Particulate Organic Carbon Fluxes on the Slope of the Mackenzie Shelf

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Journal of Marine Systems 68 (2007) 39–54 www.elsevier.com/locate/jmarsys

Particulate organic carbon fluxes on the slope of the Mackenzie Shelf (Beaufort Sea): Physical and biological forcing of shelf-basin exchanges ⁎ Alexandre Forest a, , Makoto Sampei a, Hiroshi Hattori b, Ryosuke Makabe c, Hiroshi Sasaki c, Mitsuo Fukuchi d, Paul Wassmann e, Louis Fortier a

a Québec-Océan, Université Laval, Québec, QC, Canada, G1K 7P4 b Hokkaido Tokai University, Minamisawa, Minamiku Sapporo, Hokkaido 005-8601, Japan c Senshu University of Ishinomaki, Ishinomaki, Miyagi 986-8580, Japan d National Institute of Polar Research, 9-10, Kaga 1-chome, Itabashi-ku, Tokyo 173-8515, Japan e Norwegian College of Fishery Science, University of Tromsø, N-9037, Tromsø, Norway Received 27 July 2006; received in revised form 25 October 2006; accepted 27 October 2006 Available online 12 December 2006

Abstract

To investigate the mechanisms underlying the transport of particles from the shelf to the deep basin, sediment traps and oceanographic sensors were moored from October 2003 to August 2004 over the 300- and 500-m isobaths on the slope of the Mackenzie Shelf (Beaufort Sea, Arctic Ocean). Seasonal variations in the magnitude and nature of the vertical particulate organic carbon (POC) fluxes were related to sea-ice thermodynamics on the shelf and local circulation. From October to April, distinct increases in the POC flux coincided with the resuspension and advection of shelf bottom particles by thermohaline convection, windstorms, and/or current surges and inversions. Once resuspended and incorporated into the Benthic Nepheloid Layer (BNL), particles of shelf origin were transported over the slope by the isopycnal intrusion of the BNL into the Polar-Mixed Layer off-shelf. Offshore transport of the resuspended particles allowed them to settle over the slope. The resulting vertical POC flux at the shelf-basin boundary amounted to 1.0 g C m−2 y−1 or 58% of the annual POC flux over the 300-m isobath. Consistent with the resuspension of shelf sediments, POC fluxes in fall/winter were characterized by a high terrigenous fraction (25–60%), the dominance of small flagellate cells, and increasingly degraded fecal pellets with time. In late May– early June, a short-duration POC flux maximum characterized by high POC:PON ratio and more positive δ13C resulted from the direct sinking of ice algae and transparent exopolymeric matter flushed from melting sea-ice. In July, a last sedimentation event coincided with the retreat of the sea-ice cover, phytoplankton production from a subsurface bloom, and the sinking of the intact fecal pellets of large herbivorous copepods and appendicularians. Our results confirm the importance of sea-ice thermodynamics and BNL resuspension in promoting the transfer of POC from the shelf to the deep basin in fall/winter. The actual contribution of the summer biological production to the shelf–basin flux of POC remains uncertain. © 2006 Elsevier B.V. All rights reserved.

Keywords: Shelf–basin exchange; Particulate flux; Thermohaline convection; Benthic Nepheloid Layer; Biological production; Sediment traps; Arctic Ocean; Beaufort Sea; Mackenzie Shelf; 71°N; 133°W

⁎ Corresponding author. Tel.: +1 418 656 5917; fax: +1 418 656 2339. E-mail address: [email protected] (A. Forest).

1. Introduction advected offshore (e.g. Backhaus et al., 1997; Ivanov et al., 2004). Based on transmissometer and sequential- Assessing the sequestration of atmospheric CO2 on trap data, O'Brien et al. (2006) suggested that shelf continental shelves is central to our understanding of the sediments are episodically transported to the edge of the role of oceans in regulating climate (e.g. de Haas et al., Mackenzie Shelf in bottom and mid-water nepheloid 2002; Dittmar and Kattner, 2003; Muller-Karger et al., layers. The processes underlying this transport could not 2005). This assessment is particularly important for the be resolved however. extensive shelves of the Arctic Ocean where biogeo- In the present study, sequential sediment traps and chemical cycling and carbon fluxes could be altered oceanographic sensors (temperature, salinity, current dramatically by the on-going reduction of the sea-ice speed, current direction, and turbidity) were moored cover (e.g. Stein and MacDonald, 2004; Macdonald from October 2003 to August 2004 on the slope of the et al., 2005; ACIA, 2005). Documenting the intensity Mackenzie Shelf to document the physical and biological and nature of present fluxes and modeling the response factors that promote the transport of biogenic carbon from of these fluxes to variability and change in sea-ice the continental shelf to the deep Arctic Ocean. The origin regime are central objectives of international programs by source (marine, terrigenous) and nature (protistal, fecal, such as the Canadian Arctic Shelf Exchange Study detrital) of the fluxes recorded on the slope were assessed (CASES) and the American-led Shelf–Basin Interaction through chemical (total POC, POC terrigenous fraction, Study (SBI). C:N ratios and δ13C) and microscopic analyses (protists The immense (ca. 6×106 km2), seasonally ice-covered taxonomy and volume, fecal pellets volume, shape and continental shelves of the Arctic Ocean receive allochtho- degradation state). In particular, we tested the hypothesis nous POC from river runoff, coastal erosion and the that the advection of the BNL on the Mackenzie Shelf melting of landfast ice (Rachold et al., 2004; Wassmann contributes significantly to the transfer of POC from the et al., 2004). In summer, the production of autochthonous shelf to the slope. POC by microalgal photosynthesis is favored along the circum-Arctic coastal polynya system that separates the 1.1. Study area landfast ice and the mobile central ice pack (Stirling, 1980; AMAP, 1998). The allochthonous and autochtho- As part of the Canadian Arctic Shelf Exchange Study nous POC of arctic shelves can be transferred to the food (CASES), the CCGS Amundsen research icebreaker web, to the shelf seabed, or to the deep Arctic Ocean completed a one-year over-wintering expedition to the basins (Belicka et al., 2002; Muller-Karger et al., 2005; Mackenzie Shelf of the Beaufort Sea from September O'Brien et al., 2006). Because of the resulting long-term 2003 to August 2004. The rectangular (120×530 km) sequestration of atmospheric CO2, the transfer of POC Mackenzie Shelf is relatively narrow compared to the from the shelf to the deep basin is of particular relevance broad Siberian shelves (∼500 km width). It is influenced in the present context of Global warming. by the Mackenzie River, the third largest river dischar- In a recent review, McPhee-Shaw (2006) pointed to ging into the Arctic Ocean (330 km3 y− 1; Macdonald the potential importance of the Benthic Nepheloid Layer et al., 1998) and the first in terms of sediment load (BNL) in transferring sediments and POC horizontally (124×106 ty− 1; Holmes et al., 2002). Sea-ice typically from the shelf to the slope and ultimately to the deep starts to form on the shelf in October and reaches its basin. Once bottom particles are resuspended and in- maximum thickness (2–3 m) in March. During winter, corporated into a BNL by some mixing process, the the landfast ice is bounded offshore by the stamukhi, a turbid layer can intrude the ocean interior along its new linear hummock formed along the 20-m isobath by the isopycnal, thereby moving particulate matter horizon- collision of the offshore mobile ice pack onto the landfast tally over the slope (McPhee-Shaw, 2006 and references ice edge. In winter, the stamukhi contains the turbid therein). On ice-free shelves, internal waves, coastal waters of the Mackenzie River, forming the seasonal eddies, hyperpycnal flows caused by coastal storms, Mackenzie freshwater lake under the landfast ice cover boundary currents rushing into topographic disconti- of the coastal zone. Offshore, an intermittent coastal nuities, and other turbulent processes can resuspend polynya (or flaw lead) is formed when winds and/or sediments and mix the BNL with the overlaying water, surface circulation push the central ice pack away from hence modifying its density. At high-latitudes in winter, the stamukhi (e.g. Carmack and MacDonald, 2002; thermohaline convection of dense water resulting from Barber and Hanesiak, 2004). The ice break-up develops ice formation in leads and polynyas is an additional from the coastal polynya around June. The Mackenzie mechanism by which the BNL can be produced and River plume invades the top 5–10 m of the surface layer A. Forest et al. / Journal of Marine Systems 68 (2007) 39–54 41

