Hanna Shoal Research Articles

Analysis of a Halocline Mesoscale Eddy in the Northwind Basin, Arctic Ocean

Eddies have been observed at all depths and in all regions of the Arctic Ocean. However, given the complex geographic conditions and dynamic environments of this ocean, the synchronized observations of temperature, salinity, and currents, and the detailed analysis of individual eddies are still lacking in the Northwind Basin. Our study aims to address these research gaps. We observed an eddy from a mooring in the Northwind Basin in late October 2017. It is a large anticyclonic cold eddy within the Arctic halocline, with a maximum azimuthal velocity reaching 52.63 cm/s and a horizontal scale (∼56 km) that significantly exceeds the first local baroclinic Rossby radius of deformation, i.e., it is in mesoscale. Its azimuthal velocity and scale are larger compared with those of nearby eddies, suggesting a relatively young state. This eddy possibly originated from the northern Chukchi Sea shelf, converging near Hanna Shoal with the Chukchi Slope Current before being advected northward into the Northwind Basin. Our study outlines detailed steps for extracting and analyzing eddies from mooring data and contributes to improving the understanding of the characteristics of Arctic Ocean eddies, providing a typical case for the investigation of eddies in the Northwind Basin.

Validation of OSCAR Surface Currents in the Western Arctic Marginal Seas Against Saildrone Observations

AbstractThe western Arctic marginal seas undergo large seasonal variation, but are very challenging to observe directly due to sea ice and shallow depths. Deployments of several saildrone uncrewed surface vehicles in the summers of 2018 and 2019 provided unique opportunities to validate the satellite‐derived near surface currents, Ocean Surface Current Analysis Real‐time (OSCAR), in the western Arctic marginal seas against in situ upper ocean current measurements. Overall, OSCAR current is biased low (by 5.3 cm/s) with significant noise. Higher vector correlation estimated by the cosine similarity and speed differences often occur where stronger currents (often topography‐steered) are observed. Such differences reveal that the data set resolvability depends on spatial and temporal resolutions, smoothing, and latitudes, suggesting that OSCAR is able to depict the major current systems but significantly underestimates their strength. Poorer vector correlation occurs at weaker current regimes (<10 cm/s), over the shallow Hanna Shoal, near fresher water due to ice melt and river discharge. The latter two water class regimes highlight the importance of salinity contribution to the buoyancy force which is neglected in the OSCAR formulation.

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.

Representative range of acoustic point source measurements in the Chukchi Sea

The use of stationary, active acoustics provides an effective approach to characterize and monitor temporal variability in the abundance and behavior of pelagic organisms, especially in seasonally ice-covered waters of high latitude marine ecosystems. However, point measurements from stationary echosounders are limited in their spatial coverage. A quantification of the spatial area represented by point measurements (i.e., representative range) is required to ensure effective biological characterization and monitoring. Here, concurrent mobile and stationary active acoustic data collected during summers of 2015 and 2017 are used to assess the representative range of fish and zooplankton density measurements from the Chukchi Ecosystem Observatory located at Hanna Shoal, Northeast Chukchi Sea. Six methods used to calculate representative ranges of backscatter means and variances resulted in representative ranges between approximately 0.3 and 86 km, depending on the year and calculation method. Such relatively large representative ranges reflect the tight bio-physical associations and large characteristic environmental length scales of the NE Chukchi Sea. Between years, up to 10-fold variations in representative ranges were attributed to interannual changes in water mass characteristics and associated species assemblages. Differences of 1–2 orders of magnitude in our calculated ranges among methods are attributed to differences in the rationale and associated assumptions of each approach. The choice of method and resulting representative range depends on monitoring goals: detection of change, mapping of spatial distributions, characterization of spatial variance, or interpolation of temporal variability over space. Our comparison of stationary acoustic to mobile surveys extends the understanding of spatiotemporal variability of marine organism distributions in the NE Chukchi Sea and informs cost-effective design of observing systems to monitor and predict impacts of environmental change.

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.

