Seasonal and spatial patterns in mass and organic matter sedimentation in the North Water

Deep-Sea Research II 49 (2002) 5227–5244

Seasonal and spatial patterns in mass and organic matter sedimentation in the North Water B.T. Hargravea,*, I.D. Walshb, D.W. Murrayc a

Marine Environmental Sciences Division, Department of Fisheries and Oceans, Bedford Institute of Oceanography, Dartmouth, Nova Scotia B2Y 4A2, Canada b Department of Oceanography, Oregon State University, Corvallis, OR 97331-5503, USA c Center for Environmental Studies, 135 Angell St. Brown University, Box 1943, Providence, RI 02912-1943, USA Accepted 23 February 2002

Abstract Sedimentation rates in the North Water between September 1997 and July 1999 were calculated from integrated monthly values for dry and organic matter, organic carbon, nitrogen, and sum-chloropigments, using pairs of multi-cup sediment traps moored at two depths >200 m at five stations. Highest fluxes in upper traps (144 and 297 g m
2 yr
1, 8 and 12 g C m
2 yr
1) occurred near Ellesmere Island at the western end of a cross-polynya transect (761N). Rates to the east under the West Greenland Current (35 and 124 g m
2 yr
1, 4 and 7 g C m
2 yr
1) were higher than those in the central area of the polynya (13 and 39 g m
2 yr
1, 1.4 and 2.6 g C m
2 yr
1) and to the north at the entrance to Kane Basin (781N), where levels of inorganic matter in settled material were relatively high (50 g m
2 yr
1, 1.8 g C m
2 yr
1). Minimum fluxes in upper traps (4.2 g m
2 yr
1, 0.7 g C m
2 yr
1) were measured at the southern limit of the polynya in northern Baffin Bay (751N). Annual fluxes of particulate matter in the polynya between 1997 and 1999 (4–308 g m
2 yr
1, 1–14 g C m
2 yr
1) are higher than previously reported for ice-covered arctic regions. There was a similar seasonal progression at all locations with sedimentation rates increasing from winter minima during March followed by a larger secondary peak of maximum particle flux between June and September. Maximum sedimentation occurred earlier (April–June) on the eastern side of the polynya than off the coast of Ellesmere Island (July–September). High fluxes were sustained through late fall and early winter months at sites close to the coasts of both Greenland and Ellesmere Island. At most stations 40–50% of the annual organic matter sedimentation occurred during a prolonged period of high sedimentation between June and October. Sedimentation rates at 50 m above bottom were 30–50% higher than at shallower depths and settled material contained higher amounts of inorganic matter. Annual fluxes for all variables at four stations where between-year comparisons could be made showed that rates were two to five times higher in the second year of the study. r 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction *Corresponding author. Tel.: +1-902-426-3188; fax: +1902-426-6695. E-mail address: [email protected] (B.T. Hargrave).

Seasonal variations in sedimentation of particulate organic matter have been observed successfully in ice-covered polar waters using long-term moorings containing multi-cup sediment traps

0967-0645/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 2 ) 0 0 1 8 7 - X

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B.T. Hargrave et al. / Deep-Sea Research II 49 (2002) 5227–5244

(Wefer, 1989; Honjo, 1990; Hargrave et al., 1994; Bodungen et al., 1995; Bauerfeind et al., 1997). When deployed on moorings with current meters, particle traps collect material that can be compared to time-series measurements of variables such as temperature, current velocity and direction, and suspended particulate matter to provide information useful in interpreting changing seasonal and spatial patterns in flux rates and composition of settled particulate matter. Observations with moored and drifting sediment traps in polar oceans during the past two decades have shown that rapid short-term changes and inter-annual variations in particle fluxes occur. The changes may be associated with variations in ice cover, stratification/upwelling, enhancement of phytoplankton production due to upwelling, and variability in grazer populations (Wassman et al., 1990, 1991; Bodungen et al., 1995; Ramseier et al., 1997). The North Water, a polynya at the northern extension of Baffin Bay, is a region where large gradients in biological production, sedimentation rates of particulate matter and composition of settled material might be expected. Water mass characteristics show the presence of waters of Arctic origin advected by the West Greenland Current. These waters, each with their unique physical and geochemical characteristics, meet and mix within the domain of the North Water (B#acle, 2000; B#acle et al., 2002). Rates of primary production and sediment transported by resuspension and lateral movement in these different water masses would be expected to be reflected in differences in vertical fluxes and particle composition. This study reports mass and organic matter fluxes measured with sediment traps moored at various locations in the North Water between 1997 and 1999. The data allow regional and interannual comparisons of vertical fluxes of particulate matter into deep (>200-m depth) water from traps at two depths at most mooring sites with samples collected over two consecutive years at five locations. The data provide the most complete description of regional and seasonal changes in particle sedimentation rates in any arctic or subarctic region to date. Observations for mass, bulk organic/inorganic matter, carbon, nitrogen, and

sum-chloropigments in settled material presented here provide a broad outline of spatial and annual temporal patterns of particulate matter sedimentation into deep (>200 m) water of the polynya. Details of microscopic observations and analyses for specific inorganic elements, isotopes and plant pigments in deposited material will be presented elsewhere (see also Sampei et al., 2002; Booth et al., 2002).

