Bering Sea Shelf Research Articles

Euphausiids in the eastern Bering Sea: A synthesis of recent studies of euphausiid production, consumption and population control

Euphausiids are an important component of the eastern Bering Sea marine ecosystem. We synthesized information on the ecological roles of two species, Thysanoessa raschii, which predominates over the Middle and Inner Shelf Domains, and Thysanoessa inermis, which predominates over the Outer Shelf Domain. Although estimates of euphausiid biomass across the shelf are not well constrained, we estimated that, between April and July, 2004–2010, euphausiid biomass was 3.08–5.25gCm−2 on the outer shelf and 1.95–3.92gCm−2 on the middle shelf. Modeled estimates of euphausiid production, for spring and summer combined, varied between 0.043gCm−2d−1 and 0.051gCm−2d−1, depending on location, with a mean of 0.048gCm−2d−1. Recently reported field measurements of annual primary production over the southeastern Bering Sea in 2008–2009 vary between 0.06 and 6.65gCm−2d−1, with a mean of 1.262gCm−2d−1±2.049gCm−2d−1 in spring and summer combined, a level sufficient to support euphausiids, at least on an annualized basis. Walleye pollock (Gadus chalcogrammus, hereafter pollock) is the single most important consumer of euphausiids over the eastern Bering Sea shelf. We estimated that pollock consumed between 0.0042 and 0.019.7gCm−2d−1 of euphausiids, depending on year, with a mean of 0.011gCm−2d−1 in summer averaged over 1999–2009. This consumption is equivalent to between 17% and 29% of summer euphausiid production, depending on location.Over the period for which data were available (2004–2012), we observed a strong negative relationship between euphausiid biomass as determined in acoustic surveys and pollock biomass as estimated in the eastern Bering Sea pollock stock assessment (r2=0.82). During this time period, sea-surface temperature was the second strongest predictor of euphausiid biomass, (r2=0.63). However, for the period 2004–2010, bottom temperature (r2=0.94) was the strongest predictor, followed by pollock biomass from the pollock stock assessment (r2=0.82), and sea-surface temperature (r2=0.81). Mean pollock density in the acoustic surveys was not a powerful predictor of euphausiid biomass during either period. In spatially explicit multiple regression analyses for the periods 2004–2012 and 2004–2010 those formulations that included sea-surface and bottom temperatures as well as survey estimates of pollock had the greatest explanatory value. However, when either or both temperature terms were dropped, the explanatory value of the models dropped considerably. When pollock biomass was dropped from the models, there was little change in explanatory value compared to the full model. Euphausiid production and pollock consumption data coupled with a negative relationship between euphausiid biomass and stock assessment estimates of pollock biomass indicate a top-down predation effect. However, strong negative relationships between euphausiid biomass and water temperatures indicate the influence of a bottom-up mechanism. The apparent differences in these results may relate to the different spatial and temporal scales used to assess the pollock biomass used in the analyses. Alternatively, euphausiid biomass may be strongly controlled during a restricted portion of the year, such as spring, if critical food needs are not met in some years. We lack the data necessary to resolve these alternative hypotheses.

Projected future biophysical states of the Bering Sea

Three global climate simulations from the Intergovernmental Panel on Climate Change Fourth Assessment (AR4) were used as physical forcing to drive a regional model that includes both physical and biological elements of the Bering Sea. Although each downscaled projection indicates a warming of 1–2°C between 2010 and 2040 on the Bering Sea shelf, the interannual and interdecadal details of this trend vary considerably among the three realizations. In each case, the magnitude of presently observed interannual variability of bottom temperatures and ice cover is found in the models to be maintained out to at least 2040, but with a steadily increasing probability of warm years with less ice on the southern shelf. The overall trends indicate warmer temperatures and the retreat of ice in the southeastern Bering Sea, but continued ice cover in the northeastern Bering Sea. Sensitivity analyses suggest both increasing air temperature and northward wind stress as primary drivers of higher water-column temperatures. Based on currently available models, changes in shortwave radiation are not likely to have a significant role in this warming. Warming trends on the outer shelf may lead to decreased production of large crustacean zooplankton at that location, but could increase such production on the inner shelf.

