Phyllis J. Stabeno

Climate change and control of the southeastern Bering Sea pelagic ecosystem

We propose a new hypothesis, the Oscillating Control Hypothesis (OCH), which predicts that pelagic ecosystem function in the southeastern Bering Sea will alternate between primarily bottom-up control in cold regimes and primarily top-down control in warm regimes. The timing of spring primary production is determined predominately by the timing of ice retreat. Late ice retreat (late March or later) leads to an early, ice-associated bloom in cold water (e.g., 1995, 1997, 1999), whereas no ice, or early ice retreat before mid-March, leads to an open-water bloom in May or June in warm water (e.g., 1996, 1998, 2000). Zooplankton populations are not closely coupled to the spring bloom, but are sensitive to water temperature. In years when the spring bloom occurs in cold water, low temperatures limit the production of zooplankton, the survival of larval/juvenile fish, and their recruitment into the populations of species of large piscivorous fish, such as walleye pollock (Theragra chalcogramma), Pacific cod (Gadus macrocephalus) and arrowtooth flounder (Atheresthes stomias). When continued over decadal scales, this will lead to bottom-up limitation and a decreased biomass of piscivorous fish. Alternatively, in periods when the bloom occurs in warm water, zooplankton populations should grow rapidly, providing plentiful prey for larval and juvenile fish. Abundant zooplankton will support strong recruitment of fish and will lead to abundant predatory fish that control forage fish, including, in the case of pollock, their own juveniles. Piscivorous marine birds and pinnipeds may achieve higher production of young and survival in cold regimes, when there is less competition from large piscivorous fish for coldwater forage fish such as capelin (Mallotus villosus). Piscivorous seabirds and pinnipeds also may be expected to have high productivity in periods of transition from cold regimes to warm regimes, when young of large predatory species of fish are numerous enough to provide forage. The OCH predicts that the ability of large predatory fish populations to sustain fishing pressure will vary between warm and cold regimes. The OCH points to the importance of the timing of ice retreat and water temperatures during the spring bloom for the productivity of zooplankton, and the degree and direction of coupling between zooplankton and forage fish. Forage fish (e.g., juvenile pollock, capelin, Pacific herring [Clupea pallasii]) are key prey for adult pollock and other apex predators. In the southeastern Bering Sea, important changes in the biota since the mid-1970s include a marked increase

Recent shifts in the state of the North Pacific

[1] The winters of 1999–2002 for the North Pacific were characterizedbyspatialpatternsinsealevelpressureanomaly (SLPA) and sea surface temperature anomaly (SSTA) with little resemblance to those of the leading pattern of North Pacific climate variability, the Pacific Decadal Oscillation (PDO). In essence, the southeastern (northern) portion of the North Pacific was subject to atmospheric forcing characteristic of that before (after) the major regime shift of 1976–77. Recent major changes in the ecosystems off the west coast of the United States and continued conditions similar to those of the early 1990s in the Gulf of Alaska, Bering Sea, and Sea of Okhotsk are consistent with these SLPA and SSTA patterns. Our result illustrates that a single indicator such as the PDO is incomplete in characterizing North Pacific climate. INDEX TERMS: 1635 Global Change: Oceans (4203); 1610 Global Change: Atmosphere (0315, 0325); 4215 Oceanography: General: Climate and interannual variability (3309); 3339 Meteorology and Atmospheric Dynamics: Ocean/ atmosphere interactions (0312, 4504).Citation: Bond, N. A., J. E. Overland, M. Spillane, and P. Stabeno, Recent shifts in the state of the North Pacific, Geophys. Res. Lett., 30(23), 2183, doi:10.1029/ 2003GL018597, 2003.

Is the climate of the Bering Sea warming and affecting the ecosystem

Observations from the Bering Sea are good indicators of decadal shifts in climate, as the Bering is a transition region between the cold, dry Arctic air mass to the north, and the moist, relatively warm maritime air mass to the south. The Bering Sea is also a transition region between Arctic and sub-Arctic ecosystems; this boundary can be loosely identified with the extent of winter sea-ice cover. Like a similar transition zone in the eastern North Atlantic [Beaugrand et al., 2002], the Bering Sea is experiencing a northward biogeographical shift in response to changing temperature and atmospheric forcing. If this shift continues over the next decade, it will have major impacts on commercial and subsistence harvests as Arctic species are displaced by sub-Arctic species. The stakes are enormous, as this rich and diverse ecosystem currently provides 47% of the U.S. fishery production by weight, and is home to 80% of the U.S. sea bird population, 95% of northern fur seals, and major populations of Steller sea lions, walrus, and whales.

Climate change and the control of energy flow in the southeastern Bering Sea

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.

