Chirikov Basin Research Articles

Spatiotemporal Distribution of Nitrous Oxide on the Northeastern Bering Sea Shelf

Rapid warming and loss of sea ice in the Arctic Ocean could play an important role in the dissolution and emission of greenhouse gas nitrous oxide (N2O). We investigated dissolved N2O in spatiotemporal distribution on the northeastern Bering Sea shelf (NEBS) in the summer of 2012. The results showed that N2O concentrations were higher in the Chirikov Basin (mean ± SD, 14.8 ± 2.4 nmol/L) than in the south of St. Lawrence Island (mean ± SD, 17.7 ± 2.3 nmol/L). In the Chirikov Basin, N2O displayed a decreasing distribution pattern from west (~20.4 nmol/L) to east (~12.9 nmol/L). In the area south of St. Lawrence Island, N2O almost presented a two-layer structure, although it showed a vertically homogeneous distribution in the inner shelf. In the cold bottom water, the N2O was affected mainly by in situ production or sediment emission. Longer resident time may cause N2O accumulation in the cold bottom water. The calculated sea–air flux (−1.6~36.2 μmol/(m2·d)) indicates that the NEBS is an important potential source of atmospheric N2O and could play an important role in global oceanic N2O emission with intensifying global issues.

Effects of Early Sea-Ice Reduction on Zooplankton and Copepod Population Structure in the Northern Bering Sea During the Summers of 2017 and 2018

A remarkable early sea-ice reduction event was observed in the northern Bering Sea during 2018. In turn, this unusual hydrographic phenomenon affected several marine trophic levels, resulting in delayed phytoplankton blooms, phytoplankton community changes, and a northward shift of fish stocks. However, the response of the zooplankton community remains uncharacterized. Therefore, our study sought to investigate the zooplankton community shifts in the northern Bering Sea during the summers of 2017 and 2018 and evaluate the effects of early sea-ice melt events on the zooplankton community, population structure of large copepods, and copepod production. Five zooplankton communities were identified based on cluster analysis. Further, annual changes in the zooplankton community were identified in the Chirikov Basin. In 2017, the zooplankton community included abundant Pacific copepods transported by the Anadyr water. In 2018, however, the zooplankton community was dominated by small copepods and younger stages of large copepods (Calanus glacialis/marshallaeandMetridia pacifica), which was likely caused by reproduction delays resulting from the early sea-ice reduction event. These environmental abnormalities increased copepod production; however, this higher zooplankton productivity did not efficiently reach the higher trophic levels. Taken together, our findings demonstrated that zooplankton community structure and production are highly sensitive to the environmental changes associated with early sea-ice reduction (e.g., warm temperatures and food availability).

Migratory Behavior of an Eastern North Pacific Gray Whale From Baja California Sur to Chirikov Basin, Alaska

Eastern gray whales undertake annual migrations between summer feeding grounds in the Bering and Chukchi Seas and winter breeding and calving lagoons in the west coast of Baja California, Mexico. On February 12, 2017, three adult gray whales were sighted at San José del Cabo, Mexico, in which one individual, named “María,” was tagged using a satellite telemetry transmitter (PTT). The PTT stopped the signal on July 11, 2017. María traveled 11,387 km during 149 days from San José del Cabo to the Chirikov Basin. The migration route was aligned close to the coastline (<23 km) from February to April. After passing Kodiak Island in May, María started traveling far away from the coastline (>70 km) into the Bering Sea, including the Chirikov Basin. During March, April, and May, María traveled long distances at relatively high speeds, in contrast to the lower speed during February, early March, and the arrival time to the feeding areas in May, June, and July. The total distance traveled by María during its migration from Ojo de Liebre Lagoon to the Chirikov Basin was 8,863 km during 61.5 days with an average speed of 5.5 km h–1; this excludes the 14 days and 591 km that María spent feeding on the coast of Kodiak Island in late April. The information provided by this tagged whale documents a single whales’ migration, which is consistent with previous studies and constitutes the most complete northbound reported migration of an eastern gray whale.