Mesozooplankton grazing during spring sea-ice conditions in the eastern Bering Sea

Mesozooplankton (copepods and euphausiids) grazing rates and prey preferences were determined during a series of three research cruises to the eastern Bering Sea in spring 2008, 2009, and 2010. Chlorophyll was dominated by large cells (>5µm), especially at bloom locations where they usually comprised greater than 90% of the total chlorophyll biomass. The relative importance of microzooplankton to the prey field biomass decreased with increasing chlorophyll concentration, and was less than 10% of the total prey biomass in ice-edge bloom regions. Overall, microzooplankton was the preferred prey of the mesozooplankton, although phytoplankton/ice algae were the dominant component of the diet because of their much greater biomass, especially during blooms. There were differences between mesozooplankton species in their prey preferences: Metridia pacifica, Pseudocalanus spp. and Calanus spp. had the strongest preference for microzooplankton prey, while euphausiids (Thysanoessa spp.) and Neocalanus flemingeri/plumchrus appeared to feed non-selectively on all prey items. Mesozooplankton exhibited a saturating feeding response to chlorophyll concentration (Holling׳s type II) that could be modeled by Michaelis–Menten equations. Taxa-specific maximum ingestion rates generally followed allometric theory, with smaller zooplankton having higher feeding rates than larger zooplankton, and ranged from about 4–30% body carbon day−1. Trophic cascades during grazing experiments could result in a substantial underestimate of chlorophyll ingestion rates, especially for those taxa that had a strong preference for microzooplankton. Grazing impacts by mesozooplankton on the integrated chlorophyll biomass and primary production were 2.7±4.4 and 26±48% day−1, respectively. Impacts increased significantly with increasing mesozooplankton biomass, which increased from early to late spring. However, grazing impacts were extremely low in ice-edge bloom regions. Our findings suggest that even when grazing by microzooplankton is included in our grazing impact estimates, about 50% of the primary production in phytoplankton blooms during spring on the eastern Bering Sea shelf is not grazed and is available for direct export to the benthic community.

Sedimentation in the submarine Shirshov Ridge area (Bering Sea) during the last 180–185 ka (Penultimate glaciation-Holocene)

The lithological analysis of sediments from Core SO201-2-85KL (18 m long) taken from the Shirshov Ridge in the western part of the Bering Sea (57°30.30′ N, 170°24.79′ E, water depth 968 m), which recovered the section spanning from the penultimate glaciation till Holocene, revealed their mostly terrigenous composition with several intercalations of diatomaceous ooze. The latter was accumulated mainly during relatively warm epochs (last interglacial and Holocene) with elevated bioproductivity of surface waters. Sedimentation during the penultimate glaciation was strongly influenced by bottom currents. Ice rafting of detrital material was intensified during cold marine isotope stages (MIS 6, MIS 4, MIS 2). Glaciations were accompanied by increased sedimentation rates probably due to the glacioeustatic sea level falls, desiccation of the Bering Sea shelf, and enhanced influx of sedimentary material transported by large rivers immediately to the deepwater basin.

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.

What processes contribute to the spring and fall bloom co-variability on the Eastern Bering Sea shelf?