Summer Surface CO2 Dynamics on the Bering Sea and Eastern Chukchi Sea Shelves From 1989 to 2019

AbstractBy compiling boreal summer (June to October) CO2 measurements from 1989 to 2019 on the Bering and eastern Chukchi Sea shelves, we find that the study areas act as a CO2 sink except when impacted by river runoff and wind‐driven upwelling. The CO2 system in this area is seasonally dominated by the biological pump especially in the northern Bering Sea and near Hanna Shoal, while wind‐driven upwelling of CO2‐rich bottom water can cause episodic outgassing. Seasonal surface ΔfCO2 (oceanic fCO2 – air fCO2) is dominantly driven by temperature only during periods of weak CO2 outgassing in shallow nearshore areas. However, after comparing the mean summer ΔfCO2 during the periods of 1989–2013 and 2014–2019, we suggest that temperature does drive long‐term, multi‐decadal patterns in ΔfCO2. In the northern Chukchi Sea, rapid warming concurrent with reduced seasonal sea‐ice persistence caused the regional summer CO2 sink to decrease. By contrast, increasing primary productivity caused the regional summer CO2 sink on the Bering Sea shelf to increase over time. While additional time series are needed to confirm the seasonal and annual trajectory of CO2 changes and ocean acidification in these dynamic and spatially complex ecosystems, this study provides a meaningful mechanistic analysis of recent changes in inorganic carbonate chemistry. As high‐resolution time series of inorganic carbonate parameters lengthen and short‐term variations are better constrained in the coming decades, we will have stronger confidence in assessing the mechanisms contributing to long‐term changes in the source/sink status of regional sub‐Arctic seas.

Organic carbon source variability in Arctic bivalves as deduced from the compound specific carbon isotopic composition of amino acids

In this study we used compound-specific carbon isotope analysis of amino acids (δ13CAA) to determine organic carbon sources utilized by the dominant benthic bivalve species collected along a latitudinal gradient in the northern Bering and Chukchi Seas, specifically at productivity hotspots identified within the Distributed Biological Observatory (DBO) program, and over Hanna Shoal in the northern Chukchi Sea. The recent shift to earlier sea-ice melt is one of the climate change consequences influencing Pacific Arctic ecosystems, which we integrate within our observations. Our goals included investigating the utilization of organic matter resources by several dominant Arctic bivalves and their trophic elasticity to changes in primary productivity patterns following changes in the onset of the annual productive season. Based upon δ13CAA patterns observed, these species utilized different carbon sources along the latitudinal gradient, including a strong input of bacterially reworked material and microalgae, mainly in particulate organic matter mixtures. Species type and the sampling location both played roles in δ13CAA variability, suggesting the influence of local production and decomposition processes. Macoma calcarea and Ennucula tenuis were shown to utilize organic matter of different quality, suggesting they may switch their feeding preferences to more detrital sources on a seasonal basis, but this was also affected by geographical location. These observations may have important implications for the benthic populations as microbial reworking of organic material is expected to increase with climate warming and likely shifts in food web structure.

Mechanisms of Persistent High Primary Production During the Growing Season in the Chukchi Sea

Persistent high primary production during the growing season in the Chukchi Sea (Arctic Ocean) plays a key role in maintaining an efficient biological carbon pump and diverse Arctic ecosystem. We used a three-dimensional ocean–sea ice–biogeochemical model to simulate monthly averaged net primary production from 1998 to 2015. The results show that the growing season in the Chukchi Sea lasts more than 150 days, with an annual net primary production of 30.85 ± 3.67 Tg C y−1. The mechanisms for maintaining high primary production differ in the southern and northern Chukchi Sea biological hotspots. Nutrient-rich Pacific Winter Water triggers phytoplankton blooms in both hotspots as light intensity increased in spring. After these initial blooms, Bering Summer Water and remnant Pacific Winter Water are the main contributors to nutrient levels and drive primary production during the growing season (May to September) in the southern and northern hotspots, respectively. Nitrate budget estimations in the euphotic zone reveal that after the spring blooms, persistent high primary production in the southern hotspot is mainly fueled by advecting Bering Summer Water through the Bering Strait. In the northern area, vertical mixing plays a critical role in upwelling nutrient-rich Pacific Winter Water from around the Hanna Shoal, where Pacific Winter Water is trapped for a long duration as a result of topography-influenced ocean circulation. Hence, high primary production exists in the northern Chukchi Sea during the summer and early autumn.