2. Materials and methods 2.1. Sediment trap moorings Sediment traps were placed on long-term moorings with current meters at six stations in northern Baffin Bay (75–781N; Fig. 1) to collect settling particles over 23 months between September 1997 and August 1999. Five moorings were in place from late August–early September 1997 to midJuly 1998 at stations D1, S2, S4, S5 and N2 (Fig. 1). Sample cups and batteries were replaced on recovered traps in July 1998, with re-deployments at approximately the same locations at all sites except the most northerly one (N2). Only the upper sections of moorings at N2 and at the central station (S4) were recovered. In the absence of a re-deployment at site N2 (due to logistic reasons), a mooring with two traps was deployed at a new north-central site (D2). All moorings placed in July 1998 and the lower part of the initial mooring placed at the central site S4 were recovered in mid-September 1999. Pairs (A and B) of time-programmed, multicupped sediment traps were placed at shallow (198–259 m) and deep (315–515 m) depths on each mooring. Actual depths of upper traps (A) varied from 200 m as planned due to variations in actual bottom depth at final mooring locations. Lower traps (B) were 50 m above bottom at all sites. Only upper traps were recovered at the northern (N2) and central (S4) locations in July 1998 due to acoustic release failures. Traps on the mooring at the southern location, re-deployed on 3 July 1998, functioned as programmed, but mooring flotation was lost after 1 November 1998 (determined by the

B.T. Hargrave et al. / Deep-Sea Research II 49 (2002) 5227–5244

Fig. 1. Map of mooring stations in the North Water, northern Baffin Bay, during 1997–1999. Pairs of multi-cup sediment traps were placed with current meters on long-term moorings at two depths (see Table 1) at locations that formed a north–south transect along 751W longitude (stations N2, D2, S4 and D1) and an east–west transect along 761N latitude (stations S2, S4 and S5).

date after which no particulate matter was collected by either trap). Traps at the north central site (D2, deployed on 20 July 1998) only collected particles during the first two sampling intervals. The record of carousel rotation indicated that the trap had performed as programmed leading to the conclusion that blockage of the funnel neck with aggregated material occurred after 15 August 1998. Similar blockage is believed to have occurred at the central mooring site (S4), where samples (n ¼ 3) were

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collected between 26 August and 15 October 1997, but the remaining 11 cups were empty. Pooling equipment from different sources was required to obtain sufficient numbers of traps for our study. Four different designs of sediment traps were used with pairs of traps on each mooring of similar design (Table 1). Anderra current meters were placed 10 m above traps to provide a record of current speed and direction throughout mooring periods. Current speeds at depths of traps on moorings (200–550 m) were generally low (3– 5 cm s
1), with slightly stronger flows (7 cm s
1) in southward flowing water from Smith Sound (Ingram et al., 2002; Melling et al., 2001). Baffled lids covered the upper open end of all traps to reduce internal turbulence (US GOFS Working Group, 1989). Cells within each baffle lid all had openings with a height/diameter (aspect) ratio (AR) of 5 except the McLean design (AR of 2.6; Table 1). Intervals selected for trap cup rotation (2–4 weeks during spring, summer, and fall months and bi-monthly intervals during winter months) depended on the number of cups available in each trap (Table 1). Interpolation was used to calculate integral fluxes over two consecutive annual cycles (1997–1998, 1998–1999). Due to the long deployment periods, buffered formalin was used to prevent decomposition of organic matter in settled material (US GOFS Working Group, 1989). Before deployment, cups were filled with a preservative solution prepared by diluting 6 l of 37% formaldehyde with 38 l of filtered (0.22 mm) seawater collected at 400 m (site D1; Fig. 1) to make 44 l of cup-filling solution (approximately 5% w/v formalin). Borax (100 g) and NaCl (22.5 g) were added as a buffer to maintain a neutral pH and to create a brine solution approximately +0.5% above ambient salinity. 2.2. Sample analyses Sample cups removed on trap retrieval were kept cool and allowed to settle for 24 h before 1 ml was removed for salinity determination using an optical refractometer. Formalin alters the refractive index of seawater, but comparison of samples with freshly prepared filling solution showed that

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Table 1 Technical design specifications for four types of multi-cup sediment traps deployed at various locations (shown in Fig. 1) on six longterm current meter moorings in the North Water during 1997–1999. OSU indicates Oregon State University; BIO, Bedford Institute of Oceanography Manufacturer name Model

OSU Tracer-15

Technicap PPS.3/3

BIO Mark II

McLean Mark 78G-21

Mooring Site (Fig. 1) Weight in air (kg) Weight in water (kg)

S4 75 35

S2, S5 44 20

N2 68 29.5

D1, D2 75 55

Frame material Frame height (cm) Frame width (cm)

Titanium 228 101

Fiberglass 188 61

Aluminum 6061-T6 156 62

Stainless steel 152 91

Collector material Collector shape Collector height (cm) Collector width (cm) Collector aspect ratio Collection area (m2) Cone neck height (cm) Cone neck width (cm)

Fiberglass Funnel 160 79.8 2 0.5 12.7 3.6

Fiberglass Cylinder-to-funnel 160 40 4 0.125 15 3.4

Fiberglass Funnel 75 38 2 0.112 18 2.5

Polyethylene Funnel 110 80 1.4 0.5 11 2.85

Baffle Baffle Baffle Baffle

Hexcel aramid fiber 5 1 5

Hexcel aramid fiber 5 1 5

PVC 5 1 5

Polycarbonate 6.4 2.5 2.6

Acrylic 15 43.8 3.8 500 3.9

Polyethylene 12 16 4.9 250 3.2

Polymethylpentene 14 11 4 140 2.5

HD polyethylene 20 16 5 250 3

Cup Cup Cup Cup Cup Cup

material height (cm) cell width (cm) aspect ratio

material number height (cm) width (cm) volume (mL) neck dia. (cm)

supernatant samples in cups contained >35%. A strong formalin odour indicated that cups had not been flushed during deployments. Ten milliliter aliquots of supernatant solution were placed in acid-rinsed glass containers and stored refrigerated (51C) for dissolved organic carbon (DOC) analyses. DOC was measured using a high temperature combustion method (D. Hirschberg—pers. comm.) on selected supernatant samples (n ¼ 20) from cups representative of a wide variation in exposure times (28–406 d) and sample weight cup
1 (0.48–4.18 g). If solubilization of preserved particulate organic matter occurred during longterm sample storage in cups, DOC concentrations

would be expected to increase with time and sample weight. Levels of DOC were high (up to 24.5 mg l
1) due to the presence of formalin. After correction for the initial concentration in the filling solution, regression analyses did not show significant relationships between DOC concentrations and days of exposure (p ¼ 0:03) or sample weight (p ¼ 0:01). No determinations of dissolved ¼ + + inorganic nutrients (NO
3 , NO2 , PO4 , NH4 ) s using a Technicon Autoanalyzer were possible due to interference by formalin. As a consequence, mass and organic fluxes reported here refer to particles in cups on retrieval with no correction for dissolution/solubilization of organic and inorganic material.