Fig. 1. Bathymetric map of the Mackenzie Shelf in the eastern Beaufort Sea (Arctic Ocean) with the position of moorings CA-04 and CA-07 deployed from October 2003 to August 2004 on the slope. The bold south–north line indicates the oceanographic sections conducted in October 2003 and June 2004. The stippled large rectangle and small square represent the areas over which sea-ice cover percentage was estimated from satellite imagery. The position of the over-wintering station of the CCGS Amundsen in Franklin Bay is shown. of the shelf when the stamukhi breaks. Northwesterly Canadian Ice Service of Environment Canada (http:// winds maintain the plume inshore, whereas easterlies ice-glaces.ec.gc.ca/). Wind speeds and direction at Tuk- push it seaward (Macdonald and Yu, 2006). toyaktuk were retrieved from the Weather Archive of The water masses on the slope of the Mackenzie Environment Canada (http://www.climate.weatheroffice. Shelf are typical of the oligotrophic Canadian Basin ec.gc.ca/). Time-series of weekly-averaged percent ice (Carmack and Kulikov, 1998; McLaughlin et al., 2005). cover in the Mackenzie Shelf area (mean of all pixels over The Polar-Mixed Layer (PML, 0–50 m) sits over the the shelf, large rectangle in Fig. 1) and in the specific pixel Pacific Halocline (PH, 50–200 m) that overlays the that included the sediment traps (small square in Fig. 1) Atlantic Waters (AW, N200 m). The surface circulation were extracted from the Special Sensor Microwave over the slope is dominated by the westward branch of Imager (SSM/I) by the Centre for Earth Observation the anti-cyclonic Beaufort Gyre, but increasingly Science (U. Manitoba, Winnipeg, Canada). Satellite frequent reversals of this gyre have been observed in imagery of total suspended material (TSM, μgL−1) the last 30 years in both summer and winter (Lukovich was used to identify structures in the surface layer such as and Barber, 2006). The Beaufort Shelfbreak Jet, a the extent of the Mackenzie River plume. TSM was narrow (20 km width) eastward countercurrent carrying derived from Sea-viewing Wide Field-of-View Sensor Bering Sea Water (BSW) flows at 150–200 m depth (SeaWiFS) data provided by the NASA (http://oceancolor. along the slope (Pickart, 2004). Cyclonic reversals of gsfc.nasa.gov/). Level 1a data were processed to Level the Beaufort Gyre likely result in the amplification of the 2 by applying the turbid water atmospheric correction BSW inflow (Pickart, 2004). implemented within the SeaWiFS Data Analysis System (SeaDAS 4.6). An ocean color algorithm (Lee et al., 2002) 2. Material and methods was used to derive the particulate backscattering coef- ficient [bbp (555 nm)] from the satellite data (Bélanger, in 2.1. Environmental conditions and ship-based data preparation). Total suspended material was calculated using an empirical equation linking remotely sensed back- General information on regional climate and ice scattering and the dry weight of particles retained on conditions during 2003–2004 were obtained from the 0.2 μm Anodisc© filters (TSM=101.46 + 0.64*log [bbp (555 nm)], 42 A. Forest et al. / Journal of Marine Systems 68 (2007) 39–54 r 2 =0.85, N=40; S. Bélanger and P. Larouche, unpublished Before deployment, sediment traps were thoroughly data). rinsed with freshwater and seawater following the As part of the CASES program, the CCGS Amundsen JGOFS protocol (Knap et al., 1996). Sample cups over-wintered from 4 December 2003 to 31 May 2004 in were filled with filtered seawater (GFF 0.7 μm) adjusted the landfast ice of Franklin Bay (Fig. 1). The station to 35 PSU with NaCl. Formalin was added to preserve depth was 232 m. The CTD system was deployed daily the material collected (5% v/v, sodium borate buffered). through the moon-pool of the ship, providing a high- After retrieval, sample cups were checked for salinity resolution time-depth section of oceanographic vari- and put aside 24 h to allow particles to settle down. ables. Two N–S oceanographic sections along the Quantitative splitting into several fractions was com- mooring locations (Fig. 1) were completed by the pleted onboard using a McLane Wet Samples Divider©. Amundsen on 20–22 October 2003 and on 28–30 June Zooplankton N 5 mm were removed from the samples 2004 respectively. A profiler carrying a CTD (Seabird before splitting to ensure equal splits. No difference in SBE-911+©), a transmissometer (WETlabs©) and a the state of particles was observed after splitting. fluorometer (Seapoint©) was deployed at intervals of Zooplankton swimmers b 5 mm were removed later in 10 km or less along the section. Validated data were the laboratory with a 1-mm sieve and by handpicking averaged over 1-m bins. under a stereomicroscope. The mesozooplankton was sampled near the mooring Sub-samples for the determination of particulate locations during the October and June oceanographic organic carbon (POC) and nitrogen (PON) were filtered sections. In October, an automated multi-net vertical in triplicates trough pre-weighted Whatman glass fiber sampler (Kiel Hydrobios©, 0.5 m2, 200 μm mesh) filters (GFF 0.7 μm, 25 mm, combusted 4 h at 450 °C). equipped with a flowmeter was towed vertically from Filters were dried for 12 h at 60 °C and weighted again bottom to surface at 70° 56.1′ N 133° 40.7′ W, 16 km S for dry weight. After exposure for 12 h to concentrated of CA-04, and at 71° 12.4′ N 133° 48.0′ W, 7 km N of HCl fumes to remove inorganic carbon fraction, the CA-07. In June, a 1-m2 aperture square net (200 μm samples were analyzed with a Perkin Elmer CHNS 2600 mesh) was deployed at the same two stations. Zooplank- Series II. Discrimination of biogenic POC produced in ton data from the two stations were averaged to provide the aquatic environment (marine POC) from residual an estimate of abundance for the fall and summer (mean POC of terrestrial origin (terrigenous POC) was per- water column depth of 375 m) and abundance of each formed following O'Brien et al. (2006) which make use taxon was transformed into biomass following Hopcroft of a terrigenous POC:Aluminum ratio of 0.16 for the et al. (2005). Mackenzie Shelf sediments. Sub-samples for the determination of Al were analyzed by inductively coupled 2.2. Mooring measured variables (sediment traps and plasma atomic emission spectrometry. Total, marine and oceanographic sensors) terrigenous POC fluxes were expressed as daily fluxes (mg C m− 2 d− 1). Annual POC fluxes were obtained by Moorings CA-04 (71° 05.16′ N, 133° 43.39′ W) and extrapolating the mean daily POC fluxes from October CA-07 (71° 8.99′ N, 133° 53.88′ W) were deployed to August. Stable isotope composition of particulate respectively over the 300- and 500-m isobaths of the organic carbon (δ13C) was determined using a mass Mackenzie slope from October 2003 to August 2004 spectrometer (Europa PPZ GEO 20-20) following Sato (Fig. 1). The two moorings were separated by a distance et al. (2002). In our study, fluxes were not corrected for of 5 nautical miles. Each mooring line was equipped dissolution losses which were assumed to be constant with a Nichiyu Giken Kogyo SMD26S-6000 conical among sample cups (Fischer et al., 1996). Thus, the automatedsedimenttrap(0.5m2 aperture and 26-cup fluxes presented here are minimum estimates. turntable) attached at 200-m depth. Sampling periods Protists were counted under the light microscope varied between 4 and 28 d. Aanderaa© RCM-11 and following Wassmann et al. (1999). The profusion of Compact Alec-Electronics© multi-sensors were detrital material in the samples prevented any exhaustive deployed above the sediment traps (16, 36 and 185 m taxonomic investigation. The most abundant taxa were depths) to record temperature, salinity, current speed, enumerated and measured in a Fuchs–Rosenthal count- current direction and turbidity. Physical data from ing chamber at a magnification of 400× and classified moored oceanographic sensors were processed by the into four groups (diatoms, flagellates, ciliates and other Institute of Ocean Sciences (IOS, Fisheries and Oceans protists). Protistal POC was estimated based on carbon Canada, Sydney, Canada). Validated data were aver- to volume relationships for phytoplankton and protozoa aged over one-day periods. (Menden-Deuer and Lessard, 2000). A. Forest et al. / Journal of Marine Systems 68 (2007) 39–54 43