Surface CO2 system dynamics in the Gulf of Anadyr during the open water season

The Gulf of Anadyr (GA), one of the largest and most poorly investigated gulfs of the Bering Sea, is an important part of the Pacific Arctic. Drivers and mechanisms responsible for the spatial variations in surface seawater CO2 system parameters and associated air–sea CO2 fluxes were investigated during three oceanographic cruises conducted in the GA during the open water season of 1992, 2002, and 2011. It was shown that the surface waters of the GA were moderate to strong sinks for atmospheric CO2 during this season: mean air-sea CO2 fluxes for the investigated areas of the GA, for the three cruises, were −31, −15, and −30 mmol CO2 m−2 d−1, respectively. The summer air-sea CO2 flux was attributed to intensive phytoplankton bloom, and fall CO2 uptake coincided with a period of strong winds and seasonal enhanced primary production. The surface waters of the GA were supersaturated with respect to aragonite during the investigated period. Anadyr Strait, a region of high hydrodynamic activity, was the only area where CO2 efflux was observed. Surface waters of the strait were supersaturated with respect to CO2 and in quasi-equilibrium with respect to aragonite. We found that during the late summer/fall season, the Anadyr Current exported a well-mixed, relatively cold, salty, nutrient- and CO2-rich water through the western Anadyr Strait into the Chirikov Basin. Our research highlights the need for mesoscale studies of the western Bering Sea to produce the most accurate assessment of the regional CO2 budget.

Distributional shifts among seabird communities of the Northern Bering and Chukchi seas in response to ocean warming during 2017–2019

In the northern Bering Sea and eastern Chukchi Sea, 2017–2019 were record-breaking years for warm ocean temperatures and lack of sea ice. The region supports millions of seabirds that could be affected by shifts in prey distribution and availability caused by changing environmental drivers. However, seabirds are highly mobile and often flexible in diet, and might alter their foraging distributions accordingly. To determine if there was evidence of long-term changes in abundance of seabirds, or if seabirds used the offshore habitat differently during recent warm years, we compared species richness, community composition, and distribution and abundance of selected species and Total seabirds (all species combined) between two periods, 2007–2016 and 2017–2019. We also evaluated annual changes in abundance during 2007–2019. We used 79,426 km of transects from vessel-based surveys conducted July through September. Total seabird density for the entire study area increased by ~20% during 2017–2019, but changes were not consistent across the study area, nor among species, and species richness declined except for a slight increase in the northern Chukchi Sea. Total seabird density declined most in the northern Bering Sea (−27%), although it increased in the Chirikov Basin by 73%. During 2017–2019, abundance of piscivorous murres (Uria spp.) decreased everywhere, whereas planktivorous Aethia auklet density increased by 70% in Chirikov Basin; auklets apparently abandoned their post-breeding migration to the Chukchi Sea. Short-tailed shearwaters (Ardenna tenuirostris) expanded farther into the northern Chukchi Sea, with nearly twice the density of the previous decade. We identified five seabird community types, three of which (all dominated by an alcid species) contracted spatially in the later period, and shifted south or near colonies. In contrast, a short-tailed shearwater dominated community expanded northward, and a community defined by low seabird density expanded throughout the eastern portion of both the northern Bering and Chukchi seas, suggesting higher-density communities had shifted westward. The variable responses among species correspond to documented changes in the environment as well as their natural history.

Spatial changes in the summer diatom community of the northern Bering Sea in 2017 and 2018

