Mackenzie Shelf Research Articles

Synoptic evaluation of carbon cycling in the Beaufort Sea during summer: contrasting river inputs, ecosystem metabolism and air–sea CO<sub>2</sub> fluxes

Abstract. The accelerated decline in Arctic sea ice and an ongoing trend toward more energetic atmospheric and oceanic forcings are modifying carbon cycling in the Arctic Ocean. A critical issue is to understand how net community production (NCP; the balance between gross primary production and community respiration) responds to changes and modulates air–sea CO2 fluxes. Using data collected as part of the ArcticNet–Malina 2009 expedition in the southeastern Beaufort Sea (Arctic Ocean), we synthesize information on sea ice, wind, river, water column properties, metabolism of the planktonic food web, organic carbon fluxes and pools, as well as air–sea CO2 exchange, with the aim of documenting the ecosystem response to environmental changes. Data were analyzed to develop a non-steady-state carbon budget and an assessment of NCP against air–sea CO2 fluxes. During the field campaign, the mean wind field was a mild upwelling-favorable wind (~ 5 km h−1) from the NE. A decaying ice cover (< 80% concentration) was observed beyond the shelf, the latter being fully exposed to the atmosphere. We detected some areas where the surface mixed layer was net autotrophic owing to high rates of primary production (PP), but the ecosystem was overall net heterotrophic. The region acted nonetheless as a sink for atmospheric CO2, with an uptake rate of −2.0 ± 3.3 mmol C m−2 d−1 (mean ± standard deviation associated with spatial variability). We attribute this discrepancy to (1) elevated PP rates (> 600 mg C m−2 d−1) over the shelf prior to our survey, (2) freshwater dilution by river runoff and ice melt, and (3) the presence of cold surface waters offshore. Only the Mackenzie River delta and localized shelf areas directly affected by upwelling were identified as substantial sources of CO2 to the atmosphere (> 10 mmol C m−2 d−1). Daily PP rates were generally < 100 mg C m−2 d−1 and cumulated to a total PP of ~ 437.6 × 103 t C for the region over a 35-day period. This amount was about twice the organic carbon delivery by river inputs (~ 241.2 × 103 t C). Subsurface PP represented 37.4% of total PP for the whole area and as much as ~ 72.0% seaward of the shelf break. In the upper 100 m, bacteria dominated (54%) total community respiration (~ 250 mg C m−2 d−1), whereas protozoans, metazoans, and benthos, contributed to 24, 10, and 12%, respectively. The range of production-to-biomass ratios of bacteria was wide (1–27% d−1), while we estimated a narrower range for protozoans (6–11% d−1) and metazoans (1–3% d−1). Over the shelf, benthic biomass was twofold (~ 5.9 g C m−2) the biomass of pelagic heterotrophs (~ 2.4 g C m−2), in accord with high vertical carbon fluxes on the shelf (956 ± 129 mg C m−2 d−1). Threshold PP (PP at which NCP becomes positive) in the surface layer oscillated from 20 to 152 mg C m−2 d−1, with a pattern from low-to-high values as the distance from the Mackenzie River decreased. We conclude that (1) climate change is exacerbating the already extreme biological gradient across the Beaufort shelf–basin system; (2) the Mackenzie Shelf acts as a weak sink for atmospheric CO2, suggesting that PP might exceed the respiration of terrigenous and marine organic matter in the surface layer; and (3) shelf break upwelling can transfer CO2 to the atmosphere, but CO2 outgassing can be attenuated if nutrients brought also by upwelling support diatom production. Our study underscores that cross-shelf exchange of waters, nutrients and particles is a key mechanism that needs to be properly monitored as the Arctic transits to a new state.

Pre-winter distribution and habitat characteristics of polar cod (Boreogadus saida) in southeastern Beaufort Sea

Polar cod was shown to form dense under-ice winter aggregations at depth in the Amundsen Gulf (southeastern Beaufort Sea). In this paper, we verify the premises of the aggregation mechanism by determining the distribution and habitat characteristics of polar cod prior to the formation of winter aggregations. Multifrequency split-beam acoustic data collected in October–November 2003 revealed that polar cod split into two distinct layers. Age-0 polar cod formed an epipelagic layer between 0 and ~60 m depth without any clear large-scale biomass trend. In contrast, adult polar cod tended to distribute into an offshore mesopelagic layer between ~200 and 400 m that shoaled into a denser (1–37 g m−2) benthopelagic layer on sloping bottoms (between 150 and 600-m isobaths) along the Mackenzie shelf and into the Amundsen Gulf basin. Concentrations peaked in the Amundsen Gulf where estimated total biomass reached ~250 kt. Both age-0 and adult polar cod distributed in the warmer waters (>−1.4 °C). We hypothesise that polar cod concentration over slopes is governed by the combined actions of (1) local currents concentrating both depth-keeping zooplankton and polar cod at the shelf-break and basin slopes and (2) trophic association with these predictable topographically trapped aggregations of zooplankton prey. During freeze-up, these slope concentrations of polar cod are thought to constitute the main source of the observed dense under-ice winter aggregations. The hypothesis of active short-distance displacements combined with prevailing mean currents is retained as the likely aggregation mechanism.

