The 500-km-wide continental shelf of the eastern Bering Sea supports some of the United States’ most productive and valuable fisheries, and immense populations of marine birds and mammals that contribute to the subsistence of communities of Native American peoples. It is a major economic, social, and environmental resource for the United States. Like all high-latitude ecosystems, the Bering Sea is sensitive to shifts in climate on temporal scales ranging from interannual (e.g., El Niño/Southern Oscillation (ENSO)) through decadal (e.g., Pacific Decadal Oscillation (PDO) and the Arctic Oscillation (AO)) to long-term secular trends. In fact, although the Bering Sea is dominated by year-to-year variability, dramatic shifts in the physical, and biological environment of the southeastern Bering Sea have occurred recently (e.g., Stabeno and Overland, 2001; Overland and Stabeno, 2004; Overland and Wang, 2005a, b). It is an open question whether these changes are due more to regional effects or to fluctuations in hemispheric modes of variability.
Sea-ice cover, a defining characteristic of any arctic or subarctic system, is decreasing in duration and concentration over the southeastern Bering Sea shelf (Overland and Stabeno, 2004), and is characterized by a faster melt back in spring over the northern shelf (Grebmeier et al., 2006). There are multiple possible causes for this decrease related to changes in atmospheric forcing and oceanic conditions. Changes in the timing and spatial extent of sea-ice impact the temperature of the Bering Sea shelf and, in particular, the extent of the cold pool (the region where bottom temperatures are <2°C during the summer) over the middle shelf. They also affect the timing of the spring-phytoplankton bloom (Stabeno and Hunt, 2002; Hunt et al., 2002).
Such changes in the physical environment are capable of reorganizing the ecosystem (Hunt et al., 2002; Hunt and Stabeno, 2002), and there is evidence that this ecosystem is changing. For instance, certain cold-water species such as Greenland turbot (Reinhardtius hippoglossoides) and certain amphipods are no longer found in great numbers in the southern Bering Sea (Boldt, 2004). The biomass of jellyfish (medusae) rose markedly in the 1990s (Brodeur et al., 1999) and then declined rapidly beginning about 2001, with probable linkages to regional climate variations. The central feeding location of the gray whale (Eschrichtius robustus) has shifted from the northern Bering Sea to the Chukchi Sea (Moore et al., 2003). At the same time, there has been a decline in the productivity and overall benthic standing stock over the northern Bering Sea (Grebmeier et al., 2006). These changes have occurred at the same time as the warming of the shelf and the decrease in sea-ice cover.
Oceanographic observations in the Bering Sea have been limited by its remoteness, its size and the harsh weather that dominates this area, especially in the winter. To increase the year-round observations over the southeastern Bering Sea shelf, we established three biophysical mooring sites in 1995. Two of the sites were only occupied for a few years, but Site 2 (M2) has been occupied nearly continuously since 1995. The location of M2 was selected as an area where sea ice occurred virtually every winter for at least a short period (Stabeno et al., 1998). In 1996, a biophysical mooring was deployed farther to the northwest in a region of the middle shelf that appeared to have a weak cross-shelf flow. In 1999, we selected this location (M4) as a second monitoring site. Data from both these sites have provided critical quantification of the changing physical environment over the southeastern Bering Sea shelf.
In this article, we first present information about the changing extent and duration of sea ice over the southeastern shelf. Next, we relate these changes to data on temperature, currents and fluorescence collected at several mooring sites on the southeastern Bering Sea shelf. Finally, we discuss four possible mechanisms that likely contributed to the observed warming over the Bering during the last decade.
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