Fecal pellets were measured and counted in known sub-samples at 100 or 200× magnification using an inverted microscope coupled to an image analyzer. For each sub-sample, all fecal pellets (from ca. 100 to 500) were counted, measured (width, length) and classified according to shape (cylindrical, ellipsoidal) and degradation state (intact or degraded). Cylindrical pellets were assumed to be produced by calanoid copepods, large ellipsoidal pellets by appendicularians, and small ellipsoidal pellets by small appendicularians or cyclopoid copepods (Martens, 1978; Fortier et al., 2002). Three series of 50 fecal pellets of each shape (cylindrical, ellipsoidal) were isolated from the samples, measured and analyzed by CHN (as described above) to determine volume to carbon conversion factors specific to this study (28.1 μgCmm− 3 for cylindrical and 9.4 μg Cmm− 3 for ellipsoidal). Both protistal POC and fecal POC were expressed as daily fluxes (mg C m− 2 d− 1). Detrital POC flux was estimated as total POC flux — (protistal POC+fecal POC fluxes) and corresponded to unidentified and aggregated material.

3. Results

3.1. Ice, winds, and thermohaline convection on the shelf

In 2003, landfast ice started to build-up in October and consolidation of the Mackenzie Shelf by first-year ice was completed by late December. Offshore winds along the Alaskan Coast in January moved the central arctic ice pack northward and, by February, it laid about 200 miles north of the Tuktoyaktuk Peninsula. A lead opened along the landfast ice edge in May, and gradually expanded and widened over the summer (Fig. 2). The turbid waters of the Mackenzie River invaded Mackenzie Bay following the break-up of the stamukhi during the second week of June (Fig. 2b). Landfast ice still occurred along the Tuktoyaktuk

Fig. 2. Sea-viewing Wide Field-of-View Sensor (SeaWiFS) images of the study area showing the evolution of the ice cover (grey areas) and Fig. 3. Time-series of sea-ice cover density (percent) over the Mackenzie the distribution of total suspended matter (colour scale at the bottom) Shelf and over the mooring sites from October 2003 to August 2004. The in the ice-free area in the spring and summer of 2004. areas over which sea-ice cover was averaged are shown in Fig. 1. 44 A. Forest et al. / Journal of Marine Systems 68 (2007) 39–54

Fig. 4. Monthly wind roses at Tuktoyaktuk (NWT, Canada) from October 2003 to July 2004. The percent frequency corresponding to the outer circle is given.

