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A 2-year-long time series of currents and acoustic backscatter from an acoustic Doppler current profiler, moored over the eastern Beaufort Sea continental slope from October 2003 to September 2005, were used to assess the dynamics and variability of the sound-scattering layer. It has been shown that acoustic backscatter is dominated by a synchronized diel vertical migration (DVM) of zooplankton. Our results show that DVM timings (i) were synchronous with sunlight and (ii) were modified by moonlight and sea ice, which attenuates light transmission to the water column. Moreover, DVM is modified or completely disrupted during highly energetic current events. Thicker ice observed during winter–spring 2005 lowered the backscatter values but favored extending DVM toward the midnight sun. In contrast to many previous studies, DVM occurred through the intermediate water layer during the ice-free season of the midnight sun in 2004. In 2005, the midnight-sun DVM was likely impacted by a high acoustic scattering generated by suspended particles. During full moon at low cloud cover, the nighttime moonlight illuminance led to zooplankton avoidance of the subsurface layer, disrupting DVM. Moreover, DVM was disrupted by upwelling, downwelling, and eddy passing. We suggest that these deviations are consistent with DVM adjusting to avoid enhanced water dynamics. For upwelling and downwelling, zooplankton likely respond to the along-slope water dynamics dominated by surface- and depth-intensified flow, respectively. This drives zooplankton to adjust DVM by aggregating in the low or upper intermediate water layer for upwelling and downwelling, respectively. The baroclinic eddy reversed DVM below the eddy core. [ABSTRACT FROM AUTHOR]

Estimating the erosion potential of seabed sediments and the magnitude of the resulting suspended load in relation to current dynamics near the shelf break is a key issue for better understanding shelf-slope sediment transport. On the outer Mackenzie Shelf (Canadian Beaufort Sea, Arctic Ocean), a thin and discontinuous veneer of recent surficial clays overlie old glaciomarine sediments that further pinch out at the shelf edge. Gas and fluid venting is known to underlie part of sediment instability in the area, but recent mooring-based measurements also indicate that sediments near the shelf break are recurrently remobilized by strong subsurface currents. Here, we relate storms to the development of current surges that resulted in the abrupt resuspension of sediments at two locations along the shelf break. Near-bottom concentrations of suspended sediments were estimated using the acoustic backscatter of high-frequency acoustic Doppler current profilers deployed from September 2011 to September 2013 as part of the Beaufort Regional Environmental Assessment (BREA) program. Near-bottom currents near the shelf edge (140 to 150 m isobaths) were characterized by recurring episodes of elevated velocities (instantaneous speeds up to ~ 40-50 cm s -1 ) that were extensions of current surges (~ 60-80 cm s -1 ) occurring in the core of the shelfbreak jet located at ca. 90-120 m. Sudden peaks in suspended sediments (above 100 g m -3 ) corresponded closely with current surges in the near-bottom boundary layer (< 10 m) implying the local erosion of surficial sediments and the rapid advection or redeposition of the resuspended sediments. A range of apparent threshold velocities from 18 to 36 cm s -1 was calculated based on the relationship between suspended sediment concentrations and near-bottom current speeds. Two meteorological scenarios were identified to explain the current surges underlying these erosion events at the shelf edge: (1) Pacific or Arctic-born low pressure systems that propagate into the southern Beaufort Sea and are associated with intense downwelling-favorable westerly winds. These amplify the eastward-setting shelfbreak jet along the upper slope, such as in September 2012 when successive cyclones developed during the record-low sea ice extent. (2) The persistence of a high-pressure system over the Beaufort Sea that causes strong upwelling-favorable easterly winds and a strengthening of anti-cyclonic ice motion. This induces a reversal of the shelfbreak jet to the west and a subsequent eastward-directed relaxation flow when the winds fade, such as during the major ice fracturing events of January to March 2013 that were associated with strong and persistent current speeds. [ABSTRACT FROM AUTHOR]