B.T. Hargrave et al. / Deep-Sea Research II 49 (2002) 5227–5244

After decanting to reduce the volume to approximately 50 ml, the slurry in each cup was rinsed through 0.5-mm Nitexs mesh using freshly prepared filling solution. Macroscopic (>2 cm) organisms and those between 0.5 and 2 cm were transferred to separate vials containing fresh filling solution. All organisms removed from trap cups were intact and well preserved indicating the effectiveness of the formalin solution. Seasonal patterns in ‘‘swimmers’’ that accumulated in cups will be reported elsewhere, but numbers of organisms were highest during winter months when adult Calanus hyperboreous and chaetognaths (Sagitta elegans) were the dominant species >5 mm. Fewer individuals were present in trap cups during the May to October period of higher particle flux. Material passing through the mesh was transferred to a stainless steel sorting tray for manual removal of zooplankton o5 mm in size. Microcalanus spp. and early copepodite stages of C. hyperboroeus and Pseudocalanus spp. dominated this size fraction. All visible zooplankton >1 mm were removed and placed in vials containing filling solution for later identification, enumeration and analysis. For most samples, the entire slurry was transferred to a graduated cylinder using filling solution to bring the final volume to approximately 75 ml. Samples collected during periods of high sedimentation had particle concentrations that required dilution (1:10 or 1:100) to achieve a final concentration suitable for subsampling. A wide-mouth pipette was used to take subsamples for dilution for these samples. The slurry was quantitatively transferred from the graduated cylinder to a round-bottom flask using 25 ml of filling solution (100 ml final volume). A glass stirrer rotated by a variable speed motor and a magnetic stirring bar (30 rpm) turning in the opposite direction were used for gentle homogenization and a wide bore pipette was used to withdraw subsamples. Mass fluxes were calculated from dry weights of triplicate subsamples (0.01 and 0.02 fractions; 1% and 2% by total cup volume, respectively) placed on one pre-weighed (75 mg) 25-mm Millipore HA filter (0.45-mm pore size) and two Whatman GFF pre-ashed, pre-weighed 25-mm filters (1-mm pore size). All filters were rinsed with two aliquots

5231

(0.5 ml) of Super-Qs water. Average coefficients of variation (s/mean) ranged from 0.05 to 0.20 with highest variance in samples containing mucous-rich, aggregated material. GFF filters were dried (601C, 48 h), re-weighed and combusted (5501C, 24 h) to determine weight loss as a measure of total organic matter (and by difference bulk inorganic content). Two additional fractions (0.01) were placed on 13-mm pre-baked Whatman quartz microfiber QM-A filters (0.3-mm pore size). An elemental analyzer was used to measure total carbon and nitrogen (PN) by combustion of QMA filters. Fuming (1 h over concentrated HCL) did not significantly decrease total carbon measurements which were therefore assumed to represent organic carbon (POC). Duplicate 0.01 fractions were filtered onto 13mm quartz microfiber filters and stored frozen (
181C) prior to HPLC pigment determinations (Mantoura and Llewellyn, 1983). Sample size was always sufficient to exceed the detection limit (10 ng sample
1), and in many cases high concentrations required sample dilution. Undegraded chlorophylls, phaeophytins, phaeophytin-like pigments, phaeophorbides, and carotenoids were quantified separately (Head et al., 1994). These results will be reported separately; only concentrations of total pigment, referred to as sumchloropigments (sCHL), are reported here. Chlorophyll a concentrations may decrease (transformation to phaeopigments) when stored in formalin (Dell’Anno et al., 1999). This transformation would not affect the sum-chloropigment concentrations that we report here. Results from analysis of a 0.1 fraction filtered onto a 47-mm Whatman GFF filter used for inorganic (opal, carbonates) and trace metals also will be reported elsewhere. Samples (0.01 fractions) from the last cup of each trap were filtered onto 1-mm GFF filters for 234Th and POC analysis presented in Amiel et al. (2002). Only preliminary results from microscopic examination of 0.02 fractions removed for examination of phytoplankton are reported here, but observations of fecal pellet abundance for the first year of this study (1997–1998) are reported by Sampei et al. (2002). The slurry of particles remaining after all aliquots were removed was divided into three equal

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volumes as archived samples. Statistical analyses were performed using Systats software (Wilkinson, 1996).

3. Results 3.1. Spatial and depth variations in fluxes of particulate matter Annual fluxes for sedimentation for mass and organic matter, POC, PN, and sCHL (Table 2) were calculated from integrated monthly fluxes (Fig. 2A–E). Missing data for August–September at the northern site (N2) were calculated as the mean of fluxes in July and October. Large differences in fluxes occurred among the mooring sites, which generally corresponded to the regional grouping of water mass structures based on CTD surveys of the area in 1997 and 1998 (B#acle, 2000; B#acle et al., 2002). Highest annual particle fluxes, as recorded in upper traps, occurred in the western part of the polynya near Ellesmere Island (S5; Table 2, Fig. 2D). Mass fluxes at the center of the east–

west transect (S4) and under the West Greenland Current closer to Greenland (S2) were intermediate to those in the central and western regions. Excluding the western site, a north–south gradient was apparent during the period of increased sedimentation in June–July 1998 (Fig. 2A–C and E): mass fluxes in upper traps to the north (340 mg m
2 d
1) and at central transect sites (16–358 mg m
2 d
1) were higher than at the southern location (50–59 mg m
2 d
1). Anderra current meters placed above traps at the most northerly mooring site were not recovered. However, data from an ADCP at 108 m that was recovered showed a southward flow of surface water in the upper 100 m at high velocities (between 130 and 155 cm s
1; Melling et al., 2001). High current velocities caused the mooring to be tilted up to 451, moving semi-diurnally over a depth range of almost 100 m (200–300 m) during spring tides (H. Melling, pers. Comm.). Thus particle collection rates at this location are likely to be under-estimated (Gardner, 1985; Gust et al., 1994). Data for 1997–1998 from this site were only obtained from the upper trap. Although mass fluxes were intermediate to the range at other sites,