Peninsula in late June, but had almost completely melted pack in response to wind. Starting in mid-May, sea-ice by mid-July (Fig. 2c, d). concentrations over the mooring sites declined regularly At the mooring sites, sea-ice cover ramped up from a to 0% in late July (Fig. 3), as the central ice pack melted minimum of 25% in mid-October to full cover by mid- and moved offshore. The season of N50% ice cover lasted December (Fig. 3). Ice cover b 100% in winter cor- for 253 d or 83% of the year at the mooring sites, responded to the northward movement of the central ice compared to 211 d (69%) for the Mackenzie Shelf as a whole. Winds in the coastal area (Tuktoyaktuk) blew primarily along the east–west axis for most of the sampling period (Fig. 4). The strong and variable winds during late October and November were caused by storms blowing from the WNW on 28–31 October and 7 November, and from the SSE on 27 November. These

Fig. 6. TS diagram for the data recorded at different depths on mooring lines CA-04 and CA-07 deployed from October 2003 to August 2004 on the slope of the Mackenzie Shelf. The TS signatures obtained from Fig. 5. Time-depth section of temperature at the over-wintering station the CTD casts at CA-04 during the oceanographic sections of October of the CCGS Amundsen in Franklin Bay (see Fig. 1 for location). 2003 and June 2004 are also plotted. A. Forest et al. / Journal of Marine Systems 68 (2007) 39–54 45

storms (with velocities in excess of 30 km h− 1) occurred when the ice cover on the shelf was still only partly consolidated (80–90%). Easterlies dominated from December to July, with frequent westerly gales in January, February and March. In June, easterlies pro- moted the seaward expansion of the Mackenzie River plume (Fig. 2b, c). In July, recurrent spells of stronger winds from the NW (N30 km h− 1) forced the plume to expand alongshore on the shelf (Fig. 2d). There was no evidence from the satellite images that the Mackenzie River plume reached the positions of the sediment trap moorings in 2003–2004. The time-depth section of temperature at the over- wintering site of the Amundsen provided a record of surface water cooling from early December to late May on the shelf (Fig. 5). A first cooling event began on December 20th as surface water quickly reached its freezing point (−1.8 °C) and the −1.8 °C isotherm sank from the surface to 30 m by January 1st. Assuming that simple diffusion played a minor role, the deepening of the −1.8 °C isotherm likely corresponded to thermohaline convection resulting from brine rejection as sea-ice thickened. The simultaneous disruption of the isotherms down to 220 m in early January supports this interpretation and suggests that thermohaline convection and mixing reached the bottom (Fig. 5). Water at freezing Fig. 7. Daily-averaged current vector and turbidity recorded at 185 m temperature further sank to depths of 40 to 60 m during depth on moorings CA-04 (a, b) and CA-07 (c, d). short bouts on January 10, 17 and 20. A second deepening of the −1.8 °C isotherm from 15 to 40 m occurred

Fig. 8. The distribution of transmissivity (a, c) and chlorophyll fluorescence (b, d) along the south–north sections conducted across the Mackenzie −3 Shelf in October 2003 and June 2004. Vertical lines represent the position of profiles along the section. Isopycnals are labeled in σt units (kg m ). Positions of the moored sediment traps are represented by the yellow triangles. 46 A. Forest et al. / Journal of Marine Systems 68 (2007) 39–54

Table 1 Zooplankton abundance and biomass on the slope of the Mackenzie Shelf in late October 2003 and late June 2004 Specie Abundance (ind. m− 2) Biomass (mg DW m− 2) October June October June Calanus hyperboreus 2010 (3%) 1157 (3%) 2576 (47%) 661 (41%) Calanus glacialis 4384 (7%) 1311 (4%) 1398 (25%) 322 (20%) Metridia longa 4732 (7%) 1430 (4%) 328 (6%) 128 (8%) Pseudocalanus sp. 4188 (6%) 1442 (4%) 39 (1%) 8 (0%) Microcalanus pygmaeus 11,666 (17%) 6171 (17%) 82 (1%) 43 (3%) Oithona similis 12,916 (19%) 12,246 (33%) 39 (1%) 37 (2%) Oncaea borealis 7120 (11%) 2654 (7%) 14 (0%) 5 (0%) Cyclopina sp. 14,280 (21%) 3679 (10%) 36 (1%) 9 (1%) Appendicularians 438 (1%) 1327 (4%) 9 (0%) 159 (10%) Other copepods 3160 (5%) 971 (3%) 432 (8%) 67 (4%) Other species 2177 (3%) 4881 (13%) 550 (10%) 173 (11%) Total 67,071 (100%) 37,268 (100%) 5501 (100%) 1611 (100%)

from March 1st to March 25th, indicating massive westward flow inversions between October and January cooling of the surface layer. Sporadic deepening of (Fig. 7a). A strong and brief eastward current surge freezing surface water (−1.8 °C) was also observed in was recorded from 6 to 9 of January, with sustained May. speeds N30 cm s−1 reaching a maximum of 83 cm s−1 on January 7. This acceleration of the Beaufort Shelfbreak Jet 3.2. Water masses and circulation at the slope coincided with sharp increase in turbidity (Fig. 7b). Following this peak in turbidity there was a period of The temperature–salinity signatures recorded at dif- sustained high particle concentration that corresponded to ferent depths on the moorings were consistent with the several eastward–westward flow inversions. From Feb- water masses distribution previously reported for the ruary to mid-June, circulation was sluggish with the flow Canada Basin: the Polar Mixed Layer (PML) sat on the directed mostly toward the west. Relatively strong Pacific Halocline (PH) which itself overlaid Atlantic eastward currents (20 cm s−1) in late June and July Water (AW) (Fig. 6). The sediment traps moored at brought less saline (Sb33.5) and cooler waters (Tb 200 m depth were located in the lower part of the PH. −1.2 °C), suggesting again an amplification of the Water at 16 m progressively cooled and freshened over Beaufort Shelfbreak Jet (data not shown). the month of October, reflecting the downward con- Currents were slightly weaker over the mid-slope vection and/or mixing of cold, low-salinity superficial (CA-07; 6 cm s− 1 on average over the recording period) waters. Water at 16 m was at freezing temperature from than on the upper slope, but presented the same general November to April, consistent with thermohaline con- seasonal pattern (Fig. 7c). The current surge observed in vection in the surface layer of the PML. early January on the upper slope was detected at mid- Currents recorded at 185 m on the upper slope (CA-04) slope, but speeds there did not exceed 20 cm s− 1. averaged 8 cm s−1 over the recording period. Circulation Maximum turbidity values were reached during the next was characterized by frequent (week to 10 d) eastward– westward inversion of the flow (Fig. 7d).