Are Hotspots Always Hotspots? The Relationship between Diversity, Resource and Ecosystem Functions in the Arctic

The diversity-ecosystem function relationship is an important topic in ecology but has not received much attention in Arctic environments, and has rarely been tested for its stability in time. We studied the temporal variability of benthic ecosystem functioning at hotspots (sites with high benthic boundary fluxes) and coldspots (sites with lower fluxes) across two years in the Canadian Arctic. Benthic remineralisation function was measured as fluxes of oxygen, silicic acid, phosphate, nitrate and nitrite at the sediment-water interface. In addition we determined sediment pigment concentration and taxonomic and functional macrobenthic diversity. To separate temporal from spatial variability, we sampled the same nine sites from the Mackenzie Shelf to Baffin Bay during the same season (summer or fall) in 2008 and 2009. We observed that temporal variability of benthic remineralisation function at hotspots is higher than at coldspots and that taxonomic and functional macrobenthic diversity did not change significantly between years. Temporal variability of food availability (i.e., sediment surface pigment concentration) seemed higher at coldspot than at hotspot areas. Sediment chlorophyll a (Chl a) concentration, taxonomic richness, total abundance, water depth and abundance of the largest gallery-burrowing polychaete Lumbrineristetraura together explained 42% of the total variation in fluxes. Food supply proxies (i.e., sediment Chl a and depth) split hot- from coldspot stations and explained variation on the axis of temporal variability, and macrofaunal community parameters explained variation mostly along the axis separating eastern from western sites with hot- or coldspot regimes. We conclude that variability in benthic remineralisation function, food supply and diversity will react to climate change on different time scales, and that their interactive effects may hide the detection of progressive change, particularly at hotspots. Time-series of benthic functions and its related parameters should be conducted at both hot- and coldspots to produce reliable predictive models.

Nutrient dynamics in the western Canadian Arctic. II. Estimates of new and regenerated production over the Mackenzie Shelf and Cape Bathurst Polynya

Uptake of 15 N-labelled nitrate, ammonium, and urea was measured over a quasi-annual cycle in the Cape Bathurst Polynya in the Amundsen Gulf and on the Mackenzie Shelf, during the Canadian Arctic Shelf Exchange Study (CASES) in 2003 and 2004. Before the phytoplankton bloom and in autumn, nitrogen uptake was slow, representing less than 5% of annual consumption. Uptake rates increased exponentially after ice retreat and within 3 wk reached a maximum of 38.6 mmol N m ―2 d ―1 . During spring, NO 3 ― uptake supported new production of 166 mmol N m ―2 and f-ratios rose from 0.1―0.2 to 0.6―0.9. Filter fractionation showed that GF/F filters retained 93.1 ± 1.3% of the 15 N incorporated into particulate matter, suggesting that phytoplankton were responsible for the majority of the N uptake. Although free-living bacteria took up relatively more 15 N in autumn and in the lower part of the euphotic zone than phytoplankton, their assimilation of inorganic N had little effect on water column integrated f-ratios or new production. Urea supplied almost half the N assimilated by phytoplankton annually and about 80 % of the regenerated production during the spring bloom. Total new production, estimated from water column integrated 15 N-nitrogen uptake rates and linear models that interpolated rates over unsampled periods, was 342―415 mmol N m ―2 yr ―1 . Total annual N production for the region was 1.24-1.48 mol N m ―2 yr ―1 .

