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Nitrogen constrains biomass across the Arctic Ocean, with nitrate (NO3) supply to the surface waters fuelling new primary production and net carbon drawdown. In this Review, we explore the physical mechanisms driving NO3 fluxes to the euphotic zone across the Arctic Ocean and how biological processes respond. The volume and inflow depth of Atlantic and Pacific Ocean waters, together with sea ice and halocline dynamics, govern internal physical mixing of NO3. Respectively, these inflows supply ~34 ± 5 kmol NO3 s−1 and 9 ± 1 kmol NO3 s−1, spreading at mid-depth. NO3 from below the euphotic zone is mixed upwards via several mechanisms. Overall, NO3 fluxes associated with diffusive and turbulent mixing, submesoscale fronts and cyclonic mesoscale eddies are relatively low (on the order of ~0.1–0.7 mmol m−2 per day) but cover a large area, with peaks associated with wind events or individual strong eddies. By comparison, upwelling-driven fluxes are much stronger (on the order of ~1 mmol m−2 per day) but are more localized. Near-inertial and tidal mixing over the Arctic Ocean’s complex bathymetry drives perhaps the strongest NO3 fluxes, for example, reaching 4.5 mmol m−2 per day in the Barents Sea. Comparing these fluxes with observed biological NO3 uptake rates indicates that the internal physical supply of NO3 only limits primary productivity in 9 of the 17 cases considered. Thereafter, light limitation and lagged growth responses can result in excess NO3 remaining in the surface waters. Future research should prioritize linking NO3 supply and uptake at corresponding spatiotemporal scales. 

The biologically productive Northern Gulf of Alaska (NGA) continental shelf receives large inputs of freshwater from surrounding glaciated and non-glaciated watersheds, and a better characterization of the regional salinity spatiotemporal variability is important for understanding its fate and ecological roles. We here assess synoptic to seasonal distributions of freshwater pathways of the Copper River discharge plume and the greater NGA continental shelf and slope using observations from ship-based and towed undulating conductivity-temperaturedepth (CTD) instruments, satellite imagery, and satellite-tracked drifters. On the NGA continental shelf and slope we find low salinities not only nearshore but also 100–150 km from the coast (i.e. average 0–50 m salinities less than 31.9, 31.3, and 30.8 in spring, summer, and fall respectively) indicating recurring mid-shelf and shelfbreak freshwater pathways. Close to the Copper River, the shelf bathymetry decouples the spreading river plume from the direct effects of seafloor-induced steering and mixing, allowing iron- and silicic acid-rich river outflow to propagate offshore within a surface-trapped plume. Self-organized mapping analysis applied to true color satellite imagery reveals common patterns of the turbid river plume. We show that the Copper River plume is sensitive to local wind forcing and exerts control over water column stratification up to ~100 km from the river mouth. Upwelling-favorable wind stress modifies plume entrainment and density anomalies and plume width. Baroclinic transport of surface waters west of the river mouth closely follow the influence of alongshore wind stress, while baroclinic transport east of the river mouth is additionally modified by a recurring or persistent gyre. Our results provide context for considering the oceanic fate of terrestrial discharges in the Gulf of Alaska. 

The Central Arctic Ocean remains profoundly understudied, particularly with respect to carbon cycling, ecosystem alteration, and associated changes in atmospheric, ice and ocean physics that drive those biological and biogeochemical systems. The region is expected to experience continued marked changes over the coming decades, driven by ongoing climate warming. Yet, because of relatively limited understanding of fundamental characteristics and processes in the region, predicting these changes and their Pan-Arctic linkages remains difficult. The Synoptic Arctic Survey (SAS) is organized around three major research areas: (1) physical drivers of importance to the ecosystem and carbon cycle; (2) the ecosystem response and (3) the carbon cycle. The overarching questions are: “What is the present state, and what are the major ongoing transformations of the Arctic marine system?” The overall objective of this expedition was to quantify the present states of the physical, biological, and biogeochemical systems of the Pacific Arctic (here defined as the Chukchi Sea, Beaufort shelf/slope, Chukchi Borderlands) and Canadian Basin (i.e., the Makarov and Canada basins) during summer 2022. A key goal is to document temporal changes where possible by comparison with historical data and to quantify linkages among adjacent shelves, slopes, and deep basins on a Pan-Arctic scale. These objectives are part of the International Synoptic Arctic Survey (SAS; 2021-2022) that seeks Pan-Arctic understanding of core ocean variables on a quasi-synoptic, spatially distributed basis using coordinated, international efforts. The findings of this expedition, a US contribution to the SAS, will be a foundation and legacy for future, quasi-decadal assessments of rapid and evolving Arctic Ocean system change." - Cruise Report USCGC Healy HLY2202/AWS2022 [Prepared by Carin Ashjian (cashjian@whoi.edu) and the HLY2202 Science Team] This data set contains measurements of water properties such as temperature, conductivity, chlorophyll fluorescence, Photosynthetically Available Radiation (PAR), oxygen, beam attenuation, and beam transmission. These measurements were collected by a Seabird 9 conductivity, temperature, and depth (CTD) and associated sensors on a CTD rosette lowered from the ship at discrete stations during cruise HLY2202. 

