlantic, but a population of 15,000
or so still migrates between the Chukchi Sea and California (to the great profit of the tourboat operators there). These curious whales occupy the whole shelf habitat of the Chukchi
Sea in summer and fatten by feeding on the benthic invertebrate infauna. These they
dredge—it is the only word to use—from the bottom, and then filter the organisms from
the soft deposits with their baleen plates. Nevertheless, their principal item of food is not
so different from that of the baleen whales that feed on krill in the Southern Ocean, for
example. Recall that one of the main benthic species associations on the Chukchi shelf
is that of the Ampelisca community, which is characterized by crowded masses of small
tubicolous, filter-feeding amphipods, and it is these that form the principal diet of the
whales. Feeding heavily but moving little, they are able to fuel themselves not only for
reproduction, but also for the long southern migration to their wintering grounds. They
take little food there, or on the journeys to and fro.
Synopsis
Case 1—Polar irradiance-mediated production peak. Because some parts of the province
are south of the Arctic Circle, the mean seasonal cycle does not include winter darkness;
midwinter data that are incorporated into the seasonal cycle represent regions where
sunlight occurs year-round. Nevertheless, mean regional photic depth is strongly affected
by ice cover and the winter darkness that does occur over most of this province. Z m
is shallow at all seasons in open water, and Z eu is usually deeper (at least in the sunlit
regions), so that the response of productivity to the seasonal light field is very rapid
(Fig. 9.2) and the seasonal evolution of P rate is symmetrical about midsummer peak
rate. Chlorophyll tracks the linear increase of P rate to a midsummer peak, then declines
(with P) but subsequently accumulates again to reach the annual chlorophyll maximum
in October. Thus, consumption and sinking do not constrain accumulation during the
vernal increase in P and the late summer renewal of plant biomass accumulation is
consistent with relaxation of the loss term as the large herbivorous copepods descend to
overwintering depths in July–September.
Atlantic Arctic Province (ARCT)
Extent of the Province
This is an entirely oceanic province that lies between the edge of the Greenland coastal
currents and the Oceanic Polar or Subarctic Front (Dietrich, 1964) that crosses the
ocean diagonally from Flemish Cap to Spitzbergen. The province thus excludes the East
Greenland Coastal Current but includes the central part of the Labrador Sea and a broad
zone between eastern Greenland and Spitzbergen including the Iceland and Greenland
Seas. The limits of the province are rather variable, because set by oceanic and shelf-edge
fronts, and its southern boundary probably cannot often be traced in satellite imagery.