Northeastern Chukchi Sea Research Articles

Understanding ocean stratification and its interannual variability in the northeastern Chukchi Sea

Understanding ocean stratification and its interannual variability in the northeastern Chukchi Sea

Transport-driven seasonal abundance of pelagic fishes in the Chukchi Sea observed with seafloor-mounted echosounders

Abstract Recent summer surveys of the northeastern Chukchi Sea found pelagic fishes were dominated by large numbers of age-0 Arctic cod (Boreogadus saida, Gadidae) and walleye pollock (Gadus chalcogrammus, Gadidae), while adult fishes were comparatively scarce. The source and fate of these young fishes remain unclear, as sampling in this region is impeded by seasonal ice cover much of the year. Seafloor-mounted echosounders were deployed at three locations in the northeastern Chukchi Sea from 2017 to 2019 to determine the movement and seasonal variability of these age-0 gadids. These observations indicated that the abundance of pelagic fishes and community composition on the Chukchi Sea shelf were highly variable on seasonal time scales, with few fish present in winter. Tracking indicated that fish movements were strongly correlated with local currents. Fishes were primarily displaced to the northeast in summer and fall, with periodic reversals towards the southwest driven by changes in regional wind patterns. The flux of fishes past the moorings indicated that the prevailing northward currents transport a large proportion of the age-0 pelagic fishes present on the Chukchi shelf in summer to the northeast by fall, leading to relatively low abundances of age-1+fishes in this environment.

Semidiurnal Internal Tides Observed on the Eastern Flank of Hanna Shoal in the Northeastern Chukchi Sea

AbstractThis paper investigates the role of semidiurnal tides in the Hanna Shoal region on the northeastern Chukchi Sea shelf to evaluate their impact on the regional shelf dynamics. The study is based on 2‐year long velocity time series from five oceanographic moorings. These records indicate the dominance of wind‐generated near‐inertial energy during the summer season with low ice cover. However, when the ocean is fully covered by sea ice, tides dominate the variability in the semidiurnal energy band. The records reveal considerable seasonal variability as well as regional differences, where barotropic tides dominate in the well‐mixed waters west of Hanna Shoal while bottom‐trapped internal (depth‐dependent) tides are observed east of Hanna Shoal, where stratification can persist year‐round. Resulting tide‐driven shear in winter east of Hanna Shoal under stratified conditions can occasionally reach the level of a Richardson number below 1 and can be as low as ∼0.25, implying the likelihood of shear instability and potentially eroding lower water column stability. This may lead to upward fluxes of near‐bottom nutrient‐rich Winter Water and thus carries ecosystem implications. Our study indicates that the internal tides east of Hanna Shoal are modulated by the spring‐neap cycle and result from the interaction of barotropic tides with local bathymetry and stratification.

Phytoplankton photophysiology varies depending on nitrogen and light availability at the subsurface chlorophyll maximum in the northern Chukchi Sea

Vertical distributions of phytoplankton in the Arctic Ocean are characterized by a very narrow subsurface chlorophyll maximum (SCM) layer formed every summer after the sea ice retreats. Despite the prevalence of this narrow SCM layer, phytoplankton photosynthetic response to climate change remains to be elucidated. Here, we examined the photophysiological properties of phytoplankton in the SCM layer in the northern Chukchi Sea during the summers of 2015–2018. There was a significant difference in the SCM depth between the northwestern and northeastern Chukchi Sea determined by the distribution of Pacific Summer Water (PSW) around the SCM layer (34 ± 14 m vs. 49 ± 10 m, respectively). The maximum quantum yield of photochemistry in photosystem II (Fv/Fm) in the SCM phytoplankton was high (Fv/Fm ≥ 0.54) and similar in both regions until 2016; however, since then, Fv/Fm in the northeastern Chukchi Sea has decreased by approximately 10%. This decrease was accompanied by a marked decrease in the fraction of microplankton, which are known to be susceptible to nutrient limitation. This result suggests a reduction in nitrogen availability in the SCM layer in the northeastern Chukchi Sea. Meanwhile, the maximum electron transfer rate (ETRmax) did not have a significant relationship with the nitrogen availability and phytoplankton community size structure in the SCM layer; however the improved light conditions (with an approximately two-fold increase in the relative ratio of surface PAR reaching the SCM layer) increased ETRmax by up to 30% in the SCM phytoplankton in the northwestern Chukchi Sea. Therefore, these results provide a better understanding of how changes in nitrogen and light availability could affect phytoplankton photosynthesis and primary production in the Arctic Ocean.