Observations indicate that spring and fall phytoplankton blooms on the Eastern Bering Sea (EBS) continental shelf tend to co-vary on inter-annual scales – that is, a year with a strong spring bloom also tends to have a strong fall bloom. Similar co-variability of primary production is also seen in the multi-year (1987–2007) integration of a coupled physical–biological model. Moreover, the modeled seasonal amplitudes of 10-meter chlorophyll-a concentrations at the EBS middle shelf mooring locations, computed using the canonical Redfield ratio and a mean carbon-to-chlorophyll-a ratio, are generally consistent with the in situ mooring measurements. The coupled physical–biological model simulation is used to examine the relative contributions of wind mixing, local nutrient recycling/regeneration, horizontal nutrient advection, and water-column stability to this co-variability. There is no significant correlation between the spring and fall surface wind mixing. Although wind mixing is an important mechanism for bringing nutrients in the lower water column to the surface layers, it is not the mechanism tying the two seasons׳ productivity together. Local regeneration/recycling of the nutrients initially fueling spring production is an important mechanism for spring-to-fall nutrient accumulation in the bottom layers at the middle shelf. Horizontal advection does not appear to be the dominant factor for supplying nutrients to the middle shelf during the spring-to-fall period. Fall primary production in the model is strongly influenced by the lower water-column stability/stratification. Taken together, these results highlight the importance of local recycling/regeneration of nutrients assimilated by spring phytoplankton bloom in linking together the spring and fall primary productions on EBS middle shelf.

Spatial and temporal changes in assemblage structure of zooplankton and pelagic fish in the eastern Bering Sea across varying climate conditions

Zooplankton and pelagic fish samples collected on the eastern Bering Sea shelf in late summer 2003-2010 were used to evaluate spatial and temporal changes in the plankton and nekton community structure. The zooplankton were sampled by vertical towing of a Juday net (168 μm mesh) and oblique towing of a Bongo net (505 μm mesh), and pelagic fish were caught by midwater rope trawl. The communities were compared across climate regimes (in relatively warm and cold years), by latitude (in the northern and southern parts of the shelf), and by water depth (in the inner, middle, and outer domains of the shelf). Zooplankton were dominated by the small copepod Oithona spp. in warm years but relatively larger copepods Pseudocalanus spp. and Acartia spp. in cold years. Notably, the large copepod Calanus spp., an important energy-rich prey for fish, were more abundant in cold years than warm years. Age-0 walleye pollock Gadus chalcogrammus were more abundant in warm years, while capelin Mallotus villosus were abundant within cold-year communities over the northern shelf. Latitudinal variations in communities were more prominent in the cold years, particularly in 2007 and 2010. Cross-shelf variations were evident, particularly for large zooplankton and fish, with communities corresponding to specific oceanographic domains. Outer shelf communities varied less than inner and middle shelf communities between warm and cold periods, suggesting that this region may be less impacted by climate variability. An understanding of the overlap of zooplankton (prey) and fish communities within specific shelf regions or climate regimes may provide information for ecosystem-based approaches to fisheries management.

Divergent patterns of recent sea ice cover across the Bering, Chukchi, and Beaufort seas of the Pacific Arctic Region

Over the past three decades of the observed satellite record, there have been significant changes in sea ice cover across the Bering, Chukchi, and Beaufort seas of the Pacific Arctic Region (PAR). Satellite data reveal that patterns in sea ice cover have been spatially heterogeneous, with significant declines in the Chukchi and Beaufort seas, yet more complex multi-year variability in the Bering Sea south of St. Lawrence Island. These patterns in the Chukchi and Beaufort seas have intensified since 2000, indicating a regime shift in sea ice cover across the northern portion of the PAR. In particular, satellite data over 1979–2012 reveal localized decreases in sea ice presence of up to −1.64days/year (Canada Basin) and −1.24days/year (Beaufort Sea), which accelerated to up to −6.57days/year (Canada Basin) and −12.84days/year (Beaufort Sea) over the 2000–2012 time period. In contrast, sea ice in the Bering Sea shows more complex multi-year variability with localized increases in sea ice presence of up to +8.41days/year since 2000. The observed increases in sea ice cover since 2000 in the southern Bering Sea shelf region are observed in wintertime, whereas sea ice losses in the Canada Basin and Beaufort Sea have occurred during summer. We further compare sea ice variability across the region with the National Centers for Environmental Prediction (NCEP) North American Regional Reanalysis (NARR) wind and air temperature fields to determine the extent to which this recent variability is driven by thermal vs. wind-driven processes. Results suggest that for these localized areas that are experiencing the most rapid shifts in sea ice cover, those in the Beaufort Sea are primarily wind driven, those offshore in the Canada Basin are primarily thermally driven, and those in the Bering Sea are influenced by elements of both. Sea ice variability (and its drivers) across the PAR provides critical insight into the forcing effects of recent shifts in climate and its likely ultimate profound impacts on ecosystem productivity across all trophic levels.

