Southeastern Bering Sea Shelf Research Articles

Long-Term Biophysical Observations and Climate Impacts in US Arctic Marine Ecosystems

In 1995, the first of a nearly continuous sequence of biophysical moorings was deployed at a site (M2) on the southeastern Bering Sea shelf. Over the next 15 years, 10 additional mooring sites were initiated. The resultant long-term biophysical mooring array extends over 1,800 km from the southern Bering Sea to the northern Chukchi Sea, covering most of the US Arctic. It provides a full range of oceanographic data for researchers, stakeholders, and managers. In addition, these data sets have been critical for the validation of regional ocean models. The ocean temperature data have quantified regional warming and formed the basis for understanding how warmer temperatures and loss of sea ice are modifying these high-latitude marine ecosystems. Changes observed in the context of observations from the mooring program include delayed spring bloom, low abundances of large crustacean zooplankton and crab species, seabird die-offs, changes in ocean acidification, northward expansion of subarctic fish species, and shifts in the ranges of marine mammal species.

The northern Bering Sea zooplankton community response to variability in sea ice: evidence from a series of warm and cold periods

Recent, unprecedented losses of sea ice have resulted in widespread changes in the northern Bering Sea ecosystem, and this study explores the zooplankton community response. Time-series observations were used to identify zooplankton community changes in the northern (>60°N) Bering Sea (NBS) over a 17 yr period (2002-2018). The overall objective was to determine if the changes in zooplankton populations previously described for the southeastern Bering Sea shelf (<60°N) were also observed in the NBS over alternating warm and cold periods. Particular attention was paid to more recent (2014-2018) years that showed significant losses of sea ice in the NBS (2017/2018) in comparison to a prior warm period (2003-2005) and an intervening cold period (2006-2013). A multivariate framework (redundancy analysis) was used to explore correlations with environmental conditions, and differences in mean abundance across the differing warm and cold periods were tested. The NBS zooplankton community had different responses across each warm and cold period, and the primary driver for the differences in response was sea ice. Redundancy analysis demonstrated that the zooplankton community during the second warm period experienced greater variability compared to the prior warm period. The zooplankton community had higher abundances of small copepods and meroplankton and reduced abundances of Calanus spp. and chaetognaths during the most recent warm period. This suggests that the NBS zooplankton will not be impacted by reduced sea ice when the ice coverage extends south of 60°N, but show community change once a minimum threshold in ice extent and timing of retreat is reached. Shifts in the zooplankton community may have had cascading effects on higher trophic levels that were evident during the latter warm period.

Changes in the vertical distribution of age‐0 walleye pollock (Gadus chalcogrammus) during warm and cold years in the southeastern Bering Sea

AbstractIn the last 20 years, the southeastern Bering Sea has shifted its thermal variability to longer‐term (4–6 years) ocean–ecosystem temperature stanzas. Age‐0 walleye pollock (Gadus chalcogrammus) populations respond to thermal changes with horizontal (east–west) shifts in spatial distribution over the continental shelf, though there are limited data on whether thermally mediated vertical shifts in distribution also occur. Vertical shifts may impact predator–prey overlap between age‐0 pollock and their lipid‐rich prey, calanoid copepods and euphausiids, resulting in different feeding conditions that ultimately affect fish body condition prior to winter onset. For this study, we analyzed acoustic backscatter measured during acoustic trawl surveys over the southeastern Bering Sea shelf in cold years (2011, 2012) and warm years (2014 and 2016) to determine the vertical distribution of age‐0 pollock. This study presents evidence that age‐0 pollock changes in vertical distribution were related to changes in ocean annual temperature. Age‐0 pollock went from occurring deeper in the water column during cold periods to being surface‐oriented during warm periods, potentially exacerbating spatial mismatches between pollock and prey. We relate patterns in a vertical position to physical water column properties, feeding, and bioenergetic condition of collected pollock and discuss implications for recruitment success during different thermal oceanographic stanzas.

