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Hudson Bay is a large seasonally ice-covered Canadian inland sea connected to the Arctic Ocean and North Atlantic through Foxe Basin and Hudson Strait. This study investigates zooplankton distribution, dynamics, and factors controlling them during open-water and ice cover periods (from September 2016 to October 2017) in Hudson Bay. A mooring equipped with two acoustic Doppler current profilers (ADCPs) and a sediment trap was deployed in September 2016 in Hudson Bay ∼190 km northeast from the port of Churchill. The backscatter intensity and vertical velocity time series showed a pattern typical for zooplankton diel vertical migration (DVM). The sediment trap collected five zooplankton taxa including two calanoid copepods (Calanus glacialis and Pseudocalanus spp.), a pelagic sea snail (Limacina helicina), a gelatinous arrow worm (Parasagitta elegans), and an amphipod (Themisto libellula). From the acquired acoustic data we observed the interaction of DVM with multiple factors including lunar light, tides, and water and sea ice dynamics. Solar illuminance was the major factor determining migration pattern, but unlike at some other polar and subpolar regions, moonlight had little effect on DVM, while tidal dynamics are important. The presented data constitute the first-ever observed DVM in Hudson Bay during winter and its interaction with the tidal dynamics. [ABSTRACT FROM AUTHOR]

Hudson Bay is a large, seasonally-ice covered Canadian inland sea, connected to the Arctic Ocean and North Atlantic through Foxe Basin and Hudson Strait. This study investigates zooplankton distribution, dynamics and factors controlling them during open water and ice cover periods (from September 2016 to October 2017) in Hudson Bay. A mooring equipped with two Acoustic Doppler Current Profilers (ADCP) and a sediment trap was deployed in September 2016 in Hudson Bay ~ 190 km north-east from the port of Churchill. The backscatter intensity and vertical velocity time series showed a pattern typical for the zooplankton diel vertical migration (DVM). Zooplankton collected by the sediment trap allowed for the identification of migrating scatters during the study period. From the acquired acoustic data we observed the interaction of DVM with multiple factors including lunar light, tides, as well as water and sea ice dynamics. Solar illuminance was the major factor determining migration pattern, but unlike at some other polar and sub-polar regions, moonlight had a little effect on DVM, while tidal dynamics is important. The presented data constitutes a first-ever observed presence of DVM in Hudson Bay during winter as well as its interaction with the tidal dynamics. [ABSTRACT FROM AUTHOR]

Highlights: [•] Rock-Eval pyrolysis was used to characterize recent marine sediment from Hudson Bay. [•] Marine vs. terrigenous and fresh vs. degraded organic matter patterns were observed. [•] These patterns fit those inferred from lignin and δ13C data. [•] Sediment recycling, high Mn, and grain size were related to oxidized organic matter. [•] Hydrogen and oxygen indices and δ13C fully characterize Hudson Bay sediment. [ABSTRACT FROM AUTHOR]

Abstract: Distributions of sea-ice melt (SIM) and runoff (RW) were determined in Hudson Bay (Canada) using salinity and oxygen isotope (δ18O) tracers. In late summer/early fall, stations throughout the Bay exhibited a summer surface mixed layer (SSML) 30–60m deep, which contained the seasonal freshwater inputs. The SSML overlies a cold subsurface water, extending up to 125m deep in the water column, indicative of the previous winter''s surface mixed layer (WSML). Our data show that most of the RW remains in a nearshore coastal regime in summer, with limited exchange into the interior of the Bay. At depth, brine (negative SIM) accumulation is closely associated with excess RW, implying that deep water formation occurs when sufficient brine is rejected from the growing ice to overcome RW buoyancy in surface water. The inventory of RW at depths below the WSML is equivalent to almost one year''s runoff inputs which, given a residence time of deep water of 4–14years, implies that 6–16% of the annual river discharge is shunted into deeper waters each winter. The balance between the RW buoyancy in surface waters and densification due to ice formation appears to control where and when deep-water formation can occur. Although not directly measured, regions forming deep water are likely small (<10% of the Bay area) and associated with flaw leads extending around the perimeter of the landfast ice edge in the Bay. The processes evident in δ18O and salinity data must be incorporated into ocean models to obtain a realistic view of freshwater cycling and its sensitivity to climate perturbations. [Copyright &y& Elsevier]

