Hunt, G. L., Coyle, K. O., Eisner, L. B., Farley, E. V., Heintz, R. A., Mueter, F., Napp, J. M., Overland, J. E., Ressler, P. H., Salo, S., and Stabeno, P. J. 2011. Climate impacts on eastern Bering Sea foodwebs: a synthesis of new data and an assessment of the Oscillating Control Hypothesis. – ICES Journal of Marine Science, 68: 1230–1243. Walleye pollock (Theragra chalcogramma) is an important component of the eastern Bering Sea ecosystem and subject to major fisheries. The Oscillating Control Hypothesis (OCH) predicted that recruitment of pollock year classes should be greatest in years with early ice retreat and late blooms in warm water, because more energy would flow into the pelagic (vs. benthic) community. The OCH further predicted that, with pollock population growth, there should be a shift from bottom-up to top-down regulation. New data support the predictions that in those years with early ice retreat, more primary production accrues to the pelagic compartment and that large numbers of age-0 pollock survive to summer. However, in these years, production of large crustacean zooplankton is reduced, depriving age-0 pollock of lipid-rich prey in summer and autumn. Consequently, age-0 pollock energy reserves (depot lipids) are low and predation on them is increased as fish switch to age-0 pollock from zooplankton. The result is weak recruitment of age-1 recruits the following year. A revised OCH indicates bottom-up constraints on pollock recruitment in very warm periods. Prolonged warm periods with decreased ice cover will likely cause diminished pollock recruitment and catches relative to recent values.

Concern about impacts of climate change in the Bering Sea prompted several research programs to elucidate mechanistic links between climate and ecosystem responses. Following a detailed literature review, Hunt et al. (2011) (Deep-Sea Res. II, 49, 2002) developed a conceptual framework, the Oscillating Control Hypothesis (OCH), linking climaterelated changes in physical oceanographic conditions to stock recruitment using walleye pollock (Theragra chalcogramma) as a model. The OCH conceptual model treats zooplankton as a single box, with reduced zooplankton production during cold conditions, producing bottom-up control of apex predators and elevated zooplankton production during warm periods leading to top-down control by apex predators. A recent warming trend followed by rapid cooling on the Bering Sea shelf permitted testing of the OCH. During warm years (2003-06), euphausiid and Calanus marshallae populations declined, post-larval pollock diets shifted from a mixture of large zooplankton and small copepods to almost exclusively small copepods, and juvenile pollock dominated the diets of large predators. With cooling from 2006-09, populations of large zooplankton increased, post-larval pollock consumed greater proportions of C. marshallae and other large zooplankton, and juvenile pollock virtually disappeared from the diets of large pollock and salmon. These shifts in energy flow were accompanied by large declines in pollock stocks attributed to poor recruitment between 2001 and 2005. Observations presented here indicate the need for revision of the OCH to account for shifts in energy flow through differing food-web pathways due to warming and cooling on the southeastern Bering Sea shelf.

The biological and physical processes contributing to planktonic thin layer dynamics were examined in a multidisciplinary study conducted in East Sound, Washington, USA between June 10 and June 25, 1998. The temporal and spatial scales characteristic of thin layers were determined using a nested sampling strategy utilizing 4 major types of platforms: (1) an array of 3 moored acoustical instrument packages and 2 moored optical instrument packages that recorded distributions and intensities of thin layers; (2) additional stationary instrumentation deployed outside the array comprised of meteorological stations, wave-tide gauges, and thermistor chains; (3) a research vessel anchored 150 m outside the western edge of the array; (4) 2 mobile vessels performing basin-wide surveys to define the spatial extent of thin layers and the physical hydrography of the Sound. We observed numerous occurrences of thin layers that contained locally enhanced concentrations of material; many of the layers persisted for intervals of several hours to a few days. More than one persistent thin layer may be present at any one time, and these spatially distinct thin layers often contain distinct plankton assemblages. The results suggest that the species or populations comprising each distinct thin layer have responded to different sets of biological and/or physical processes. The existence and persistence of planktonic thin layers generates extensive biological heterogeneity in the water column and may be important in maintaining species diversity and overall community structure.

26Survey data from the northern Bering and Chukchi sea continental shelves in August-September 27 2012 and 2013 reveal inter-annual differences in the spatial structure of water masses along with 28 statistically significant differences in thermohaline properties, chemical properties, and 29 phytoplankton communities. We provide a set of water mass definitions applicable to the northern 30 Bering and Chukchi continental shelves, and we find that the near-bottom Bering-Chukchi Summer 31 Water (BCSW) was more saline in 2012 and Alaskan Coastal Water (ACW) was warmer in 2013. 32Both of these water masses carried higher nutrient concentrations in 2012, supporting a larger 33 chlorophyll a biomass that was comprised primarily of small (<10 µm) size class phytoplankton, so 34 the classical relation between higher nutrient loads and larger phytoplankton does not hold for this 35 region in late summer. The distributions of phytoplankton biomass and size structure reveal 36 linkages between the wind fields, seafloor topography, water mass distributions and the pelagic 37 production. The water mass structure, including the strength and location of stratification and 38 fronts, respectively, differed primarily because of the August regional wind field, which was more 39 energetic in 2012 but was more persistent in direction in 2013. High concentrations of ice in 40 winter and early spring in 2012 and 2013 resembled conditions of the 1980s and early 1990s but 41 the regional ice retreat rate has accelerated in the late 1990s and 2000s so the summer and fall ice 42 concentrations more closely resembled those of the last two decades. Our data show that wind 43 forcing can shut down the Alaskan Coastal Current in the NE Chukchi Sea for periods of weeks to 44 months during the ice-covered winter and during the summer when buoyancy forcing is at its 45 annual maximum. We hypothesize that a decrease in salinity and nutrients from 2012 to 2013 was 46 a consequence of a decreased net Bering Strait transport from 2011 to 2012. Biological 47 ramifications of an accelerated ice melt-back, restructuring of shelf flow pathways, and inter-48 annually varying Bering Strait nutrient fluxes are mostly unknown but all of these variations are 49 potentially important to the Arctic ecosystem. Our results have implications for the total magnitude 50 3 and seasonal evolution of primary productivity, secondary production, and the fate of fresh water, 51 heat, and pelagic production on the Bering-Chukchi shelves. 52 53

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