Dynamically downscaled projections of ocean acidification for the Bering Sea

A regional ocean biogeochemical model for the Bering Sea is used to dynamically downscale three Earth System Models from the CMIP5 archive under the RCP 8.5 and RCP 4.5 scenarios. These continuous model runs, completed in conjunction with the Alaska Climate Integrated Modeling Project (ACLIM), span the 2006–2100 timeframe and project continued warming, freshening, and ocean acidification (OA) for the Bering Sea shelf region over the 21st Century, with larger magnitude changes in the RCP 8.5 scenario. The downscaled projections suggest that annual average surface seawater aragonite saturation state (Ωarag) for the Bering Sea shelf will decrease by 0.63–0.86 under RCP 8.5 and 0.18–0.43 under RCP 4.5 by 2100. Surface pH values decrease by 0.31–0.35 under RCP 8.5 and 0.07–0.13 under RCP 4.5. Seasonally, Ωarag < 1 conditions start to emerge for ∼2 months per year during winter between 2015 and 2030 under both climate change scenarios. Under RCP 8.5, the duration of these undersaturated conditions grows to ∼5 months per year by 2100, occurring from mid-October through mid-March. Under RCP 4.5, these conditions remain constrained to 2–3 months per year by 2100. In both scenarios, summer months maintain conditions of Ωarag > 1 due to primary productivity, though the maximum in Ωarag is greatly reduced under RCP 8.5. Spatially, the regions of greatest pH and Ωarag decline are the southeastern Bering Sea shelf and the outer shelf domain near the shelf break. Linear trends in carbonate variables between our downscaled simulations and the Earth System Model (ESM) output are comparable and indistinguishable compared to the model spread. However, bottom water trends differ somewhat between the ESM and our downscaled simulations, with the latter more consistently resolving the different shelf domains. The OA information provided by these downscaled simulations can help inform biological sensitivity experiments and longterm strategic planning for marine fisheries management.

A coupled pelagic–benthic–sympagic biogeochemical model for the Bering Sea: documentation and validation of the BESTNPZ model (v2019.08.23) within a high-resolution regional ocean model

Abstract. The Bering Sea is a highly productive ecosystem, supporting a variety of fish, seabird, and marine mammal populations, as well as large commercial fisheries. Due to its unique shelf geometry and the presence of seasonal sea ice, the processes controlling productivity in the Bering Sea ecosystem span the pelagic water column, the benthic sea floor, and the sympagic sea ice environments. The Bering Ecosystem Study Nutrient-Phytoplankton-Zooplankton (BESTNPZ) model has been developed to simulate the lower-trophic-level processes throughout this region. Here, we present a version of this lower-trophic-level model coupled to a three-dimensional regional ocean model for the Bering Sea. We quantify the model's ability to reproduce key physical features of biological importance as well as its skill in capturing the seasonal and interannual variations in primary and secondary productivity over the past several decades. We find that the ocean model demonstrates considerable skill in replicating observed horizontal and vertical patterns of water movement, mixing, and stratification, as well as the temperature and salinity signatures of various water masses throughout the Bering Sea. Along the data-rich central portions of the southeastern Bering Sea shelf, it is also able to capture the mean seasonal cycle of primary production. However, its ability to replicate domain-wide patterns in nutrient cycling, primary production, and zooplankton community composition, particularly with respect to the interannual variations that are important when linking variation in productivity to changes in longer-lived upper-trophic-level species, remains limited. We therefore suggest that near-term application of this model should focus on the physical model outputs, while model development continues to elucidate potential mechanisms controlling nutrient cycling, bloom processes, and trophic dynamics.