Abstract: The remineralization of organic matter (OM) in Hudson Bay was examined using redox element (Mn, Fe, S, Cd, Re, Mo and U) profiles in six sediment cores. Variation in thickness of surface Mn enrichments and increases in Fe and S in subsurface sediments provide a means to sort sediment cores according to metabolic intensity. The data imply a relatively high flux of labile OM in Hudson Strait, weaker labile OC fluxes in inshore regions of Hudson Bay and weakest fluxes in central Hudson Bay. Enrichments of Cd, U, and Re, which precipitate in reducing sediments, are present at depth in all sediments, whereas Mo enrichments occur only in the more strongly reducing sediments. Highest rates of authigenic element accumulation in the inshore regions generally reflect the higher flux of labile OM inferred from Mn profiles. After accounting for variation in water depth, we find Hudson Bay to be the most oligotrophic among ocean margins for which there are comparable data. Redox element profiles in sediments provide a way to compare and thus possibly monitor change in labile OC forcing over different continental margins, including those in Arctic seas, where other monitoring methods are scarce. [Copyright &y& Elsevier]

PCB concentrations, congener patterns, and fluxes were examined in 13 dated and organically characterized (C, N, δ13C, δ15N) marine sediment cores from Hudson Bay, Canada, to investigate the importance of organic matter (OM) supply and transport to PCB sequestration. Drawdown of PCBs, supported by marine primary production, is reflected in elevated ΣPCB concentrations and more highly chlorinated PCB signatures in surface sediments underlying eutrophic regions. Sediments in oligotrophic regions, which are dominated by "old" marine OM, have lower PCB concentrations and weathered signatures. For the surface of Hudson Bay, average atmospheric deposition appears to be very low (ca. 1.4 pg ΣPCBs cm-2 a-1) compared to fluxes reported for nearby lakes (ca. 44 pg ΣPCBs cm-2 a-1). 210Pb fails to provide a means to normalize the fluxes, highlighting important differences in the biocycling of 210Pb and PCBs. Unlike 210Pb, atmospheric PCB exchange with the water's surface is partially forced by the aquatic organic carbon cycle. The extremely low atmospheric deposition of PCBs to the surface of Hudson Bay is likely a reflection of the Bay's exceptionally low productivity and vertical carbon fluxes. If future marine production and vertical flux of carbon increase due to loss of ice cover or change in river input as consequences of global warming, PCB deposition would also increase. [ABSTRACT FROM AUTHOR]

Abstract: Elemental (carbon and nitrogen) ratios and stable carbon and nitrogen isotope ratios (δ13C and δ15N) are examined in sediments and suspended particulate matter from Hudson Bay to study the influence of river inputs and autochthonous production on organic matter distribution. River-derived particulate organic matter (POM) is heterogeneous, nitrogen-poor and isotopically depleted, consistent with expectations for OM derived from terrestrial C3 vascular plant sources, and distinct from marine OM sources. Both δ13C and C/N source signatures seem to be transmitted to sediments with little or no modification, therefore making good tracers for terrigenous OM in Hudson Bay. They suggest progressively larger contributions from marine sources with distance from shore and secondarily from south to north, which broadly corresponds to the distribution of river inputs to Hudson Bay. Processes other than mixing of marine and terrigenous OM influence sedimentary δ15N values, including variability in the δ15N of phytoplankton in the Bay''s surface waters due to differences in relative nitrate utilization, and post-production processes, which bring about an apparently constant 15N-enrichment between surface waters and underlying sediments. Variability in the δ15N of phytoplankton in the Bay''s surface waters, in contrast, seems to be organized spatially with a pattern that suggests an inshore–offshore difference in surface water nitrogen conditions (open- vs. closed-system) and hence the δ15N value of phytoplankton. The δ15N patterns, supported by a simple nitrate box-model budget, suggest that in inshore regions of Hudson Bay, upwelling of deep, nutrient-rich waters replenishes surface nitrate, resulting in ‘open system’ conditions which tend to maintain nitrate δ15N at low and constant values, and these values are reflected in the sinking detritus. River inflow, which is constrained to inshore regions of Hudson Bay, appears to be a relatively minor source of nitrate compared to upwelling of deep waters. However, river inflow may contribute indirectly to enhanced inshore nutrient supply by supporting large-scale estuarine circulation and consequently entrainment and upwelling of deep water in this area. In contrast to previous proposals that Hudson Bay is oligotrophic because it receives too much fresh water (), our results support most of the primary production being organized around the margin of the Bay, where river flow is constrained. [Copyright &y& Elsevier]