Tight association between microbial eukaryote and giant virus communities in the Arctic Ocean

AbstractViruses are important regulatory factors of the marine microbial community including microeukaryotes. However, little is known about their role in the northern Chukchi Sea in the Arctic basin, which has oligotrophic conditions in summer. To clarify the link between microbial eukaryotic communities and viruses as well as environmental conditions, we investigated the community structures of microeukaryotes (from 3 to144 μm and from 0.23 μm size bio‐particles collected from seawater) and Imitervirales (from 0.23 μm size bio‐particles collected from seawater), a dominant group of viruses infecting marine microeukaryotes. To the best of our knowledge, no study has investigated both Imitervirales and eukaryotic communities in the Arctic Ocean. Surface water samples were collected at 21 ocean stations located in the northeastern Chukchi Sea and an adjacent area outside the Beaufort Gyre (Adjacent Sea), and at two melt ponds on sea ice in the summer of 2018. At the ocean stations, nutrient concentrations were low in most of the locations, except the shelf in the adjacent sea. The community variations were significantly correlated between eukaryotes and Imitervirales, even within the northeastern Chukchi Sea characterized by relatively homogeneous environmental conditions. The association of the eukaryotic community with the viral community was stronger than that with geographical and physicochemical environmental factors. These results suggest that Imitervirales actively infect their hosts even in the cold and oligotrophic seawater in the Arctic Ocean.

Species composition, structure and biomass of zooplankton in the Chukchi Sea in August-September 2019

Plankton survey was conducted in the Chukchi Sea in August-September 2019 by two research vessels: RV Professor Levanidov in the western part of the sea and RV Ocean Starr in its eastern part. Parasagitta elegans dominated everywhere in zooplankton. Beside these arrowworms, species of the arctic and arctic-boreal complex were abundant in the northwestern Chukchi Sea, as Calanus glacialis and other species typical for the adjacent East-Siberian Sea. C. glacialis was numerous in the northeastern Chukchi Sea, too, together with species of the Arctic waters and the Beaufort Sea. The southern Chukchi Sea was occupied by the waters invaded from the Bering Sea, so species of the Bering Sea complex were abundant there, as Eucalanus bungii and Neocalanus plumchrus/flemingeri, at Alaska coast together with neritic species typical for the Alaska coastal water mass. The zooplankton community structure corresponded to the spring phase of seasonal succession in the northern area, whereas to the summer phase in the southern area. Total density of zooplankton was estimated as 65 t/km2 for the northwestern part of the sea, presented mostly by large-sized fraction (56 t/km2) including arrowworms (35 t/km2) and copepods (13 t/km2), and as 48 t/km2 for the southwestern part of the sea, also mostly large-sized fraction (40 t/km2). So high zooplankton density in the western Chukchi Sea is considered as result of the Bering Sea waters advection (previously such abundance was observed in 2008 — 40 and 36 t/km2, when the advection was also active). In the eastern Chukchi Sea, the total density of zooplankton was 44 t/km2 in the northeastern area (including 29 t/km2 of large-sized fraction) and 43 t/km2 in the southeastern area, that was higher than in 2017 (15 t/km2) but lower than in 2007 (49 t/km2). However, different phases of seasonal succession should be noted: summer season in 2019, spring season in 2017, and late-summer season in 2007.

Long: Influence of water masses on the summer structure of the seabird community in the northeastern Chukchi Sea.