Copepod dynamics across warm and cold periods in the eastern Bering Sea: Implications for walleye pollock (Gadus chalcogrammus) and the Oscillating Control Hypothesis

AbstractDifferences in zooplankton populations in relation to climate have been explored extensively on the southeastern Bering Sea shelf, specifically in relation to recruitment of the commercially important species walleye pollock (Gadus chalcogrammus). We addressed two research questions in this study: (i) Does the relative abundance of individual copepod species life history stages differ across warm and cold periods and (ii) Do estimated secondary production rates for copepods differ across warm and cold periods? For most copepod species, warmer conditions resulted in increased abundances in May, the opposite was observed in colder conditions. Abundances of smaller‐sized copepod species did not differ significantly between the warm and cold periods, whereas abundances of larger‐sized Calanus spp. increased during the cold period during July and September. Estimated secondary production rates in the warm period were highest in May for smaller‐sized copepods; production in the cold period was dominated by the larger‐sized Calanus spp. in July and September. We hypothesize that these observed patterns are a function of temperature‐driven changes in phenology combined with shifts in size‐based trophic relationships with primary producers. Based on this hypothesis, we present a conceptual model that builds upon the Oscillating Control Hypothesis to explain how variability in copepod production links to pollock variability. Specifically, fluctuations in spring sea‐ice drive regime‐dependent copepod production over the southeastern Bering Sea, but greatest impacts to upper trophic levels are driven by cascading July/September differences in copepod production.

Regionalizing indicators for marine ecosystems: Bering Sea–Aleutian Island seabirds, climate, and competitors

Seabirds are thought to be reliable, real-time indicators of forage fish availability and the climatic and biotic factors affecting pelagic food webs in marine ecosystems. In this study, we tested the hypothesis that temporal trends and interannual variability in seabird indicators reflect simultaneously occurring bottom-up (climatic) and competitor (pink salmon) forcing of food webs. To test this hypothesis, we derived multivariate seabird indicators for the Bering Sea–Aleutian Island (BSAI) ecosystem and related them to physical and biological conditions known to affect pelagic food webs in the ecosystem. We examined covariance in the breeding biology of congeneric pelagic gulls (kittiwakes Rissa tridactyla and R. brevirostris) and auks (murres Uria aalge and U. lomvia), all of which are abundant and well-studied in the BSAI. At the large ecosystem scale, kittiwake and murre breeding success and phenology (hatch dates) covaried among congeners, so data could be combined using multivariate techniques, but patterns of response differed substantially between the genera. While data from all sites (n=5) in the ecosystem could be combined, the south eastern Bering Sea shelf colonies (St. George, St. Paul, and Cape Peirce) provided the strongest loadings on indicators, and hence had the strongest influence on modes of variability. The kittiwake breeding success mode of variability, dominated by biennial variation, was significantly related to both climatic factors and potential competitor interactions. The murre indicator mode was interannual and only weakly related to the climatic factors measured. The kittiwake phenology indicator mode of variability showed multi-year periods (“stanzas”) of late or early breeding, while the murre phenology indicator showed a trend towards earlier timing. Ocean climate relationships with the kittiwake breeding success indicator suggest that early-season (winter–spring) environmental conditions and the abundance of pink salmon affect the pelagic food webs that support these seabirds in the BSAI ecosystem.

Differential responses of seabirds to environmental variability over 2 years in the continental shelf and oceanic habitats of southeastern Bering Sea

Abstract. Seasonal sea-ice cover has been decreasing in the southeastern Bering Sea shelf, which might affect ecosystem dynamics and availability of food resources to marine top predators breeding in the region. In this study, we investigated the foraging responses of two seabird species, surface-foraging red-legged kittiwakes Rissa brevirostris (hereafter, RLKI) and pursuit-diving foraging thick-billed murres Uria lomvia (TBMU) to different marine environmental conditions over 2 years. At-sea distributions of RLKI and TBMU breeding on St. George Island, the largest seabird colony in the region, were recorded using GPS loggers, and blood samples were taken to examine their physiological condition and isotopic foraging niche in a given year. Between the study years, winter ice retreated earlier and summer water temperatures were relatively warmer in 2014 compared to those in 2013. RLKI foraging occurred mostly over the oceanic basin in both years. TBMU, however, foraged mostly over the shelf but showed a relatively higher use of the shelf break and oceanic basin in 2013. The foraging distances from the colony peaked at 250–300 km in 2013 and bimodally at 150–250 and 300–350 km in 2014 for RLKI and tended to be farther in 2013 compared to those in 2014 for TBMU. Plasma levels of corticosterone did not differ between the years in RLKI but differed in TBMU, showing higher levels of physiological stress incurred by murres in 2013, the year of relatively cooler sea surface temperatures with later sea-ice retreat. δ13N (a proxy of trophic level of prey) did not differ between the years in either RLKI or TBMU. These results suggest that the response of ecosystem dynamics to climate variability in the southeastern Bering Sea may differ between the ocean basin and continental shelf regions, which, in turn, may generate differential responses in seabirds relying on those habitats for foraging.

