The MALINA oceanographic campaign was conducted during summer 2009 to investigate the carbon stocks and the processes controlling the carbon fluxes in the Mackenzie River estuary and the Beaufort ...Sea. During the campaign, an extensive suite of physical, chemical and biological variables were measured across seven shelf–basin transects (south–north) to capture the meridional gradient between the estuary and the open ocean. Key variables such as temperature, absolute salinity, radiance, irradiance, nutrient concentrations, chlorophyll a concentration, bacteria, phytoplankton and zooplankton abundance and taxonomy, and carbon stocks and fluxes were routinely measured onboard the Canadian research icebreaker CCGS Amundsen and from a barge in shallow coastal areas or for sampling within broken ice fields. Here, we present the results of a joint effort to compile and standardize the collected data sets that will facilitate their reuse in further studies of the changing Arctic Ocean. The data set is available at https://doi.org/10.17882/75345 (Massicotte et al., 2020).
The phytoplankton community of the Mackenzie shelf and the Amundsen Gulf (southeastern Beaufort Sea) was characterized (e.g. chlorophyllabiomass, primary production and taxonomy) during autumn 2002 ...(23 September to 14 October) and 2003 (30 September to 14 November). Spatial differences were evident, particularly in early autumn. Total phytoplankton biomass and the contribution of large cells (>5 μm) to biomass were higher in the Amundsen Gulf than on the Mackenzie shelf. The community of autotrophic cells (>10 μm) was numerically dominated by diatoms in the Amundsen Gulf and by dinoflagellates on the Mackenzie shelf. The abundance of chlorophytes revealed the influence of the Mackenzie River on the Mackenzie shelf. Contrary to 2002, when all measurements were from early October, the phytoplankton community of the Amundsen Gulf in 2003 presented the characteristics of a late bloom, which presumably peaked in late September. In early autumn, however, primary production rates were similar for both years, averaging 75 mg C m–2d–1. High primary production-to-biomass ratios and overall dominance of small cells (<5 μm) suggest that pelagic production in the southeastern Beaufort Sea was sustained by active recycling. During autumn 2003, a temporal decrease in phytoplankton biomass and primary production likely resulted from decreasing light availability. Overall, the autumnal primary production estimated in this study, from mid-September to the end of October, could increase the annual primary production previously estimated for the Beaufort Sea by 15%.
We examined the seasonal variability of water mass distributions in the southeastern Beaufort Sea from data collected between September 2003 and August 2004. Salinity, total alkalinity (TA) and ...isotopic composition (delta O-18) of seawater were used together as tracers of freshwater input, i.e., meteoric water and sea ice meltwater. We used an optimum multiparameter analysis to identify the different water masses, including the Mackenzie River, sea ice melt (SIM), winter polar mixed layer (PML), upper halocline water (UHW) with core salinity of 33.1 psu (Pacific origin) and Atlantic Water. Computed values of CO2 fugacity in seawater (fCO(2)-sw) show that the surface mixed layer (SML) remains mostly undersaturated (328 +/- 55 mu atm, n = 552) with respect to the average atmospheric CO2 concentration (380 +/- 5 mu atm) over the study period. The influence of the Mackenzie River (fCO(2-SW) > 500 mu atm) was relatively small in the southeastern Beaufort Sea, and significant fractions were only observed on the inner Mackenzie Shelf. The contribution of sea ice melt (fCO(2-SW) < 300 mu atm) to the SML could reach 30% beyond the shelf break and close to the ice pack in autumn. The density of the PML increased through the winter due to cooling and brine rejection. The winter PML reached a maximum depth of 70 m in late April. The UHW (fCO(2-SW) > 600 mu atm) was usually located between 120 and 180 m depth, but could contribute to the SML during wind-driven upwelling events, in summer and autumn, and during brine-driven eddies, in winter.
Spatial patterns in prokaryotic biodiversity and production were assessed in the Mackenzie shelf region of the Beaufort Sea during open-water conditions. The sampling transect extended 350 km ...northwards, from upstream freshwater sites in the Mackenzie River to coastal and offshore sites, towards the edge of the perennial arctic ice pack. The analyses revealed strong gradients in community structure and prokaryotic cell concentrations, both of which correlated with salinity. Picocyanobacterial abundance was low (10 super(2) to 10 super(3) cells ml super(- 1)), particularly at the offshore stations that were least influenced by the river plume. Analysis by catalyzed reporter deposition for fluorescence in situ hybridization (CARD-FISH) showed that the dominant heterotrophic cell types were beta -Proteobacteria at river sites, shifting to dominance by alpha- Proteobacteria offshore. Cells in the Cytophaga-Flavobacter-Bacteroides and gamma-Proteobacteria groups each contributed <5% of total counts in the river, but >10% of counts in the marine samples. Archaea were detected among the surface-water microbiota, contributing on average 1.3% of the total DAPI counts in marine samples, but 6.0% in turbid coastal and riverine waters. super(3)H- leucine uptake rates were significantly higher at 2 stations influenced by the river (1.5 pmol l super(-1) h super(-1)) than at other marine stations or in the river itself (<0.5 pmol super(-1) h super(-1)). Size-fractionation experiments at 2 coastal sites showed that >65% of heterotrophic production was associated with particles >3 mum. These results indicate the importance of particle-attached prokaryotes, and imply a broad functional diversity of heterotrophic microbes that likely facilitates breakdown of the heterogeneous dissolved and particulate terrestrial materials discharged into arctic seas.