We used data collected during a variety of research cruises in the northeastern Chukchi Sea and contributed to the Distributed Biological Observatory to explore the influence of the seasonal change in water masses on the development of the seabird community during the summer. Surveys that included seabird observations and hydrographic sampling were conducted from Alaska’s northwestern coast to ~220 km offshore during 2008–2018. Species composition varied geographically, shifting from a nearshore community that included short-tailed shearwaters, loons, and seaducks to an offshore community dominated by crested auklets. Crested auklets were remarkably consistent in their occupation of Hanna Shoal among years and remained in the area throughout the summer. Short-tailed shearwaters exhibited the greatest seasonal and interannual variation in abundance and distribution of the 35 species recorded. They were concentrated south of 71°N and within 50 km of shore in August and tended to spread throughout the region in September. Surface-feeding species like gulls, fulmars, and phalaropes were 1–2 orders of magnitude less abundant and had wider distributions than birds that feed by diving. Including information about hydrography improved the fit of models of seabird density. Seabirds, especially those that breed in the Bering Sea, generally were more abundant in areas dominated by moderate-salinity Bering Sea Water than nearshore in low-salinity Alaska Coastal Water. The distribution of seabirds across the northeastern Chukchi Sea reflected the heterogeneity of oceanic habitats and prey availability over the shallow shelf. Our results will inform efforts to develop ecosystem models that incorporate oceanographic conditions to predict ongoing consequences of climate change.

Changes in gray whale phenology and distribution related to prey variability and ocean biophysics in the northern Bering and eastern Chukchi seas.

Changes in gray whale (Eschrichtius robustus) phenology and distribution are related to observed and hypothesized prey availability, bottom water temperature, salinity, sea ice persistence, integrated water column and sediment chlorophyll a, and patterns of wind-driven biophysical forcing in the northern Bering and eastern Chukchi seas. This portion of the Pacific Arctic includes four Distributed Biological Observatory (DBO) sampling regions. In the Bering Strait area, passive acoustic data showed marked declines in gray whale calling activity coincident with unprecedented wintertime sea ice loss there in 2017-2019, although some whales were seen there during DBO cruises in those years. In the northern Bering Sea, sightings during DBO cruises show changes in gray whale distribution coincident with a shrinking field of infaunal amphipods, with a significant decrease in prey abundance (r = -0.314, p<0.05) observed in the DBO 2 region over the 2010-2019 period. In the eastern Chukchi Sea, sightings during broad scale aerial surveys show that gray whale distribution is associated with localized areas of high infaunal crustacean abundance. Although infaunal crustacean prey abundance was unchanged in DBO regions 3, 4 and 5, a mid-decade shift in gray whale distribution corresponded to both: (i) a localized increase in infaunal prey abundance in DBO regions 4 and 5, and (ii) a correlation of whale relative abundance with wind patterns that can influence epi-benthic and pelagic prey availability. Specifically, in the northeastern Chukchi Sea, increased sighting rates (whales/km) associated with an ~110 km (60 nm) offshore shift in distribution was positively correlated with large scale and local wind patterns conducive to increased availability of krill. In the southern Chukchi Sea, gray whale distribution clustered in all years near an amphipod-krill 'hotspot' associated with a 50-60m deep trough. We discuss potential impacts of observed and inferred prey shifts on gray whale nutrition in the context of an ongoing unusual gray whale mortality event. To conclude, we use the conceptual Arctic Marine Pulses (AMP) model to frame hypotheses that may guide future research on whales in the Pacific Arctic marine ecosystem.

Automatic detection and classification of bearded seal vocalizations in the northeastern Chukchi Sea using convolutional neural networks.