Late summer/early fall phytoplankton biomass (chlorophyll a) in the eastern Bering Sea: Spatial and temporal variations and factors affecting chlorophyll a concentrations

The spatial and temporal variability of late summer/early fall phytoplankton biomass estimated from in situ chlorophyll a (Chla) concentrations was investigated over a 10-year time period from 2003–2012 in the eastern Bering Sea, encompassing both warm (2003–2005) and cold (2007–2012) temperature regimes. Warm temperature regimes were characterized by above average water temperatures and low seasonal sea-ice extent and by below-average temperatures and high seasonal sea-ice extent. The highest phytoplankton Chla was observed near the Pribilof Islands and the southeastern shelf break where nutrient concentrations were high due to onshore flow from Pribilof and Bering Canyons. The lowest Chla was observed on the northeastern middle and inner shelf, north of Nunivak and St. Matthew Islands and south of St. Lawrence Island (~61–63°N). Stations north of St Matthew Island (61°N) did not show significant variations in Chla between temperature regimes. To the south, total phytoplankton Chla was significantly higher in warm compared to cold years on the south-outer shelf and on portions of the middle shelf. Large phytoplankton Chla was higher in warm years over most of the southern middle-shelf. For the entire southeastern Bering Sea shelf (~30–200m bathymetry, south of Nunivak Island), the highest Chla was seen in 2005 and lowest in 2007 and 2008. On the south-middle shelf, wind mixing and temperature below the pycnocline had strong positive associations with Chla (total and large-size fraction) integrated over the top 50m, explaining 85% of the variability in mean Chla. This indicates that Chla in summer and early fall is positively affected by wind-induced upwelling of nutrients to the surface and possibly by other bottom up effects such as temperature-mediated growth. Higher bottom temperature is related to reductions in sea-ice extent, which may elicit ecosystem responses such as reduced biomass of large crustacean zooplankton grazers, potentially due to the removal of ice algae, an important food resource for zooplankton in early spring. This, in turn, could reduce or alter the grazing pressure on phytoplankton later in the growing season. Overall, spatial and temporal variations in phytoplankton Chla are due to a combination of factors, from local inputs of nutrients related to mixing or advection, up to large-scale ecosystem effects.

Climate-mediated changes in zooplankton community structure for the eastern Bering Sea

Zooplankton are critical to energy transfer between higher and lower trophic levels in the eastern Bering Sea ecosystem. Previous studies from the southeastern Bering Sea shelf documented substantial differences in zooplankton taxa in the Middle and Inner Shelf Domains between warm and cold years. Our investigation expands this analysis into the northern Bering Sea and the south Outer Domain, looking at zooplankton community structure during a period of climate-mediated, large-scale change. Elevated air temperatures in the early 2000s resulted in regional warming and low sea-ice extent in the southern shelf whereas the late 2000s were characterized by cold winters, extensive spring sea ice, and a well-developed pool of cold water over the entire Middle Domain. The abundance of large zooplankton taxa such as Calanus spp. (C. marshallae and C. glacialis), and Parasagitta elegans, increased from warm to cold periods, while the abundance of gelatinous zooplankton (Cnidaria) and small taxa decreased. Biomass followed the same trends as abundance, except that the biomass of small taxa in the southeastern Bering Sea remained constant due to changes in abundance of small copepod taxa (increases in Acartia spp. and Pseudocalanus spp. and decreases in Oithona spp.). Statistically significant changes in zooplankton community structure and individual species were greatest in the Middle Domain, but were evident in all shelf domains, and in both the northern and southern portions of the eastern shelf. Changes in community structure did not occur abruptly during the transition from warm to cold, but seemed to begin gradually and build as the influence of the sea ice and cold water temperatures persisted. The change occurred one year earlier in the northern than the southern Middle Shelf. These and previous observations demonstrate that lower trophic levels within the eastern Bering Sea respond to climate-mediated changes on a variety of time scales, including those shorter than the commonly accepted quasi-decadal time periods. This lack of resilience or inertia at the lowest trophic levels affects production at higher trophic levels and must be considered in management strategy evaluations of living marine resources.