Table 2 Annual sedimentation rates for mass dry weight (dw), organic matter (org), organic carbon (C), nitrogen (N) and sum-chloropigments (sCHL) for sediment traps moored at five sites (Fig. 1) in the North Water over two consecutive years. Values were derived by integrating bi-weekly to monthly measurements (Fig. 2) between September 1997 and July 1999 for traps moored at depths indicated Annual particle flux (m
2) Site

Depth (m)

Deployment period (month/yr)

g dw

g org

gC

gN

mg sCHL

N2A S5A S5A S5B S4A S4A S2A S2A S2B S2B D1A D1B

203 257 259 315 205 201 198 205 507 515 250 406

11/97–06/98 08/97–06/98 07/98–06/99 08/97–05/98 09/97–06/98 08/98–07/99 09/97–06/98 08/98–07/99 09/97–07/98 08/98–07/99 09/97–06/98 09/97–06/98

50.4 144.3 297.0 190.7 12.7 38.5 35.1 123.8 157.8 307.8 4.2 9.4

5.9 26.1 50.5 22.7 3.3 11.2 9.5 30.1 24.6 55.6 1.5 2.0

1.8 8.0 11.7 8.4 1.4 2.6 3.8 6.7 8.1 13.8 0.7 1.1

0.26 1.11 1.63 1.16 0.19 0.34 0.46 0.95 1.03 2.04 0.10 0.14

6.9 68.7 90.1 43.7 5.5 18.4 21.4 75.7 45.5 70.9 1.1 2.0

Fig. 2. Seasonal variations in mass (panels A–E) and organic matter (panels F–J) sedimentation in traps at five of the long-term mooring stations in the North Water (Fig. 1) between 1997 and 1999. Upper traps (unshaded histograms and solid lines) were moored at approximately 200 to 300 m; lower traps (solid histograms and dashed lines), at approximately 50 above bottom.

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organic matter and pigment fluxes were among the lowest measured (Table 2). Sites central and west on the cross-polynya transect along 761N were located within the geographical extent of the Central Northern Baffin Bay water mass type (B#acle, 2000; B#acle et al., 2002). These locations are dominated by the strong outflow of ice and cold Arctic Ocean water through Smith Sound (Melling et al., 2001; Ingram et al., 2002). Where fluxes in upper (201–259 m) traps could be compared between years, rates at the western end of the transect (S5) were approximately 10-fold higher in 1997–1998 and 5-fold higher in 1998–1999 than those at the central site (S4; Table 2). There were also higher fluxes at both locations during the second year of the study. Differences in annual sedimentation over depth could be compared at three sites, the eastern (S2), western (S5), and southern (D1) sites. Fluxes for all variables 50 m above bottom were higher (or equivalent in one case) than at the shallower depth, and settled material was enriched with inorganic matter (Table 2). Mass fluxes at the eastern site (S2) in 1997–1998 were 4.5 times higher at a depth of 507 m than at 198 m. The difference in 1998–1999 over a similar depth range (205–515 m) was 2.0 times. At the western site (S5), the change in mass fluxes over a smaller depth range (257–315 m) in 1997–1998 was lower (1.3 times). Other variables at this site showed smaller differences between depths. At the southern site (D1), annual values for mass (and sCHL) sedimentation were 1.9 times higher at the shallower depth (250 m) than at the deeper one (406 m), while other flux variables differed by smaller increments (Table 2). 3.2. Seasonal variations in particle fluxes and organic content Large seasonal changes occurred in particle mass flux at all stations, with patterns differing between upper and lower traps (Fig. 2). Highest fluxes polynya-wide occurred in early September 1998 in upper traps at the western station (1923 mg m
2 d
1), while maximum rates at the eastern (1350 mg m
2 d
1), northern (340 mg m
2 d
1) and southern (95 mg m
2 d
1)

locations occurred in June–July. At the eastern and western locations (S2, S5) high mass fluxes were sustained through fall and winter months, while at the others (S4 and D1) maximum rates did not extend beyond October. For upper traps >70% of total annual massflux matter occurred between June and October, but the pattern was not the same at all stations. High proportions of annual flux (as a percent of total mass in1997–1998) occurred in May and June in the south (36% at D1) and east/central (33% at S2, 34% at S4) regions of the polynya. Peak values occurred in June and July to the north (39% at N2) and in July and August along the central east coast of Ellesmere Island (34% at S5). The difference between seasonal minima and maxima (approximately 20-fold) was similar at stations with low (1–59 mg m
2 d
1 at D1A), intermediate (13–340 mg m
2 d
1 at N2A) and high (112–1923 mg m
2 d
1 at S5A) mass fluxes. Seasonal variations in organic matter, POC, PN and sCHL fluxes were even greater due to changes in organic composition throughout each year (Figs. 2 and 3). These changes magnified seasonality in fluxes of organic constituents. In general lowest seasonal values for all organic constituent variables occurred during the winter when mass fluxes were low. Maximum concentrations of organic matter in settled material occurred during times of highest mass flux. Periods of high mass fluxes during summer/fall months were associated with seasonal maxima in organic content. A small spring/early summer peak (April–June) at most stations (Fig. 2) coincided with the opening of the polynya (Be! langer, 2001; Be! langer et al., 2001). Ribbon colonies of species of pennate diatoms (Fragilariopsis, Navicula, Nitzschia, Pauliella, and Fossula) dominated biomass of settled phytoplankton (B. Booth, pers. comm.; see also Booth et al., 2002). This peak occurred in both shallow and deep traps where samples were collected simultaneously at two depths (Figs. 2B, D and E) and was apparent in rapid increases in organic matter, POC, PN and sCHL in settled material (Fig. 2F–J, Fig. 3). A much larger late summer/fall (July–early October) peak in sedimentation occurred at all sites (Fig. 2). Phytoplankton biomass in settled