Ecosystem function and particle flux dynamics across the Mackenzie Shelf (Beaufort Sea, Arctic Ocean): an integrative analysis of spatial variability and biophysical forcings

Abstract. A better understanding of how environmental changes affect organic matter fluxes in Arctic marine ecosystems is sorely needed. Here we combine mooring times series, ship-based measurements and remote sensing to assess the variability and forcing factors of vertical fluxes of particulate organic carbon (POC) across the Mackenzie Shelf in 2009. We developed a geospatial model of these fluxes to proceed to an integrative analysis of their determinants in summer. Flux data were obtained with sediment traps moored around 125 m and via a regional empirical algorithm applied to particle size distributions (17 classes from 0.08–4.2 mm) measured by an Underwater Vision Profiler 5. The low fractal dimension (i.e., porous, fluffy particles) derived from the algorithm (1.26 ± 0.34) and the dominance (~ 77%) of rapidly sinking small aggregates (< 0.5 mm) in total fluxes suggested that settling material was the product of recent aggregation processes between marine detritus, gel-like substances, and ballast minerals. Modeled settling velocity of small and large aggregates was, respectively, higher and lower than in previous studies within which a high fractal dimension (i.e., more compact particles) was consequential of deep-trap collection (~400–1300 m). Redundancy analyses and forward selection of abiotic/biotic parameters, linear trends, and spatial structures (i.e., principal coordinates of neighbor matrices, PCNM) were conducted to partition the variation of the 17 POC flux size classes. Flux variability was explained at 69.5% by the addition of a temporal trend, 7 significant PCNM, and 9 biophysical variables. The first PCNM canonical axis (44.5% of spatial variance) reflected the total magnitude of POC fluxes through a shelf-basin gradient controlled by bottom depth and sea ice concentration (p < 0.01). The second most important spatial structure (5.0%) corresponded to areas where shelf break upwelling is known to occur under easterlies and where phytoplankton was dominated by diatoms. Among biophysical parameters, bacterial production and northeasterly wind (upwelling-favorable) were the two strongest corollaries of POC fluxes (r2 cum. = 0.37). Bacteria were correlated with small aggregates, while northeasterly wind was associated with large size classes (> 1 mm ESD), but these two factors were weakly related with each other. Copepod biomass was overall negatively correlated (p < 0.05) with vertical POC fluxes, implying that metazoans acted as regulators of export fluxes, even if their role was minor given that our study spanned the onset of diapause. Our results demonstrate that on interior Arctic shelves where productivity is low in mid-summer, localized upwelling zones (nutrient enrichment) may result in the formation of large filamentous phytoaggregates that are not substantially retained by copepod and bacterial communities.

Carbon sources in suspended particles and surface sediments from the Beaufort Sea revealed by molecular lipid biomarkers and compound-specific isotope analysis

Abstract. Molecular lipid biomarkers (hydrocarbons, alcohols, sterols and fatty acids) and compound-specific isotope analysis of suspended particulate organic matter (SPM) and surface sediments of the Mackenzie Shelf and slope (southeast Beaufort Sea, Arctic Ocean) were studied in summer 2009. The concentrations of the molecular lipid markers, characteristic of known organic matter sources, were grouped and used as proxies to evaluate the relative importance of fresh algal, detrital algal, fossil, C3 terrestrial plants, bacterial and zooplankton material in the organic matter (OM) of this area. Fossil and detrital algal contributions were the major fractions of the freshwater SPM from the Mackenzie River with ~34% each of the total molecular biomarkers. Fresh algal, C3 terrestrial, bacterial and zooplanktonic components represented much lower percentages, 17, 10, 4 and <1%, respectively. In marine SPM from the Mackenzie slope, the major contributions were fresh and detrital algal components (>80%), with a minor contribution of fossil and C3 terrestrial biomarkers. Characterization of the sediments revealed a major sink of refractory algal material mixed with some fresh algal material, fossil hydrocarbons and a small input of C3 terrestrial sources. In particular, the sediments from the shelf and at the mouth of the Amundsen Gulf presented the highest contribution of detrital algal material (60–75%), whereas those from the slope contained the highest proportion of fossil (40%) and C3 terrestrial plant material (10%). Overall, considering that the detrital algal material is marine derived, autochthonous sources contributed more than allochthonous sources to the OM lipid pool. Using the ratio of an allochthonous biomarker (normalized to total organic carbon, TOC) found in the sediments to those measured at the river mouth water, we estimated that the fraction of terrestrial material preserved in the sediments accounted for 30–40% of the total carbon in the inner shelf sediments, 17% in the outer shelf and Amundsen Gulf and up to 25% in the slope sediments. These estimates are low compared to other studies conducted 5–20 yr earlier, and they support the increase in primary production during the last decade mainly because of the increase in the number of ice-free days and due to the strength and persistence of winds favouring upwelling.