Seasonal variation in the cross-shelf distribution of seabirds in the southeastern Bering Sea

We tested the hypothesis that the distribution of seabird species' associations across the southeastern Bering Sea shelf reflects the underlying ecology of four bathymetrically-defined hydrographic domains: the Inner or Coastal Shelf Domain (depth (Z)<50m), the Middle Shelf Domain (50m<Z<100m), the Outer Shelf Domain (100m<Z<200m), and the Shelf-Slope Domain (200m<Z<3000m). The domains differ in stratification, which intensifies from winter to summer and breaks down in the fall. To examine seabird distributions with respect to these domains in multiple seasons, we quantified the cross-shelf distribution of species with respect to water depth using a 37-year database. We then used a multivariate tree analysis to group species with similar depth-use distributions, and mapped these clusters against the hydrographic domains. There were three patterns of seabird depth use: an inshore, shallow-water group in summer and fall, but not winter and spring, which conformed roughly to the Inner Shelf Domain; a group of species that were distributed widely across the Middle and Outer Shelf Domains, and a third group of species that occupied the outer portion of the Outer Shelf Domain and the Shelf-Slope Domain. The multivariate tree analysis revealed close correspondence between the seabird-derived domains and the bathymetrically-defined Outer Shelf and Shelf-Slope domains in spring and to a lesser extent in summer. In summer and fall, and to a lesser extent in spring, the seabird groupings showed a differentiation between the Inner Shelf Domain and the Middle Shelf Domain. Seabird-derived differentiation between the Shelf-Slope Domain and the Outer Shelf Domain was strongest in spring and summer. These seasonal patterns likely reflected the seasonal variation in the hydrographic differentiation of the bathymetrically-defined domains. Cross-validation of the multivariate tree analysis showed that the portion of seabird distribution patterns explained by the tree analysis was smallest in winter (when there is no stratification on the middle and inner shelves) and greatest in summer (when stratified water columns result in hydrographically defined domains), as would be expected under our hypothesis. We also examined hypotheses predicting why pursuit diving seabirds most often forage in shallow water whereas surface-foraging (surface-seizing) seabirds are more common over deep offshore waters. The hypothesis for regionally enhanced primary production as a driving factor was not supported for the inshore foraging seabirds but was supported for those foraging over shelf-slope waters.

Conservative and non-conservative variations of total alkalinity on the southeastern Bering Sea shelf

