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On 4 August 2014, a catastrophic breach of the Mount Polley mine tailings impoundment released ~25 M m3of tailings and water and scoured an unknown quantity of overburden into the West Basin of Quesnel Lake. We document Quesnel Lake and Quesnel River observations for 2 months postspill. Breach inflows raised Quesnel Lake by 7.7 cm, equivalent to ~21 M m3. The West Basin hypolimnion was modified immediately, exhibiting increased temperature (~5°C to 6–7.5°C), conductivity (110 to 160 μS/cm), and turbidity (<1 to 200–1000 nephelometric turbidity units (NTU)). Cooscillating seiches moved West Basin hypolimnetic water both westward and eastward contaminating the Main Basin. Postspill, high‐turbidity water propagated eastward (~1 cm/s), introducing a persistent ~20 m thick layer below the thermocline and an ~30 m thick layer at the bottom. The contaminant introduction, mobilization, and bioaccumulation may pose risks to resident and anadromous fish stocks, which support recreational, commercial, and First Nations fisheries. Hypolimnetic increase in temperature and turbidity due to a mine tailings spillStratification and seiching promote and distribute fine‐grained sediment plumePhysical, biological and chemical implications for a near‐pristine lake

Phytoplankton and nutrient dynamics were investigated during the 2007 and 2008 summers in the euphotic zone of five broad domains across Subarctic and Arctic Seas: the Eastern Subarctic North Pacific Ocean, ESNP; Bering and Chukchi Seas, BE‐CH; Beaufort Sea and Canada Basin, BS‐CB; Canadian Arctic Archipelago, CAA; and Baffin Bay and Labrador Sea, BB‐LS. Average concentrations of nutrients (NO3−, NH4+, Si(OH)4, and PO43−) decreased markedly from west to east, with minima in NO3−and NH4+in surface BS‐CB waters, but relatively invariant urea‐N concentrations across the entire region. In the BS‐CB domain, low uptake rates of nitrate (ρNO3−) and ammonium (ρNH4+) were exceeded by uptake of urea (ρUrea‐N). Whereas average ρNO3−was highest in the BE‐CH domain, ρUrea‐N was maximal in BB‐LS. Average depth‐integrated f‐ratios ranged from 0.27 in the BS‐CB domain to 0.57 in BE‐CH, while chlorophyll a(chl a) and primary productivity (ρC) were highest in BE‐CH and BB‐LS, and consistently low in the BS‐CB domain. The >5 µm phytoplankton fraction dominated ρC and ρNO3−in the BE‐CH and CAA domains, whereas ESNP and BS‐CB were dominated by the <5 µm fraction. In the BB‐LS domain, the larger cells were responsible for ∼50% of ρC, ρNO3−, and ρUrea‐N. This study highlights the contrast in ice‐corrected average new production between the BE‐CH (396 mg C m−2d−1) and BS‐CB (5.50 mg C m−2d−1) domains in summer, and the larger contribution of urea‐N uptake to total N uptake in central and eastern regions where NO3−concentrations were lower. Canada Basin had the lowest primary and new production ratesHigh new production in the Chukchi Sea was dominated by >5 um phytoplanktonUrea was an important nitrogen source for phytoplankton when nitrate was low

The Arctic Ocean is changing rapidly but there are no long‐term time series observations on the state of the phytoplankton community that could allow a link to be made from physical/chemical pressures to the impact on marine ecosystems. Here, we test the idea that space‐for‐time (SFT) substitution might predict temporal change in the Canada Basin premised on differences in the present state of phytoplankton in other geographic zones, specifically the ratio in the abundance of picophytoplankton to nanophytoplankton (Pico:Nano). We compared the change in Pico:Nano observed in the Canada Basin from 2004 to 2012 to the different average states of this ratio in 26 other ocean ecological regions. Our results show that as upper ocean nitrate concentration changed in the Canada Basin from year to year, the concomitant change in Pico:Nano was statistically commensurate with the difference that this ratio exhibits between Longhurst ecological provinces in relation to nitrate concentration. Lower average concentration of nitrate in the upper water column is associated with a higher value of Pico:Nano, a result consistent with resource control of phytoplankton size structure in the ocean. We suggest that SFT substitution allows an explanation of temporal progression from spatial pattern as a test of mechanism, but such statistical prediction is not necessarily a projection of future states. Space is substituted for time to statistically predict phytoplankton changes