Photoproduction of ammonium in the southeastern Beaufort Sea and its biogeochemical implications

Abstract. Photochemistry of dissolved organic matter (DOM) plays an important role in marine biogeochemical cycles, including the regeneration of inorganic nutrients. DOM photochemistry affects nitrogen cycling by converting bio-refractory dissolved organic nitrogen to labile inorganic nitrogen, mainly ammonium (NH4+). During the August 2009 Mackenzie Light and Carbon (MALINA) Program, the absorbed photon-based efficiency spectra of NH4+ photoproduction (i.e. photoammonification) were determined using water samples from the SE Beaufort Sea, including the Mackenzie River estuary, shelf, and Canada Basin. The photoammonification efficiency decreased with increasing wavelength across the ultraviolet and visible regimes and was higher in offshore waters than in shelf and estuarine waters. The efficiency was positively correlated with the molar nitrogen:carbon ratio of DOM and negatively correlated with the absorption coefficient of chromophoric DOM (CDOM). Combined with collateral measurements of CO2 and CO photoproduction, this study revealed a stoichiometry of DOM photochemistry with a CO2 : CO : NH4+ molar ratio of 165 : 11 : 1 in the estuary, 60 : 3 : 1 on the shelf, and 18 : 2 : 1 in the Canada Basin. The NH4+ efficiency spectra, along with solar photon fluxes, CDOM absorption coefficients and sea ice concentrations, were used to model the monthly surface and depth-integrated photoammonification rates in 2009. The summertime (June–August) rates at the surface reached 6.6 nmol l−1 d−1 on the Mackenzie Shelf and 3.7 nmol l−1 d−1 further offshore; the depth-integrated rates were correspondingly 8.8 μmol m−2 d−1 and 11.3 μmol m−2 d−1. The offshore depth-integrated rate in August (8.0 μmol m−2 d−1) was comparable to the missing dissolved inorganic nitrogen (DIN) source required to support the observed primary production in the upper 10-m layer of that area. The yearly NH4+ photoproduction in the entire study area was estimated to be 1.4 × 108 moles, with 85% of it being generated in summer when riverine DIN input is low. Photoammonification could mineralize 4% of the annual dissolved organic nitrogen (DON) exported from the Mackenzie River and provide a DIN source corresponding to 7% of the riverine DIN discharge and 1400 times the riverine NH4+ flux. Under a climate warming-induced ice-free scenario, these quantities could increase correspondingly to 6%, 11%, and 2100 times. Photoammonification is thus a significant nitrogen cycling term and may fuel previously unrecognized autotrophic and heterotrophic production pathways in the surface SE Beaufort Sea.

Biogeochemistry of near-bottom suspended particulate matter of the Beaufort Sea shelf (Arctic Ocean): C, N, P, δ 13C and fatty acids

The influence of the Mackenzie River on the source, composition, and distribution of dissolved nutrients and suspended particulate matter (SPM) in near-bottom waters on the Beaufort Sea shelf was determined by measuring particulate organic carbon (POC), particulate nitrogen (PN), particulate phosphorus (PP), elemental ratios (C:N, C:P, N:P), chlorophyll a, bulk δ13C, and fatty acids. The Mackenzie River had a strong influence on the composition of SPM in near-bottom waters across the entire Canadian Beaufort Sea shelf, including the Amundsen Gulf. This influence was strongest at stations near the river mouth and decreased offshore and northeastward towards the Amundsen Gulf, as seen in SPM elemental concentrations, δ13C, and terrestrial plant fatty acid markers. Low C:N ratios and high relative levels of odd-numbered carbon and branched-chained fatty acids (bacterial fatty acid markers) indicated that bacteria were an important contributor to organic matter at stations near the river and on the Mackenzie shelf. Fatty acid analysis allowed detection of a phytoplankton sinking event in the Amundsen Gulf in which polyunsaturated fatty acid (PUFA) levels in near-bottom waters significantly increased from June to July. However, this change in PUFA was not associated with any observed temporal change in chlorophyll a, δ13C, or C:N ratios. These results show that a multiple biomarker approach is necessary in ecosystem studies of dynamic environments such as near-bottom waters or river-influenced shelves.

Size distribution of particles and zooplankton across the shelf-basin system in southeast Beaufort Sea: combined results from an Underwater Vision Profiler and vertical net tows