Recent observations of calcium carbonate (CaCO3) mineral undersaturations on the Bering Sea shelf have prompted new interest in the physical and biological factors that control the inorganic carbon system in the region. Understanding of the dynamics that influence the spatio-temporal variability of total alkalinity (TA) – one major component of the seawater carbonate system – has been constrained by limited historical data collected across the shelf, and the consensus has been that TA is largely conservative. However, the recently documented undersaturated conditions have the potential to cause substantial non-conservative variability in TA in this region through the dissolution of carbonate minerals. In order to quantify the contribution of carbonate mineral precipitation and dissolution to variability in TA on the southeastern Bering Sea shelf, we examined seasonal observations of TA that were made between 2008 and 2010 as part of the BEST-BSIERP Bering Sea Project. Conservative influences accounted for most of the variability in TA concentrations, with well-constrained mixing dominating in spring and summer of 2008. Bering Shelf Water (BSW) contained a constant ratio of TA to salinity, while river discharge (RW) added TA relative to salinity at a predictable rate. Although substantial organic carbon production and denitrification can cause some non-conservative variation in TA concentrations (a maximum of ~15μmolkgSW−1 combined), carbonate mineral dissolution and precipitation were shown to be the most important processes responsible for non-conservative TA–salinity relationships. CaCO3 uptake by the dominant pelagic phytoplankton calcifier (i.e., coccolithophores) was shown to alter TA concentrations by as much as 59μmolkgSW−1. Evidence for shallow-water CaCO3 mineral dissolution was also observed, which caused TA concentrations to increase by as much as 36μmolkgSW−1. Therefore, contrary to our previous understanding, the non-conservative physico-biogeochemical factors observed in this study play an important role in controlling the ocean carbon cycle of the Bering Sea shelf.

Emergence of the Arctic Themisto libellula (Amphipoda: Hyperiidae) on the southeastern Bering Sea shelf as a result of the recent cooling, and its potential impact on the pelagic food web

AbstractPinchuk, A. I., Coyle, K. O., Farley, E. V., and Renner, H. M. 2013. Emergence of the Arctic Themisto libellula (Amphipoda: Hyperiidae) on the southeastern Bering Sea shelf as a result of the recent cooling, and its potential impact on the pelagic food web. – ICES Journal of Marine Science, 70: 1244–1254. The eastern Bering Sea shelf experienced a sequence of warm years after a regime shift in the late 1970s. Following a series of unusually warm years in the early 2000s, the climate shifted again in 2007 to a series of extremely cold years that were marked by intense ice coverage and late ice retreat. Spatial and temporal variability in zooplankton communities during the recent cold period was investigated as part of the collaborative BEST-BSIERP program. An increasing presence of the Arctic hyperiid Themisto libellula, which had not been reported from the southeastern Bering Sea since the 1970s, was observed in the Middle Shelf Domain, indicating a developing structural shift in the zooplankton community in response to continuous cold conditions. Simultaneously, T. libellula became an increasingly dominant prey in the diets of zooplanktivorous fish and seabirds, demonstrating the important role for T. libellula in the pelagic food web. Our analysis suggests that T. libellula is capable of controlling copepod populations, thus it may become a potential contributor to top-down regulation of Calanus spp. in the eastern Bering Sea.

A multivariate analysis of observed and modeled biophysical variability on the Bering Sea shelf: Multidecadal hindcasts (1970–2009) and forecasts (2010–2040)

Coupled physical/biological models can be used to downscale global climate change to the ecology of subarctic regions, and to explore the bottom-up and top-down effects of that change on the spatial structure of subarctic ecosystems—for example, the relative dominance of large vs. small zooplankton in relation to ice cover. Here we utilize a multivariate statistical approach to extract the emergent properties of a coupled physical/biological hindcast of the Bering Sea for years 1970–2009, which includes multiple episodes of warming and cooling (e.g. the recent cooling of 2005–2009), and a multidecadal regional forecast of the coupled models, driven by an IPCC global model forecast of 2010–2040. Specifically, we employ multivariate empirical orthogonal function (EOF) analysis to derive the spatial covariance among physical and biological timeseries from our simulations. These are compared with EOFs derived from spatially gridded measurements of the region, collected during multiyear field programs. The model replicates observed relationships among temperature and salinity, as well as the observed inverse correlation between temperature and large crustacean zooplankton on the southeastern Bering Sea shelf. Predicted future warming of the shelf is accompanied by a northward shift in both pelagic and benthic biomass.