In recent years, the northern Bering Sea has experienced changes in the timing of sea-ice retreat and in hydrographic conditions during the summer. The influence of these environmental changes on the diatom community has not been examined. In this study, we investigated the spatial changes in the diatom community of the northern Bering Sea during the summers of 2017 and 2018, and evaluated the effects of environmental variability on these communities. We found that the diatom cell density and diversity varied with water masses. A cluster analysis based on cell density revealed that the diatom communities were separated into four groups, and that the distributions of three of these groups were different spatially between 2017 and 2018. In the Bering Strait and the Chirikov Basin regions, the diatom communities differed between 2017 and 2018. In 2017, these diatom communities were dominated by cold-water species such as Chaetoceros gelidus and Chaetoceros spp. (subgenus Hyalochaetae), while in 2018, the community was dominated by cosmopolitan species such as Thalassionema nitzschioides and Chaetoceros spp. (subgenus Phaeoceros). NMDS and multiple regression analysis indicated that the timing of the sea-ice retreat was the most important contributor to the differences in the diatom community. In contrast, there was no year-to-year difference south of St. Lawrence Island, possibly because nutrients were depleted and phytoplankton types other than diatoms were dominant.

Recent change in benthic macrofaunal community composition in relation to physical forcing in the Pacific Arctic

There is growing evidence that increased Pacific water transport into the Arctic affects the marine ecosystem. One of the theoretical predictions for a future Arctic characterized by such environmental change is that subarctic taxa will expand northward and invade the native Arctic ecosystem. This study focuses on variation in macrofaunal community composition and the influence of changing physical drivers at known benthic hotspots in the Pacific Arctic. The average number of macrofaunal family-level taxa has increased significantly south of St. Lawrence Island and in the Chirikov Basin, whereas the number of macrofaunal taxa in the southeastern Chukchi Sea showed no significant trend over the 2000–2013 time period. However, the Shannon–Weaver diversity index, based on abundance, did not mirror these regional changes in the number of macrofaunal taxa, indicating that the abundance of newly present taxa was negligible compared to the entire abundance already present. We also investigated temporal variations in meridional sea level gradient and local winds, which contribute 2/3 and 1/3 of the variation in northward volume transport at Bering Strait, respectively. There were significant increasing trends in the meridional sea level gradient and local winds, suggesting the increased northward seawater volume transports over the benthic hotspots could contribute to the expansion of subarctic taxa into these northern Arctic regions. Our data suggest an increase in macrofaunal taxa type with increasing current transport northward into the Pacific Arctic region that could have a strong influence in restructuring the benthic ecosystem in this region in the future.

Carbonate Characteristics of the Gulf of Anadyr Waters

The first field data describing the dynamics of the carbonate system, the aragonite saturation state, and CO2 fluxes between the ocean and the atmosphere in the Gulf of Anadyr in the late autumn season are presented. It was established that during this period the gulf waters absorbed carbon dioxide from the atmosphere at a rate of –22.5 mmol m–2 day–1, which determined the “classical” mechanism of seawater acidification due to uptake of excess atmospheric CO2. In general, surface waters of the gulf were supersaturated with respect to aragonite. The exception was the highly dynamic region of Anadyr Strait, where the vertical distribution of the investigated parameters was homogeneous, the surface waters were close to equilibrium with respect to aragonite, and the CO2 flux was directed to the atmosphere. The bottom waters of the gulf, in contrast, were characterized by significant seasonal corrosivity due to remineralization of organic matter. It was shown that, during the late fall, relatively salty and acidic, quasi-equilibrium with respect to aragonite, and oxygen-depleted waters with high concentrations of nutrients and CO2 enter into the Chirikov Basin and further to the Arctic Ocean with Navarin Current.

A decade of summertime measurements of phytoplankton biomass, productivity and assemblage composition in the Pacific Arctic Region from 2006 to 2016