Sound velocity and related properties of seafloor sediments in the Bering Sea and Chukchi Sea

The Bering Sea shelf and Chukchi Sea shelf are believed to hold enormous oil and gas reserves which have attracted a lot of geophysical surveys. For the interpretation of acoustic geophysical survey results, sediment sound velocity is one of the main parameters. On seven sediment cores collected from the Bering Sea and Chukchi Sea during the 5th Chinese National Arctic Research Expedition, sound velocity measurements were made at 35, 50, 100, 135, 150, 174, 200, and 250 kHz using eight separate pairs of ultrasonic transducers. The measured sound velocities range from 1 425.1 m/s to 1 606.4 m/s and are dispersive with the degrees of dispersion from 2.2% to 4.0% over a frequency range of 35–250 kHz. After the sound velocity measurements, the measurements of selected geotechnical properties and the Scanning Electron Microscopic observation of microstructure were also made on the sediment cores. The results show that the seafloor sediments are composed of silty sand, sandy silt, coarse silt, clayey silt, sand-silt-clay and silty clay. Aggregate and diatom debris is found in the seafloor sediments. Through comparative analysis of microphotographs and geotechnical properties, it is assumed that the large pore spaces between aggregates and the intraparticulate porosity of diatom debris increase the porosity of the seafloor sediments, and affect other geotechnical properties. The correlation analysis of sound velocity and geotechnical properties shows that the correlation of sound velocity with porosity and wet bulk density is extreme significant, while the correlation of sound velocity with clay content, mean grain size and organic content is not significant. The regression equations between porosity, wet bulk density and sound velocity based on best-fit polynomial are given.

Faunal features of submarine canyons on the eastern Bering Sea slope

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 526:21-40 (2015) - DOI: https://doi.org/10.3354/meps11201 Faunal features of submarine canyons on the eastern Bering Sea slope Michael F. Sigler1,*, Christopher N. Rooper2, Gerald R. Hoff2, Robert P. Stone1, Robert A. McConnaughey2, Thomas K. Wilderbuer2 1NOAA Alaska Fisheries Science Center, 17109 Point Lena Loop Road, Juneau, Alaska 99801, USA 2NOAA Alaska Fisheries Science Center, 7600 Sand Point Way NE, Seattle, Washington 98115, USA *Corresponding author: mike.sigler@noaa.gov ABSTRACT: Some of the largest submarine canyons in the world incise the eastern Bering Sea shelf break. Here, we examine whether 5 large canyons (Bering, Pribilof, Zhemchug, Pervenets and Navarin) are faunally distinct features within the eastern Bering Sea. We compiled data from the eastern Bering Sea that included trawl survey data on fish and invertebrate distributions and observations of ocean conditions and benthic habitat. These data were analyzed using multivariate techniques to determine if the canyons are distinguishable from the adjacent continental slope. We found that the 5 canyons are not faunally distinct features of the eastern Bering Sea slope but that the major characteristics structuring fish and invertebrate communities were depth, latitude and sediment rather than submarine canyons. One notable feature of these canyons is that about one quarter of the coral habitat predicted for the eastern Bering Sea slope occurs in Pribilof Canyon, an area that comprises only ~10% of the total slope area. The predicted coral habitat also extends westward to the adjacent slope, indicating that this coral habitat concentration is not unique to Pribilof Canyon but rather that it is typical for a larger area (Pribilof Canyon and westward). KEY WORDS: Eastern Bering Sea slope · Canyons · Habitat · Fish · Coral · Sponge Full text in pdf format PreviousNextCite this article as: Sigler MF, Rooper CN, Hoff GR, Stone RP, McConnaughey RA, Wilderbuer TK (2015) Faunal features of submarine canyons on the eastern Bering Sea slope. Mar Ecol Prog Ser 526:21-40. https://doi.org/10.3354/meps11201 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 526. Online publication date: April 22, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