Resolving the mechanisms that support the transfer of particulate matter across the shelf–slope interface is a key issue for the sustainable development of marine resources along continental margins. A better comprehension of shelf–slope exchanges is particularly needed in the Arctic Ocean given the intensification of human activities and rapid environmental changes in this region. Here, we use three years of physical and biogeochemical data collected with tautline moorings deployed from September 2009 to August 2012 over the slope of the Mackenzie Shelf to identify the processes that drive the lateral transport of particulate matter off the shelf. The main dataset consists of particle flux time-series collected with automated sediments traps deployed on moorings at ~80 and ~180 m depth over the mid-slope. We detected a strong vertical discrepancy in the magnitude of particulate mass fluxes that were 20–600% higher at ~180 m than at ~80 m, and up to ~1500% greater during the winter season alone. The high fluxes at ~180 m depth were linked to several sedimentation events occurring from November to May each year, which were not captured by the upper ~80 m traps. These differences corroborate previous studies that documented active transport of resuspended material near the bottom across the shelf-break and in the mid-water column over the slope. Consideration of particle fluxes along with synchronous current time-series, water column properties and meteorological data revealed that thermohaline convection and storm winds act as the main mechanisms underlying resuspension and transport processes. Their combination drives mesoscale eddy formation, downwelling flows and current surges that are characterized by moderate to high velocities (~20–80 cm s −1 ) sufficient to mobilize sediments. Turbidity near the shelf-break and particle fluxes over the slope were particularly enhanced in winter 2011 (mass fluxes up to ~2 g m −2 d −1 ) when a persistent downwelling-favorable wind regime and a large production of winter water were observed. Overall, the amount of winter water events correlated significantly ( R 2 =0.76) with the magnitude of mass fluxes collected at ~180 m. Our analysis revealed a complex pattern of mean currents over the slope facilitating instabilities, frontal structures, shear and eddying motion. Additional work is needed on erosion mechanisms in the bottom boundary layer and their relationship to regional and mesoscale circulation and eddy activity over the upper slope. [ABSTRACT FROM AUTHOR]

Arctic Ocean microbial eukaryote phytoplankton form subsurface chlorophyll maximum (SCM), where much of the annual summer production occurs. This SCM is particularly persistent in the Western Arctic Ocean, which is strongly salinity stratified. The recent loss of multiyear sea ice and increased particulate-rich river discharge in the Arctic Ocean results in a greater volume of fresher water that may displace nutrient-rich saltier waters to deeper depths and decrease light penetration in areas affected by river discharge. Here, we surveyed microbial eukaryotic assemblages in the surface waters, and within and below the SCM. In most samples, we detected the pronounced SCM that usually occurs at the interface of the upper mixed layer and Pacific Summer Water (PSW). Poorly developed SCM was seen under two conditions, one above PSW and associated with a downwelling eddy, and the second in a region influenced by the Mackenzie River plume. Four phylogenetically distinct communities were identified: surface, pronounced SCM, weak SCM and a deeper community just below the SCM. Distance-decay relationships and phylogenetic structure suggested distinct ecological processes operating within these communities. In the pronounced SCM, picophytoplanktons were prevalent and community assembly was attributed to water mass history. In contrast, environmental filtering impacted the composition of the weak SCM communities, where heterotrophic Picozoa were more numerous. These results imply that displacement of Pacific waters to greater depth and increased terrigenous input may act as a control on SCM development and result in lower net summer primary production with a more heterotroph dominated eukaryotic microbial community. [ABSTRACT FROM AUTHOR]

Given the volumetric nature of the ocean, 3D spatial modeling and interpolation could be a key to a better understanding of continuous abiotic and biotic phenomena that compose the marine ecosystem, although such techniques are rarely used and their actual performance is poorly studied. Here, we evaluate the performance of 3D spatial interpolation for five pelagic variables derived from a typical oceanographic campaign conducted in the southeastern Beaufort Sea (Canadian Arctic) in 2009. Our main objective is to evaluate and compare the performance of a deterministic interpolation method (inverse distance, IDW) and a geostatistical method (ordinary kriging, OK) with a variation of method input parameters (search neighborhood, weights) for variables with increasing complexity in terms of data anisotropy and sampling configuration. Performance of different 3D interpolation strategies is evaluated by cross-validation and a qualitative comparison of 3D spatial models. Our results show that OK was the optimal method. However, when the complexity of pelagic variables increased in terms of spatial autocorrelation and data variation, the error difference between OK and IDW was reduced. We recommend that recent advances in spatial 3D modeling tools developed primarily for geological modeling should be exploited to extend the usual interpretation of marine pelagic phenomena from a 2D to a 3D environment. [ABSTRACT FROM AUTHOR]

Calanoid copepods dominate mesozooplankton biomass in the Arctic Ocean. Variations in C content, C:N ratio and stable isotope composition (δ13C, δ15N) of Calanus hyperboreus, Calanus glacialis and Metridia longa collected from January to August 2008 in the southeast Beaufort Sea provided insights into their metabolism, feeding and reproduction. Seasonal differences in the C–prosome length relationships and C:N ratios were driven by distinct spawning strategies and changes in lipid content. Relatively high copepod biomass over the study period (2.6–9.7 g C m−2) corresponded to favorable growth conditions in 2007–2008. The mean 15N enrichment of copepods (+2.8–4.7‰) relative to particulate organic nitrogen values recorded at the ice bottom and at the chlorophyll maximum indicated a primarily herbivorous diet. In all species, δ13C and δ15N decreased markedly in April, reflecting the feeding onset on ice algae, but a rapid transition to feeding on phytoplankton occurred as a pelagic bloom was triggered by the early ice melt in May. A second decline in the δ13C and δ15N of copepods was recorded in June–July, coincident with a second increase in phytoplankton production. The two isotope depletion events in copepods were both followed by a return to high values and an increase in their C:N as a consequence of previous C fixation and nitrate limitation in phytoplankton and the likely formation of body reserves/tissue. Our results illustrate that Arctic calanoids respond quickly to any increase in primary production and can cope with changes in its nature and timing. [ABSTRACT FROM AUTHOR]