Bearded seals vocalizations are often analyzed manually or by using automatic detections that are manually validated. In this work, an automatic detection and classification system (DCS) based on convolutional neural networks (CNNs) is proposed. Bearded seal sounds were year-round recorded by four spatially separated receivers on the Chukchi Continental Slope in Alaska in 2016-2017. The DCS is divided in two sections. First, regions of interest (ROI) containing possible bearded seal vocalizations are found by using the two-dimensional normalized cross correlation of the measured spectrogram and a representative template of two main calls of interest. Second, CNNs are used to validate and classify the ROIs among several possible classes. The CNNs are trained on 80% of the ROIs manually labeled from one of the four spatially separated recorders. When validating on the remaining 20%, the CNNs show an accuracy above 95.5%. To assess the generalization performance of the networks, the CNNs are tested on the remaining recorders, located at different positions, with a precision above 89.2% for the main class of the two types of calls. The proposed technique reduces the laborious task of manual inspection prone to inconstant bias and possible errors in detections.

Where do the fish come from, and where do they go? Seasonal observations of pelagic fishes from echosounder moorings in the Chukchi sea

Recent summer surveys of the Chukchi Sea determined that pelagic fishes were dominated by large numbers of age-0 Arctic cod and walleye pollock, while adult fishes are comparatively scarce. Modeling based on regional currents indicates that these age-0 fishes are likely advected to the north in fall; however, the source and fate of these fishes remains unclear as this region is seasonally ice-covered. To determine the movement and variability of this age-0 gadid population, bottom-moored multifrequency echosounders were deployed at three locations in the northeastern Chukchi Sea from 2017-2019. These observations indicate that the abundance and composition of the pelagic community on the Chukchi Sea shelf is highly variable over seasonal time scales. Fish abundance was very low in winter, increased in May, and reached peak abundance in late summer. Target strength and diel vertical migration of fishes increased in summer, indicating that this is a key period for growth. Age-0 gadids were displaced to the northeast, consistent with the dominant advection on the shelf. Fish speeds and headings were strongly correlated with local currents, providing evidence that these small age-0 fishes are primarily being passively transported and behavior plays a limited role in population distribution.

Springtime renewal of zooplankton populations in the Chukchi Sea

Although considerable work has been done in the Chukchi Sea during summer, much less has been done during other seasons. This has limited our ability to fully understand seasonal cycles and transformations of the Chukchi Sea zooplankton, particularly the key copepod species Calanus glacialis. Abundance and distributions of large zooplankton and of all life stages of C. glacialis in the northeastern Chukchi Sea during May-June 2014 are described. Three main zooplankton communities are identified; “arctic oceanic” along the Chukchi slope associated with off-shelf water masses; Chukchi Sea “overwintering” associated with cold winter water in the northern part of the study area; and Chukchi Sea “spring” associated with early season summer water in the southern portion of the study area. The overwintering and spring communities were distinguished by the near total absence of younger copepodid (CI-CIII) C. glacialis stages and meroplankton in the overwintering community while older (CV-adult) C. glacialis, amphipods, and chaetognaths were present in both. The distributions of the communities followed the major circulation pathways in the northeastern Chukchi Sea. Water and plankton flooding in from the northern Bering Sea was filling the Chukchi Sea and replenishing the zooplankton communities as the remnants of the overwintering community was being advected northwards and into the Canadian Basin. A conceptual model of the seasonal evolution of C. glacialis populations in the Chukchi Sea, based on the interaction of C. glacialis phenology and advective drivers, enfolds both these spring observations and summer observations from numerous previous studies. Because the flushing time of the Chukchi Sea is shorter than the C. glacialis generation time, the copepod is unable to establish an endemic population in the Chukchi Sea, hence the population there must be renewed annually from the northern Bering Sea.