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.

Observational and Modeling Evidence of Seasonal Trends in Sediment‐Derived Material Inputs to the Chukchi Sea

AbstractBenthic inputs of nutrients help support primary production in the Chukchi Sea and produce nutrient‐rich water masses that ventilate the halocline of the western Arctic Ocean. However, the complex biological and redox cycling of nutrients and trace metals make it difficult to directly monitor their benthic fluxes. In this study, we use radium‐228, which is a soluble radionuclide produced in sediments, and a numerical model of an inert, generic sediment‐derived tracer to study variability in sediment inputs to the Chukchi Sea. The 228Ra observations and modeling results are in general agreement and provide evidence of strong benthic inputs to the southern Chukchi Sea during the winter, while the northern shelf receives higher concentrations of sediment‐sourced materials in the spring and summer due to continued sediment‐water exchange as the water mass traverses the shelf. The highest tracer concentrations are observed near the shelfbreak and southeast of Hanna Shoal, a region known for high biological productivity and enhanced benthic biomass.

Annual cycle of export fluxes of biogenic matter near Hanna Shoal in the northeast Chukchi Sea

The Chukchi Ecosystem Observatory (CEO), a mooring array of subsurface oceanographic instruments, was established on the northeast Chukchi Sea continental shelf to obtain time-series measurements of physical, biogeochemical, and biological parameters. A sequential sediment trap was deployed on a CEO mooring 8 m above seafloor to measure export fluxes of chlorophyll a (chl a), microalgal cells, zooplankton fecal pellets, total particulate matter (TPM), particulate organic carbon (POC), and zooplankton actively entering the trap from August 2015 to July 2016. These time-series measurements allowed us to monitor sympagic and pelagic algal production, the seasonal development of the zooplankton community, pelagic-benthic coupling, and particulate matter export in relation to snow and sea-ice cover on the shallow Chukchi Sea continental shelf. Notably, chl a and algal fluxes were nearly as high from August to October 2015 as in June–July 2016, indicating substantial autumn production. Autumn algal fluxes were dominated by the epipelic Cylindrotheca closterium while summer fluxes were dominated by pennate diatoms, including Fossula arctica and Neodenticula seminae. Peaks in the export of the exclusively sympagic diatom Nitzschia frigida in May and June 2016 indicated the release of ice algae due to snow and ice melt events. While pelagic copepods Calanus glacialis/marshallae, Pseudocalanus spp. and Oithona similis were the dominant copepods collected in the sediment trap, meroplanktonic stages of benthic organisms displayed the largest abundances and reflected mixing of pelagic stages and resuspension events on the shallow Chukchi Sea shelf. Enhanced fecal pellet carbon fluxes reflected zooplankton grazing in August and September 2015 and in July 2016. Despite the grazing pressure, high chl a, diatom and POC fluxes during these periods allowed strong pelagic-benthic coupling in the northeast Chukchi Sea. Persistent summer and autumn production also suggest that the local benthic community benefits from a sustained food supply rather than episodic flux events. Overall, these observations demonstrate the importance of year-round monitoring for fully understanding the phenology of marine processes and set a baseline for understanding the impact of environmental changes on Arctic marine ecosystems.

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.