Abstract. The size distribution and mean spatial trends of large particles (>100 μm, in equivalent spherical diameter, ESD) and mesozooplankton were investigated across the Mackenzie Shelf (southeast Beaufort Sea, Arctic Ocean) in July–August 2009. Our main objective was to combine results from an Underwater Vision Profiler 5 (UVP5) and traditional net tows (200 μm mesh size) to characterize the structural diversity and functioning of the Arctic shelf-basin ecosystem and to assess the large-scale correspondence between the two methodological approaches. The core dataset comprised 154 UVP5 profiles and 29 net tows conducted in the shelf (<100 m isobath), slope (100–1000 m) and basin (>1000 m) regions of the study area. The mean abundance of total particles and zooplankton in the upper water column (<75 m depth) declined exponentially with increasing distance from shore. Vertical and latitudinal patterns in total particle concentration followed those of chlorophyll a (chl a) concentration, with maximum values between 30 and 70 m depth. Based on the size-spectra derived from the UVP5 dataset, living organisms (0.1–10 mm ESD) accounted for an increasingly large proportion of total particle abundance (from 0.1 % to >50 %) when progressing offshore and as the ESD of particles was increasing. Both the UVP5 and net tows determined that copepods dominated the zooplankton community (~78–94 % by numbers) and that appendicularians were generally the second most abundant group (~1–11 %). The vertical distribution patterns of copepods and appendicularians indicated a close association between primary production and the main grazers. Manual taxonomic counts and ZooScan image analyses shed further light on the size-structure and composition of the copepod community – which was dominated at ~95 % by a guild of 10 typical taxa. The size distributions of copepods, as evaluated with the 3 methods (manual counts, ZooScan and UVP5), showed consistent patterns co-varying in the same order of magnitude over the upper size range (>1 mm ESD). Copepods <1 mm were not well quantified by the UVP5, which estimated that only ~13–25 % of the assemblage was composed of copepods <1 mm ESD compared with ~77–89 % from the net tow estimates. However, the biovolume of copepods was overwhelmingly dominated (~93–97 %) by copepods >1 mm ESD. Our results illustrate that the combination of traditional sampling methods and automated imaging techniques is a powerful approach that enabled us to conclude on the prevalence of a relatively high productivity regime and dominant herbivorous food web over the shelf when compared with the low-productive recycling system detected offshore.

Annual cycles of pCO2sw in the southeastern Beaufort Sea: New understandings of air‐sea CO2 exchange in arctic polynya regions

From 23 October 2007 to 1 August 2008, we made continuous measurements of sea surface partial pressure of CO2 (pCO2sw) in three regions of the southeastern Beaufort Sea (Canada): the Amundsen Gulf, the Banks Island Shelf, and the Mackenzie Shelf. All three regions are seasonally ice covered, with mobile winter ice and an early spring opening that defines them as polynya regions. Amundsen Gulf was characterized by undersaturated pCO2sw (with respect to the atmosphere) in the late fall, followed by an under‐ice increase to near saturation in winter, a return to undersaturation during the spring, and an increase to near saturation in summer. The Banks Island Shelf acted similarly, while the Mackenzie Shelf experienced high supersaturation in the fall, followed by a spring undersaturation and a complex, spatially heterogeneous summer season. None of these patterns are similar to the annual cycle described or proposed for other Arctic polynya regions. We hypothesize that the discrepancy reflects the influence of several previously unconsidered processes including fall phytoplankton blooms, upwelling, winter air‐sea gas exchange, the continental shelf pump, spring nutrient limitation, summer surface warming, horizontal advection, and riverine input. In order to properly predict current and future rates of air‐sea CO2 exchange in such regions, these processes must be considered on a location‐by‐location basis.

Seasonal variability of water mass distribution in the southeastern Beaufort Sea determined by total alkalinity and δ18O

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 (δ18O) 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 (fCO2‐sw) show that the surface mixed layer (SML) remains mostly undersaturated (328 ± 55μatm, n = 552) with respect to the average atmospheric CO2 concentration (380 ± 5 μatm) over the study period. The influence of the Mackenzie River (fCO2‐SW > 500 μatm) 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 (fCO2‐SW < 300 μatm) 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 (fCO2‐SW > 600 μatm) 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.

The role of eddies and energetic ocean phenomena in the transport of sediment from shelf to basin in the Arctic

[1] Particle fluxes over the Beaufort continental slope (70.548°N/140.045°W; 710 m) were measured at two depths (290 and 490 m) during the fall 1992 to spring 1993 period. Sequential sediment traps recorded a peak in particle flux under heavy ice cover in midwinter. The fine-grained, highly terrigenous material was consistent in composition with bottom sediments from the shelf and upper slope. The peak occurred coincident with high current speed and isopycnal displacements characteristic of cyclonic eddies, and upwelling-downwelling (UW-DW) cycles driven by winds and drifting ice. Such phenomena may be the operative mechanisms for transporting sediments from shelf to basin in the Arctic. Available evidence favors the origin of the midwinter peak to be rainout of particles from the cyclonic eddies; we suggest that the particles were earlier entrained into the eddy when it impinged upon the upper slope. Estimates show that sediment-laden water in an eddy could sustain rainout at 250 mg m−2 d−1 for ∼120 days, so that sediment could have been transported 100–500 km with the eddy before reaching our site. Current speeds generated by wind-driven UW-DW events were insufficient to resuspend sediment on the outer shelf, and the timing of strong events was unconvincing as a link to the enhanced sediment flux over the slope 50 km to the north. Bottom current must exceed 19–36 cm s−1 depending on the sediment type to resuspend material from the seabed of the Mackenzie shelf.