Table 2 Comparison of annual POC fluxes in different regions of the Arctic Ocean Study area Coordinates (average) Time-window Depth (m) POC flux (g C m− 2 y− 1) Source North Water Polynya 75°N 75°W 1997–98 200 1.0–13.8 Sampei et al. (2004) Cape Bathurst Polynya 71°N 126°W 2003–04 100 2.8–12.8 Sampei et al. (in prep.) Mackenzie Shelf Edges 70°N 132°W 1987–88 125 1.7–5.8 O'Brien et al. (2006) North-East Water Polynya 80°N 11°W 1992–93 130 1.0–2.7 Bauerfeind et al. (1997) Mackenzie Shelf Slope 71°N 133°W 2003–04 200 1.0–1.7 Present study Greenland Continental Shelf 75°N 13°W 1994–95 245 1.6 Bauerfeind et al. (2005) Amundsen Basin 81°N 138°E 1995–96 150 1.0–1.5 Zernova et al. (2000) Canadian Archipelago 79°N 102°W 1989–90 130 0.2 Hargrave et al. (1994) A. Forest et al. / Journal of Marine Systems 68 (2007) 39–54 47

3.3. Inshore–offshore oceanographic sections in October 3.5. Integrated annual POC fluxes at 200 m depth on and June the slope

In late October, the transmissivity section displayed a When integrated between October 2003 and August well defined BNL extending vertically from ca. 40 m to 2004 and pro-rated for the missing days of August and the bottom over the shelf (Fig. 8a). The suspended September, estimates of annual POC fluxes on the particulate matter load was highest inshore. The low Mackenzie slope during 2003–2004 were within the fluorescence signal at the depth corresponding to the range of estimates based on sequential sediment traps BNL indicated that the particulate matter corresponded deployments in other Arctic regions (Table 2). Fluxes to resuspended sediments rather than freshly sunk microalgal material (Fig. 8b). The BNL extended horizontally over the slope along the 26.1 σt isopycnal, forming a mid-depth Intermediate Nepheloid Layer (INL). The suspended particulate load decreased as the INL spread over the slope (Fig. 8a). Interestingly, a distinct and weak INL was detected further offshore (71.2°N) along the same isopycnal, suggesting that the production and/or offshore spreading of the INL is a discontinuous process. A subsurface (15–25 m depth) lens of low transmissivity was also observed in October, sitting on the pycnocline and centered over the shelf break (Fig. 8a). This subsurface lens of higher turbidity corresponded with the base of the chlorophyll-rich surface layer (Fig. 8b). There was no evidence of a BNL or INL in the transmissivity section at the end of June (Fig. 8c). However, the Deep Chlorophyll Maximum (DCM) observed on the shelf (30–45 m depth) extended far offshore along the 25.6 σt isopycnal (Fig. 8d). Except for the difference in the depth of occurrence, the resulting distribution of phytoplankton along a precise isopycnal was analogous to the BNL/INL structure observed in October.

3.4. Mesozooplankton on the slope in October and June

By numbers and biomass, copepods contributed over 75% of the mesozooplankton collected on the slope in late October 2003 and late June 2004 (Table 1). The low- diversity assemblage of copepods was typical of Arctic Seas with three large species (Calanus hyperboreus, C. glacialis and Metridia longa) making up the bulk (N65%) of the biomass and small calanoids (Pseudoca- Fig. 9. Magnitude and characteristics of the particulate organic carbon lanus sp., Microcalanus pygmaeus) and cyclopoids (POC) flux recorded at 200 m over the upper slope of the Mackenzie Shelf (mooring CA-04, 300-m isobath) from October 2003 to August (Oithona similis, Oncaea borealis, Cyclopina sp.) 2004. (a) Total POC flux divided into its marine and terrigenous dominating by numbers (N70%). Cyclopina sp. was components (peaks in total POC flux are labeled I to V), and fraction of the most abundant species (21%) in October and O. total dry weight (DW) made of POC. (b) Protistal flux divided into similis (33%) in June. Appendicularians clogged our nets major taxonomic groups, and POC:PON ratio of total flux. (c) Fecal in late June–early July and their abundance is certainly flux with contribution of cylindrical and ellipsoidal pellets, and proportion of degraded pellets. (d) Detrital flux and the δ13C value of underestimated in Table 1 given the low efficiency of the total POC flux. (e) Relative contribution of the protistal, fecal and conventional nets in collecting these fragile organisms detrital fluxes to the total POC flux and terrigenous fraction of the total (Maar et al., 2004). POC flux. 48 A. Forest et al. / Journal of Marine Systems 68 (2007) 39–54

to V for convenience (Figs. 9 and 10). The difference in the total amount of POC collected could be traced back to the wintertime (Maxima II and III), when POC fluxes were higher on the upper slope (CA-04) than at mid- slope (CA-07). The terrigenous fraction of the total POC flux tended to be less at mid-slope (2–30%) than over the upper slope (6–50%). Otherwise, seasonal variations in the nature of the POC flux (proportion of protistal, fecal and detrital material, cell composition, POC:PON ratio, fecal pellet type and degradation, δ13C) were similar at the two locations (Figs. 9–11). Starting in October, total POC flux increased to a first maximum in late November and then declined to a minimum in late December (Figs. 9a and 10a). The POC terrigenous fraction of the fluxes was 40% on average from early October to mid-December. Maximum I resulted from a simultaneous increase in the protistal (30% of total) and fecal (45% of total) POC fluxes (Figs. 9b–e and 10b–e). The flux of protists was dominated by flagellates and other small species (Figs. 9b and 10b). Fecal pellets were small and abundant (Fig. 11) and dominated by cylindrical shapes (Figs. 9c and 10c). Over 90% of the fecal pellets were intact, indicating recent production (Figs. 9c and 10c). Maximum I followed the onset of ice formation on the shelf (Fig. 3) and generally coincided with the flow inversions (Fig. 7), the first cooling of the surface layer to freezing temperature in October (Fig. 6) and the windstorms of November (Fig. 4). Maximum II in January and February comprised from 30 to 60% of terrigenous POC (Figs. 9a–e and 10a–e). This maximum resulted mainly from an Fig. 10. Magnitude and characteristics of the particulate organic carbon (POC) flux recorded at 200 m over the mid-slope of the increase in the detrital POC flux and, to a lesser extent, Mackenzie Shelf (mooring CA-07, 500-m isobath) from October 2003 to August 2004. Details for panels a–easinFig. 9. were in the higher range of estimates for permanently ice-covered regions, and in the lower range of values measured in recurrent arctic polynyas. Fluxes were higher over the upper slope (CA-04, 1.7 g C m− 2 y− 1) than at mid-slope (CA-07, 1.0 g C m− 2 y− 1). Annual POC flux over the upper part of the slope was exactly the same as the one measured at 125 m depth in 1987– 1988 at about a same location (O'Brien et al., 2006; their SS3 trap).