Polar bear behavioral response to vessel surveys in northeastern Chukchi Sea, 2008–2014

Evaluating the impacts of anthropogenic activities on Arctic wildlife is a key issue in current management and conservation strategies. With global climate change, expanding shipping routes, and increasing industrial development in the Arctic, the polar bear (Ursus maritimus) faces new challenges to its survival. Polar bear behavioral response to vessel presence is not well-documented. During the 2008–2014 Chukchi Sea Environmental Studies Program (CSESP), polar bear occurrence and behavioral data were collected during summer–autumn vessel surveys. We examined polar bear occurrence and behavioral response to vessel presence by distance. During this study, 56,901 km of observation effort occurred from 3 survey vessels and 42 groups (50 individuals) of polar bears were recorded. Over half of the groups exhibited a behavioral response (i.e., vigilance or flee) including all groups of mothers with cubs. The mean distance at which bears responded to vessels (805 ± 648 m) was closer than the mean distance at which no response was observed (2,001 ± 1,368 m). Logistic regression analysis revealed that response was associated with distance and our model indicated the estimated distance at which 50% of the polar bears would exhibit a behavioral response to be 1,645 m. Our findings are relevant to assess potential impacts of increasing vessel activity in the Arctic and to assist in the development of effective monitoring and mitigation strategies for polar bears.

Fin whale (Balaenoptera physalus) song pattern in the southern Chukchi Sea

The number of migrating fin whales (Balaenoptera physalus) appears to be increasing in the Pacific Arctic after changes in the marine ecosystem and recovering from depletion by commercial whaling. Fin whale songs are sequences of sounds produced repeatedly, and they may be used for population structure assessments. However, little is known about song types and population structures in the Pacific Arctic. We recorded fin whale songs using a fixed passive acoustic monitoring system from July 2012 to June 2015 in the southern Chukchi Sea. We randomly selected one hour of data from each week of the study period and measured the inter-pulse intervals (IPIs) of the songs. Songs were detected from August to November 2012–2014. All songs had a sequence of doublets with two IPIs (10.5 and 19.6 s), and they were similar to previously reported songs of whales from the eastern North Pacific in 2001–2013 but differed from those of whales from the Bering Sea and northeastern Chukchi Sea in 2007–2010. These results suggest that one group of fin whales migrated to the southern Chukchi Sea from the eastern North Pacific in 2012–2014, or the song changed. Comparing songs from different areas of the Pacific Arctic during the same years will reveal whether this group dispersed in this area.

Circulation and Thermohaline Variability of the Hanna Shoal Region on the Northeastern Chukchi Sea Shelf

AbstractWe analyzed velocity and hydrographic data from 23 moorings in the northeast Chukchi Sea from 2011 to 2014. In most years the eastern side of Hanna Shoal was strongly stratified year‐round, while weakly stratified regions prevailed on the shelf south and west of the Shoal. Stratification differences cause differential vertical mixing rates, which in conjunction with advection of different bottom water properties resulted in seasonally varying along‐isobath density gradients. In agreement with numerical models, we find that bottom waters flow anticyclonically around the Shoal. Whereas most of the shelf responded barotropically to wind‐forcing, there was a strong baroclinic component to the flow field northeast of Hanna Shoal, resulting in no net vertically integrated transport on average. In contrast there is a net eastward transport from west of the Shoal, which implies convergence north of the Shoal. Convergence and along‐isobath density gradients may foster cross‐shelf exchange north of Hanna Shoal. Modal analyses indicate that the shelf south of the Shoal and Barrow Canyon responded coherently to local and remote winds, whereas the wind‐current response around Hanna Shoal was less coherent. Barotropic topographic waves, of ~3‐day period, were generated episodically northeast of the Shoal and propagate clockwise around Hanna Shoal, but are blocked from entering Barrow Canyon and are possibly scattered by the horizontally sheared flow and converging isobaths on the western side of the Shoal. Analysis of water properties on the western side of Hanna Shoal suggests that these include contributions from the western and southern portions of the Chukchi Sea.