Circulation in the Eastern Bering Sea: Inferences from a 2-km-resolution model

A 2-km-resolution model of the eastern Bering Sea is developed to capture dynamical processes on the scale of the Rossby radius of deformation on tidal to seasonal time scales. The model spans the region from 178°E to the Alaskan coast and from roughly 50° to 66°N, including the Aleutian Islands in the south and the Bering Strait in the north. The high resolution throughout ensures that the mesoscale dynamics of significant subregions of the domain, such as the Aleutian Island passes, Bering Sea slope, and the shelf canyons, are captured simultaneously without the concern for loss of interconnectivity between regions. Simulations are performed for the ice-free season (June–October) of 2009, with tidal and atmospheric forcing. The model compares favorably with observations from AVHRR and Envisat satellites, Argo drifters, and Bering Sea shelf moorings. The mesoscale dynamics of the mixing and exchange flow through the eastern Aleutian Island passes, which exhibit strong diurnal and two-week variability, are well represented. The two-week oscillation in volume flux through the largest of these passes, Amukta Pass, is found to be out of phase with the transport through the neighboring passes (e.g., Seguam and Samalga passes). Mesoscale structure is also found to be ubiquitous along the mixing front of the cold pool. Structures at the scale of O(20km) persist and play a role in determining the pattern of erosion of the water mass as the shelf warms and mixes. On the Bering Sea shelf, tidal motions are dominant, and variability on the horizontal scale of the first-mode internal tide develops (O(30km)) from the shelf break to the onshore edge of the Bering shelf cold pool.

Size, diet, and condition of age-0 Pacific cod (Gadus macrocephalus) during warm and cool climate states in the eastern Bering sea

The revised Oscillating Control Hypothesis for the Bering Sea suggests that recruitment of groundfish is linked to climatic processes affecting seasonal sea ice that, in turn, drives the quality and quantity of prey available to young fish for growth and energy storage during their critical life history stages. We test this notion for age-0 (juvenile) Pacific cod (Gadus macrocephalus) by examining the variability in size, diet, and energetic condition during warm (2003–2005), average (2006), and cool (2007–2011) climate states in the eastern Bering Sea. Juvenile cod stomachs contained high proportions of age-0 walleye pollock (by wet weight) during years with warm sea temperatures with a shift to euphausiids and large copepods during years with cool sea temperatures. Juvenile cod were largest during years with warm sea temperatures and smallest during years with cool sea temperatures. However, energetic status (condition) of juvenile cod was highest during years with cool sea temperatures. This result is likely linked to the shift to high quality, lipid-rich prey found in greater abundance on the shelf and in the stomach contents of juvenile cod during cool years. Our examination of juvenile cod size, diet, and energetic status provided results that are similar to those from studies on juvenile pollock, suggesting that the common mechanisms regulating gadid recruitment on the eastern Bering Sea shelf are climate state, prey quality and quantity, and caloric density of gadids prior to winter.

Interannual summer variability in euphausiid populations on the eastern Bering Sea shelf during the recent cooling event (2008–2010)