Abstract: A preliminary sediment and organic carbon budget is constructed for the Hudson Bay marine system based on published literature and new data collected from nine rivers and 13 sediment boxcores. The budget considers the main inputs of terrestrial and marine components of sediment and organic carbon and the main sinks (sediment burial) and losses (oxidation, export to Hudson Strait). Sedimentation rates (0.032–0.23 g cm−2 yr−1) were estimated from 210Pb profiles and a sediment advection–diffusion model in our cores and in two cores collected previously by Lockhart et al. [Lockhart, W.L. et al., 1998. Fluxes of mercury to lake sediments in central and northern Canada inferred from dated sediment cores. Biogeochemistry 40, 163–173]. Sedimentation rates implied by the 210Pb profiles were verified against profiles of 137Cs and, for some of the cores, contaminant Pb. The sediment sink in Hudson Bay was estimated at 138 (±64)×106 t a−1 by applying the average sedimentation rate to the area of active sedimentation in the Bay (~15% of the seafloor), calculated according to a regional sedimentation map developed from seismic data [Josenhans, H.W. et al., 1988. Preliminary seismostratigraphic and geomorphic interpretations of the Quaternary sediments of Hudson Bay, Natural Resources Canada]. The known sediment sources to Hudson Bay include river input (11.6 (±6)×106 t a−1), subaerial coastal erosion (8 (±4)×106 t a−1), marine primary production (24.2 (±17)×106 t a−1), and atmospheric deposition (0.5×106 t a−1). Based on the available data, the modern sediment input (44.3 (±18)×106 t a−1) represents only about one-third of the apparent sediment sink in Hudson Bay. Resuspension and lateral transport of shallow-water deposits (primarily glacigenic) and winnowing from topographic highs, likely consequences of the exceptional, continuous relative sea-level (RSL) fall in Hudson Bay, provides a plausible source for the ~116×106 t a−1 imbalance. Using organic carbon and δ13C values in the cores, we estimate the burial of particulate organic carbon (POC) in sediments at 1.3 (±0.6)×106 t C a−1, of which ~80% is marine. The importance of reworked glaciomarine carbon to the burial flux is difficult to estimate because modern (autochthonous) primary production provides a large source of POC to the system (16.1 (±7)×106 t C a−1) and its degradation is not well-constrained. The 0.58×106 t C a−1 terrestrial carbon, estimated to be transported as part of the redistributed sediment load, accounts for almost one-half the total terrigenous POC inputs to the Bay. If sediment and OC supply and burial in the Hudson Bay system are presently responding to the history of isostatic rebound, as proposed here, the task of predicting and measuring the additional consequences of river diversions and climate change (e.g., sea ice, permafrost, runoff) is rendered considerably more difficult. [Copyright &y& Elsevier]

