Pacific Polar Biome
329
occurred in the last half-century or so. Because of the availability of this work, I have not
emphasized the aspect of changes of regional ecology in what follows, but have rather
concentrated my text on the structural aspects of each region. “Marine Ecosystems of the
North Pacific” may be downloaded from www.pices.int/publications.
Pacific Polar Biome
North Pacific Epicontinental Sea Province (BERS)
Extent of the Province
This province comprises the Bering Sea and the Sea of Okhotsk, respectively enclosed by
the arcs of the offshore Kuril and Aleutian Islands. There is considerable commonality
between these two seas, but in a more dispersed classification of coastal provinces it
would be logical to separate them. BERS is placed in the Polar, rather than in the
Coastal biome, as a matter of convenience; marginal seas, such as the Mediterranean, with
significant areas of both shelf and deep water, are perhaps better discussed holistically
than partitioned.
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
In each of these marginal seas, the continental shelf occupies about half the total area.
The shelf of the Bering Sea is a flat, featureless plain (as was noted in 1778 by Captain
James Cook) and occupies almost one half of the area of the basin, the continental
edge lying NW-SE across the basin (Coachman, 1986; Schumacher and Stabeno, 1998).
A strong break of slope occurs at the shelf edge, with depths of 1000 m only tens of
kilometers beyond the 200-m contour. The Okhotsk Sea shelf encircles the entire basin
and is approximately the same width everywhere. It is narrower than the Bering Sea shelf,
and the continental slope is less steep. These facts have significance for the occurrence of
a relatively strong slope current in the Bering Sea and an unusually complex system of
shelf break and shelf fronts there; tidal fronts are known to occur also on the continental
shelf of the Okhotsk Sea (Zhabin et al., 1990).
As Schumacher and Stabeno (1998) point out, tidal currents provide the dominant
source of kinetic energy over very extensive regions of the inner shelf and may generate the
observed residual flows. Although there is much variability in the exact tidal mechanism
that dominates local forcing in different shelf regions: shoaler than 40–50 m, there is only
very weak stratification so that tidally mixed conditions dominate.
Three convergent fronts, defining three shelf domains, occur in the open-water season
over the continental shelf of the Bering Sea (Coachman et al., 1980; Coachman, 1986).
The depths of wind and tidal mixing overlap along an inner, coastal front that lies along
the 50-m isobath for a distance of 1000 km from Bristol Bay to about 60
N, where it
passes east of St. Lawrence Island and so north to the Bering Straits. This front encloses
unstratified, low-salinity, high-nutrient water of the coastal domain that originates at the
Umiak Pass. Flow in this domain is strongest adjacent to the front, where pumping of
nutrients into the euphotic zone may occur (Kachel et al., 2002).
A middle shelf front lies along the 100-m isobath and defines the central shelf domain,
where mean flow is weak and variable, strongly influenced by subtidal fluctuations of
several days duration. Here, in the absence of ice cover, winter wind stress mixes the
water column to 100 m although, in spring, ice melt may provide sufficient stability
for shallow stratification, before summer heating at the surface establishes stratification
more securely. Stratification may break down in the middle shelf domain in summer
after severe summer storms (Schumacher and Reed, 1983) even though there is a very
329
occurred in the last half-century or so. Because of the availability of this work, I have not
emphasized the aspect of changes of regional ecology in what follows, but have rather
concentrated my text on the structural aspects of each region. “Marine Ecosystems of the
North Pacific” may be downloaded from www.pices.int/publications.
Pacific Polar Biome
North Pacific Epicontinental Sea Province (BERS)
Extent of the Province
This province comprises the Bering Sea and the Sea of Okhotsk, respectively enclosed by
the arcs of the offshore Kuril and Aleutian Islands. There is considerable commonality
between these two seas, but in a more dispersed classification of coastal provinces it
would be logical to separate them. BERS is placed in the Polar, rather than in the
Coastal biome, as a matter of convenience; marginal seas, such as the Mediterranean, with
significant areas of both shelf and deep water, are perhaps better discussed holistically
than partitioned.
Continental Shelf Topography and Tidal and Shelf-Edge Fronts
In each of these marginal seas, the continental shelf occupies about half the total area.
The shelf of the Bering Sea is a flat, featureless plain (as was noted in 1778 by Captain
James Cook) and occupies almost one half of the area of the basin, the continental
edge lying NW-SE across the basin (Coachman, 1986; Schumacher and Stabeno, 1998).
A strong break of slope occurs at the shelf edge, with depths of 1000 m only tens of
kilometers beyond the 200-m contour. The Okhotsk Sea shelf encircles the entire basin
and is approximately the same width everywhere. It is narrower than the Bering Sea shelf,
and the continental slope is less steep. These facts have significance for the occurrence of
a relatively strong slope current in the Bering Sea and an unusually complex system of
shelf break and shelf fronts there; tidal fronts are known to occur also on the continental
shelf of the Okhotsk Sea (Zhabin et al., 1990).
As Schumacher and Stabeno (1998) point out, tidal currents provide the dominant
source of kinetic energy over very extensive regions of the inner shelf and may generate the
observed residual flows. Although there is much variability in the exact tidal mechanism
that dominates local forcing in different shelf regions: shoaler than 40–50 m, there is only
very weak stratification so that tidally mixed conditions dominate.
Three convergent fronts, defining three shelf domains, occur in the open-water season
over the continental shelf of the Bering Sea (Coachman et al., 1980; Coachman, 1986).
The depths of wind and tidal mixing overlap along an inner, coastal front that lies along
the 50-m isobath for a distance of 1000 km from Bristol Bay to about 60
N, where it
passes east of St. Lawrence Island and so north to the Bering Straits. This front encloses
unstratified, low-salinity, high-nutrient water of the coastal domain that originates at the
Umiak Pass. Flow in this domain is strongest adjacent to the front, where pumping of
nutrients into the euphotic zone may occur (Kachel et al., 2002).
A middle shelf front lies along the 100-m isobath and defines the central shelf domain,
where mean flow is weak and variable, strongly influenced by subtidal fluctuations of
several days duration. Here, in the absence of ice cover, winter wind stress mixes the
water column to 100 m although, in spring, ice melt may provide sufficient stability
for shallow stratification, before summer heating at the surface establishes stratification
more securely. Stratification may break down in the middle shelf domain in summer
after severe summer storms (Schumacher and Reed, 1983) even though there is a very