The recent cooling in the eastern Bering Sea resulted in a series of cold years (2008–2010) marked by extensive ice coverage and late ice retreat. In the present study, we examined the spatial and temporal variability of three predominant euphausiid species Thysanoessa raschii, Thysanoessa inermis, and Thysanoessa longipes in the summers of 2008–2010. Simple box-and-whisker plots as well as generalized additive models (GLMs) were applied to examine habitat selection for three euphausiid species. The coefficients of dispersion were calculated for three species to examine potential changes in spatial patterns. Results showed that T. raschii was broadly distributed in the inner and middle domains. The abundance of T. raschii was related to water temperature, salinity and chlorophyll a concentration. T. inermis was primarily distributed in the middle domain and was related to water temperature and salinity. T. longipes was mostly distributed in the outer domain and was only related to salinity. The proportion of T. raschii, a coastal species in the inner and middle domains, showed large interannual variation with the highest in 2008 and the lowest in 2009 (a >50% decline). This same species did not show large interannual variation in the spatial distribution. T. longipes, a shelf species in the outer domain, did not show large interannual variation in abundance, but the center of mass distribution consistently shifted northward and offshore from 2008 to 2010. T. inermis, a coastal species in the middle and coastal domains, did not show large interannual variation in abundance and spatial distribution. In summary, euphausiid populations showed large spatial and temporal variability among cold years. Different species could be affected by different processes in the southeastern Bering Sea.

Annual sea‐air CO2 fluxes in the Bering Sea: Insights from new autumn and winter observations of a seasonally ice‐covered continental shelf

AbstractHigh‐resolution data collected from several programs have greatly increased the spatiotemporal resolution of pCO2(sw) data in the Bering Sea, and provided the first autumn and winter observations. Using data from 2008 to 2012, monthly climatologies of sea‐air CO2 fluxes for the Bering Sea shelf area from April to December were calculated, and contributions of physical and biological processes to observed monthly sea‐air pCO2 gradients (ΔpCO2) were investigated. Net efflux of CO2 was observed during November, December, and April, despite the impact of sea surface cooling on ΔpCO2. Although the Bering Sea was believed to be a moderate to strong atmospheric CO2 sink, we found that autumn and winter CO2 effluxes balanced 65% of spring and summer CO2 uptake. Ice cover reduced sea‐air CO2 fluxes in December, April, and May. Our estimate for ice‐cover corrected fluxes suggests the mechanical inhibition of CO2 flux by sea‐ice cover has only a small impact on the annual scale (<2%). An important data gap still exists for January to March, the period of peak ice cover and the highest expected retardation of the fluxes. By interpolating between December and April using assumptions of the described autumn and winter conditions, we estimate the Bering Sea shelf area is an annual CO2 sink of ∼6.8 Tg C yr−1. With changing climate, we expect warming sea surface temperatures, reduced ice cover, and greater wind speeds with enhanced gas exchange to decrease the size of this CO2 sink by augmenting conditions favorable for greater wintertime outgassing.

Understanding the relationship between ice, primary producers, and consumers in the Bering Sea

Technology has progressed to the level of allowing for investigations of trophic level interactions over time scales of months to years which were previously intractable. A combination of active and passive acoustic technology has been integrated into sub-surface moorings on the Eastern Bering Sea shelf, and seasonal transition measurements were examined to better understand how interannual variability of hydrographic conditions, phytoplankton biomass, and acoustically derived consumer abundance and community structure are related. Ocean conditions were significantly different in 2012 compared to relatively similar conditions in 2009, 2010, and 2011. Differences were largely associated with variations in sea ice extent, thickness, retreat timing, and water column stratification. There was a high degree of variability in the relationships between different classes of consumers and hydrographic condition, and evidence for intra-consumer interactions and trade-offs between different size classes was apparent. Phytoplankton blooms in each year stimulated different components of the consumer population. Acoustic technology now provides the opportunity to explore the ecosystem dynamics in a remote, ice-covered region that was previously limited to ship-board measurements during ice-free periods. The new knowledge we are gaining from remote, long-term observations is resulting in a re-examination of previously proposed ecosystem theories related to the Bering Sea.