Abstract: Hudson Bay is a large, estuarine, shelf-like sea at the southern margin of the Arctic, where changes in seasonal ice cover and river discharge appear already to be underway. Here we present lignin data for dated sediments from eleven box cores and evaluate sources of terrigenous carbon, transport pathways, and whether terrigenous organic matter has been influenced by recent environmental change. Lignin yields (0.04 to 1.46 mg/100 mg organic carbon) decreased from the margin to the interior and from south to north, broadly reflecting the distribution of river inputs. Lignin compositional patterns indicated distinct regional sources with boreal forest (woody gymnosperm) vegetation an important source in the south, vs. tundra (non-woody angiosperm) in the north. Lignin patterns suggest redistribution of a fine-grained, mineral-associated fraction of the southern-derived terrigenous carbon to the northeast part of the Bay and ultimately into west Hudson Strait with the Bay''s cyclonic coastal circulation. A small component of the carbon makes it to the central basins of Hudson Bay but most of the terrigenous organic material in that area appears to derive from resuspension of older, isostatically-rebounding coastal and inner shelf deposits. Most modern plant debris appears to be retained near river mouths due to hydrodynamic sorting, with the exception of the southwest inner shelf, where these materials extend >30 km from shore. Temporal changes in the composition of terrigenous organic carbon recorded in most of the southern Hudson Bay cores perhaps reflect increases in erosion and cross-shelf transport from coastal deposits, possibly mediated by change in ice climate. In contrast, temporal changes in the northwest may relate to changes in the supply of modern plant debris under recent warmer conditions. On the western shelf, changes may relate to ice climate and the distribution of northern coastal water and/or changes in the delivery of materials by the Churchill River due to water diversion. Although the cores show evidence of change related to the ice climate, there is little evidence that ice itself transports terrigenous organic carbon within the system. [Copyright &y& Elsevier]

• Terrestrial carbon remineralization reduces aragonite-saturation and increases pCO 2. • Export production and terrestrial CaCO 3 dissolution offset remineralization effects. • Changes in marine pCO 2 and Ω Ar depend on watershed geology and organic matter C:N. Recent research has focused on the changing ability of oceans to absorb atmospheric CO 2 and the consequences for ocean acidification, with Arctic shelf seas being among the most sensitive regions. Hudson Bay is a large shelf sea in northern Canada whose location at the margin of the cryosphere places it in the vanguard of global climate change. Here, we develop a four-compartment box-model and carbon budget using published and recently collected measurements to estimate carbon inputs, transformations, and losses within Hudson Bay. We estimate the annual effects of terrestrial carbon remineralization on aragonite saturation (Ω Ar , a proxy for ocean acidification) and on the partial pressure of CO 2 (pCO 2 , a proxy for air-sea CO 2 flux) within each compartment, as well as the effects of marine primary production, marine organic carbon remineralization, and terrestrial calcium carbonate dissolution. We find that the remineralization of terrestrial dissolved organic carbon is the main driver of CO 2 accumulation and aragonite under-saturation in coastal surface waters, but this is largely offset by marine primary production. Below the surface mixed layer, marine organic carbon remineralization is the largest contributor to CO 2 accumulation and aragonite under-saturation, and is partially offset by terrestrial CaCO 3 dissolution. Overall, the annual delivery and processing of carbon reduces Ω Ar of water flowing through HB by up to 0.17 units and raises pCO 2 by up to 165 µatm. The similarities between Hudson Bay and other Arctic shelf seas suggest these areas are also significantly influenced by terrestrial carbon inputs and transformation. [ABSTRACT FROM AUTHOR]

Twelve marine sediment cores from Hudson Bay, Canada, were collected to investigate the response of sub-Arctic marine sediments to atmospherically transported anthropogenic mercury (Hg). Modeling by a two-layer sediment mixing model suggests that the historical Hg deposition to most of the sediment cores reflects the known history of atmospheric Hg deposition in North America, with an onset of increasing anthropogenic Hg emissions in the late 1800s and early 1900s and a reduction of Hg deposition in the mid- to late-1900s. However, although anthropogenic Hg has contributed to a ubiquitous increase in Hg concentrations in sediments over the industrial era, the most elevated industrial-era sedimentary Hg concentrations only marginally exceed the upper preindustrial sedimentary Hg concentrations. Analysis of δ13C and relationship between Hg and organic matter capture suggests that the response of Hudson Bay sediments to changes in atmospheric Hg emissions is largely controlled by the particle flux in the system and that natural changes in organic matter composition and dynamics can cause variation in sedimentary Hg concentrations at least to the same extent as those caused by increasing anthropogenic Hg emissions. [ABSTRACT FROM AUTHOR]