Typically, all undecomposed metazoans found in formalin-poisoned cups of sediment traps are considered to be active intruders (or 'swimmers') and are removed to prevent an overestimation of the downward particle flux. However, intact metazoans dead before entering the trap should be included in the estimation of downward flux of carbon. Arctic copepods collected in the field were killed either by formalin to mimic death by actively swimming into the formalin-poisoned cups, or by crowding or high temperature and then preserved in formalin to simulate death before entering the trap. In the crowding and heat treatments, 64% of Calanus hyperboreus and C. glacialis and 44% of Pareuchaeta glacialis differed from copepods killed by formalin in the postmortem posture of the antennules or swimming legs. These frequencies were used to estimate the contribution of passively sinking copepods (PSCs) to the particulate organic carbon (POC) flux measured by a sediment trap moored at 70 m in the Beaufort Sea (Canadian sector of the Arctic Ocean). PSCs represented only a small fraction (<5%) of the copepods collected in the trap, thus justifying the removal of swimmers from samples to avoid a massive overestimation of fluxes. Nevertheless, PSCs contributed 36% of the overall POC flux of 6.8 g C m-2 yr-1, and discarding them along with the swimmers would have resulted in a significant underestimation of downward export. Given their rich carbon content (up to 50%), PSCs could be an important food resource for pelagic and benthic scavengers. [ABSTRACT FROM AUTHOR]

Abstract: The ongoing regression of sea ice cover is expected to significantly affect the fate of organic carbon over the Arctic continental shelves. Long-term moored sediment traps were deployed in 2005–2006 in the Beaufort Sea, Northern Baffin Bay and the Laptev Sea to compare the annual variability of POC fluxes and to evaluate the factors regulating the annual cycle of carbon export over these continental shelves. Annual POC fluxes at 200m ranged from 1.6 to 5.9gCm−2 yr−1 with the highest export in Northern Baffin Bay and the lowest export over the Mackenzie Shelf in the Beaufort Sea. Each annual cycle exhibited an increase in POC export a few weeks before, during, or immediately following sea ice melt, but showed different patterns over the remainder of the cycle. Enhanced primary production, discharge of the Lena River, and resuspension events contributed to periods of elevated POC export over the Laptev Sea slope. High POC fluxes in Northern Baffin Bay reflected periods of elevated primary production in the North Water polynya. In the Beaufort Sea sediment resuspension contributed to most of the large export events. Our results suggest that the outer shelf of the Laptev Sea will likely sustain the largest increase in POC export in the next few years due to the large reduction in ice cover and the possible increase in the Lena River discharge. The large differences in forcing among the regions investigated reinforce the importance of monitoring POC fluxes in the different oceanographic regimes that characterize the Arctic shelves to assess the response of the Arctic Ocean carbon cycle to interannual variability and climate change. [Copyright &y& Elsevier]

The Arctic is warming and losing sea ice. Happening at a much faster rate than previously expected, these changes are causing multiple ecosystem feedbacks in the Arctic Ocean. The Arctic in Rapid Transition (ART) initiative was developed by early-career scientists as an integrative, international, multidisciplinary, long-term pan-Arctic network to study changes and feedbacks among the physical and biogeochemical components of the Arctic Ocean and their ultimate impacts on biological productivity on different timescales. In 2012, ART jointly organized with the Association of Polar Early Career Scientists their second science workshop—Overcoming Challenges of Observation to Model Integration in Marine Ecosystem Response to Sea Ice Transitions—at the Institute of Oceanology, Polish Academy of Sciences, in Sopot. This workshop aimed to identify linkages and feedbacks between atmosphere–ice–ocean forcing and biogeochemical processes, which are critical for ecosystem function, land–ocean interactions and productive capacity of the Arctic Ocean. This special thematic cluster of Polar Research brings together seven papers that grew out of workgroup discussions. Papers examine the climate change impacts on various ecosystem elements, providing important insights on the marine ecological and biogeochemical processes on various timescales. They also highlight priority areas for future research. [ABSTRACT FROM AUTHOR]