Production and retention of biogenic matter in the southeast Beaufort Sea during 2003–2004: insights from annual vertical particle fluxes of organic carbon and biogenic silica

Regional variability in the annual fluxes of particulate organic carbon (POC) and biogenic silica (Si) at the periphery of the Mackenzie Shelf (Beaufort Sea) was investigated using eight long-term sediment traps moored at ~100-m depth. Relatively high autochthonous POC and Si fluxes were recorded in the Mackenzie Trough (4.1 and 8.9 g m−2 year−1 respectively) and off Cape Bathurst (6.6 and 79 g m−2 year−1), two areas where upwelling events are frequently observed. Diatomaceous new production was minimum on the mid-slope of the Mackenzie Shelf (2.8 g C m−2 year−1), moderate in the Mackenzie Trough (14.5 g C m−2 year−1), and highest off Cape Bathurst (128.7 g C m−2 year−1). High annual autochthonous POC flux corresponded to high diatom production. Among sites, the vertical attenuation of the POC flux increased with diatomaceous new production. Hence, the retention of autochthonous POC in the surface layer (<100 m) was highest (95%) at the highly productive site off Cape Bathurst, intermediate (72%) in the moderately productive Mackenzie Trough, and low (4%) at the unproductive mid-slope of the shelf. Our results indicate that, on Arctic shelves, upwelling and the production of diatoms increase the fraction of the POC which is retained in the surface layer and diverted to the pelagic food web. In the relatively unproductive waters of the Arctic Ocean, biological hot spots such as the one identified off Cape Bathurst where the food web promotes retention rather than vertical export could be disproportionately important as feeding grounds for higher trophic levels.

Sinking export of particulate organic material from the euphotic zone in the eastern Beaufort Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 410:55-70 (2010) - DOI: https://doi.org/10.3354/meps08608 Sinking export of particulate organic material from the euphotic zone in the eastern Beaufort Sea Thomas Juul-Pedersen1,2,3,*, Christine Michel1, Michel Gosselin2 1Freshwater Institute, Fisheries and Oceans Canada, 501 University Crescent, Winnipeg, Manitoba R3T 2N6, Canada 2Institut des sciences de la mer (ISMER), Université du Québec à Rimouski, 310 Allée des Ursulines, Rimouski, Québec G5L 3A1, Canada 3Present address: Greenland Climate Research Centre, Greenland Institute of Natural Resources, PO Box 570, DK-3900 Nuuk, Greenland *Email: thpe@natur.gl ABSTRACT: This paper presents an extensive spatial and temporal study of the sinking export of particulate organic material below the euphotic zone in the eastern Beaufort Sea. Free-drifting short-term particle interceptor traps were deployed, generally at 50 m, during fall 2002 and 2003, and summer 2004. The different regions of the sampling area, i.e. the Amundsen Gulf and the Mackenzie shelf and slope, showed similar ranges in the sinking export of chlorophyll a (chl a) and particulate organic carbon (POC) in fall, while regional differences were observed in summer. The 2 regions showed a general decreasing trend in sinking fluxes towards fall. The highest chl a and POC sinking fluxes during this study were therefore recorded during summer (3.6 and 258 mg m–2 d–1, respectively). A high retention of suspended biomass was observed throughout this study, i.e. low daily loss rates of suspended chl a and POC (both averaging ca. 1% d–1) were observed. Still, the POC sinking export accounted for, on average, half of the particulate primary production throughout this study. Zooplankton, primarily copepods, played an important role in the sinking export of particulate organic material, particularly in the Amundsen Gulf. A cluster-based analysis of the sinking protist cell assemblage revealed a seasonal succession that prevailed over spatial and interannual differences between the stations sampled in the eastern Beaufort Sea. Flagellates dominated throughout the study area, while diatoms, dominated by Fragilariopsis cylindrus, showed a decreasing contribution to the sinking protist cell assemblage towards fall. The presence of the sea ice related pennate diatoms Nitzschia frigida and Navicula vanhoeffenii in the material collected during summer reflected an input of organic material from sea ice. Results from particle interceptor traps deployed at a station in Franklin Bay during ice-covered and ice-free conditions showed the importance of taking into account under-ice sinking fluxes (up to 115 mg C m–2 d–1 for POC) for sinking export estimates on Arctic shelves. KEY WORDS: Arctic · Beaufort Sea · Sedimentation · Particulate organic matter · Phytoplankton · Fecal pellets · Carbon · Sea ice · Shelf Full text in pdf format PreviousNextCite this article as: Juul-Pedersen T, Michel C, Gosselin M (2010) Sinking export of particulate organic material from the euphotic zone in the eastern Beaufort Sea. Mar Ecol Prog Ser 410:55-70. https://doi.org/10.3354/meps08608 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 410. Online publication date: July 14, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Parasitic infection of the hyperiid amphipod Themisto libellula in the Canadian Beaufort Sea (Arctic Ocean), with a description of Ganymedes themistos sp. n. (Apicomplexa, Eugregarinorida)