Abstract The location of the amphidromic point of the M2 tide in Hudson Bay roughly coincides with Belcher Islands, a region where the surface mixed layer stays relatively fresh throughout summer and winter due to significant ice melt and river discharge. High-resolution satellite radar imagery for the ice-free season revealed that the coastal region in the south-east Belcher Islands is a hot spot for short-period internal wave activity. For a first investigation of tidal dynamics in the region, we took advantage of the sea ice platform to deploy an ice-tethered mooring consisting of nine conductivity and temperature sensors and an acoustic Doppler current profiler. The mooring was deployed at 65 m depth in January–March 2014 in a narrow channel between Broomfield and O′Leary islands located in the south-east tip of the Belcher Islands group in Hudson Bay (56°20′N, 79°30′W), northeast Canada. The surface mixed layer under the land-fast ice in this area stays relatively fresh through winter presumably because of significant winter river discharge in nearby James Bay. The mooring recorded oscillations of temperature and salinity throughout the whole water column, which were attributed to vertical displacement caused by internal tidal waves. The tidal harmonic analysis performed for the M 2 tidal constituent showed the dominance of the baroclinic tide with maximum velocity amplitudes at the surface and decreasing with depth. Vertical displacements of water parcels derived from both temperature and salinity were statistically similar and displayed the maximum values of 11.9 m at 35 m (instrument depth). The combination of winter hydrographic data and summer satellite observations confirmed that the observed internal waves were generated by the interaction of strong tides, typical for Hudson Bay, with highly variable bottom topography south-east of the Belcher Islands archipelago. Highlights • South-east Hudson Bay is a hot spot for internal waves. • An ice-tethered mooring recorded the pattern characteristic of internal tidal waves. • SAR imagery revealed short-period internal waves during the ice-free season. [ABSTRACT FROM AUTHOR]

Surface sediment samples from the Hudson Bay system were analysed in order to examine the role of key regulators of arctic marine productivity — light and nutrients as affected by freshwater stratification and sea-ice cover — on the spatial distribution and production of dinoflagellate cysts. Total cyst fluxes vary from 0.2×106 to 30.6×106 cysts m−2 a−1, with the highest values observed in eastern Hudson Bay. A total of 24 cyst taxa, representing 11 genera of five orders, were identified and distribution maps of the most common taxa have been produced. This is the first record of Echinidinium aculeatum, Echinidinium karaense, cf. Echinidinium delicatum, Islandinium brevispinosum, Selenopemphix quanta, cysts of Protoperidinium americanum, cysts of cf. Biecheleria sp. and Polarella glacialis in the Hudson Bay system. Dinoflagellate cyst assemblages show distinct spatial patterns revealing three compositional domains: eastern Hudson Bay, western-central Hudson Bay and Hudson Strait. The eastern domain is characterised by a dominance of autotrophic cysts of Pentapharsodinium dalei whereas the western-central domain is characterised by autotrophic Operculodinium centrocarpum with some contribution by heterotrophic Polykrikos sp. var. arctic morphotype and Polykrikos spp. Sites from Hudson Strait are distinguished by an overwhelming prevalence of heterotrophic Protoperidiniaceae cysts, mainly Islandinium minutum, and have the highest values of sedimentary biogenic silica, used as a proxy for diatom productivity. Sediment geochemical tracers are used as proxies for freshwater inputs (lignin and its biomarkers) and nitrate availability (nitrogen isotopes), and sea-ice concentrations derived from passive microwave data as a proxy for light availability. Sea-ice regulated length of the dark season has a negligible influence on the proportion and production of heterotrophic (dark-adapted) versus autotrophic (light-dependent) dinoflagellate cysts, perhaps due to the location of our study area on the southern fringe of the Arctic. Instead, cyst populations in Hudson Bay are primarily regulated by vertical stratification and nitrate availability, while in Hudson Strait the pivotal mechanism constitutes food availability. [ABSTRACT FROM AUTHOR]