Bering Strait is the only ocean gateway connecting the Pacific and Arctic oceans. The ∼1 Sv northward flow of Pacific water through the strait to the Arctic Ocean has been increasing by ∼0.01 Sv/yr since 1990. Monthly dynamic ocean topography (DOT), wind, and sea‐ice data at Bering Strait are analyzed in context with the long‐term record of flow through the strait to investigate local drivers. Ocean transport is found to be proportional to the across‐strait slope in DOT, suggesting some component of the flow is in geostrophic balance. Along‐strait ocean surface stresses, which modulate the across‐strait DOT slope via Ekman transport, are analyzed in the presence of a seasonally varying ice cover. It is shown that northward interior ocean flow under sea ice in winter results in southward surface stresses, and westward Ekman transport that slows the geostrophic component of the northward ocean flow. As the number of open water days local to Bering Strait increase each year, we find no trend in the annual mean surface stress, that is, the loss of sea ice is not leading to increased northward wind stress input that would enhance northward ocean flow. This analysis is consistent with the theory that changes in both the atmosphere and ocean non‐local to Bering Strait are likely driving the increased transport from the Pacific into the Arctic via Bering Strait. 

Abstract A 25‐year (1996–2020) hindcast from a coupled physical‐biogeochemical model is evaluated with nutrients, phytoplankton and zooplankton field data and is analyzed to identify mechanisms controlling seasonal and interannual variability of the northern Gulf of Alaska (NGA) planktonic food web. Characterized by a mosaic of processes, the NGA is a biologically complex and productive marine ecosystem. Empirical Orthogonal Function (EOF) analysis combining abiotic and biotic variables averaged over the continental shelf reveals that light intensity is a main driver for nanophytoplankton variability during spring, and that nitrate availability is a main driver for diatoms during spring and for both phytoplankton during summer. Zooplankton variability is a combination of carry‐over effects from the previous year and bottom‐up controls from the current year, with copepods and euphausiids responding to diatoms and microzooplankton responding to nanophytoplankton. The results also demonstrate the effect of nitrate availability and phytoplankton community structure on changes in biomass and energy transfers across the planktonic food web over the entire growing season. In particular, the biomass of large copepods and euphausiids increases more significantly during years of higher relative diatom abundance, as opposed to years with higher nitrate availability. Large microzooplankton was identified as the planktonic group most sensitive to perturbations, presumably due to its central position in the food web. By quantifying the combined variability of several key planktonic functional groups over a 25‐year period, this work lays the foundation for an improved understanding of the long‐term impacts of climate change on the NGA shelf. 

Abstract The global rate of ocean deoxygenation may be influenced by the molar ratio of dissolved oxygen to organic carbon consumed during microbial respiration. However, respiration stoichiometry and its environmental controls are not fully understood due to sparse direct measurements and limited global coverage. Here, we measured surface concentrations of particulate chemical oxygen demand, particulate organic carbon, and particulate organic nitrogen from the Bering Sea (54°N) to the North Pole (90°N). Unlike previous measurements in other regions, the respiration quotient (−O₂:C) was not primarily associated with sea surface temperature or nitrate availability. Instead, it was more strongly correlated with sea surface salinity, with oxygen isotope measurements (δ¹⁸O) supporting sea ice formation as a contributor to salinity variability. Based on DNA sampling and short‐read metagenomic sequencing, we found that the relative abundance of heterotrophic bacteria positively correlated with sea surface salinity and the respiration quotient. Additionally, gene modules involved in nitrogen metabolism and lipid synthesis were relatively more common when the respiration quotient was higher. These results suggest that Arctic respiration stoichiometry is linked to shifts in plankton community composition, which may help constrain projections of oxygen demand in a warming ocean. 