We present phytoplankton and nutrient observations from a period of ten years within five biological ‘hotspots’ in the Bering and Chukchi Seas, as identified by the Distributed Biological Observatory (DBO). Nitrate (NO3-) and total and size-fractionated (< and >5 µm) chlorophyll a (Chl a) concentrations, and rates of carbon (ρC, ‘primary productivity’) and NO3- utilization (ρNO3) were measured throughout the euphotic zone during eight cruises in July 2006, 2008 and yearly from 2011 to 2016. Samples were collected at one station within each of these five hotspots, which were located south of St. Lawrence Island (DBO1), south of the Bering Strait in the Chirikov Basin (DBO2), in the southeastern (DBO3) and northeastern (DBO4) Chukchi Sea, and in Barrow Canyon (DBO5). Nitrate concentrations averaged over the 10 years increased with depth and euphotic-zone integrated values were highest in the Chirikov Basin. Subsurface maxima in Chl a were present at about 30 m depth at most locations during every cruise, although the maximum ρC and ρNO3 rates were shallower, within the top 10 m of the water column. The f-ratio (calculated as ρNO3/ρC) averaged for all DBO regions and for the 10-year study-period was 0.41 (± 0.24). Similarly, phytoplankton > 5 µm in size accounted for 65 (± 23) % of total Chl a for all regions over the 10 years. Taxonomic analysis done in 2013 showed that diatoms were the dominant taxa throughout all of the DBO regions, with the exception of areas influenced by low-nutrient waters on the eastern side of the Chukchi shelf near the Alaska coast. These coastal waters were dominated by coccolithophores and small (< 7 μm) flagellates and had much lower Chl a concentrations, ρC and ρNO3 than farther west. In addition, the proportion of pennate diatoms to total diatom abundance was found to be elevated relative to centric diatoms when sea-ice was present. Our measurements of phytoplankton biomass and ρC indicated that the higher abundance of pennate diatoms in the euphotic zone was the result of phytoplankton blooms happening below the ice, rather than pennate diatoms being supplied by a sea-ice diatom bloom. The dynamic nature of the Pacific Arctic Region (PAR) resulted in strong interannual variability within each DBO region for all parameters, with no clear increasing or decreasing trends from 2006 to 2016. Spatial variations were more consistent, with the highest rates of ρC and ρNO3 occurring in the nutrient-rich waters of the southeastern Chukchi Sea (away from shore), and decreasing in regions further north as NO3- concentrations were lower. An east-west gradient in phytoplankton biomass and productivity was also observed in the southeastern Chukchi Sea, which can be attributed to differences in the nutrient content of the water masses along the gradient. This study shows that the observed strong interannual variability in phytoplankton biomass and productivity cannot be attributed to differences in methodology or sampling time. It also highlights the need for better temporal and spatial sampling resolution such that the long-term effects of climate-induced changes can be identified against the backdrop of the naturally-strong interannual variability in the PAR.

Decadal trends in phytoplankton production in the Pacific Arctic Region from 1950 to 2012

This paper provides a synthesis of available in situ primary production (PP) measurements from the Pacific Arctic Region (PAR), collected between 1950 and 2012. Seasonal integrated primary production (IPP) across the PAR was calculated from 524 profiles, 340 of which were also analyzed to determine the average vertical distribution of PP rates for spring, summer and fall months. The Chirikov Basin and Chukchi Shelf were the most productive areas, with the East Siberian Sea, Chukchi Plateau and Canada Basin the lowest. Decadal-scale changes were indicated in the southern Chukchi Sea, and across Hanna Shoal. In the southern Chukchi Sea in August, IPP increased significantly from 113±35mgCm-2 d-1 in 1959 and 1960 to 833±307mgCm-2 d-1 in the 2000s. Increases in the magnitude of IPP were accompanied by variations in the vertical distribution, the subsurface peak observed in the 1959/60 was not present in the 2000s. The mechanism behind this change was undetermined but could have included changes in stratification, mixing or surface distribution of water masses as well as methodological differences. Over Hanna Shoal, the phytoplankton surface bloom now occurs earlier by several weeks compared to 1993, linked to increases in light due to earlier sea- ice retreat. In 1993 with sea ice still present in the region the surface bloom occurred in August, in 2002 and 2004 this same period was characterized by open water and low surface PP and strong subsurface production. This dataset provides a region-wide quantification of IPP and decadal trends and highlights the need for a cooperative monitoring program to observe the long-term impacts of climate change in the Arctic ecosystem.