Sea ice and water column structure on the eastern Bering Sea shelf

Seasonal sea ice is a defining characteristic of the eastern Bering Sea shelf, and plays a critical role in determining the vertical structure of temperature and salinity over this shelf. Ice movement relative to local winds, ice composition, and the impact of both arrival and retreat of ice on the water column at four mooring sites over the middle shelf are examined. Ice forms primarily in coastal regions and is advected over the southern and outer shelves. Ice drift from satellite data for two representative years, 2003 and 2007, was ~2% of local NCEP wind speed and oriented 44° to the right of the winds (r2=0.25). Measurements from 30 ice cores collected in 2006–2009 gave an average salinity of 5.62±0.88, and an average nitrate concentration of 0.99±0.83µM. Time series data collected at the biophysical moorings in the Bering Sea (1995–2012) were used to explore the evolution of the water column under ice. At the northern mooring, M8, the water column had mixed and cooled to ~−1°C prior to the arrival of ice. Little melt occurred after ice arrival. At the other three moorings, the ocean temperature was 2–4°C when ice arrived, resulting in extensive melt. Melting ice freshened and cooled the upper water column, resulting in stratification, which persisted for 10–25 days. Wind-induced water-column mixing occurred more slowly under the ice than in ice-free waters. An estimated 1.4m of ice melted with the first arrival of ice at the three southern moorings where the latent heat of fusion accounted for approximately half the observed cooling. During ice retreat, there appeared to be little ice melt around the southern two moorings, but an estimated 0.8m at M5 and M8. The extent of ice melt sets up the water column for the following summer.

Diffuse spectral reflectance of surficial sediments indicates sedimentary environments on the shelves of the Bering Sea and western Arctic

Abstract Visible diffuse spectral reflectance (DSR) has been described by previous workers as an efficient tool for extracting lithological compositions: clay minerals, iron oxyhydroxides, carbonates and diatoms in sedimentary material. The spatial patterns of these components are essential to understanding the processes driving their distribution and accumulation in the marine environment. Some of these processes include sea ice variability, nutrient supply, current flow patterns, current strength, sediment supply, provenance and bathymetry. The DSR derivatives and Varimax-rotated principal component analysis (VPCA) of the spectral data collected from 234 core samples identified five leading components that account for variations in sediment lithology of the surface samples from the Bering Sea and western Arctic. These spectral components were then matched to spectral standards to identify the mineral assemblages responsible for them. The components responsible for the VPCA trends are chlorite + muscovite; goethite + phycoerythrin + phycocyanin; smectite; calcite + dolomite; and illite + chlorophyll a. The spatial pattern of the inorganic components is indicative of provenance and processes controlling their transportation and depositional patterns, while the organic components are indicative of areas of primary productivity and driving mechanisms. The chlorite + muscovite assemblage which is sourced dominantly from the Yukon River has maximum values on the Bering Sea shelf; this is the source of the chlorite + muscovite transport to the Chukchi Sea. The chlorite + muscovite assemblage also has a main transport route to the Aleutian basin in the northern Bering Sea shelf. The Chukchi Sea on the western Arctic Ocean shelf is a region of highly reducing condition. This is based on the lowest values for the goethite + phycoerythrin + phycocyanin assemblage, supported by the high carbonate concentrations at this location. The carbonate distribution pattern on the Bering Sea shelf also appears to be tidally influenced. This is because their spatial pattern divides the Bering shelf into three domains, which are similar to the three hydrographic domains on the Bering shelf—a function of tidal influence. The smectite assemblage reaches maximum values along the coast of the Chukotka Peninsula in the Chukchi Sea, denoting the Chukotka Peninsula as a major source area of smectite, and supporting the notion for volcanically derived sediment to the Chukchi Sea. Another likely source of smectite to the Chukchi Sea is via the Bering Strait. The illite in the illite + chlorophyll a assemblage shows maximum values and distribution on the western Arctic Ocean shelf, indicating the western Arctic Ocean shelf to be a main province of illite. All the high values of the illite + chlorophyll a component on the Bering Sea shelf are chlorophyll a from diatoms. Our results support the usefulness of the DSR method as an effective and efficient tool for provenance analysis, and understanding the processes controlling sediment and biologic distribution, in this case, in the marine environment.