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Fig. 3. Seasonal variations in organic carbon (POC, solid lines), nitrogen (PN, dashed lines) (panels A–E) and in sum-chloropigments (sCHL) (panels F–J) in particulate matter from moored traps at stations depicted in Fig. 1. Bold lines indicate samples from traps between 200 and 300 m; light lines from traps approximately 50 m above bottom (see Table 2).

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material during periods of highest particle flux between June and mid-September along the east– west transect (at S2, S4 and S5) was composed of mucous-rich mats containing aggregates of the centric diatoms, Chaetoceros socialis and Thalassiosira spp. (Booth et al., 2002). Sedimentation of aggregated mats of material into trap northcentral D2B (154 mg m
2 d
1) during the first two weeks of August 1998 and central trap S4B (>400 mg m
2 d
1) during the first two weeks of September was so rapid that the necks of trap funnels were blocked, preventing further sample collection. Maximum percentages of organic matter (>20%), POC (>10%), and PN (>1%) in settled material occurred in southern trap D1A (Figs. 2J and 3E), while highest sCHL amounts (>2000 ng mg
1) were present in the eastern (S2A) and western (S4A) traps (Figs. 3G and H. Organic content of material collected by traps on single moorings over the same period at two depths was also most similar during periods of high particle flux at the two sites where the comparison could be made (S2 and D1; Figs. 2G, J and 3B, E, G, J). Ratios of POC:PN (C:N) and POC:sCHL show that organic matter collected at the two depths at each site were most similar between late May-early June and September (Fig. 4B, E, G and J).

4. Discussion 4.1. Comparison with previous studies and empirical models The range of fluxes for annual sedimentation in different regions of the North Water (4–308 g m
2 yr
1 and 1–15 g C m
2 yr
1; Table 2) is higher than previously observed at polar latitudes. Measurements of mass flux at >701N and depths >100 m prior to 1991 vary over a wide range (1 g m
2 yr
1 off Axel Heiberg Island to

5237

124 g m
2 yr
1 at the shelf edge near Spitzbergen; Hargrave et al., 1994). POC fluxes generally scale with mass and thus reflect a similar range (0.01 to 5.6 g C m
2 yr
1). More recent observations in deep water in the Jan Mayen Current, Barents Sea, and Northeast Water polynya between 72 and 801N (Bodungen et al., 1995) vary within similar ranges (9 to 60 g m
2 yr
1and 0.4 to 4 g C m
2 yr
1). The pattern of increased flux beginning in March-April followed by a much larger increase during summer/fall months (Fig. 2) has been observed in many previous studies at locations >751N (Wefer, 1989; Honjo, 1990; Wassmann et al., 1991; Hargrave et al., 1994; Ramseier et al., 1997). Absolute rates and seasonal patterns of mass flux measured at the southern North Water site (D1) are remarkably similar to observations in the Northeast Water polynya (1992–93; Bauerfeind et al., 1997; Schuler . et al., 1997). In both cases low fluxes during winter increased in March with annual maxima in August and September. Hargrave (1985) calculated annual mass and POC fluxes for various oceanic regions >200 m, including seasonally ice-covered sub-arctic areas, away from continental margins. An empirical equation combined with observations of changes in organic content with depth was used to predict sedimentation from estimates of annual phytoplankton production and depth using data from moored trap studies prior to 1980. Estimated mass and POC fluxes for sub-arctic waters are similar to values measured at our sites most distant from shore (D1 and S4) in 1997–1998 (Table 2). Empirical models predict that particle fluxes should decrease exponentially below 200 m (Sampei et al., 2002). Decreases of this magnitude have been observed in the Norwegian Atlantic Current (1 to 2.5 g C m
2 yr
1) and the East Greenland Current (0.5 to 1 g C m
2 yr
1; Wassmann et al., 1991). Similarly, data from the Greenland Sea followed empirical relationships between sedimentation, total primary production and depth (Bod-

Fig. 4. Seasonal variations in ratios (derived from data in Figs. 2 and 3) of POC:PN (panels A–E) and POC:sCHL (panels F–J) in settled material from traps moored at two depths at five sites (Fig. 1). Bold lines indicate samples from traps between 200 and 300 m; light lines, from traps approximately 50 m above bottom (see Table 2).