Gray whale sightings in the Canadian Beaufort Sea, September 2014

Gray whales (Eschrichtius robustus) are distributed within the productive neritic and estuarine waters of the North Pacific Ocean, the Bering Sea, and adjacent waters of the Arctic Ocean. They migrate to high-latitude feeding grounds each spring. Their main feeding grounds in the Arctic include the Chirikov Basin, the northeastern Chukchi Sea from Pt. Hope to Cape Lisburne and Pt. Lay to Pt. Barrow, and the northwestern Chukchi Sea along the Chukotka coast. Although sightings are rare in the Canadian Beaufort Sea, we observed three gray whales in two groups in this area in September 2014. A mud plume was observed near one of the whales, suggesting the animal had been feeding. In the Alaskan Beaufort Sea, large-scale monitoring of the distributions of marine mammals has been continuously conducted since 1979; however, there has been less monitoring in the Canadian Beaufort Sea. Therefore, it is necessary to record opportunistic sightings, such as those described here.

Late summer zoogeography of the northern Bering and Chukchi seas

Ocean currents, water masses, and seasonal sea ice formation contribute to determining relationships among the biota of the Bering and Chukchi seas. The Bering Sea communicates with the Chukchi Sea via northward advection of water, nutrients, organic matter, and plankton through Bering Strait. We used data from concurrent surveys of zooplankton, pelagic fishes and jellyfish, epibenthic fishes and invertebrates, and seabirds to identify faunal distribution patterns and environmental factors that are related to these faunal distributions within the US portions of the Chukchi Sea shelf and Bering Sea shelf north of Nunivak Island. Regional differences in late summer (August–September) distributions of biota largely reflected the underlying hydrography. Depth, temperature, salinity, stratification, and chlorophyll a, but less so sediment-related or nutrient-related factors, were related to the distributions of the assemblages (zooplankton: depth, salinity, stratification; pelagic fishes and jellyfish: depth, stratification, chlorophyll a; epibenthic fishes and invertebrates: depth, temperature, salinity; seabirds: temperature, salinity, stratification). These six environmental factors that most influenced distributions of zooplankton, pelagic fishes/jellyfish, epibenthic fishes and invertebrate, and seabird assemblages likely can be simplified to three factors reflecting bottom depth, water mass, and their stratification and productivity (which are tightly linked in the study region). The assemblages were principally structured from nearshore to offshore and from south to north. The nearshore to offshore contrast usually was stronger in the south, where the enormous discharge of the Yukon River is more apparent and extends farther offshore, influencing zooplankton, pelagic fish/jellyfish, and seabird assemblages. Some assemblages overlapped spatially (e.g., seabird and zooplankton), indicating shared influential environmental factors or trophic linkages among assemblages. The gradients in assemblage composition were gradual for epibenthic taxa, abrupt for zooplankton taxa, and intermediate for pelagic fish/jellyfish and seabird taxa, implying that zooplankton assemblage structure is most strongly tied to water mass, epibenthic least, with the other two taxa intermediates. Three communities (i.e., cross-assemblage groupings) emerged based on maps of ordination axes and core use areas by taxa; one associated with Alaska Coastal Water (warmer, fresher, nutrient depauperate), second associated with Chirikov Basin and the southern Chukchi Sea (colder, saltier, nutrient rich), and third associated with the northern Chukchi shelf (colder and saltier but not as nutrient rich). Gradients in species composition occurred both within and between these communities. The Chirikov Basin/southern Chukchi Sea community was characterized by distinct zooplankton and seabird taxa, but was not strongly associated with distinct pelagic or epibenthic fish and invertebrate taxa. Although comprehensive data were only available for a single year and annual variation may affect the generality of our results, our comprehensive ecosystem survey approach yielded new insights into the ecological relationships (specifically, gradients in assemblage composition and identification of communities) of this Arctic region.