Spatial patterns, environmental correlates, and potential seasonal migration triangle of polar cod (Boreogadus saida) distribution in the Chukchi and Beaufort seas

Polar cod (Boreogadus saida) is a key forage fish in the Arctic marine ecosystem and provides an energetic link between lower and upper trophic levels. Despite its ecological importance, spatially explicit studies synthesizing polar cod distributions across research efforts have not previously been conducted in its Pacific range. We used spatial generalized additive models to map the distribution of polar cod by size class and relative to environmental variables. We compiled demersal trawl data from 21 cruises conducted during 2004–2017 in the Chukchi and Beaufort seas, and investigated size-specific patterns in distribution to infer movement ecology of polar cod as it develops from juvenile to adult life stages. High abundances of juvenile polar cod (≤ 70 mm) in the northeastern Chukchi Sea and western Beaufort Sea were separated from another region of high abundance in the eastern Beaufort Sea, near the US and Canadian border, suggesting possible population structure in the Pacific Arctic. Relating environmental correlates to polar cod abundance demonstrated that temperature and salinity were related to juvenile distribution patterns, while depth was the primary correlate of adult distribution. A comparison of seasonal 2017 abundances of polar cod in the southern Chukchi Sea found low demersal abundance in the spring when compared to the summer. Seasonal differences in polar cod abundance suggest that polar cod migration may follow a classical ‘migration triangle’ route between nursery grounds as juveniles, feeding grounds as subadults, and spawning grounds as adults, in relation to ice cover and seasonal production in the Chukchi Sea.

Favorable Conditions for Suspension Freezing in an Arctic Coastal Polynya

AbstractArctic sea ice incorporates and transports sediment, releasing it back into the water column during the melting season. This process constitutes an important aspect of marine sediment transport and biogeochemical cycling. Sediment incorporation into sea ice is considered to occur mainly through underwater interaction between frazil ice and resuspended sediment, referred to as suspension freezing. However, harsh environmental conditions have greatly limited field observations of this phenomenon. Analysis of mooring data from a coastal polynya in the northeastern Chukchi Sea, in conjunction with coastal ice radar and meteorological data, indicates that suspension freezing is a key mechanism for sediment entrainment into sea ice. During polynya episodes, acoustic backscatter data obtained by an Acoustic Doppler Current Profiler showed the presence of frazil ice from the surface down to 20‐ to 25‐m depth, coinciding with in situ and potential supercooling. Underwater frazil ice persisted over 1 week under windy, turbulent water column conditions. A combination of the turbidity and Acoustic Doppler Current Profiler backscatter data revealed upward sediment dispersion associated with strong currents during the polynya episodes. The fact that frazil ice and resuspended sediment were detected at the same depth and time strongly suggests the interaction between ice crystals and sediment particles, that is, suspension freezing.

Composite Underwater Noise Footprint of a Shallow Arctic Exploration Drilling Project

In this paper, we present the results of a comprehensive acoustic monitoring and modeling study of the composite underwater noise footprint of an exploration drilling project in shallow waters (46 m) in the northeastern Chukchi Sea. Measured acoustic source levels of multiple vessels and drilling equipment were used as inputs to theoretical propagation models. The models generated time-dependent wide-area noise fields, accounting for oceanographic and weather conditions and the time-dependent drilling activities and positions of multiple vessels. Modeling in increments of 1 h was performed over the full operations period, from July 7, 2015 to October 16, 2015. Model validation was carried out by comparing simulated results with high-resolution acoustic data acquired at multiple stations in the area. The study shows that noise radiating from up to 19 supporting vessels is usually dominant, but noise from certain drilling-related activities occasionally rises above vessel noise within a radius of 8 km from drill sites, limited to a few hours in durations. Modeled scenarios show that by constraining vessel positions to within a few kilometers of the drilling location, the extent of the 120 dB re 1 μPa noise footprint, as defined for single-well and concurrent double-well drilling operations, is contained within a nominal radius of 20 km. Broadband noise from drilling platforms and support vessels reached wind-generated ambient noise levels at ranges between 60 and 85 km from the drill site. Still, some tones and narrowband sounds may remain above the natural ambient noise at the same frequencies to longer ranges.