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ungen et al., 1995). This was not the case in the North Water, where particle fluxes 50 m above bottom were higher than at shallower depths (Table 2). The increased flux was predominantly in mass, with typically smaller increases for organic constituents. The importance of processes such as resuspension and lateral transport that increase sedimentation with depth is discussed below. 4.2. Factors affecting measured fluxes Our measurements of particulate material fluxes may be underestimates, since we have not corrected for dissolution of inorganic matter or conversion of POC to DOC during prolonged collection periods. Preservatives are necessary to stop decomposition of organic matter, but even in the presence of formalin, which is effective in stopping microbial degradation (U.S. Global Ocean Flux Study, 1989), loss of organic matter may occur through hydrolysis and solubilization (Gardner et al., 1983). Several studies (Bodungen et al., 1995; Murray et al., 1996; Bauerfeind et al., 1997; Noji, 1998) have shown that from 10–20% of POC collected in long-term moored traps can be solubilized. However, DOC in cup-supernatant water in our study did not show any systematic variation with sample weight or preservation time. If solubilization of organic matter occurred, it could not be measured against the high background DOC concentrations created by formalin. The fact that we used four different designs of sediment traps must be considered when comparing fluxes at various sites. Hydrodynamic effects involving turbulence and current speed, the size and organic composition of settled material, trap design, mooring configuration and movements influence both rates and types of particles accumulated in collectors (Butman et al., 1986; US GOFS, 1989; Gust et al., 1994; Gust and Kozerski, 2000). Our most northerly site (N2) was in an area of high current velocity (>100 cm s
1) at the entrance to Smith Sound (Melling et al., 2001). This mooring was occasionally tilted >301, which would have decreased particle collection efficiency (Gardner, 1985). Currents measured close to traps at all other sites were generally o5 cm s
1. While

hydrodynamic biases might still occur, underestimates of particle fluxes would be less at other locations where moorings were not subject to such high currents. The collection efficiency of sediment traps has been shown to be related to collector shape as described by trap aspect ratio (AR), defined as collector height-width ratio (Butman et al., 1986; U.S. Global Ocean Flux Study, 1989). Baffled lids are usually placed in upper trap openings to reduce turbulence and retain deposited particles within collectors (Table 1). While there is evidence to the contrary, trap collection efficiency is generally highest for cylindrical collectors with AR>5 when current speeds are o10 cm s
1. For the traps used in our study, collector AR varied from 1.4 (McLean funnel shape) to 4 (Technicap cylindrical shape; Table 1). The AR for baffle cells was lower (2.6) in the McLean design compared to other trap types (5). Fluxes measured in McLean traps (at D1 and D2) were lower than at other locations for comparable times, while organic matter, POC and PN content in settled material were higher (Table 2, Figs. 2J and 3E). The assumption is that real spatial differences in fluxes were recorded, but it is also possible that lower sedimentation rates at D1 and D2 reflect reduced trap collection efficiency due to the low AR. Mass and organic matter fluxes in traps at the southern site (D1) during 1997–98 were similar to rates measured at the central site (S4A) in the same year (Table 2, Fig. 2C and E). Yet, in the following year, with the same trap types at the same locations, fluxes at S4A exceeded those at D1A by about five times (Table 2). Clearly, if trap design had an influence on measured fluxes, the effect was less than interannual variability. 234 Th measurements on material collected in the last sample cup were used to compare POC fluxes derived from traps and water-column Th deficits using pumps in the upper 100 m (Amiel et al., 2002). Agreement for stations sampled in July 1998 and August 1999 between calculated and measured Th fluxes was best (within a factor of 4– 8) for the cylindrically shaped Technicap traps while pump/trap ratios in fluxes for the OSU (>22) and the McLane (60) designs were higher.

B.T. Hargrave et al. / Deep-Sea Research II 49 (2002) 5227–5244

Similar discrepancies have been observed in other studies comparing of flux estimates from moored traps and water column Th measurements (Amiel et al., 2002). There are many reasons why calculated and measured estimates of Th flux might not agree. Water column observations were offset by at least two weeks from moored trap sample collection. Also, particle collection efficiency may have been lower in the OSU and McLane funnel-shaped traps. The importance of particle type and composition in affecting particle collection rate in different traps was apparent in our study when mucous-rich diatoms mats clogged lower funnel openings of these trap types during September and August, respectively, in two successive years. Blockages occurred following periods of relatively low mass fluxes (12–403 mg m
2 d
1). At the same time of year at other sites where Technicap traps were deployed, fluxes were ten-fold higher yet no blockages occurred. The Technicap design had the highest collector AR (4) of any trap type used in our study. The smaller mouth opening and cylindrical shape with vertical walls leading to an internal funnel may have been effective in preventing accumulation of mucous-rich aggregated material on inner trap surfaces. Another indication that some trap designs may have underestimated particle fluxes is provided by comparisons of organic carbon sedimentation and benthic respiration, where concurrent data from moored traps 50 m above bottom and core incubations were available. Grant et al. (2002) showed that carbon demand exceeded supply by 5–40 times at central site S4 where the OSU trap design was moored. At western site S5 (Technicap trap), supply and demand were approximately equivalent. Although organic matter stored in sediments could support benthic respiration rates in excess of organic carbon supply for short periods, the discrepancy also could indicate that organic carbon flux at S4 was underestimated. 4.3. Spatial and temporal changes in particle fluxes The spatial and temporal patterns of fluxes measured at different sites resembled the seasonal progression of changes in phytoplankton biomass

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and production described for the North Water using SeaWiFS imagery (Be! langer, 2001; Be! langer et al., 2001) and by direct measurements (Klein et al., 2002; Odate et al., 2002). High chlorophyll concentrations spread from the east in April, to the north in May–June and to the west in June. Although collection of samples with moored traps ended in July 1999, chlorophyll concentrations along the coast of Ellesmere Island remained high throughout August, suggesting that the pattern of prolonged bloom conditions through late summer observed at western site S5 in 1998 (Fig. 3I) reoccurred in 1999. As observed with particle fluxes, the lowest levels of biomass and rates of phytoplankton production occurred in southern areas of the polynya. Patterns in phytoplankton production and particle flux are consistent with a counterclockwise gyre of nutrient-rich water along the coast of Greenland that stimulates phytoplankton growth during early summer (Klein et al., 2002; Tremblay et al., 2002). The smaller initial peak in sedimentation (April– May) at most sites coincided with the onset of phytoplankton production. Maximum ice cover (>90%) peaked during March and decreased rapidly during April-May to a minimum (10– 30%) in June (Mundy, 2000; Miller et al., 2002). Particle fluxes increased rapidly during June and July to the south and east, followed by the north. Maxima occurred along the central transect (at S4 and S5) during August and September and the period of high sedimentation was prolonged through fall and winter months at each end of the transect (sites S2 and S5; Fig. 2). This prolonging was not due to enhanced organic matter flux, since concentrations of organic constituents decreased during this time (Figs. 2 and 3). Since the effect occurred at peripheral locations (east and western sides of the polynya), it could reflect resuspension and lateral transport of particulate inorganic matter discussed below. Other studies at northern latitudes have recorded episodic mass sedimentation events as we observed. Wassmann et al. (1990) measured maximum fluxes (1 g C m
2 d
1) during a dense Phaeocystis pouchetii bloom during May-June in the Barents Sea. Clogging of lower funnel trap openings also was reported between July and