3.6. Seasonal variations in the magnitude and nature of the POC fluxes on the slope

Over the sampling period, variations in the magni- Fig. 11. Number and volume of the fecal pellets collected in the tude of the POC flux on the slope were characterized by sediment traps deployed from October 2003 to August 2004 over the five periods of increased flux that we labeled Maxima I upper part (CA-04) and mid-part (CA-07) of the slope. A. Forest et al. / Journal of Marine Systems 68 (2007) 39–54 49 in the protistal and fecal fluxes (Fig. 9b–e). The flux of sediments in the mid-water and bottom layers. In the protists was dominated by small flagellates (N90%). present work, several lines of evidence also indicated Fecal pellets were small and up to 75% of them were that the POC collected by the traps on the slope in winter degraded. The development of Maximum II followed originated from the shelf Benthic Nepheloid Layer the intense convection event of late December–early (BNL) rather than locally. First, the bulk (97%) of the January (Fig. 6) and coincided with the eastward current POC resulting from autochthonous primary production surge, the repeated flow inversions and the increased on the shelf is recycled and not preserved in the turbidity recorded above the traps (Fig. 7). sediments (Macdonald et al., 1998). Hence, the POC in Maximum III in March–April (upper slope only, the BNL of the Mackenzie Shelf is likely to be rich in Figs. 9a and 10a) comprised up to 50% of terrigenous terrigenous material, as supported by the observation of POC (Fig. 9e). It corresponded with an increase in the terrestrial bacteria in this particle-rich layer (Wells et al., flux of intact cells which made up to 40% of the total 2006). Therefore, the high terrigenous content (25– POC flux and was again dominated (N90%) by small 60%) of the POC fluxes recorded on the slope from flagellates (Fig. 9b). There was no increase in the fecal October to April is consistent with a shelf origin. flux during Maximum III (Fig. 9c). This maximum Second, sediments on the Mackenzie Shelf consist coincided with the cooling and deepening of the surface essentially (85%) of silt and clay discharged by the layer observed in March (Fig. 6). Mackenzie River or released by coastal erosion (Hill The total POC flux peaked briefly in late May and et al., 1991). The protists found in the traps in winter early June (Figs. 9a and 10a), when ice still covered the were comprised mainly of small heterotrophic flagellates mooring region (Fig. 2). Maximum IV resulted from associated with silt and clay aggregates (T. Ratkova, increased protistal and detrital fluxes (Figs. 9b–d and Shirshov Institute of Oceanology, Academy of Sciences 10b–d). It differed from the previous maxima (I to III) in of Russia, personal communication), also consistent with many respects. The sinking POC was almost exclusively a shelf origin for the POC sinking on the slope. Third, of marine origin (N90%) and represented a larger copepod and appendicularian fecal pellets sink to the fraction (up to 22% at mid-slope) of the total vertical bottom in a matter of hours and days (see Fortier et al., mass flux. The flux of protists was dominated by 1994 for a review) and their bacterial degradation is slow diatoms and ciliates over the upper slope (Fig. 9b) and (b1% d− 1) at low temperatures (Hansen et al., 1996). mostly diatoms at mid-slope (Fig. 10b). The POC:PON Accordingly, the fecal pellets produced during the brief ratio and δ13C of the sinking material increased sharply arctic summer can be expected to accumulate quickly on during Maximum IV (Figs. 9b–d and 10b–d). The fecal the shelf and then to slowly degrade and disappear over POC flux remained weak and dominated by small and the winter months. Both the progressive reduction in the degraded pellets (Figs. 9c, 10c and 11). number of pellets and the increasing frequency of Maximum V in mid-July was also primarily of degraded pellets in the traps over the slope are coherent marine origin (Figs. 9a–e and 10a–e) and coincided with the advection of progressively fewer and increas- with the regression of the ice cover over the moorings ingly degraded fecal pellets from the shelf in winter. (Fig. 2). It resulted from an increase in the protistal and Finally, the small cyclopoid Cyclopina sp. is associated fecal fluxes (Figs. 9b–c and 10b–c). The flux of protists with epibenthic habitats (Grainger, 1991). Its dominance was dominated by flagellates and diatoms over the in the zooplankton of the slope in late October (21% by upper slope (Fig. 9b) and mostly flagellates at mid-slope number) is also consistent with the resuspension and (Fig. 10b). The fecal flux increased in early July and was advection offshore of the shelf BNL. made of large intact cylindrical and ellipsoidal pellets, The spreading above the slope of an Intermediate indicating recent production by large copepods and Nepheloid Layer (INL) following the resuspension and appendicularians (Figs. 9c, 10c and 11). detachment of the shelf BNL is a relatively well docu- mented phenomenon (e.g. Thomsen and Weering, 1998; 4. Discussion Wong et al., 2000; McPhee-Shaw, 2006). Depending on stratification, lateral fluxes may exceed vertical fluxes, a 4.1. Shelf sediment resuspension and the shelf-basin situation that results in a significant shelf-basin transport POC flux in fall/winter of sediments and POC (Thomsen and Weering, 1998; Wong et al., 2000). Our results support the hypothesized In their comprehensive study of particle fluxes on the importance of the intrusion over the slope of the nepheloid Mackenzie Shelf in 1987–1988, O'Brien et al. (2006) layer in transporting POC from the shelf to the deep basin. found strong evidence for shelf-slope transport of The cross-section of transmissivity available for October 50 A. Forest et al. / Journal of Marine Systems 68 (2007) 39–54 clearly illustrates the resuspension of the BNL on the shelf immediately over the upper slope. That this amplifica- and its isopycnal intrusion over the slope as an INL. This tion of the Jet was hardly detected 10 km away at mid- BNL/INL event in October coincided with a first increase slope indicates its local nature and suggests a relatively in POC flux on the slope that culminated in November modest role in effecting the transfer of POC from the (Maximum I). Because of logistic constraints, similar shelf to the slope. cross-shelf sections could not be obtained during the heavy ice winter months to confirm that Maxima II and III 4.2. Limited primary production and the weakness of also corresponded to the detachment and intrusion of the summer POC fluxes at the slope shelf BNL over the slope. The exact mechanism or combination of mechanisms By attenuating solar radiation and by limiting wind that produce and mix the BNL during the fall and winter mixing and the replenishment of nutrients in the surface on the Mackenzie Shelf has proven difficult to elucidate layer, sea-ice, haline stratification, and the Mackenzie (O'Brien et al., 2006). In addition to wind mixing, River plume all contribute to limit primary production in thermohaline convection following rapid ice growth and the southern Canadian Beaufort Sea (Macdonald, 2000; brine rejection in wind-generated lee-polynyas is a Macdonald and Yu, 2006). Based on the statistical potential mechanism for the resuspension of bottom relationship linking vertical POC flux (J ) and primary sediments on shallow arctic shelves. The modeling study production (PP) in the Canada Basin (J=PP/[0.024z+ of Backhaus et al. (1997) suggests that thermohaline 0.21]; Carmack et al., 2004), the POC fluxes of 1.0– convection reaching the BNL should predominate late in 1.7 g C m− 2 y− 1 measured at 200 m over the slope in winter after the erosion of the summer stratification. 2003–2004 reflected an annual PP of 5–9gCm− 2 y− 1. However, the recent review by Ivanov et al. (2004) Using the local non-terrigenous (i.e. autochthonous indicates that dense water overflow (cascading) at arctic marine POC) fraction of the total flux (80–90%) over shelf edges can take place in any month of the year. In the the period of biological production covered by our trap present study, the three maxima in terrigenous POC flux deployment (April–early August) yields a more conser- on the slope in fall/winter corresponded to distinct vative estimate of 2–3gCm− 2 y− 1 for primary pro- cooling events in the area. Maximum I (November) duction at the slope. This is roughly six times less than followed the rapid formation of ice on the shelf in late the measured annual production of 12–16 g C m− 2 y− 1 October. With a delay of a week or two, the development on the Mackenzie Shelf (Carmack et al., 2004), of Maxima II (late January) and III (early April) indicating severe limitation of autochthonous produc- coincided with ice growth and the intense formation of tion over the slope. Our estimates for the slope are more freezing-temperature surface water at the over-wintering consistent with recent PP estimates (5 g C m− 2 y− 1) for station of Franklin Bay in January and March respec- the deep Canada Basin where light and nutrients tively. This covariance from October to April between availability both control photosynthetic activity (Lee surface cooling and POC fluxes on the slope points to and Whitledge, 2005). thermohaline convection as a significant mechanism for With ice cover N50% for 253 d (or 83%) of the the resuspension and advection of the shelf BNL. As- annual cycle, light likely limited primary production suming that landfast ice growth in Franklin Bay was most of the time beyond the shelf break. But there was paralleled by intense ice formation and brine rejection in also evidence of limitation by nutrients in spring and the adjacent lee-polynya, we propose that thermohaline summer. Maximum IV in late May–early June