Pathways, timing, and evolution of Pacific Winter Water through Barrow Canyon

Observations from a ship-based campaign in July-August 2009, combined with idealized numerical simulations, are used to investigate the seasonal delivery of Pacific Winter Water to Barrow Canyon and the subsequent adjustment of the flow down the canyon. As the current advects dense water, it transitions from a nearly barotropic structure near the canyon head to a strongly baroclinic flow with a subsurface maximum near the canyon mouth. Both the data and model indicate that the transit times along the three Chukchi Shelf pathways feeding Barrow Canyon – a coastal pathway, a southern Hanna Shoal pathway, and a northern Hanna Shoal pathway – modulate the mode of winter water that occupies the canyon at a given time. In particular, remnant Pacific Winter Water carried along the rapid coastal pathway can precede the arrival of newly ventilated Pacific Winter Water carried along the two interior pathways. The observations and model indicate that the transition between water types draining from the canyon can occur rapidly over time scales of days to weeks. We also demonstrate that mixing along the path of the current is unlikely to result in the observed down-canyon transition from newly ventilated Pacific Winter Water to remnant winter water, further supporting the dominant role of advection. While the focus here is on the transition of winter water modes, the implication that seasonality within Barrow Canyon is tied to seasonality of the Bering Strait inflow, together with the relative transit times along advective pathways, should hold for other water types as well.

Deposition patterns on the Chukchi shelf using radionuclide inventories in relation to surface sediment characteristics

Forty sediment cores collected on the Chukchi shelf and adjacent portions of the East Siberian Sea have been assayed for the sedimentation tracer 137Cs and 34 were also assayed for 210Pb. While both sedimentation and bioturbation influence how these tracers are distributed vertically in sediment cores, only about half of the cores had distinct, single mid-depth or subsurface maximum activity peaks associated with 137Cs originating from bomb fallout. For the same reasons, only 14 of the 34 cores assayed showed a consistent decline in excess sedimentary 210Pb with depth in the core. Furthermore, sedimentation rate estimates from 210Pb assays were only consistent with estimated 137Cs sedimentation rates in 4 of the 14 cores from north of Bering Strait. A high degree of bioturbation on the shelf is primarily responsible for these patterns, but the influence of sedimentation on vertical profiles is also important, particularly in areas of low accumulation where shallow burial of maximum burdens of 137Cs in high current areas such as Herald Canyon can be observed. Shallow burial of radiocesium is also observed in comparatively low sedimentation areas such as Hanna Shoal, on the northeast Chukchi Shelf. By contrast, elsewhere on the northeast Chukchi Shelf and in productive benthic “hotspots,” the stronger influence of bioturbation leads to radiocesium that is more evenly distributed vertically within sediments, i.e., no distinct mid- or subsurface depth maximum activity associated with bomb fallout. These sediment profiles of radiocesium reflect several other sediment characteristics that are affected by current flow. These sediment characteristics in turn impact biological activity, including grain size, carbon to nitrogen ratios of the organic fraction of surface sediments and total organic carbon content. The distribution patterns of the radionuclides, particularly the depth where 137Cs reaches maximum activity, reflects sedimentation under both quiescent and strong currents. The activity of 137Cs at that depth of maximum activity provide insights on how much the vertical distribution of the radionuclide has been impacted by bioturbation, as well as the characteristics of the sediments that play a role in influencing deposition and total inventories of radiocesium on continental shelves.

Trophodynamics of the Hanna Shoal Ecosystem (Chukchi Sea, Alaska): Connecting multiple end-members to a rich food web

Predicting how alterations in sea ice-mediated primary production will impact Arctic food webs remains a challenge in forecasting ecological responses to climate change. One top-down approach to this challenge is to elucidate trophic roles of consumers as either specialists (i.e., consumers of predominantly one food resource) or generalists (i.e., consumers of multiple food resources) to categorize the dependence of consumers on each primary producer. At Hanna Shoal in the Chukchi Sea, Alaska, we used stable carbon and nitrogen isotope data to quantify trophic redundancy with standard ellipse areas at both the species and trophic guild levels. We also investigated species-level trophic plasticity by analyzing the varying extents that three end-members were assimilated by the food web using the mixing model simmr (Stable Isotope Mixing Model in R). Our results showed that ice algae, a combined phytoplankton and sediment organic matter composite (PSOM), and a hypothesized microphytobenthos (MPB) component were incorporated by consumers in the benthic food web, but their importance varied by species. Some primary consumers relied heavily on PSOM (e.g, the amphipods Ampelisca sp. and Byblis sp.; the copepod Calanus sp.), while others exhibited generalist feeding and obtained nutrition from multiple sources (e.g., the holothuroidean Ocnus glacialis, the gastropod Tachyrhynchus sp., the sipunculid Golfingia margaritacea, and the bivalves Ennucula tenuis, Nuculana pernula, Macoma sp., and Yoldia hyperborea). Most higher trophic level benthic predators, including the gastropods Buccinum sp., Cryptonatica affinis, and Neptunea sp, the seastar Leptasterias groenlandica, and the amphipod Anonyx sp. also exhibited trophic plasticity by coupling energy pathways from multiple primary producers including PSOM, ice algae, and MPB. Our stable isotope data indicate that consumers in the Hanna Shoal food web exhibit considerable trophic redundancy, while few species were specialists and assimilated only one end-member. Although most consumers were capable of obtaining nutrition from multiple food sources, the timing, quantity, and quality of ice-mediated primary production may still have pronounced effects on food web structure.