Based on extensive sampling of the rivers, troposphere, seawater and sediments, mercury (Hg) mass budgets are constructed for both contemporary and preindustrial times in the Hudson Bay Marine System (HBS) to probe sources and pathways of Hg and their responses to the projected climate change. The contemporary total Hg inventory in the HBS is estimated to be 98 t, about 1% of which is present in the biotic systems and the remainder in the abiotic systems. The total contemporary Hg influx and outflux, around 6.3 t/yr each, represent a 2-fold increase from the preindustrial fluxes. The most notable changes are in the atmospheric flux, which has gone from a nearly neutral (0.1 t/yr) to source term (1.5 t/yr), increased river inputs (which may also reflect increased atmospheric deposition to the HBS watershed) and in the sedimentary burial flux which has increased by 2.4 t/yr over preindustrial values, implying that much of the modern Hg loading entering this system is buried in the sediments. The capacity to drive increased Hg loading from the atmosphere to sediment burial may be supported by the resuspension of an extraordinarily large flux (120 Mt/yr) of shallow water glacigenic sediments uncontaminated by anthropogenic Hg, which could scavenge Hg from the water column before being transported to the deeper accumulative basins. Under the projected climate warming in the region, the rate of the sediment recycling pump will likely increase due to enhanced Hg scavenging by increasing biological productivity, and thus strengthen atmosphere–ocean Hg exchanges in the HBS. [Copyright &y& Elsevier]

Hudson Bay (Canada) is the world's largest inland sea, which receives upward of ~700 km3 of river discharge annually. Cyclonic water circulation transports this riverine water along the coast toward Hudson Strait and into the Labrador Sea. Yearlong observations of the current velocity profile, collected from an array of oceanographic moorings deployed in western Hudson Bay from September 2016 to September–October 2017, show that cyclonic wind forcing amplifies cyclonic water circulation in Hudson Bay and favours cross-shelf exchange. Cyclonic storms generate synoptic variability of salinity in the coastal regions, which remains poorly understood. Here we use temperature, salinity and Colored Dissolved Organic Matter (CDOM) fluorescence data from the same mooring array to examine the role of atmospheric forcing on variability of temperature and salinity in western Hudson Bay. We find that in terms of the storm-driven variability, the Nelson River estuary is impacted by river runoff year round because the Nelson River flow is regulated. Our data show that cyclonic storms intensify outflow from the Nelson River estuary. This water is also high in CDOM originating from terrestrial organic matter. Thus, the storm-driven cross-shelf displacement of the estuarial water generates a negative correlation between salinity and CDOM. The upstream Hudson Bay area ~250 km northwest of the Nelson River estuary is found to be marine dominated. In this area, cyclonic storms force the surface marine water onshore, generating storm surge along the coast. During summer, the offshore water is saline and higher in CDOM than the inshore water presumably because of a marine DOM component. Thus, the storm-driven onshore displacement of this saline (marine dominated) water generates positive anomalies of salinity and CDOM, resulting in positive correlation between salinity and CDOM during summer-fall. In contrast, during winter the onshore water becomes saltier than the offshore water due to the southward alongshore transport of saline water generated in the upstream Northwest Hudson Bay polynya. The onshore displacement of water from the Hudson Bay interior during winter cyclonic storms generates negative salinity anomalies recorded by our mooring. Since there is no marine primary production during winter, the CDOM is not impacted significantly by winter cyclonic wind forcing. • We used data from oceanographic moorings deployed in western Hudson Bay in 2016–17. • Cyclonic storms facilitate cross-shelf exchange in western Hudson Bay. • They force the surface water onshore, generating storm surge along the coast. • This intensifies outflow of the riverine water from the Nelson River estuary. [ABSTRACT FROM AUTHOR]