Distributions and air-sea fluxes of CO2 in the summer Bering Sea

The 3rd Chinese National Arctic Research Expedition (CHINARE-Arctic III) was carried out from July to September in 2008. The partial pressure of CO2 (pCO2) in the atmosphere and in surface seawater were determined in the Bering Sea during July 11–27, 2008, and a large number of seawater samples were taken for total alkalinity (TA) and total dissolved inorganic carbon (DIC) analysis. The distributions of CO2 parameters in the Bering Sea and their controlling factors were discussed. The pCO2 values in surface seawater presented a drastic variation from 148 to 563 μatm (1 μatm = 1.013 25×10−1 Pa). The lowest pCO2 values were observed near the Bering Sea shelf break while the highest pCO2 existed at the western Bering Strait. The Bering Sea generally acts as a net sink for atmospheric CO2 in summer. The air-sea CO2 fluxes in the Bering Sea shelf, slope, and basin were estimated at −9.4, −16.3, and −5.1 mmol/(m2·d), respectively. The annual uptake of CO2 was about 34 Tg C in the Bering Sea.

Phytoplankton composition and its ecological effect in subsurface cold pool of the northern Bering Sea in summer as revealed by HPLC derived pigment signatures

CHEMTAX analysis of high-performance liquid chromatography (HPLC) pigment was conducted to study phytoplankton community structure in the northern Bering Sea shelf, where a seasonal subsurface cold pool emerges. The results showed that fucoxanthin (Fuco) and chlorophyll a (Chl a) were the most abundant diagnostic pigments, with the integrated water column values ranging from 141 to 2 160 μg/m2 and 477 to 5 535 μg/m2, respectively. Moreover, a diatom bloom was identified at Sta. BB06 with the standing stock of Fuco up to 9 214 μg/m3. The results of CHEMTAX suggested that the phytoplankton community in the northern Bering Sea shelf was dominated by diatoms and chrysophytes with an average relative contribution to Chl a of 80% and 12%, respectively, followed by chlorophytes, dinoflagellates, and cryptophytes. Diatoms were the absolutely dominant algae in the subsurface cold pool with a relative contribution exceeding 90%, while the contribution of chrysophytes was generally higher in oligotrophic upper water. Additionally, the presence of a cold pool would tend to favor accumulation of diatom biomass and a bloom that occurred beneath the halocline would be beneficial to organic matter sinks, which suggests that a large part of the phytoplankton biomass would settle to the seabed and support a rich benthic biomass.

Changes in the distribution and abundance of albatrosses in the eastern Bering Sea: 1975–2010

A number of marine species are showing poleward shifts in their distributions in response to climate warming. Three albatross species frequent the Bering Sea, the Laysan (Phoebastria immutabilis), the black-footed (Phoebastria nigripes), and the endangered short-tailed albatross (Phoebastria albatrus). To determine if albatrosses changed their distribution or abundance in the eastern Bering Sea between 1975 and 2010, we examined at-sea survey data using the North Pacific Pelagic Seabird Database. Within our study area, all three species of albatross occurred most frequently in the waters of the Aleutian Islands. In the eastern Bering Sea, all three species were most abundant near the shelf break, and in particular in the vicinity of the major submarine canyons in the shelf slope. Starting in the 1990s, population densities increased for all three albatross species, with a marked increase in the 2000s. In the 2000s, there was also an increase in the frequency at which albatrosses were recorded in the central and northern Bering Sea. Both black-footed and short-tailed albatrosses shifted the centers of their Bering Sea distributions northward. The Laysan albatross center of distribution shifted southward due to increased numbers along the southern shelf break, but densities also increased northward. We suggest that the observed changes in distribution and abundance of the three albatross species in the eastern Bering Sea may have been responses to an increase in the availability of squid, their primary prey, there. Additionally, the expansion of the distribution of the short-tailed albatross in the eastern Bering Sea may represent the reclamation of its previous range, now that the population is beginning to recover from near extinction caused by over harvesting. We suggest that predicted increases in ocean temperatures and northward movement of prey could result in albatrosses and other marine apex predators foraging farther north along the Bering Sea shelf and staying later in fall.