Abstract
The near‐surface temperature structure in the southeastern Beaufort Sea is shown to have been largely dependent on frontal dynamics in spring 2004, which may be typical for the region. ...Easterly wind events generated coastal upwelling along the Cap Bathurst peninsula; a recurring event in that area. Further west, a large mesoscale anticyclone simultaneously developed and subsequently controlled the sea‐surface circulation in the central Amundsen Gulf. Sharp temperature and density fronts were created at the surface at both eastern and western ends of the domain. Sampling north of Cape Bathurst and Cape Parry showed evidence of frontal intensification. Frontal features were detected near the 50–200 m isobaths, at the mouth of the gulf, where density‐compensated near‐surface intrusions driven by agesotrophic vertical circulation were identified. These warm water tongues intruded into the outcropping isopycnal layers, which dipped down between 5 and 25 m over the Mackenzie Shelf. They then crossed the density surfaces with an inverse slope consistent with
N
/
f
as predicted for quasi‐geostrophic flows. The front event ended prior to the breakup of the landfast‐ice bridge in late June with sea‐surface temperature undergoing quick and widespread changes throughout the Amundsen Gulf.
Key Points:
A frontal event was observed in spring 2004, in the southeastern Beaufort Sea
Sampling showed evidence of frontal intensification at both ends of the domain
Vertical temperature structure will be shown to be largely dependent on fronts
Recent measurements of dissolved oxygen (DO) along the Laurentian Channel in Eastern Canada revealed the presence of hypoxic waters in the bottom 50 m of the water column. At hypoxic oxygen levels, ...many fish species cannot survive or reproduce, and the microbial life community undergoes significant modifications. The cumulative effect of a substantial sediment oxygen demand along the Lower St. Lawrence Estuary (LSLE) is proposed as the possible cause of this DO depletion. To verify this hypothesis, a laterally integrated, two-dimensional model of the DO distribution was implemented for the bottom waters of the Laurentian Channel along a transect of stations sampled in July 2003. The fluid transport was parameterized in a simple advection–diffusion finite element grid where sedimentation of organic matter (OM) feeds the processes that lead to O
2 depletion in the deep waters. Two major types of OM are considered in this study: a fast-reacting marine component that originates from autochthonous material produced in surface waters, and a more refractory terrestrial component originating from continental river discharges. To counterbalance the OM oxygen sink, the deep, landward mean circulation continuously brings O
2-rich waters from the Atlantic. Both the DO and the early diagenesis model parameters were calibrated using field data collected between 1985 and 2003. Our physical parameter sensitivity study reveals that vertical diffusion from the oxygenated upper water column has the greatest impact on deep DO concentrations. The diagenetic model reproduces the oxygen penetration depths and fluxes very well along the Gulf of St. Lawrence portion of the Channel but overestimates the sediment oxygen demand in the LSLE. We propose that the sediment oxygen demands calculated from DO gradients, measured by voltammetric micro-electrodes, across the sediment–water interface of cores retrieved in the Lower St. Lawrence Estuary are underestimated.
We examined the seasonal variability of water mass distributions in the southeastern Beaufort Sea from data collected between September 2003 and August 2004. Salinity, total alkalinity (TA) and ...isotopic composition (
δ
18
O) of seawater were used together as tracers of freshwater input, i.e., meteoric water and sea ice meltwater. We used an optimum multiparameter analysis to identify the different water masses, including the Mackenzie River, sea ice melt (SIM), winter polar mixed layer (PML), upper halocline water (UHW) with core salinity of 33.1 psu (Pacific origin) and Atlantic Water. Computed values of CO
2
fugacity in seawater (
f
CO
2
‐sw) show that the surface mixed layer (SML) remains mostly undersaturated (328 ± 55
μ
atm, n = 552) with respect to the average atmospheric CO
2
concentration (380 ± 5
μ
atm) over the study period. The influence of the Mackenzie River (
f
CO
2‐SW
> 500
μ
atm) was relatively small in the southeastern Beaufort Sea, and significant fractions were only observed on the inner Mackenzie Shelf. The contribution of sea ice melt (
f
CO
2‐SW
< 300
μ
atm) to the SML could reach 30% beyond the shelf break and close to the ice pack in autumn. The density of the PML increased through the winter due to cooling and brine rejection. The winter PML reached a maximum depth of 70 m in late April. The UHW (
f
CO
2‐SW
> 600
μ
atm) was usually located between 120 and 180 m depth, but could contribute to the SML during wind‐driven upwelling events, in summer and autumn, and during brine‐driven eddies, in winter.