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September during a 3-year study with moored Kiel traps in the Greenland Sea (Bodungen et al., 1995). However, no clogging of the Kiel trap or the BIO design used in the present study occurred during a year-long deployment of both trap types at 791N off Ellef Ringes Island (Hargrave et al., 1994) where fluxes were very low (1 g m
2 yr
1; 0.1 g C m
2 yr
1). No mucous-rich material was collected in this permanently ice-covered shelf area. High fluxes of radiolarians (Sagenoscena spp.) in the study of Bodungen et al. (1995) were thought to have formed nuclei for formation of large aggregates. Radiolarians were not abundant in any of our samples. Mucous-rich aggregates were tentatively identified as arising from breakdown products of the diatom Chaetoceros socialis * et al., (Booth et al., 2002; but see also Acuna 2002). However the aggregated material also contained pennate diatoms (Fragilariopsis, Navicula, and Nitzschia) in addition to C. socialis. High settling velocity of mucous-rich material could effectively remove other phytoplankton species from the water column by scavenging during rapid sinking (Sampei et al., 2002). 4.4. Changes in flux with depth and primary production POC fluxes in surface layers (o100 m) were observed with floating sediment traps (Michel et al., 2002; Huston and Deming, 2002) and calculated from 234Th/POC profiles (Amiel et al., 2002) at transect stations S5 and S4 in July 1998 over the 5–174 m depth range. Rates (100–1100 mg POC m
2 d
1) were five to ten times higher than values measured with moored traps (Table 2). Maximum chlorophyll a fluxes (5–19 mg m
2 d
1) in floating traps at 50 m during June were 10–100 times greater than rates measured in moored traps at depths >200 m (Michel et al., 2002). The comparative results indicate a strong attenuation of particle flux between 100 and 200 m in the North Water. Increased particle fluxes in deeper traps in our study are consistent with the view that resuspended particles comprise an increasing proportion of settling material with proximity to the bottom (Monaco et al., 1987; Walsh et al., 1988;

Bodungen et al., 1991). The two sites in the North Water with the highest fluxes of inorganic material were located in areas of steep bottom gradients where particle rebound and horizontal advection of resuspended sediment would be expected. Observations during our cruises in 1998 (I. Walsh, unpubl. obs.) indicated the presence of a nearbottom nepheloid layer with increased suspended particulate matter concentrations at some stations that extended up to >50 m above bottom. Klein et al. (2002) combined observations over three years (August 1997, April–July 1998 and September and October 1999) to estimate total (POC+DOC) primary production by phytoplankton in various regions of the North Water for the April to September production period. Highest rates (377 g C m
2 yr
1) occurred in eastern regions of the polynya within the Carey Island regional water mass (B#acle, 2000) with maximum values (>5 g C m
2 d
1) during May and June. SeaWiFS imagery showed that phytoplankton blooms (primarily Thalassiosira spp.) began in the south and east regions of the polynya in May 1998 and 1999 at the time of a reduction in ice cover (Be! langer, 2001; Be! langer et al., 2001; Booth et al., 2002). This scenario was followed in June by a more intense bloom dominated by Chaetoceros socialis that progressively dominated phytoplankton biomass in northern and western areas in July. Sedimentation of POC in the eastern polynya (S2), with rates that increased rapidly during June and early July (20–120 mg m
2 d
1), represented a small fraction (0.4–2.4%) of estimated values for total primary production. As observed in other studies of temporal changes in phytoplankton and ice-algal populations in different arctic regions, release of dissolved extracellular products accounted for a large proportion of total production. An average of 44% (maximum 82%) of total primary production was estimated to be released as DOC in the North Water (Klein et al., 2002). If phytoplankton production and particle fluxes are compared on the basis of POC production, from 1.7–5.7% of carbon fixed in particulate form during the growing season was collected in traps between 198 and 515 m (Table 2). Lower rates of phytoplankton production were measured to the west along Ellesmere Island

B.T. Hargrave et al. / Deep-Sea Research II 49 (2002) 5227–5244

(76 g C m
2 yr
1; Klein et al., 2002). This was the area, however, where the highest particle fluxes (6.9–11.5 g C m
2 yr
1, equivalent to from 9–15% of POC production) were observed. Water depth was shallower (365 m) at this location in comparison to other stations along the east-west transect (443–581 m). The influence of lateral transport of resuspended material and proximity to steep bottom gradients in increasing fluxes has been mentioned above; counter-clockwise advective transport of post-bloom biomass may also pertain. 4.5. Compositional changes in settled material Floristic descriptions of dominant phytoplankton in suspended and sedimented material showed that the smaller spring/early summer (April-June) peak in particle flux occurred when loose cells of Thalassiosira spp. and chains of ribbon-pennate diatoms dominated biomass and cell numbers in settled material (Booth et al., 2002). Melosira arctica, a centric diatom commonly associated with epontic communities of ice algae, and Phaeocystis spp., phytoplankton species commonly observed when mass sedimentation events have been measured in polar waters (Wassmann et al., 1990; Syvertsen, 1991; Bodungen et al., 1995), were not abundant in water samples from the photic zone collected during our study (Lovejoy et al., 2002). M. arctica was present in moored trap material from the North Water (P. Mudie, pers. comm.) but Phaeocystis was not observed. Coccolithophores were also not found despite previous observations in various sub-arctic regions that these organisms predominated in settled material at certain times of the year (Honjo, 1990; Bodungen et al., 1995). The lack of carbonate in our samples is consistent with the absence of calcareous coccolithophores. Our data do not support the hypothesis of Honjo (1990) that calcareous organisms dominate particle flux at all polar latitudes. High currents through Smith Sound would prevent deposition of particulate matter as indicated by the predominance of rock and gravel substrates in northern parts of the polynya (Grant et al., 2002). While localized resuspension due to proximity to steep bottom gradients might be a