Two parasites were found in the hyperiid amphipod Themisto libellula sampled with nets and collected by sediment traps over the annual cycle in the Canadian Beaufort Sea. The trophozoites of the newly described gregarine Ganymedes themistos sp. n. infected the digestive tract of 60.2% of the T. libellula analyzed from net collections. An unidentified ciliate infected the body cavity of 4.4% of amphipods. G. themistos possessed the ball-like structure at the anterior end and the cup-like invagination at the posterior end that are typical of the genus Ganymedes. The frequency and severity (number of parasites host−1) of infection by G. themistos increased with the length of T. libellula in the range 8–20 mm, and leveled off at ca. 94% and 186 trophozoites host−1 on average in the range 20–34 mm. Spatially, gregarine infection was less severe (63 ± 100 G. themistos host−1) on the Slope than on the Mackenzie Shelf (110 ± 160) and in the Amundsen Gulf (132 ± 157). No evidence of an impact of trophozoite infection on the feeding and sexual maturation of the host was found. For a given size of T. libellula, infection by both parasites was more frequent in the traps than in the nets (G. themistos: 91.0% vs. 82.7%; ciliates: 16.3% vs. 6%). The 2.7 times higher infection frequency in the traps suggested that the ciliate parasite may kill its host.

CO2 fluxes across the air‐sea interface in the southeastern Beaufort Sea: Ice‐free period

Surface mixed layer CO2 fugacities (fCO2‐sw) calculated from carbonate system parameters in the southeastern Beaufort Sea during the ice‐free period ranged from 240 to 350 μatm in fall 2003 and from 175 to 515 μatm in summer 2004. The surface mixed layer remains mostly undersaturated with respect to atmospheric CO2 (378 μatm) and, therefore, acts as a potential CO2 sink throughout this period. Air‐sea CO2 fluxes (FCO2) were first computed assuming ice‐free conditions and ranged from −32.4 to +8.6 mmol m−2 d−1 in fall 2003 and summer 2004, respectively. Then we included a reduction factor to account for ice cover (ic) and we computed the resulting fluxes (FCO2‐ic). In fall 2003, FCO2‐ic ranged from −4.7 mmol m−2 d−1 in the relatively open water of the Cape Bathurst Polynya to −0.1 mmol m−2 d−1 in the southeastern Beaufort Sea, limited by the presence of the multiyear sea ice. In summer 2004, FCO2‐ic ranged from −13.1 mmol m−2 d−1 on the western Mackenzie Shelf to +8.6 mmol m−2 d−1 at Cape Bathurst; the variability being ascribed to competing effects of vertical mixing, temperature variations, and possibly biological production. On average, a net sink of −2.3 ± 3.5 mmol m−2 d−1 was estimated for the ice‐free period over the study area. Nevertheless, the FCO2 displays strong variability due to ice coverage, freshwater input, and upwelling events. The potential responses (direction and intensity of potential feedbacks) of the carbon cycle in the study area to a changing Arctic climate are discussed.

Variability of baroclinic tidal currents on the Mackenzie Shelf, the Southeastern Beaufort Sea