During a spring‐summer bloom in a large Arctic polynya, vertically distinct protist communities (phytoplankton and protozoa) occurred within layers caused by interleaving and entrainment of different water masses. We developed a ternary community distance index to quantify the variability in protist community structure in these heterogeneous surface layers. This index was highly correlated (r= 0.984, n= 6) with the extent of physical interleaving (quantified as the root mean square deviations in temperature between measured and smoothed profiles) indicating a high degree of physical‐biotic coupling within water columns of the polynya. Water mass layering created favorable conditions for ciliate blooms. These blooms were associated with distinct temperature‐salinity layers. These layers might act as processing traps for particulate organic matter, with highest concentrations of viruses and bacteria (specifically cells with open or leaky membranes, suggesting microbial grazing pressure) occurring in highly interleaved water columns. In contrast, viral and bacterial concentrations were lowest at a noninterleaved station where protist biomass was dominated by flagellates and small dinoflagellates that were evenly distributed down the water column. Water mass interleaving is likely to contribute to microbial biodiversity, community structure and vertical segmentation of biogeochemical processes in the upper ocean.

We describe the wintertime development and persistence of an anomalous temperature inversion layer in a deep temperate lake with emphasis on the roles of convective mixing and river‐induced circulation. The layer is about 20 m thick, 100 km long, and lies at a depth of 60–80 m. The destabilizing temperature distribution is compensated by a very weak (0.015 ppt) salinity increase across the inversion layer. We observe that neither the energy available from the wind nor the buoyancy extracted at the surface via a negative heat flux are sufficient to penetrate the depth of the inversion. Calculations of a thermal diffusion coefficient for this layer indicate a weak turbulent exchange below the halocline. Lake overturn does not follow the classical pattern of surface processes driving complete winter mixing; rather, full circulation is made possible only by the input of relatively saline river water.

Analyses of data from three shipborne surveys describe the quasi-synoptic density and velocity fields near Barrow Canyon, Alaska. The canyon parallels the northwestern coast of Alaska and contains three different water masses. These are 1) warm and fresh Alaskan coastal waters that originate from the Bering Strait; 2) cold and moderately salty waters that originate from the Chukchi shelf; and 3) warm and salty waters that originate from the Atlantic layer of the Arctic Ocean. A halocline separates the Chukchi shelf and Atlantic layer waters. The halocline slopes upward into the canyon where it is then twisted to slope across the wide canyon. An intensification of the Beaufort gyre near the shelf break just seaward of Barrow Canyon raises the halocline more than 100 m toward the surface. Locally upwelling favorable winds raise the Arctic halocline, which thus is ventilated within Barrow Canyon adjacent to the coast. In the absence of winds the halocline slopes across-canyon in the thermal wind sense due to a northward flowing coastal current.

Temperature, dissolved oxygen, nutrients, and chlorofluorocarbon‐12 data obtained from Lake Baikal are used to describe deep‐water renewal in a deep, temperate‐latitude lake. Observations are used to propose the physical mechanism governing convection and to formulate a model of deep ventilation. The key physical mechanism governing deep‐water renewal is the so‐called thermobaric instability. Because the temperature of maximum density decreases with depth, a lake can become conditionally unstable if the base of the cold mixed layer is displaced to a depth at which its temperature matches the local temperature of maximum density, thereby resulting in sinking plumes. An important consequence of this phenomenon is that deep temperate lakes such as Baikal do not completely mix twice yearly; instead, deep ventilation is episodic. A two‐dimensional model of a wind‐ and buoyancy‐driven lake shows many strong mixing events and a fairly realistic seasonal cycle, indicating that the hypothesis is physically realizable. A filling‐box model is used to deduce the annually averaged fluxes necessary to produce a steady vertical distribution of tracers as observed. Good fits are obtained to oxygen and chlorofluorocarbon distributions by this model.