Ecosystem characteristics and processes facilitating persistent macrobenthic biomass hotspots and associated benthivory in the Pacific Arctic

The northern Bering and Chukchi Seas are areas in the Pacific Arctic characterized by high northward advection of Pacific Ocean water, with seasonal variability in sea ice cover, water mass characteristics, and benthic processes. In this review, we evaluate the biological and environmental factors that support communities of benthic prey on the continental shelves, with a focus on four macrofaunal biomass “hotspots.” For the purpose of this study, we define hotspots as macrofaunal benthic communities with high biomass that support a corresponding ecological guild of benthivorous seabird and marine mammal populations. These four benthic hotspots are regions within the influence of the St. Lawrence Island Polynya (SLIP), the Chirikov Basin between St. Lawrence Island and Bering Strait (Chirikov), north of Bering Strait in the southeast Chukchi Sea (SECS), and in the northeast Chukchi Sea (NECS). Detailed benthic macrofaunal sampling indicates that these hotspot regions have been persistent over four decades of sampling due to annual reoccurrence of seasonally consistent, moderate-to-high water column production with significant export of carbon to the underlying sediments. We also evaluate the usage of the four benthic hotspot regions by benthic prey consumers to illuminate predator–prey connectivity. In the SLIP hotspot, spectacled eiders and walruses are important winter consumers of infaunal bivalves and polychaetes, along with epibenthic gastropods and crabs. In the Chirikov hotspot, gray whales have historically been the largest summer consumers of benthic macrofauna, primarily feeding on ampeliscid amphipods in the summer, but they are also foraging further northward in the SECS and NECS hotspots. Areas of concentrated walrus foraging occur in the SLIP hotspot in winter and early spring, the NECS hotspot in summer, and the SECS hotspot in fall. Bottom up forcing by hydrography and food supply to the benthos influences persistence and composition of benthic prey that then influences the distributions of benthivorous upper trophic level populations.

Contrasting trends in sea ice and primary production in the Bering Sea and Arctic Ocean

AbstractBrown, Z. W., and Arrigo, K. R. 2012. Contrasting trends in sea ice and primary production in the Bering Sea and Arctic Ocean. – ICES Journal of Marine Science, 69: . Satellite remote sensing data were used to examine recent trends in sea-ice cover and net primary productivity (NPP) in the Bering Sea and Arctic Ocean. In nearly all regions, diminished sea-ice cover significantly enhanced annual NPP, indicating that light-limitation predominates across the seasonally ice-covered waters of the northern hemisphere. However, long-term trends have not been uniform spatially. The seasonal ice pack of the Bering Sea has remained consistent over time, partially because of winter winds that have continued to carry frigid Arctic air southwards over the past six decades. Hence, apart from the “Arctic-like” Chirikov Basin (where sea-ice loss has driven a 30% increase in NPP), no secular trends are evident in Bering Sea NPP, which averaged 288 ± 26 Tg C year−1 over the satellite ocean colour record (1998–2009). Conversely, sea-ice cover in the Arctic Ocean has plummeted, extending the open-water growing season by 45 d in just 12 years, and promoting a 20% increase in NPP (range 441–585 Tg C year−1). Future sea-ice loss will likely stimulate additional NPP over the productive Bering Sea shelves, potentially reducing nutrient flux to the downstream western Arctic Ocean.

Feeding ecology of dominant groundfish in the northern Bering Sea

We investigated current diets of the six most abundant benthic fish in the northern Bering Sea. Our objective was to explore feeding strategies and potential competition with other top predators as ecosystem changes occur in the northern Bering Sea ecosystem. Our approach used stomach content data collected from field sampling during spring 2006 and 2007. Calanoid copepods and ampeliscid amphipods were important prey of Arctic cod (Boreogadus saida) but in different proportions depending upon fish size, feeding location, and local environmental conditions. Snailfish (Liparidae) occupied a broad niche and fed on a variety of benthic amphipods. Arctic alligatorfish (Ulcina olrikii) and Arctic staghorn sculpin (Gymnocanthus tricuspis) consumed ampeliscid amphipods predominantly. Shorthorn sculpin (Myoxocephalus scorpius) had a less-diverse diet, with snow crab (Chionoecetes opilio) most important by weight. Finally, all Bering flounder (Hippoglossoides robustus) sampled had empty stomachs. Our results indicate that ampeliscid amphipods, which have high biomass in the central region of the northern Bering Sea, are the most important prey for the dominant groundfish in the Chirikov Basin. Generally, all dominant benthic fish in the northern Bering Sea had narrow feeding niches, except snailfish. High diet overlap was found among many of the fish species, including Arctic cod and snailfish, snailfish and Arctic alligatorfish, and Arctic alligatorfish and Arctic staghorn sculpin. These findings are consistent with a relatively short food chain for benthic fish that are for the most part specialized feeders with narrow preferences for food and may be affected by changes in benthic prey distributions.