Semidiurnal tidal currents on the outer shelf of the Mackenzie Shelf in the Beaufort Sea were found to be strongly influenced by the locally generated baroclinic tide. Two primary factors are involved in this process: (1) the sharp shelf break along the northeastern Mackenzie Shelf, promoting the generation of vigorous internal tidal waves; and (2) the proximity to critical latitudes for M 2 and N 2 motions locking these waves and preventing them from leaving the source region. As a result, internal tides are resonantly trapped between the shelf and critical latitudes. The physical properties and temporal variations of tidal motions were examined using current meter measurements obtained from 1987–1988 at four sites (SS1, SS2, SS3, and SS4) offshore of the shelf break at depths of ∼200 m. Each mooring had Aanderaa RCM4s positioned at ∼35 m below the surface and ∼50 m above the bottom. Complex demodulation was used to compute the envelopes (amplitude modulation) of these components. A striking difference in the variability of clockwise (CW) and counterclockwise (CCW) tidal currents was found. The CW tides are highly variable, have greater amplitude, exhibit a burst-like character associated with wind events and contain about 80% of the total energy of the semidiurnal tidal currents. In contrast, the CCW components have a more regular temporal regime with distinct monthly, fortnightly and 10-day modulation at astronomical periodicities associated with frequency differences M 2–N 2 (0.03629 cpd), S 2–M 2 (0.06773 cpd), and S 2–N 2 (0.10402 cpd). Significant horizontal correlation of the CW current envelopes was found only between stations near the northeast Mackenzie Shelf, indicating this to be the main area of baroclinic internal wave generation.

Calcium carbonate saturation in the surface water of the Arctic Ocean: undersaturation in freshwater influenced shelves

Abstract. In the summer of 2005, we sampled surface water and measured pH and total alkalinity (AT) underway aboard IB Oden along the Northwest Passage from Cape Farewell (South Greenland) to the Chukchi Sea. We investigated the variability of carbonate system parameters, focusing particularly on carbonate concentration [CO32−] and calcium carbonate saturation states, as related to freshwater addition, biological processes and physical upwelling. Measurements on AT, pH at 15°C, salinity (S) and sea surface temperature (SST), were used to calculate total dissolved inorganic carbon (CT), [CO32−] and the saturation of aragonite (ΩAr) and calcite (ΩCa) in the surface water. The same parameters were measured in the water column of the Bering Strait. Some surface waters in the Canadian Arctic Archipelago (CAA) and on the Mackenzie shelf (MS) were found to be undersaturated with respect to aragonite (ΩAr&lt;1). In these areas, surface water was low in AT and CT (&lt;1500 μmol kg−1) relative to seawater and showed low [CO32−]. The low saturation states were probably due to the likely the effect of dilution due to freshwater addition by sea ice melt (CAA) and river runoff (MS). High AT and CT and low pH, corresponded with the lowest [CO32−], ΩAr and ΩCa, observed near Cape Bathurst and along the South Chukchi Peninsula. This was linked to the physical upwelling of subsurface water with elevated CO2. The highest surface ΩAr and ΩCa of 3.0 and 4.5, respectively, were found on the Chukchi Sea shelf and in the cold water north of Wrangel Island, which is heavily influenced by high CO2 drawdown and lower CT from intense biological production. In the western Bering Strait, the cold and saline Anadyr Current carries water that is enriched in AT and CT from enhanced organic matter remineralization, resulting in the lowest ΩAr (~1.2) of the area.

Variability in the annual cycle of vertical particulate organic carbon export on Arctic shelves: Contrasting the Laptev Sea, Northern Baffin Bay and the Beaufort Sea

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 200 m ranged from 1.6 to 5.9 g C m −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.

Photoreactivity of chromophoric dissolved organic matter transported by the Mackenzie River to the Beaufort Sea

The photoreactivity of chromophoric dissolved organic matter (CDOM) transported to Arctic shelf environments by rivers has only recently been studied and its quantitative role in Arctic shelf biogeochemistry has received little attention. Sunlight exposure experiments were performed on CDOM collected over a three year period (2002 to 2004) from river, estuary, shelf, and gulf regions of the Western Canadian Arctic. Decreases in CDOM absorption, synchronous fluorescence (SF), and dissolved organic carbon (DOC) concentration were followed after 3 days of exposure, and in two experiments, six optical cutoff filters were used to incrementally remove ultraviolet radiation incident on the samples. Apparent quantum yields for CDOM photobleaching (AQY ble) and for DOC photomineralization (AQY min) were computed, as were two AQY spectra (ϕ ble and ϕ min) for the Mackenzie River and a sample from the Mackenzie Shelf. The photoreactivity of Mackenzie River CDOM was highest after break-up and peak discharge and lowest in late summer. The half-lives of CDOM and DOC were estimated at 3.7 days and 4.8 days, respectively, when Mackenzie River water was exposed to full sunlight. Photobleaching of Mackenzie River CDOM fluorescence after most UV-B wavelengths were removed increased the correlation between the river and offshore waters in the Beaufort Sea. When light attenuation from particle- and CDOM-rich river water was considered for the Mackenzie Shelf, our photodegradation models estimated around 10% loss of absorption and < 1% DOC loss, suggesting that sunlight exposure does not substantially degrade CDOM on Arctic shelves.