5241

factor in increasing near-bottom particle fluxes, this process was not indicated by POC:PN ratios. Ratios (6–11) in settled material were similar at sites along the cross-polynya transect (Fig. 4B–D). Higher ratios (>11), indicative of resuspended material, only occurred to the north during winter and to the south in June (Figs. 4A and E). POC:PN ratios at all sites except near Ellesmere Island increased rapidly between April and June indicating mineralization of nitrogen (Huston and Deming, 2002). POC:sCHL ratios, however, decreased during this time as the overall supply of freshly produced organic matter was reduced (Fig. 4). Collectively, these results point to anticyclonic biomass advection into regions of lower productivity to partially account for summer-time moored trap results on the western side of the polynya. Particles collected at all sites during the winter, when fluxes decreased by an order of magnitude, were predominantly (>75%) inorganic (Fig. 2). Biogenic material was most abundant between May and October, when ratios of POC:PN (6–8) and POC:sCHL (o100–1000) indicated the presence of relatively freshly produced biogenic matter (Fig. 4). POC:sCHL ratios o100 in May and early June (Fig. 4G, H and J) are characteristic of seston enriched with phytoplankton. This was also the period when organic composition in shallow and deep traps was most similar possibly indicating high particle settling velocities. Sampei et al. (2002) found that diatoms predominated collected material from mid-May through August– September. Calculations for upper traps at eastcentral sites (S2 and S4) in June 1998 indicated that from 18% to 57% of sedimented organic carbon could have been contained in autotrophic phytoplankton (primarily diatoms; B. Booth, pers. comm.). Biomass of phytoplankton in the water column during this period was dominated by ribbon colonies of several pennate diatom species and the centric form Thalassiosira spp. (Booth et al., 2002). POC:PN ratios >10, indicative of more refractory organic material, and high values of POC:sCHL (>1000) with minimal chloropigment contribution, tended to increase in late summer and fall when fluxes were highest at most stations

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(Fig. 4). C:N ratios of sinking particulate matter in floating traps at depths o140 m had increased to 10 by July (Huston and Deming, 2002). Late summer was also the time when fecal pellet abundance increased (Sampei et al., 2002). The most extreme changes in POC:sCHL, with maximum values during winter months, occurred at southern site D1 (Fig. 4J). A similar pattern occurred at the northern site (N2), where POC:PN ratios increased from 8 to 15 during winter and early spring months (Fig. 4A). Equally high POC:PN values occurred at other locations in June (Fig. 4B and E) and August (Fig. 4C). 4.6. Interannual variability To our knowledge, only one previous study has provided data for between-year comparisons of annual particle flux in polar waters (Bodungen et al., 1995). Annual fluxes at 500 m for different variables in the Greenland Sea varied two- to fourfold between 1988 and 1991. The between-year differences in the North Water were of a similar magnitude (fluxes two to five times higher in 1998– 1999 than 1997–1998; Table 2) and are consistent with SeaWiFS observations of surface chlorophyll distribution (Be! langer, 2001; Be! langer et al., 2001). In 1998 maximum chlorophyll concentrations were reached in mid-June. In 1999, however, the bloom started later, with maximum chlorophyll concentrations occurring in mid-July and high values off Ellesmere Island persisting through mid-August. Year-to-year differences in melting of winter ice cover, vertical density and thermal stratification, and variability in drifting pack-ice may account for the interannual variation in production by epontic and phytoplankton algal communities indicated by changing patterns in chlorophyll distribution. There is also evidence for interannual variability related to seasonal differences in ice cover that affect inventories of inorganic and organic carbon in the North Water (Miller et al., 2002) and a prediction that changes in source waters, and thus nutrient inventories, to the polynya will be critical to future variability (Tremblay et al., 2002). The results show the importance of conducting seasonal studies of biological production over more than one year

and at more than one location to gain an understanding of processes controlling the formation and export of organic matter in arctic waters.

Acknowledgements We thank the Canadian Coast Guard for essential logistic support; officers and crew of the Canadian icebreakers CCGS Louis S St. Laurent and CCGS Pierre Radisson made our work possible through their dedicated and professional efforts. The moored sediment trap program would not have been successful without the assistance of H. Melling, D. Sieberg, P. Peltola, and J.-Y. Anctil who were involved with mooring design, assembly, deployment and retrieval. We thank Y. Kashino, S. Kudoh, T. Odate, M. Sampei and B. Schofield for help with trap preparation, mooring deployment and recovery, and ship-board sample processing, G. Phillips for mass determinations, M. Altabet for POC and PN analyses, T. Perry and E. Head for HPLC pigment analysis, D. Hirschberg for DOC analysis, and B. Booth, P. Mudie and A. Rochon for microscopic and SEM examination of settled material and floristic analysis. Support for this project was provided by the Government of Canada (National Science and Engineering Research Council—North Water Research Network and the Department of Fisheries and Oceans), the US National Science Foundation Office of Polar Programs and the National Institute of Polar Research of Japan. Our thanks to J.K. Cochran and an anonymous reviewer for comments on the manuscript. This is a contribution to the International North Water Polynya Study (NOW).

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