High gray whale relative abundances associated with an oceanographic front in the south-central Chukchi Sea

We describe gray whale (Eschrichtius robustus) distribution in the south-central Chukchi Sea in relation to environmental factors during two 5-day surveys in June and September of 2003. Whale counts per 10-min scan (an index of relative abundance) ranged from 0 to 41 in June and from 0 to 28 in September. CTD data showed an ocean front around 67.81N with strong horizontal gradients in temperature, salinity, chlorophyll-a concentration and water-column stability. Highest whale abundance indices occurred in or near the front in both periods. Preliminary qualitative assessment of biological communities in the study area suggests that infaunal clams, echinoderms, euphausids, chaetognaths and Arctic cod were common, while ampeliscid amphipods, the previously abundant infauna (and likely prey) in the nearby Chirikov Basin feeding area, were not dominant. Euphausids may be a prey for gray whales in this area. We suggest that frontal systems may play an important role in eastern North Pacific gray whale foraging grounds. Further study is needed to fully describe the role of frontal systems in gray whale foraging grounds. r 2007 Elsevier Ltd. All rights reserved.

Amphipod prey of gray whales in the northern Bering Sea: Comparison of biomass and distribution between the 1980s and 2002–2003

The ampeliscid amphipod community in the Chirikov Basin of the northern Bering Sea was a focus of study during the 1980s because they were a major food for the Eastern North Pacific (ENP) population of gray whales Eschrichtius robustus. Information from the 1980s benthic investigations, published accounts of ENP gray whale population trends and the occurrence in 1999–2000 of an unusual number of gray whale mortalities prompted concern that the whale population may have exceeded the carrying capacity of its food base. Therefore, during two cruises per year between June and September, 2002 and 2003, we resampled the 20 stations occupied during the 1980s, to determine if there had been any significant changes in ampeliscid abundance and biomass. During 2002–2003, average ampeliscid dry weight biomass was about 28710 g m � 2 (95% confidence interval), a decline of nearly 50% from maximum values in the 1980s. Amphipod length measurements indicated that the declines were due mainly to the absence of the larger animals (20–30 mm length). Two hypotheses were considered regarding the amphipod declines: gray whale predation and climate. Ampeliscid production (105 kcal m � 2 yr � 1 ) and gray whale energy requirements (1.6 � 10 8 kcal individual � 1 yr � 1 ) indicated that as little as 3–6% of the current estimate of the ENP gray whale population could remove 10–20% of the annual ampeliscid production from the study site in 2002–2003, a finding consistent with the hypothesis that top-down control by foraging whales was the primary cause of the observed declines. A 10-yr time series of temperature near the bottom in the Bering Strait and northward transport did not reveal a consistent trend between 1990 and 2001, suggesting that climate influences were not the major cause of the observed declines. Arctic ampeliscids have slow growth rates and long generation times; therefore the ampeliscid community may require decades to recover to densities observed in the 1980s. Predicted warming trends in the northern Bering Sea could impact ampeliscid recovery by lowering primary production or altering the community composition of the benthos. r 2007 Elsevier Ltd. All rights reserved.