Environmental and biological influences on the distribution and population dynamics of polar cod (Boreogadus saida) in the US Chukchi Sea

Polar cod (Boreogadus saida) is the most abundant fish in the US Arctic (US Chukchi and Beaufort seas), where commercial fishing is currently prohibited because data to assess the sustainability of potential fisheries are lacking. Using data from fisheries oceanographic surveys in the US Chukchi Sea during 2012 and 2013 (August–September), we examined drivers of distribution and abundance, and assessed the current status and dynamics of polar cod. We estimated age structure, abundance, biomass, natural mortality, and reproductive potential using data from these surveys, combined with available estimates of biological parameters from the literature. High densities of age-0 polar cod were observed in the northeastern Chukchi Sea, while older polar cod (age-1+) were more widely distributed throughout the survey area. Results indicate that temperature is an important driver for the distribution of both age-0 and age-1+ polar cod with age-0 polar cod less likely to be present in warmer waters (≥ 8 °C), which supports past research on drivers of fish and other ectotherm species distribution. Estimates of egg production and early survival suggest that the numbers of mature polar cod present in the survey area during summer are unlikely to produce the observed high abundances of age-0 polar cod in the US Chukchi Sea. This could imply that either mature polar cod migrate to the Chukchi Sea to spawn in the winter, that age-0 fish are advected from outside the study area, or that adult polar cod abundance is underestimated. Additional systematic surveys and further research are needed to resolve the origins of age-0 polar cod, identify nursery areas, and estimate early life survival.

Water Mass Evolution and Circulation of the Northeastern Chukchi Sea in Summer: Implications for Nutrient Distributions

AbstractSynoptic and historical shipboard data, spanning the period 1981–2017, are used to investigate the seasonal evolution of water masses on the northeastern Chukchi shelf and quantify the circulation patterns and their impact on nutrient distributions. We find that Alaskan coastal water extends to Barrow Canyon along the coastal pathway, with peak presence in September, while the Pacific Winter Water (WW) continually drains off the shelf through the summer. The depth‐averaged circulation under light winds is characterized by a strong Alaskan Coastal Current (ACC) and northward flow through Central Channel. A portion of the Central Channel flow recirculates anticyclonically to join the ACC, while the remainder progresses northeastward to Hanna Shoal where it bifurcates around both sides of the shoal. All of the branches converge southeast of the shoal and eventually join the ACC. The wind‐forced response has two regimes: In the coastal region the circulation depends on wind direction, while on the interior shelf the circulation is sensitive to wind stress curl. In the most common wind‐forced state—northeasterly winds and anticyclonic wind stress curl—the ACC reverses, the Central Channel flow penetrates farther north, and there is mass exchange between the interior and coastal regions. In September and October, the region southeast of Hanna Shoal is characterized by elevated amounts of WW, a shallower pycnocline, and higher concentrations of nitrate. Sustained late‐season phytoplankton growth spurred by this pooling of nutrients could result in enhanced vertical export of carbon to the seafloor, contributing to the maintenance of benthic hotspots in this region.

Sources of primary production to Arctic bivalves identified using amino acid stable carbon isotope fingerprinting

ABSTRACTBenthic invertebrates are a crucial trophic link in Arctic marine food webs. However, estimates of the contribution of different primary production sources sustaining these organisms are not well characterised. We measured the stable carbon isotope values (δ13C) of essential amino acids (EAAs) in muscle tissue from two common bivalve genera (Macoma spp. and Astarte spp.) collected in Hanna Shoal in the northeastern Chukchi Sea. Mixing models comparing the δ13CEAA fingerprints of the bivalves to a suite of primary production endmembers revealed relatively high contributions of EAAs from phytoplankton and bacteria in both species. We also examined whether δ13CEAA fingerprints could be produced from the EAAs preserved in bivalve shells, which could allow primary production sources to be estimated from ancient bivalve shells. The δ13CEAA fingerprints from a suite of paired modern bivalve shells and muscle from Macoma calcarea from across the Chukchi Sea revealed a correspondence between the estimates of the dominant primary production source of EAAs derived from analyses of these two tissue types. Our findings indicate that δ13CEAA fingerprinting of marine bivalves can be used to examine dominant organic matter sources in the Arctic marine benthos in recent years as well as in deeper time.