Climate change is rapidly modifying biodiversity across the Arctic, driving a shift from Arctic to more boreal ecosystem characteristics. This phenomenon, known as borealization, is mainly described for certain functional groups along sub-Arctic inflow shelves (Barents and Chukchi Seas). In this review, we evaluate the spatial extent of such alterations across the Arctic, as well as their effects on ecosystem-level processes and risks. Along the inflow shelves, borealization is driven by long-term strengthened inflow of increasingly warm waters from the south and punctuated by advection and low sea ice extreme events. A growing body of literature also points to an emerging borealization of the other Arctic shelf ecosystems, through a “spillover” effect, as local changes in environmental conditions enable movement or transport of new species from inflow shelves. These modifications are leading to changes across functional groups, although many uncertainties remain regarding under-sampled groups, such as microbes, and technical challenges of consistent, regular monitoring across regions. There is also clear consensus that borealization is affecting phenology, species composition, community traits, population structure and essential habitats, species interactions, and ecosystem resilience. Non-dynamic environmental factors, such as depth and photoperiod, are thought to limit the complete borealization of the system, and may lead to intermediate, “hybrid” ecosystems in the future. We expect current borders of Arctic and boreal ecosystems to progress further northward and ultimately reach an equilibrium state with seasonal borealization. Risks to the system are difficult to estimate, as adaptive capacities of species are poorly understood. However, ice-associated species are clearly most at risk, although some might find temporary refuge in areas with a slower rate of change. We discuss the likely character of future Arctic ecosystems and highlight the uncertainties. Those changes have implications for local communities and the potential to support Blue Growth in the Arctic. Addressing these issues is necessary to assess the full scale of Arctic climate impacts and support human mitigation and adaptation strategies. 

The productive Northern Gulf of Alaska (NGA) ecosystem is impacted by spatiotemporal variability in nutrient distributions and ratios. Due to its advective nature and wide (50–150 km) continental shelf, the NGA may act as a nutrient source to the greater NE Pacific. Using high‐resolution data from 11 seasonal (spring, summer, fall) research cruises (2018–2023), we examined regional drivers of nutrient fields. We found that variable sources of micro‐ and macronutrients influence their distributions and ratios. For example, relative to cellular nutrient quotas for coastal diatoms, surface dissolved Fe (dFe): nitrate (NO₃⁻) ratios indicated an Fe‐replete regime near river outflows throughout the year. In contrast, far from freshwater and subsurface Fe inputs, this ratio indicated potential Fe stress or limitation within outer waters during spring. Near the seafloor, a pool of high dissolved micro‐ and macronutrients highlighted variable nutrient sources (sedimentary and deep offshore waters), and its location indicated its susceptibility to wind forcing. We also observed detachment of bottom boundary layers along shoaling isopycnals within the shelf‐break front that can carry subsurface dFe to depths of ∼100 m: a depth that can be accessed by winter mixing. This represents a potential mechanism to deliver dFe from the shelf to offshore high nutrient low chlorophyll (HNLC) surface waters of the NGA, although shelf‐basin exchange can be diminished by along‐shelf flow (e.g., the Alaskan Stream). This work enhances understanding of nutrient sources in the NGA and adds to a relatively small global repository of seasonally high‐resolution micronutrient data over continental shelves. 

Surface and subsurface moored buoy, ship-based, remotely sensed, and reanalysis datasets are used to investigate thermal variability of northern Gulf of Alaska (NGA) nearshore, coastal, and offshore waters over synoptic to century-long time scales. NGA sea surface temperature (SST) showed a larger positive trend of 0.22 ± 0.10 °C per decade over 1970–2021 compared to 0.10 ± 0.03 °C per decade over 1900–2021. Over synoptic time scales, SST covariance between two stations is small (<10%) when separation exceeds 100 km, while stations separated by 500 km retain 50% of their co-variability for seasonal and longer fluctuations. Relative to in situ sensor data, remotely sensed SST data has limited accuracy in some NGA settings, capturing 60–70% of the daily SST anomaly in coastal and offshore waters, but often <25% nearshore. North Pacific and NGA leading modes of SST variability leave 25–50% of monthly variance unresolved. Analysis of the 2014–2016 Pacific marine heatwave shows that NGA coastal surface temperatures warmed contemporaneously with offshore waters through 2013, but deep inner shelf waters (200–250 m) exhibited delayed warming. Offshore surface waters cooled from 2014 to 2016, while shelf waters continued to warm from the combined effects of local air-sea and advective heat fluxes. We find that annually averaged Sitka air temperature is a leading predictor (r2 = 0.37, p < 0.05) for following-year NGA coastal water column temperature. Our results can inform future environmental monitoring designs, assist forward-looking projections of marine conditions, and show the importance of in situ measurements for nearshore studies that require knowledge of thermal conditions over time scales of days and weeks.