Abstract
We examine how coupling between physical and biological processes influences the production and transfer of energy to upper trophic-level species in the southeastern Bering Sea. We review time series that illustrate changes in the marine climate of the southeastern Bering Sea since the mid-1970s, particularly variability in the persistence of sea ice and the timing of its retreat. Time series (1995 – 2001) from a biophysical mooring in the middle domain of the southeastern shelf support the hypothesis that retreat of the winter sea ice before mid-March (or the failure of ice to be advected into a region) results in an open water bloom in May or June in relatively warm water (≥3°C). Conversely, when ice retreat is delayed until mid-March or later, an ice-associated bloom occurs in cold (≤0°C) water in early spring. These variations are important because the growth and production of zooplankton and the growth and survival of larval and juvenile fish are sensitive to water temperature. The Oscillating Control Hypothesis (OCH) recently proposed by Hunt et al. (2002), predicts that control of the abundance of forage fish, and in the case of walleye pollock ( Theragra chalcogramma), recruitment of large piscivorous fish, will switch from bottom-up limitation in extended periods with late ice retreat to top-down in warmer periods when ice retreat occurs before mid-March. In support of this hypothesis, we review recent data from the southeastern Bering Sea that show 2- to 13-fold changes in copepod abundance with changes in spring water temperatures of 3 to 5°C. We also provide indirect evidence that the abundance of adult pollock on the eastern Bering Sea shelf negatively affects the abundance forage fishes (including juvenile pollock) available to top predators. Although there is evidence that pollock year-class strength is positively related to temperature, we lack the time series of pollock populations in extended periods (8 – 10 years) of cold-water blooms necessary to test the OCH.
Highlights
In recent years, correlations between climate patterns and responses of marine ecosystems have been the focus of considerable attention
The Bering Sea, as a marginal ice zone, should be sensitive to climate change, because small changes in wind velocities can make large differences in the extent, timing and duration of wintertime sea ice. Such far-reaching signals as El Nino/Southern Oscillation (ENSO) on occasion may affect the climate of the Bering Sea (e.g. Overland, Bond & Miletta, 2001), the climate of the southeastern Bering Sea is most strongly influenced by the Pacific North American pattern (PNA), and by the Arctic Oscillation (AO) (Overland, Adams & Bond, 1999)
We briefly review some of the previously published evidence for changes in the marine climate of the southeastern Bering Sea shelf, and for ecosystem-wide changes in the ecology of organisms responding to the shifts in climate forcing that occurred in 1976/77 and 1989
Summary
Correlations between climate patterns and responses of marine ecosystems have been the focus of considerable attention. Recent work has shown that ecosystem responses to decadal-scale changes in these and other indices of North Pacific Ocean and Bering Sea climate have been pervasive and of great economic importance (Francis et al, 1998; Hare & Mantua, 2000; McFarlane et al, 2000; Hollowed et al, 2001). During periods when the bloom coincides with warm water temperatures, the control of pollock populations would become top-down This is because copepod growth and production will be high, as will be the survival of larval and juvenile fishes, including those of large piscivorous fish. We present new evidence for the OCH, including the importance of depth-averaged temperatures for zooplankton and pollock, and the relationship between the biomass of adult pollock on the shelf and the productivity of Pribilof Island-nesting black-legged kittiwakes (Rissa tridactyla), a piscivorous seabird that forages for juvenile walleye pollock and other small fishes
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