corre- convection was a major engine of sediment resuspension sponded to the sinking of ice algae, with the increased and shelf-slope transport of POC on the Mackenzie δ13C that is symptomatic of the low availability of Shelf. dissolved inorganic carbon in old ice (Kennedy et al., In addition to thermohaline convection, at least two 2002). Nutrient-limited ice algae produce transparent other mechanisms likely contributed to the resuspension exopolymeric substances (EPS) that are characterized of shelf sediments and the transport of POC to the slope. by a C:N ratio N20 (Engel and Passow, 2001). The First, coastal windstorms likely combined with thermo- increased detrital flux (likely reflecting the production haline convection to resuspend shelf bottom particles of EPS) and the high C:N ratio during Maxima IV and mix the BNL in November (Maximum I). Second, suggest that ice diatoms had previously used up the the strong maximum in POC flux on the upper slope in nutrients available in the surface layer at the outer shelf. January (Maximum II) clearly followed the brief and Consistent with this interpretation, limiting nitrogen intense amplification of the Beaufort Shelfbreak Jet that nutrients concentrations ([NO3 +NO2]b1 μM) were resuspended material and sharply increased turbidity observed in the upper water column (b40 m) of the A. Forest et al. / Journal of Marine Systems 68 (2007) 39–54 51 slope in late June (K. Simpson, University McGill, November (Maximum I) suggested that small copepods personal communication). As a consequence, the and small appendicularians were the dominant grazers summer phytoplankton developed as a Deep Chloro- on the Mackenzie Shelf in the fall. The absence of large phyll Maximum (DCM) at the nutricline between the cylindrical pellets indicated that large calanoid cope- nutrient-poor PML and the nutrient-rich PH (see also pods had already stopped feeding by November. Carmack et al., 2004; Lee and Whitledge, 2005). The Overall, freshly-produced intact fecal pellets of relatively weak protistal POC flux maximum of mid- copepods and appendicularians contributed from 25 to July resulted from the sinking of the assemblage of 75% of the vertical flux of primarily autochthonous photosynthetic and mixotrophic organisms that is POC in fall and summer. To estimate the grazing typical of a DCM (e.g. Tittel et al., 2003). Thus, pressure of copepods on the slope, we assumed (1) a indications are that nutrient availability was too low 40% carbon content (Sampei, unpublished data); (2) that when the ice retreated beyond the shelf break in late large calanoids stored about half of their carbon into June to trigger an intense surface bloom and the large lipids reserve (Seuthe et al., 2006); and (3) that they vertical fluxes of autochthonous POC that usually stopped feeding after entering diapause in the fall characterize marginal ice-edges (e.g. Wassmann et al., (Ashjian et al., 2003). These assumptions yield an 1999). estimate of 320 mg C m− 2 d− 1 in the fall and 290 mg C m− 2 d− 1 in summer for the food requirements of the 4.3. Top-down control of the autochthonous POC flux copepod assemblage (or active carbon biomass sensu by zooplankton Olli et al., in press). The primary production of 15 and 70 mg C m− 2 d− 1 measured respectively during the two By repackaging phytoplankton and other particles periods (S. Brugel, Université du Québec à Rimouski, into large fast-sinking fecal pellets, mesozooplankton personal communication) could satisfy b5% and b25% grazers such as copepods and appendicularians can of these daily requirements only. We conclude that accelerate significantly the vertical flux of POC (e.g. copepods exerted a strong grazing pressure on the weak Fortier et al., 1994; Wassmann, 1998). Appendicularians microalgal production at the shelf break and used are unselective filter feeders that indiscriminately repack additional resources to meet their energy requirements. particles in the size range from 2 to 20 μm(Flood and Although under-represented in the zooplankton counts Deibel, 1998). By contrast, the flexible feeding stra- because of a sampling bias, appendicularians were tegies of copepods enable them to shift from herbivory abundant on the slope and 35% of the fecal flux was to omnivory and coprophagy when microalgal food is in made of large ellipsoidal appendicularian pellets. The short supply and of low-quality (Turner, 2004; Sampei combined grazing pressure of copepods and appendi- et al., 2004; Olli et al., in press). cularians on the relatively weak biomass of microalgae Fecal pellets were few and small in early June on the can largely explain the apparent oligotrophic nature of slope; the percentage of degraded pellets reached its the shelf–basin boundary. annual maximum (75%) and the fecal flux its annual minimum. Thus, there was no indication in the fecal 4.4. Atmospheric CO2 sequestration and shelf–basin POC flux that copepods and appendicularians inter- fluxes of POC on the Mackenzie Shelf cepted the flux of sinking ice algae (Maximum IV in early June). Large, fresh, cylindrical pellets first ap- The weak POC fluxes measured on the slope of the peared in the POC flux in late June, followed by large Mackenzie Shelf (see also O'Brien et al., 2006) were ellipsoidal pellets in early July, indicating that large dominated by inputs from the shelf from October to calanoids and appendicularians waited until the devel- April (1.0 g C m− 2 on the upper slope), and by marine opment of the DCM to start grazing. Despite the nume- production from May to August (0.7 g C m− 2 on the rical dominance of small copepods, small pellets were upper slope). About 50% of the POC in the fall/winter scarce in the summer POC flux, suggesting intense shelf-slope flux (Maxima I, II and III) originated from recycling by coprophagy and/or coprorhexy in the upper continental erosion of already buried carbon and, 200 m. Consistent with this interpretation, the cyclopoid therefore, did not contribute to the direct extraction of copepod O. similis, which is known to feed on the slow- CO2 from the atmosphere. The remaining POC in the sinking, chlorophyll-rich fecal pellets produced by small fall/winter flux (0.5 g C m− 2) was linked to biological grazers (e.g. Reigstad et al., 2005), numerically domi- production on the shelf and, therefore, contributed to the nated the mesozooplankton in summer (33%). The direct transfer of atmospheric CO2 from the shelf to the prevalence of small intact fecal pellets in the traps in slope and, eventually, to its sequestration in the arctic 52 A. Forest et al. / Journal of Marine Systems 68 (2007) 39–54 basin. In spring, the vertical flux of POC associated with man, 2003; Sakshaug, 2004). Less ice in summer on the the flushing of ice algae and exopolymers from the ice arctic margins is likely to increase primary production, matrix (Maximum IV: 0.1–0.2 g C m− 2) was not resulting in the enrichment of the surface bottom intercepted to any significant extent by grazers and sediments in POC. The increased resuspension of sedi- apparently contributed nearly in toto to the exportation ments and the advection of a BNL richer in marine POC of atmospheric CO2 at depth. By contrast, a large would lead to a more intense shelf–basin exchange and fraction of the limited summer primary production sink- potential sequestration at depth of biogenic carbon. Our ing on the slope was intercepted by large grazers that understanding of the final fate of the marine POC accelerated the downward flux of phytoplankton and ballasted by terrigenous material into the interior Arctic other particles. Ocean remains too sketchy for any firm conclusion on Clarifying the actual origin of the phytoplankton in the time-scales of its sequestration (e.g. Dittmar and the Deep Chlorophyll Maximum (DCM) observed at the Kattner, 2003; Benner et al., 2005; O'Brien et al., 2006). nutricline over the slope in the late June section is However, given estimated ventilation periods for the important in the context of quantifying shelf-basin POC deep Canada Basin (e.g. Gregor et al., 1998), particulate fluxes. Spatially, the slope DCM was in the continuity of carbon transported beyond the slope can be expected not the more intense mid-water phytoplankton bloom that to return to the atmosphere for at least several hundred developed on the shelf (Fig. 8d). This continuous mid- years. water layer of phytoplankton was bounded by the Consistent with the ubiquitous role of sea-ice in the 24.5 σt isopycnal above and the 25.6 σt isopycnal carbon cycle of arctic shelves, this study points to ice below, suggesting some isopycnal intrusion over the thermodynamics as a major engine of the shelf-slope flux slope of the phytoplankton produced on the shelf. of POC on the Mackenzie Shelf. It also underscores the However, the dynamical processes that would drive need for the simultaneous measurement of thermohaline such a mid-water intrusion over the slope remain un- convection in the coastal polynya system and POC known. If real, the intrusion over the slope of phy- fluxes at the shelf break and beyond, if the link between toplankton produced on the shelf would contribute sea-ice thermodynamics and shelf-basin fluxes outlined directly to the shelf-basin flux of POC. The simpler here is to be quantified and modeled. Logistically, this alternative interpretation is that the DCM observed over could be achieved by the deployment of automatic daily the slope resulted from local production at the nutricline. CTD profilers (cyclers) on strategically located mooring In this case, while effective in sequestering atmospheric lines, and/or by direct icebreaker-based measurements in CO2, the summer increase in the POC flux on the slope the polynya system over the winter months. (Maximum V) would not contribute to the shelf-basin POC flux. Acknowledgements

