International Polar Year 2007–2008 - WMO

et al., 2006), since this exemplifies the reduced carbon
supply to the benthos.
The proximate cause of the change is a northwards
retraction of the subsurface cold pool, formed as
a result of ice formation in winter but persisting
beneath warmer surface waters in summer, that
normally extends near-freezing temperatures across
the Bering Sea floor. As warming caused the cold pool
to retract, the subarctic-Arctic boundary defined by its
southern margin also retracted northwards, allowing
a northward shift of the pelagic-dominated marine
ecosystem that had previously been confined to the
warmer waters of the southeastern Bering Sea.
In Fig. 2.2-12, which is unpublished, but based on
the data in Mueter and Litzow (2008), Franz Mueter
(UAF) quantifies this ecosystem shift by showing the
rate of northward movement (km/25y) in the center
of distribution of 45 species over 25 years (1982-2006).
As Mueter points out, these rates are a communitylevel
phenomenon and are similar to those recently
reported for the North Sea (Perry et al., 2005) though
we note that in the latter case, there was a parallel
tendency for species to deepen as part of their
response to warming (Dulvy et al., 2008). In agreement
with other studies including Grebmeier et al., (2006),
Mueter and Litzow conclude that the proximate cause
of these distributional changes is changing bottom
temperature and provide a figure of ~230 km for the
northward retreat of the southern edge of the summer
cold pool in the Bering Sea since the early 1980s (Fig.
2.2-12): ‘other climate variables explained little of the
residual variance not explained by bottom temperature,
which supports the view that bottom temperature is the
dominant climate parameter for determining demersal
community composition in marginal ice seas’.
Establishing a mechanism for the Influence of climatic
regime-shifts on the ecosystem of the Bering
Sea: new evidence for the Oscillating Control Hypothesis.
Though it predates these studies, the Oscillating
Control Hypothesis (OCH) of Hunt et al., (2002)
is an attempt to rationalize these changes in terms of
ecosystem function. Basically, the hypothesis predicts
that pelagic ecosystem function in the southeastern
Bering Sea will alternate between bottom-up control
in cold regimes and top-down control in warm
regimes. The timing of spring primary production is
determined mainly by the timing of ice retreat. Late
ice retreat (late March or later) leads to an early, ice-associated
bloom in cold water, whereas early ice retreat
before mid-March, leads to a late open-water bloom in
May or June in warm water. Zooplankton populations
are not closely coupled to the spring bloom, but are
sensitive to water temperature.
In years when the (early) spring bloom occurs in
cold water, low temperatures limit the production
of zooplankton, the survival of larval/juvenile fish
and thus (eventually) the recruitment of large
piscivorous fish, such as walleye pollock. Continued
for decades, this will lead to bottom-up limitation and
a decreased biomass of piscivorous fish. Alternatively,
in periods when the (late) bloom occurs in warm
water, zooplankton populations should grow rapidly,
Fig. 2.2-12. The rate of the
northward shift in the
center of distribution of 45
species in the Bering Sea,
1982-2006. Unpublished,
courtesy of Franz Mueter,
UAF, pers comm.
s C I e n C e P r o g r a m 175