Interannual changes in the zooplankton community structure on the southeastern Bering Sea shelf during summers of 1994–2009

On the southeastern Bering Sea shelf, mesozooplankton plays an important role in material transfer between primary producers and fisheries resources. The biomass of mesozooplankton in this region is known to vary annually, but little is known about annual changes in community structure and species composition. In the present study, regional and long-term changes in abundance, biomass and community structure of copepods and chaetognaths on the shelf were evaluated based on NORPAC net samples collected during summers of 1994–2009. During the study period, regime shifts occurred from high interannual variability regime (1994–1999) to low interannual variability regime with high temperature (2000–2005), then to a low interannual variability regime with low temperature (2007–2009). A total of 24 calanoid copepod species belonging to 21 genera were identified from samples. Copepod abundance ranged from 150 to 834,486inds.m−2, was greatest on the Middle shelf, and was higher in cold years, than in warm years. Copepod biomass ranged from 0.013 to 150gDMm-2, and was also higher in cold years than in warm years. Based on the results of cluster analysis, the copepod community was divided into six groups (A–F). The regional and interannual distributions of each group were distinct. Interannual changes in abundance of the dominant copepod on the Outer shelf and Middle shelf were highly significant (p<0.0001), and their abundances were negatively correlated with temperature and salinity. Interannual changes in copepod community that occurred between cold and warm years are thought to have been caused by differences in the magnitude and timing of the spring phytoplankton bloom between the two regimes. Abundance and biomass of the chaetognath Parasagitta elegans ranged from 30 to 15,180inds.m-2 and from 11 to 1559mgDMm−2, respectively. Chaetognath abundance was significantly correlated with the abundance of the dominant copepods (p<0.0001). Differences in cold and warm years may also affect recruitment of walleye pollock. We conclude that on the southeastern Bering Sea shelf, the magnitude and timing of primary production, which is related to climate change, may significantly affect how it is transferred through the food web.

Comparison of warm and cold years on the southeastern Bering Sea shelf and some implications for the ecosystem

The southeastern, middle shelf of the Bering Sea has exhibited extreme variability in sea ice extent, temperature, and the distribution and abundance of species at multiple trophic levels over the past four decades. From 1972–2000, there was high interannual variability of areal extent of sea ice during spring (March–April). In 2000, this shifted to a 5-year (2001–2005) period of low ice extent during spring, which transitioned to a 4-year (2007–2010) period of extensive sea ice. High (low) areal extent of sea ice in spring was associated with cold (warm) water column temperatures for the following 6–7 months. The ocean currents also differed between warm and cold years. During cold years, the monthly-mean currents over the shelf were largely westward, while in warm years the direction of currents was more variable, with northward flow during December–February and relatively weak flow during the remainder of the year. The types and abundance of zooplankton differed sharply between warm and cold years. This was especially true during the prolonged warm period (2001–2005) and cold period (2007–2010), and was less evident during the years of high interannual variability. During the warm period, there was a lack of large copepods and euphausiids over the shelf; however, their populations rebounded during cold period. Small crustacean zooplankton taxa did not appear to vary between and warm and cold years. For both walleye pollock and Pacific cod, year-class strength (recruitment) was low during the prolonged warm period, but improved during the following cold period. Year-class strength did not appear to vary as a function of warm and cold years during the period of high year-to-year variability. Also, recruitment of arrowtooth flounder (a predator of pollock and cod) did not appear influenced by the warm or cold years. Finally, the distribution and relative abundance of fin whales appeared to differ in warm and cold years, with fewer whales on the southeastern, middle shelf during warm years.

