Pacific Coastal Biome
391
the shelf of the Bering Sea. In so doing, it becomes a major component in the freshwater
budget of the Bering shelf and the entire Arctic Ocean. During the period of anticyclonic
winds of summer, flow in the Coastal Current may be reversed for several months along
the eastern coast.
Forced by the seasonal wind regime, the velocity of the Alaska Stream above the steepto continental slope is greater in winter than in summer as a result of intensification of the
Aleutian atmospheric low-pressure cell (Schumacher and Reed, 1983; Thompson, 1981).
The downwelling tendency at the coast is maintained by the density distribution within
the coastal flow, resulting from dilution of the surface water mass, from wind-curl stress,
and from a longshore wind-induced slope in sea level. Flow is therefore strongest during
early winter when river effluent is greatest; where flow becomes approximately zonal,
along the Alaska peninsula, instability develops and the stream may separate from the
continental edge or may regain stability by anticyclonic meandering and eddy shedding
(Okkonen, 1992).
This situation obtains along most of this coast: only off the Alaska peninsula in summer
(near Kodiak Island) do local winds force any significant upwelling of isopleths and weak
upwelling lasts only from June to August. Along the Alaska peninsula and the Aleutian
chain to the southwest, the Aleutian Stream normally flows close along the shelf-edge
topography to the end of the Aleutian Islands and is the dominant source water for the
Bering Sea gyre; anomalous separation of the current at about 170
W can occur, and this
condition may persist for some months (Stabeno and Reed, 1992).
The fractal nature of the fjord coastline results in instabilities and in the recurrent
formation of mesoscale eddies at certain localities (Ladd et al., 2005). The best-studied
are the anticyclonic Haida eddies that form off the Queen Charlotte Islands, while similar
features form off Yakutat in the northern Gulf and off Sitka on the Alaska panhandle;
these were compared by Royer to the warm-core eddies of lower latitudes, although they
have very moderate thermal signatures. Haida eddies are initiated in buoyant plumes
from Hecate Strait, usually in late winter, so that coastal water is retained at their cores
as they propagate offshore across the NE Pacific; they are of order 150–300 km diameter,
form at a rate of about one per year, and persist for about 2 years.
The permanent halocline of the Gulf of Alaska extends through this coastal province,
though it is shoaler than out in the open gulf; it restrains winter mixing to about
50–60 m, though convective cooling extends much deeper. The stability thus induced
constrains the vertical entrainment of nitrate during winter, and it is suggested that the
spring bloom would be as brief as it is offshore (see Pacific Subarctic Gyres Province) if
interaction between the Alaska Current and coastal topography, where the shelf is narrow
and steep-to, did not entrain some nitrate throughout the summer.
This is, as we shall see, a highly productive coastal region for which, until very recently,
we had no information at all concerning the nutrient regime: how, in the face of the
permanent density stratification over the shelf, was productivity maintained? What was
the basis for the rich fish ecosystem of the shelf regions of the Gulf of Alaska? These
problems have now been resolved by Childers et al. (2005), who describe a very unusual
mechanism. Repetitive cross-shelf sections of the northern shelf during summer reveal
that the upper 20 m becomes totally depleted of macronutrients, but that below this
the water column is nutrient-replete. This situation is maintained by an onshore flux of
oceanic water that is induced by the cessation of winter downwelling at the coast and
the slight upwelling tendency induced by summer winds. This subsurface reservoir of
nutrients is thoroughly mixed to the surface in winter and thus available in spring. I know
of no other region where this process is so clear although it has some of the characteristics
of an estuarine circulation—offshore at the surface, inshore along the bottom: it appears,
however, not to be forced in the same manner as an estuary.
391
the shelf of the Bering Sea. In so doing, it becomes a major component in the freshwater
budget of the Bering shelf and the entire Arctic Ocean. During the period of anticyclonic
winds of summer, flow in the Coastal Current may be reversed for several months along
the eastern coast.
Forced by the seasonal wind regime, the velocity of the Alaska Stream above the steepto continental slope is greater in winter than in summer as a result of intensification of the
Aleutian atmospheric low-pressure cell (Schumacher and Reed, 1983; Thompson, 1981).
The downwelling tendency at the coast is maintained by the density distribution within
the coastal flow, resulting from dilution of the surface water mass, from wind-curl stress,
and from a longshore wind-induced slope in sea level. Flow is therefore strongest during
early winter when river effluent is greatest; where flow becomes approximately zonal,
along the Alaska peninsula, instability develops and the stream may separate from the
continental edge or may regain stability by anticyclonic meandering and eddy shedding
(Okkonen, 1992).
This situation obtains along most of this coast: only off the Alaska peninsula in summer
(near Kodiak Island) do local winds force any significant upwelling of isopleths and weak
upwelling lasts only from June to August. Along the Alaska peninsula and the Aleutian
chain to the southwest, the Aleutian Stream normally flows close along the shelf-edge
topography to the end of the Aleutian Islands and is the dominant source water for the
Bering Sea gyre; anomalous separation of the current at about 170
W can occur, and this
condition may persist for some months (Stabeno and Reed, 1992).
The fractal nature of the fjord coastline results in instabilities and in the recurrent
formation of mesoscale eddies at certain localities (Ladd et al., 2005). The best-studied
are the anticyclonic Haida eddies that form off the Queen Charlotte Islands, while similar
features form off Yakutat in the northern Gulf and off Sitka on the Alaska panhandle;
these were compared by Royer to the warm-core eddies of lower latitudes, although they
have very moderate thermal signatures. Haida eddies are initiated in buoyant plumes
from Hecate Strait, usually in late winter, so that coastal water is retained at their cores
as they propagate offshore across the NE Pacific; they are of order 150–300 km diameter,
form at a rate of about one per year, and persist for about 2 years.
The permanent halocline of the Gulf of Alaska extends through this coastal province,
though it is shoaler than out in the open gulf; it restrains winter mixing to about
50–60 m, though convective cooling extends much deeper. The stability thus induced
constrains the vertical entrainment of nitrate during winter, and it is suggested that the
spring bloom would be as brief as it is offshore (see Pacific Subarctic Gyres Province) if
interaction between the Alaska Current and coastal topography, where the shelf is narrow
and steep-to, did not entrain some nitrate throughout the summer.
This is, as we shall see, a highly productive coastal region for which, until very recently,
we had no information at all concerning the nutrient regime: how, in the face of the
permanent density stratification over the shelf, was productivity maintained? What was
the basis for the rich fish ecosystem of the shelf regions of the Gulf of Alaska? These
problems have now been resolved by Childers et al. (2005), who describe a very unusual
mechanism. Repetitive cross-shelf sections of the northern shelf during summer reveal
that the upper 20 m becomes totally depleted of macronutrients, but that below this
the water column is nutrient-replete. This situation is maintained by an onshore flux of
oceanic water that is induced by the cessation of winter downwelling at the coast and
the slight upwelling tendency induced by summer winds. This subsurface reservoir of
nutrients is thoroughly mixed to the surface in winter and thus available in spring. I know
of no other region where this process is so clear although it has some of the characteristics
of an estuarine circulation—offshore at the surface, inshore along the bottom: it appears,
however, not to be forced in the same manner as an estuary.