Many complex fjord systems cross British Columbia's coastline. A 70 year (1951–2020) time series analysis of temperature, salinity, and oxygen in four such fjords between ∼54 and 50oN (Douglas Channel, Rivers Inlet, Knight Inlet and Bute Inlet) shows that changes were greatest in deep waters between the sill and the bottom. In Rivers, Knight and Bute Inlet, the deep water temperature increased by 1.2–1.3°C over 70 years, up to two times the global average for open ocean waters at corresponding depths, while salinity increased by 0.1–0.2, and oxygen decreased by 0.4–0.7 mLL−1. The most northern inlet, Douglas Channel, showed a temperature increase of 0.8°C from 1951 to 2016, while trends in oxygen and salinity were not statistically significant. An analysis of Apparent Oxygen Utilization suggests that the deep waters in Douglas Channel are more readily exchanged with the outer coast than the three other fjords. Glacially carved fjords draw their deep waters from adjacent offshore ocean basins through a combination of upwelling, cross‐shelf transport, tidal mixing and estuarine circulation related processes. We studied data spanning seven decades from four such systems covering four degrees of latitude along the Canadian Pacific and found that fjord deep waters below sill depth were warming two times faster than their offshore source waters. Changes in temperature were accompanied by significant trends of decreased oxygen concentrations and increased salinity. Potential processes explaining these anomalous responses to climate change are discussed. Temperature and oxygen data from four fjords (Rivers, Knight, and Bute Inlets and Douglas Channel) from 1951 to 2020 were examinedOver 70 years, deep water has warmed by up to 1.3°C and lost up to 0.7 mLL−1of oxygen, with evidence of accelerated warming since 2016Deep warming and deoxygenation trends are statistically alike in at least three of the four fjords, suggesting similar drivers Temperature and oxygen data from four fjords (Rivers, Knight, and Bute Inlets and Douglas Channel) from 1951 to 2020 were examined Over 70 years, deep water has warmed by up to 1.3°C and lost up to 0.7 mLL−1of oxygen, with evidence of accelerated warming since 2016 Deep warming and deoxygenation trends are statistically alike in at least three of the four fjords, suggesting similar drivers

A 15‐year (2004–2018) record of mooring observations from the upper 50 m ocean in the eastern Eurasian Basin reveals increased current speeds and shear, associated with an increasing coupling between wind, ice, and oceanic currents and their vertical shear over 2004–2018, particularly in summer. Substantial increases in both current speeds and shears in the upper 50 m are dominated by a two times amplification of currents in the semidiurnal band, which includes tides and wind‐forced near‐inertial oscillations. For the first time the strengthened upper ocean currents and shear are observed to coincide with weakening stratification. This coupling links the Atlantic Water heat to the sea ice, a consequence of which would be reducing regional sea ice volume. These results point to a new positive feedback mechanism in which reduced sea ice extent facilitates more energetic inertial oscillations and associated upper‐ocean shear, thus leading in enhanced ventilation of the Atlantic water. Previous studies demonstrated that in recent years density gradients above the warm and salty intermediate (~150–900 m) water of Atlantic origin in the eastern Arctic Ocean have weakened, allowing stronger upward transport heat to the bottom of the sea ice. Using mooring observations, we show that this weakening of stratification has been accompanied by stronger upper‐ocean currents and their vertical shear and by increasing coupling between the wind and sea ice with upper ocean currents and shear. Most of this enhanced energy and shear is in the semidiurnal band, which includes baroclinic tides and wind‐driven inertial oscillations. The increased shear together with the weakening stratification indicate a greater potential for shear‐driven turbulent mixing. We propose a new process, the ice/ocean‐heat positive feedback, that can accelerate current sea ice loss and impede the rate of recovery of eastern Arctic sea ice even if large‐scale climate warming conditions relax. Currents and associated shear in the upper 50 m in the eastern Eurasian Basin are increased in the 2010sIncreased currents and shear are dominated by accelerating currents in the semidiurnal (inertial and tidal) bandThere was an increasing coupling between wind, ice, and oceanic currents in the eastern Eurasian Basin over 2004–2018