Mesozooplankton abundance and distribution in association with hydrography on Hanna Shoal, NE Chukchi Sea, during August 2012 and 2013

Hanna Shoal, in the northeastern Chukchi Sea, is potentially vulnerable to ecosystem disruption under ongoing climate change, however aspects of its ecology, particularly of its zooplankton, have been poorly understood. Mesozooplankton distribution, taxonomic composition, and abundance were described from across Hanna Shoal in August 2012 and 2013 as part of the multidisciplinary COMIDA Hanna Shoal Program. Zooplankton were collected using vertical tows of paired Bongo nets equipped with 150-µm and 500-µm mesh nets; samples from the 150-µm mesh nets were enumerated to identify and count taxa, copepod species, and copepod life stages. Haplotypes of the mitochondrial cytochrome c oxidase I (mtCOI) gene were used to differentiate Bering and Arctic haplotype groups of the copepod Calanus glacialis and to differentiate C. glacialis and its close congener C. marshallae. The meroplankton, particularly bivalve larvae and in 2012 echinoderm larvae, were an important component of the zooplankton and were of greater abundance on the eastern portion of the Shoal than elsewhere. Regions identified on the basis of different taxonomic compositions were associated with different water masses and current pathways. The northeast corner of the Shoal in particular was distinct from the remainder of the Shoal and from Barrow Canyon, with both different life stage compositions and unique Arctic haplotypes of the mtCOI gene for C. glacialis/marshallae, suggesting populations at those locations originated in the Arctic Ocean rather than the Bering Sea. Bering Sea Summer water, and intrinsic plankton, was observed in the southwest portion of the Shoal. Comparisons with historic and recent studies done near Hanna Shoal demonstrated that similar plankton compositions were present across a broad region of the Chukchi Sea and that abundances of the copepod C. glacialis appear to be increasing on the time scale of decades, potentially through increased input from the northern Bering Sea. Because the Chukchi Sea is highly advective, it is likely that zooplankton populations over Hanna Shoal are lost to the Canada Basin to the north and must be re-established annually through input from the northern Bering Sea. Under this scenario, enhancement of a resident Hanna Shoal zooplankton community that would retain a significant proportion of the primary production in the water column is unlikely.

Alkane and polycyclic aromatic hydrocarbons in sediments and benthic invertebrates of the northern Chukchi Sea

The Hanna Shoal region represents an important northern gateway for transport and deposition in the Chukchi Sea. This study determined the concentration and distribution of organic contaminants (aliphatic hydrocarbon and polycyclic aromatic hydrocarbons, PAHs) in surface sediments from 34 sites across Hanna Shoal. Up to 31 total PAHs, including parent and alkyl homologues were detected with total concentrations ranging from a low of 168ngg−1 the western flank of Hanna Shoal (station H34) to 1147ngg−1 at station in Barrow Canyon (station BarC5). Alkyl PAHs were more abundant than parent structures and accounted for 53–64% of the summed concentrations suggesting overall at background levels (< 1600ngg−1) in sediments. Alkane (C15–C33) hydrocarbons ranged from 4.3µgg−1 on the southern flank of Hanna shoal to 31µgg−1 at a northern station. Sediments were often dominated by short chain (C15–C22) alkanes with overall terrestrial aquatic ratios (TAR) for the region averaging 0.20. Based on the ratio of Fl/(Fl+ Py) and BaF/(Baf+BeP) verses (BA/BA+Ch) in sediments, PAHs are largely derived from petrogenic sources with minor amounts of mixed combustion sources. A diversity of PAHs were detected in the northern whelk Neptunea heros foot muscle with total concentrations ranging from 0.14 to 1.5µgg−1 dry tissue wt. Larger (and presumably older) animals showed higher levels of PAH per unit muscle tissue, suggesting that animals may bioaccumulate PAHs over time, with low but increasing concentrations also present in internal and external eggs. Alkane hydrocarbons were also higher in whelks with distributions similar to that seen in sediments. The mussel Muscularus discors collected in Barrow Canyon showed constrained distributions and substantially lower concentrations of both PAHs and alkanes than the surrounding surface sediments.