Key Points
Total alkalinity and oxygen isotopes are used as tracers of freshwater inputs
Describe the use of an optimum multiparameter analyis
Assess the variability of water mass distribution during a full year study
It has been common practice in scientific studies to assume negligible phytoplankton production when the ocean is ice-covered, due to the strong light attenuation properties of snow, sea ice, and ice ...algae. Recent observations of massive under-ice blooms in the Arctic challenge this concept and call for a re-evaluation of light conditions prevailing under ice during the melt period. Using hydrographic data collected under landfast ice cover in Resolute Passage, Nunavut, Canada between 9 May and 21 June 2010, we documented the exponential growth phase of a substantial under-ice phytoplankton bloom. Numerous factors appeared to influence bloom initiation: (1) transmitted light increased with the onset of snowmelt and termination of the ice algal bloom; (2) initial phytoplankton growth resulted in the accumulation of biomass below the developing surface melt layer where nutrient concentrations were high and turbulent mixing was relatively low; and (3) melt pond formation rapidly increased light transmission, while spring-tidal energy helped form a surface mixed layer influenced by ice melt-both were believed to influence the final rapid increase in phytoplankton growth. By the end of the study, nitrate+nitrite was depleted in the upper 10 m of the water column and the under-ice bloom had accumulated 508 mg chl am super(-2) with a new production estimate of 17.5 g C m super(-2) over the upper 50 m of the water column. The timing of bloom initiation with melt onset suggests a strong link to climate change where sea ice is both thinning and melting earlier, the implication being an earlier and more ubiquitous phytoplankton bloom in Arctic ice-covered regions.
The near‐surface temperature structure in the southeastern Beaufort Sea is shown to have been largely dependent on frontal dynamics in spring 2004, which may be typical for the region. Easterly wind ...events generated coastal upwelling along the Cap Bathurst peninsula; a recurring event in that area. Further west, a large mesoscale anticyclone simultaneously developed and subsequently controlled the sea‐surface circulation in the central Amundsen Gulf. Sharp temperature and density fronts were created at the surface at both eastern and western ends of the domain. Sampling north of Cape Bathurst and Cape Parry showed evidence of frontal intensification. Frontal features were detected near the 50–200 m isobaths, at the mouth of the gulf, where density‐compensated near‐surface intrusions driven by agesotrophic vertical circulation were identified. These warm water tongues intruded into the outcropping isopycnal layers, which dipped down between 5 and 25 m over the Mackenzie Shelf. They then crossed the density surfaces with an inverse slope consistent with N/f as predicted for quasi‐geostrophic flows. The front event ended prior to the breakup of the landfast‐ice bridge in late June with sea‐surface temperature undergoing quick and widespread changes throughout the Amundsen Gulf.
Key Points:
A frontal event was observed in spring 2004, in the southeastern Beaufort Sea
Sampling showed evidence of frontal intensification at both ends of the domain
Vertical temperature structure will be shown to be largely dependent on fronts
α‐HCH (hexachlorocyclohexane) and the enantiomeric fraction (EF) of its mirror‐image isomers have been determined for water column profiles in the southern Beaufort Sea in 2004 and 2007. Using ...estimated rates of metabolic degradation, we have applied a simple kinetic model to convert the observed EFs to apparent ventilation ages of the water masses in the study region. We found an age of 1.7 ± 0.1 years for the Polar Mixed Layer (PML), 6.6 ± 0.6 for the core of the Pacific Layer centered at salinity 33.1, and 21.7 ± 0.5 years for the core of the Atlantic Layer identified by a Tmax of ∼0.5°C. These ages are in reasonable accord with other methods used to date water masses in the Arctic Ocean suggesting that α‐HCH has an unexploited potential as a dating tool.
Key Points
HCH EF can be used to estimate ventilation age of water in the Arctic Ocean
HCH EF tracer age is most accurate when applied to cores of water masses
Conceptually, EFs of many substances can provide ventilation age estimates
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