The catastrophic August 2014 Mount Polley tailings spill, the second largest ever documented, sent ~18 Mm3of waste plunging to the bottom of the >100 m deep West Basin of Quesnel Lake, British Columbia, a critical West Coast salmon habitat. To understand the impact of the spill on the lake, including the fate of suspended solids, we examine changes in physical water properties over 11 years (2006–2017) using water column profiles, moored timeseries, and satellite imagery. Contaminated waters were initially largely confined to the hypolimnion; however, during autumn 2014 turnover, turbid waters were mixed to the surface, resulting in the clear blue lake turning bright green. Twelve months after the spill, the lake's temperature, conductivity, and turbidity temporarily returned to pre‐spill conditions; however, initiation of mine effluent discharge in late 2015 was associated with a subsequent 15 μS cm−1conductivity increase above historic values. Importantly, a post‐spill 1–2.5 formazin turbidity unit hypolimnetic turbidity increase was observed during spring and fall turnovers of 2015–2017, which appeared to be due to resuspension of a thin layer of unconsolidated spill‐related material from the lake bed driven by large internal seiche motions. This process implies spill contaminants may be seasonally mobilized into the water column, with potentially detrimental impacts on aquatic ecology. Our findings underscore that basin‐scale physical processes, including seasonal turnover and internal seiches, must be accounted for, even in deep lakes, to understand the long‐term impact of the ever increasing number of tailings spills into aquatic ecosystems. The catastrophic failure of the Mount Polley copper and gold mine tailings pond dam in August 2014 was, at the time, the largest mine‐related spill ever documented. The turbid torrent of mine waste plunged to the bottom of the near pristine waters of Quesnel Lake, British Columbia, critical habitat for West Coast salmon stocks. Despite the scale of the spill, there was surprisingly little visible evidence of it at the surface of the lake in the months following. But what happened below the surface? Using 11 years of water column measurements we show that as the lake mixed from top to bottom (called turnover) that autumn the contaminated deep water was brought to the surface, abruptly changing the color of the clearblue lake to an abnormal bright green color—a change readily visible from space. Since the spill the turbidity of the lake has increased during spring and autumn turnover, apparently due to resuspension of spill material off the lake bottom driven by basin‐scale wave oscillations. This process, and the ongoing discharge of excess mine waste water directly into the lake, raises concerns over the seasonal mobilization of mine contaminants and their impact on aquatic ecosystems. Quesnel Lake turned bright green when turbid spill material trapped in the hypolimnion mixed to the surface during autumn 2014 turnoverSeasonal turbidity increases since the spill are likely due to resuspension of spill‐related bottom sediment by internal seichesPhysical dynamics are key to understanding the long‐term impact of acute and chronic mine spills even in deep aquatic systems

A survey of 25 coastal‐draining rivers across the Canadian Arctic Archipelago (CAA) shows that these systems are distinct from the largest Arctic rivers that drain watersheds extending far south of the Arctic circle. Observations collected from 2014 to 2016 illustrate the influences of seasonal hydrology, bedrock geology, and landscape physiography on each river's inorganic geochemical characteristics. Summertime data show the impact of coincident gradients in lake cover and surficial geology on river geochemical signatures. In the north and central CAA, drainage basins are generally smaller, underlain by sedimentary bedrock, and their hydrology is driven by seasonal precipitation pulses that undergo little modification before they enter the coastal ocean. In the southern CAA, a high density of lakes stores water longer within the terrestrial system, permitting more modification of water isotope and geochemical characteristics. Annual time‐series observations from two CAA rivers reveal that their concentration‐discharge relationships differ compared with those of the largest Arctic rivers, suggesting that future projections of dissolved ion fluxes from CAA rivers to the Arctic Ocean may not be reliably made based on compositions of the largest Arctic rivers alone, and that rivers draining the CAA region will likely follow different trajectories of change under a warming climate. Understanding how these small, coastal‐draining river systems will respond to climate change is essential to fully evaluate the impact of changing freshwater inputs to the Arctic marine system. River inputs are important for the physics and biogeochemistry of the rapidly changing Arctic Ocean. Most of our knowledge of river inputs comes from studies of the six largest rivers which drain areas reaching far south of the Arctic Circle, omitting 45% of the pan‐Arctic watershed. This has left a gap in our understanding of smaller, coastal‐draining rivers and how these are influenced by the changing climate. We studied 25 coastal‐draining rivers in the Canadian Arctic Archipelago (CAA) for geochemical comparison to the larger, more southerly rivers. Summertime survey data show that geochemical properties change from south to north following the distribution of lake cover, bedrock geology, and glacial history. Unlike the largest Arctic rivers, however, repeat observations over the annual cycle indicate that dissolved ion concentrations are not strongly influenced by changing water fluxes in the CAA region. To fully understand the impact of changing freshwater inputs to the Arctic Ocean requires consideration of smaller watersheds that may change differently compared to the largest Arctic rivers. Geochemical characteristics of Canadian Arctic Archipelago rivers reflect geological and hydrological heterogeneity across the regionTime‐series observations from two CAA rivers reveal weak, positive concentration‐discharge relationshipsMajor ion characteristics of CAA rivers are similar to large Arctic rivers, but their response to future change will likely differ