Recent Bering Sea warm and cold events in a 95-year context

The meteorology and oceanography of the southeastern Bering Sea shelf was recently dominated by a multi-year warm event (2000–2005) followed by a multi-year cold event (2007–2010). We put these recent events into the context of the 95-year air temperature record from St. Paul Island and with concurrent spatial meteorological fields. For March 2000–2005 the mean air temperature anomaly at St. Paul was 2.1°C above the long-term mean, and for March 2007–2010 the mean temperature anomaly at St. Paul was 4.7°C below the long-term mean. The only multi-year temperature deviations comparable to the first decade of the 2000s are a cold event from 1971 to 1976 followed by a warm event from 1978 to 1983. There was also a short warm event 1935–1937. The temperature transition between warm and cold events in the 1970s and 2000s took two years. While there are theoretical arguments for some physical memory processes in the North Pacific climate system, we cannot rule out that the recent warm and cold events are of a random nature: they are rare in the St. Paul temperature record, they are dominated by North Pacific-wide sea level pressure events rather than local Bering Sea processes, and they are consistent with a red noise model of climate variability. The 1970s transition appears to have an ENSO (El Niño–Southern Oscillation) influence, while the recent events are likely connected to Arctic-wide warming. Evidence provided by the 95-year St. Paul meteorological record reinforces the idea that a red-noise model of climate variability is appropriate for the North Pacific and southeastern Bering Sea. We stress the importance of relatively rare sub-decadal events and shifts, rather than multi-decadal variability associated with the Pacific Decadal Oscillation (PDO). Thus, in the future we can expect large positive and negative excursions in the region that can last for multiple years, but there is as yet little predictability for their timing and duration.

Effects of climate variations on pelagic ocean habitats and their role in structuring forage fish distributions in the Bering Sea

This paper examines how climate variations influence the boundaries of suitable ocean habitat, and how these changes affect the spatial distribution and interactions between forage fishes in the southeastern Bering Sea shelf. The study focuses on the summer distributions of forage fish age-0 and age-1 walleye pollock, Theragra chalcogramma, and capelin, Mallotus villosus, observed during National Marine Fisheries Service summer acoustic trawl, surface trawl and bottom trawl surveys conducted in the Bering Sea between 2004 and 2009. We compare the responses of these forage fish to climate-induced shifts in ocean habitats. Habitat boundaries were defined using key explanatory variables including depth, bottom temperature and surface temperature, using general additive models. Bathymetry, bottom temperature and frontal zones formed boundaries between different groups of forage fishes. Age-0 pollock were dispersed throughout the middle domain (50–100m depth) in well-stratified regions. In cold years the highest densities of age-0s were found in the southern regions of the middle domain waters in waters warmer than approximately 1°C. In contrast, age-1 pollock were observed on the sea floor over the middle domain and in midwater in the northern outer domain in cold years and more broadly dispersed across the middle and outer domain in warm years. The demersal concentrations of age-1 pollock in the middle domain shows age-1 pollock tolerate a wide range of bottom temperatures. Midwater and demersal distributions of age-1 pollock exhibited a patchier distribution than age-0 pollock. Midwater concentrations of age-1 pollock tended to be associated with the outer domain and regions where higher levels of lower trophic level production are expected. Capelin were concentrated in the inner domain, a well-mixed region. The overlap of age-1 pollock and capelin was higher in cold years than in warm years.

North Pacific right whale up-call source levels and propagation distance on the southeastern Bering Sea shelf

Call source levels, transmission loss, and ambient noise levels were estimated for North Pacific right whale (Eubalaena japonica) up-calls recorded in the southeastern Bering Sea in autumn of 2000 and 2001. Distances to calling animals, needed to estimate source levels, were based on two independent techniques: (1) arrival-time differences on three or more hydrophones and (2) shallow-water dispersion of normal modes on a single receiver. Average root-mean-square (rms) call source levels estimated by the two techniques were 178 and 176 dB re 1 μPa at 1 m, respectively, over the up-call frequency band, which was determined per call and averaged 90 to 170 Hz. Peak-to-peak source levels were 14 to 22 dB greater than rms levels. Transmission loss was approximately 15∗log(10)(range), intermediate between cylindrical and spherical spreading. Ambient ocean noise within the up-call band varied from 72 to 91 dB re 1 μPa(2)/Hz. Under average noise conditions, call spectrograms were detectable for whales at distances up to 100 km, but propagation and detection distance may vary depending on environmental parameters and anthropogenic noise. Obtaining distances to animals and acoustic detection range is a step toward using long-term passive acoustic recordings to estimate abundance for this critically endangered whale population.

Climate change in the southeastern Bering Sea: impacts on pollock stocks and implications for the oscillating control hypothesis

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, 5821) 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.