GRAY WHALE OCCURRENCE AND FORAGE SOUTHEAST OF KODIAK, ISLAND, ALASKA

Although the Bering and Chukchi seas are commonly cited as the principal summer feeding grounds of Eastern North Pacific (ENP) gray whales (e.g., Highsmith et al. in press, Swartz et al. 2006), records indicate that this population actually feeds opportunistically throughout its range from the lagoons of Baja California, Mexico, to Alaskan waters (Nerini 1984). Specifically, recent reports suggest that whales may routinely feed in the Gulf of California (Sanchez-Pacheco et al. 2001) and Bahia Magdalena, Baja California Sur (Caraveo-Patino and Soto 2005), whereas Clapham et al. (1997) noted that feeding gray whales occurred offshore California even in the 1920s when population numbers were very low. The dynamic nature of foraging in this population is best described from coastal study sites along the southeastern shore of Vancouver Island, Canada, where whales shift among pelagic, epi-benthic, and benthic prey within and between years (Darling et al. 1998; Dunham and Duffus 2001, 2002). In the 1980s the southern Chukchi Sea and the Chirikov Basin in the northern Bering Sea were considered the primary feeding grounds for ENP gray whales, based on reported high densities of both whales (Braham 1984, Kim and Oliver 1989, Moore et al. 2000) and their ampelicid amphipod prey (Grebmeier et al. 1989, Highsmith and Coyle 1990). However, by 2002, benthic productivity in the Chirikov Basin had declined precipitously, due to either whale foraging (Highsmith et al. 2006), ecosystem change (Grebmeier et al. 2006), or both, and only the southern Chukchi Sea supported dense aggregations of gray whales (Moore et al. 2003). Indeed, the

Ocean circulation and exchanges through the northern Bering Sea—1979–2001 model results

Abstract We have developed and run a model with sufficiently high resolution (∼9 km and 45 levels) and a large enough spatial domain to allow for realistic representation of flow through the narrow and shallow straits in the northern Bering Sea. This is potentially important for quantification of long-term mean and time-dependent ocean circulation, and water mass and property exchanges between the Pacific and Arctic Oceans. Over a 23 year interval (1979–2001), mean transport through Bering Strait is estimated to be 0.65 Sv. Comparison of our model results with published observations indicates that ocean circulation is not only variable at seasonal to interdecadal scales but it is also responsive to short-term atmospheric forcing. One of such events occurred during the winter of 2000–2001 with reversed oceanic flow in some areas and much reduced sea-ice cover. Analyses of eddy kinetic energy fields identify some high biological productivity regions of the Chirikov Basin coincident with persistent high energy (up to 2700 cm 2 s −2 in the surface layer and up to 2600 cm 2 s −2 at mid-depth) throughout the annual cycle. Model output in the Bering Strait region is validated against several time series of moored observations of water mass properties. Comparison with shipboard observations of near-bottom salinity from late winter through autumn indicates that the model reasonably represents the major water-mass properties in the region. The modeled vertical water-column structure in the northern Bering Sea allows increased understanding of the mechanisms of water transformation and transport northward through Bering Strait into the Chukchi and Beaufort Seas. We conclude that the long-term model results for the northern Bering Sea provide important insights into the ocean circulation and fluxes and they are a useful frame of reference for limited observations that are short-term and/or cover only a small geographic region.

Gray whale distribution relative to forage habitat in the northern Bering Sea: current conditions and retrospective summary

Hundreds of gray whales (Eschrichtius robustus) stranded dead along beaches from Mexico to Alaska in 1999 and 2000. The cause of the mortalities remains unknown, but starvation resulting from a reduction in prey, especially in the Chirikov Basin, was suggested as the cause. In the 1980s, the Chirikov Basin was considered a prime gray whale feeding area, but there has been no recent comprehensive assessment of whale or prey distribution and abundance. In 2002, a 5-day survey for gray whales revealed restricted distribution in the basin and a 3- to 17-fold decline in sighting rates. To put these data in context, a retrospective summary of gray whale and benthic fauna distribution and abundance was undertaken. During the 1980s, gray whale sighting rates in the Chirikov Basin were highly variable. Ampeliscid amphipods dominated the benthos where gray whale sighting rates were highest. Available measures of biomass suggest a downturn in amphipod productivity from 1983 to 2000, when estimates of gray whale population size were increasing, suggesting that the whales simply expanded their foraging range. We encourage long-term study of the Chirikov Basin as a location where predator–prey responses to changing ocean climate can be researched, because decadal time series data are available.