Characteristics and variability of the inner front of the southeastern Bering Sea

The inner front of the southeastern Bering Sea shows marked spatial variability in frontal characteristics created by regional differences in forcing mechanisms. Differences in forcing mechanisms (sea ice advance/retreat and storm strength and timing) and early spring water properties result in strong interannual variability in biological, chemical, and physical features near the front. We have developed a simple model based on surface heat flux and water-column mixing to explain the existence of cold belts (Cont. Shelf Res. 19(14) (1999) 1833) associated with such fronts. Hydrography, fluorescence and nutrient observations show that pumping of nutrients into the euphotic zone occurs, and this can prolong primary production at the inner front. The effectiveness of this process depends on two factors: the existence of a reservoir of nutrients in the lower layer on the middle shelf and the occurrence of sufficient wind and tidal energy to mix the water column.

The Alaska coastal current : continuity of transport and forcing

Current moorings were deployed on the continental shelf at 13 locations in the northwest Gulf of Alaska. They measured the Alaska Coastal Current at three positions along the coast during April–October 1991. The strongest currents were in the confined region of Shelikof Strait. Mean daily transport was as large as 2.5×106 m3 s−1. Mean transport over the 6-month period ranged from 0.85×106 m3 s−1 at the easternmost line (off Gore Point) to 0.64×106 m3 s1− at the westernmost section (Shelikof Strait), indicating significant transport to the south of Kodiak Island. The transports were well correlated (r > 0.8) and in phase with each other and were also correlated with wind (r ∼ 0.6). Data suggest that in Shelikof Strait and off Gore Point, baroclinic flow is ∼75% and <40%, respectively, of total transport.

Under-ice observations of water column temperature, salinity and spring phytoplankton dynamics : Eastern Bering Sea shelf

The inundation of two moored platforms by sea ice in late winter and early spring of 1995 provided unique time series of water column temperature, salinity, estimated chlorophyll-a, and phytoplankton fluorescence under advancing and retreating sea ice. One platform was located at 72 m in the weakly advective middle shelf regime. Here, chlorophyll-a concentrations began increasing shortly after the arrival of the ice (March) during the period of weak stratification and continued to increase while wind actively mixed the water column to >60 m. Changes in water column structure and properties resulted from an event of strong advection rather than vertical fluxes. During winter, such advective events can replenish the nutrients required to support the rich blooms that occur over the middle shelf during spring. The advancing ice was associated with the coldest waters and a deep (>50 m) mixed layer. The ice melt enhanced the two-layer system previously established by advection. A second mooring was located at the 120 m isobath on the more advective outer shelf. The ice reached this site on April 6, and chlorophyll-a concentrations increased as the sea ice melted. At the third mooring, located on the shelf farther south beyond the range of ice, the spring bloom began on ∼may 9.

Bering Sea shifts toward an earlier spring transition

Major changes have occurred in the northern high latitudes in the last two decades. These changes range from decreases in marine mammal populations to stratospheric cooling and permafrost warmings. Over Alaska and northwestern Canada, there is an earlier transition from winter to spring. Alaskan natives who live along the coast of the northern Bering Sea have noted warmer spring temperatures, thinner sea ice, and earlier melting of snow and ice. While winters over the northern Bering Sea are cold and dark, the long hours of daylight during spring and summer, coupled with high concentrations of nutrients, make this region among the most productive in the world. Change in timing of the transition between winter and spring is affecting the ecosystem, which in turn will impact the fishermen and natives who use the Bering Seas living resources.

Evidence of episodic on-shelf flow in the southeastern Bering Sea

Trajectories from satellite-tracked drifters released over the continental slope (depth 250–500 m) in the southeastern Bering Sea revealed an episodic event of on-shelf flow. The first set of seven drifters followed the isobaths northwestward at speeds of 20–40 cm s−1; the second set, released 12 days later, moved 60 km onto the shelf at speeds of 10–15 cm s−1. Meanders or eddies were evident in the trajectories as the drifters were advected onto the shelf. An anticyclonic eddy formed at the shoreward termination of on-shelf flow (120 m isobath). The eddy dissipated in 8–10 days. Such events can be important mechanisms in replenishing nutrients and introducing oceanic plankton and larvae onto the shelf.

Volume transport in the Alaska Coastal Current

Abstract Nine moorings were deployed in three sections in the Shelikof Strait/Semidi Islands region of the Alaskan continental shelf during the period of August 1984 to July 1985. Analysis of the resulting current and bottom pressure data, together with surface wind, provides a new understanding of transport in the Alaska Coastal Current. Using current observations, mean volume transport through the Shelikof sea valley was computed to be 0.85 × 106 m3 s−1, which is in good agreement with estimates of transport obtained from hydrographic data. Approximately 75% of this flux flowed seaward through the Shelikof sea valley, with the remainder flowing along the Alaska Peninsula. Data showed the expected increase of volume transport concomitant with maximum freshwater discharge in autumn. The greatest monthly mean transport, however, occurred in winter and was related to wind forcing. On time intervals of days, fluctuations in transport were often large (up to 3.0 × 106 m3 s−1), and generally geostrophic (r = 0.79). Some of these fluctuations resulted from convergence of flow caused by the complex interaction of storms with orography. Approximately half of the fluctuations in volume transport were accounted for by the alongshore wind.