Arctic shelves as platforms for biogeochemical activity: Nitrogen and carbon transformations in the Chukchi Sea, Alaska

Continental shelves comprise <5% of global ocean area but may account for a disproportionate 30% of primary production, 80% of organic matter burial, and >50% of marine denitrification. The Hanna Shoal region, part of the continental shelf system in the northeast Chukchi Sea, Alaska, is recognized for its high biodiversity and productivity. We investigated the role of sediments in organic matter decomposition and nutrient cycling at five stations on the shallow Hanna Shoal. In particular, we asked (1) how much sediment organic matter is remineralized in the Chukchi Sea, and what factors drive this degradation, (2) do sediments function as a net source for fixed nitrogen (thus fueling primary production in the overlying water), or as a net sink for fixed nitrogen (thereby removing it from the system), and (3) what is the balance between sediment NH4+ uptake and regeneration, and what factors drive NH4+ cycling? We conducted dark sediment core incubations to measure sediment O2 consumption, net N2 and nutrient (NH4+, NO3-, NO2-, PO43-) fluxes, and rates of sediment NH4+ cycling, including uptake and regeneration. Rates of sediment O2 consumption and NH4+ and PO43- efflux suggest that high organic matter remineralization rates occurred in these cold (−2°C) sediments. We estimated that total organic carbon remineralization accounted for 20–57% of summer export production measured on the Chukchi Shelf. Net N2 release was the dominant nitrogen flux, indicating that sediments acted as a net sink for bioavailable nitrogen via denitrification. Organic carbon remineralization via denitrification accounted for 6–12% of summer export production, which made up ~25% of the total organic carbon oxidized in Hanna Shoal sediments. These shallow, productive Arctic shelves are “hotspots” for organic matter remineralization.

The summer hydrographic structure of the Hanna Shoal region on the northeastern Chukchi Sea shelf: 2011–2013

We used shipboard and towed CTD, current meter, and satellite-tracked drifter data to examine the hydrographic structure in the northeastern Chukchi Sea in August–September of 2011, 2012, and 2013. In all years the densest winter water was around and east of Hanna Shoal. In 2012 and 2013, a ~ 15m deep layer of cold, dilute meltwater overlaid the dense water north of the shelf region between ~ 71.2 and ~ 71.5°N. A front extends from the southwest side of Hanna Shoal toward the head of Barrow Canyon, separated meltwaters from warmer, saltier Bering Sea Summer Waters to the south. Stratification was stronger and the surface density variances in the meso- and sub-mesoscale range were higher north of the front than to the south. No meltwater or surface fronts were present in 2011 due to a very early ice retreat. Differences in summer ice cover may be due to differences in the amount of grounded ice atop Hanna Shoal associated with the previous winter’s regional ice drift.Along the north side of Hanna Shoal the model-predicted clockwise barotropic flow carrying waters from the western side of the Shoal appears to converge with a counterclockwise, baroclinic flow on the northeast side. The baroclinic tendency is confined to the upper 30m and can include waters transported from the shelfbreak. The inferred zonal convergence implies that north of the Shoal: a) near-surface waters are a mixture of waters from the western and eastern Chukchi Sea and b) the cross-isobath pressure gradient collapses thereby facilitating leakage of upper layer waters northward across the shelf.