5. Conclusion We thank the officers and crew of the CCGS Amundsen and CCGS Sir Wilfrid–Laurier for enthusi- Present trends in the extent of the arctic sea-ice cover astic and professional assistance at sea. The CASES in September indicate that the summer ablation of sea-ice mooring program would not have been successful is intensifying (NSIDC, 2005). Most climate models without B. van Hardenberg, D. Sieberg, L. Michaud, S. anticipate that this trend will continue, and that, before Blondeau and G. Desmeules. Thanks to L. Létourneau, the end of the century, arctic shelves will become totally P. Lafrance, V. Perron, G. Darnis, A. Prokopowicz, S. free of ice during the summer months (ACIA, 2005). Lebel, P. Massot, for their help in sampling at sea and/or Absence of ice on the arctic shelves at the end of summer zooplankton analyses in the laboratory. We are particu- could mean that a larger volume of ice would need to be larly grateful to T. Ratkova for the enumeration and formed again in the fall and winter before the surface identification of protists. Special thanks to D. Barber and ocean becomes isolated from the atmosphere. According W. Chan for ice data; G. Ingram, B. Williams and Y. to our results, increased ice formation and thermohaline Gratton for the validated physical data; S. Bélanger and convection will intensify the resuspension of the shelf P. Larouche for SeaWiFS data processing; D. Benoît for sediment and its transport over the slope. In addition, ice the time-depth temperature section at the over-wintering determines phytoplankton production by limiting light, station; S. Brugel for primary production data; and K. wind mixing, on-shelf upwelling of nutrients, and the Simpson for nutrients data. The contribution of the vernal stratification of the upper water column (e.g. CASES network managers M. Fortier, M. Ringuette and Carmack and MacDonald, 2002; Carmack and Chap- J. Michaud to the coordination of work at sea and A. Forest et al. / Journal of Marine Systems 68 (2007) 39–54 53 workshops is gratefully acknowledged. We thank two Carmack, E.C., MacDonald, R.W., 2002. Oceanography of the anonymous reviewers for insightful comments and Canadian Shelf of the Beaufort Sea: a Setting for Marine Life. Arctic 55 (suppl. 1), 29–45. suggestions. The Research Network CASES is funded Carmack, E.C., Macdonald, R.W., Jasper, S., 2004. Phytoplankton by the Natural Sciences and Engineering Research productivity on the Canadian Shelf of the Beaufort Sea. Marine Council of Canada (NSERC) and the Canada Foundation Ecology Progress Series 277, 37–50. for Innovation. AF benefited from scholarships from de Haas, H., van Weering, T.C.E., de Stigter, H., 2002. Organic carbon NSERC and the Fonds québécois de la recherche sur la in shelf seas: sinks or sources, processes and products. Continental Shelf Research 22 (5), 691–717. nature et les technologies. 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