ContinentalShelfResearch,Vol. 14, No. 2/3,pp. 279-293,1994. Printedin GreatBritain.

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Seasonal variability in particle sedimentation under permanent ice cover in the Arctic Ocean B. T. HARGRAVE,*B. VONBODUNGEN,~"P. STOFFYN-EGLI~:and P. J. MUDIE§ (Received 26 February 1992; accepted 16 June 1992) Abstraet--A study at 79°N over the polar continental shelf off Ellef Ringes Island in 1989-1990 provided year-round measurements of particulate matter sedimentation in a permanently ice covered region of the Arctic Ocean. Mean annual flux rates of mass (1.1 g m-2), organic carbon (134 mg m-2), nitrogen (24 mg m-2), chlorophyll a (3 mg m -2) and biogenic silicon (11 mg m-2) were determined by deployment of two sediment traps from the Canadian Ice Island at a water depth of 100 m. High fluxes of mass, biogenic silicon and inorganic matter occurred between July and September during the melt-water runoff. Maximum sedimentation of organic matter and chlorophyll a occurred in August and September when centric diatoms and zooplankton fecal pellets were numerous in samples. Between February and June, when mass fluxes were low, settled particles were organically rich with low carbon:nitrogen ratios (4-8) in contrast to higher values (612) during the melt-water period. Mineralogy showed that chlorite, mica, illite and quartz were abundant in settled particles collected in August, October and December. Similar minerals, thought to be supplied as small particles by eolian transport, are present in ice cores and cryoconites on the Ice Island. The observations provide data for assessing future changes in production and particle export for this ice-covered region of the Arctic Ocean that may be altered due to global warming and related changes in ice cover.

INTRODUCTION

FOR logistic reasons, previous studies with sediment traps in offshore polar ocean regions have usually been carried out in areas where ice-free periods exist during some portion of the year (FISCHER et al., 1988; HONJO et al., 1988; HONJO, 1990; HEBBELN and WEFER, 1991). In some of these studies, for example the Weddell Sea (FISCHERet al. 1988) and Fram Strait (HEBBELIq and WEFER, 1991), seasonal variability in particle fluxes was attributable to increases during the period of ice transgression. Studies with sediment traps in shallow Arctic coastal waters (ATKINSOtq and WACASEY, 1987; CAREY, 1987; HSlAO, 1987) have also shown that increased sedimentation occurs during the period of ice melting due to physical and biological processes that result in particle generation at ice edges (TREMBLAYel HI., 1989; RIEBESELLet al., 1991). * Habitat Ecology Division, Biological Sciences Branch, Department of Fisheries and Oceans, P.O. Box 1006, Dartmouth, Nova Scotia, Canada, B2Y 4A2. tSFB 313, Institut fiir Meereskunde, Universit/it Kiel, Olshausenstrasse 40, D-2300 Kiel 1, Germany. ~Micrbchem, Geochemistry Consultants, RR # 2 Jeddore-Oyster Pond, Nova Scotia, Canada, B0J lW0. § Marine Environmental Geology, Atlantic Geoscience Center, Department of Energy, Mines and Resources, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia, Canada, B2Y 4A2. 279

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These observations suggest that sedimentation rates in seasonally ice free areas of the Canadian Arctic Archipelago Ocean may not be representative of fluxes in permanently ice covered regions. Since the average extent of sea ice cover varies from 41% of the total Arctic Ocean area in summer to 82% during the boreal winter (WALSH and JOHNSON, 1979), it is important to have annual measurements of sedimentation in areas of permanent ice cover to provide information on rates of elemental cycling under these regions of the Arctic Ocean. In 1986, a research camp established on an ice island off Axel Heiberg Island, north of the Canadian Arctic Archipelago, allowed a preliminary study of seasonality of particle fluxes in a permanently ice covered region of the Arctic Ocean, but for logistic reasons no data were collected during summer months (HARGRAVEe t al., 1989a). We now report results from mooring of two sediment traps under the ice island from September 1989 to 1990. These data provide the first year-long record of particle fluxes measured under permanent ice cover in the Arctic Ocean. MATERIALS AND METHODS An ice island, located in the Arctic Ocean on Canada's polar continental margin (HoBsoN et al., 1989), was used as a platform for mooring two mu!ticup funnel-shaped traps to collect settling particles over one year. The ice island was semi-stationary or drifted sporadically (< 1 km day-1) during the study when temporary leads opened within Peary Channel. Satellite positioning showed irregular movement of the island ( 0.05). These results might be attributable to the smaller funnel volume and steeper wall angle of the BT trap relative to the KT trap. This design difference could have decreased the relative residence time of particles within the funnel of the BT trap before they either entered a cup or were removed by hydrodynamic exchange. No fouling of inner funnel walls with an organic film was observed in either trap on retrieval that would account for the observations. HgC12 would have prevented zooplankton from grazing once particles settled into cups. It is also possible that heterogeneity in types and chemical composition particles reflects small scale variation in processes affecting sedimentation. For example, the large fluxes of mass and inorganic matter to both traps in July and August (Table 1, Fig. 1) might be due to the release of melt-water from the ice surface and drainage through leads. Freshwater lakes that form on the island during late summer have been observed to empty within a few hours, discharging melt-water through cracks in the glacial ice. High fluxes of chlorophyll a in August and September, which coincided with a seasonal maximum in suspended chlorophyll a observed during these months previously from the ice island (HARGRAVEet

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al., 1989a), could also create aggregates of flocculated organic-rich particulate matter (RIEBESELL et al., 1991). Both processes could create patchiness in suspended particle concentrations that were reflected in variations in particle composition between the traps. With the exception of the abundance of the diatom Chaetoceros sp. in trap samples during August, no other planktonic algal species was dominant throughout the year. Both chlorophyll a and biogenic silicon in settled particles were highest in August with peak values extending into September, October and November (Table 1). Diatoms (mainly Thalassiosira sp.) and silicoflagellates and chrysophytes were also common during these months when high numbers of fecal pellets occurred in traps. This contrasted the lower numbers of pellets between April and July. Thus, although there was not a typical seasonal succession of phytoplankton cell types as occurs in more southern latitude oceans, there was a pattern of increased inorganic matter sedimentation from melt-water release during July and August, followed by increases in phytoplankton-derived organic material and biogenic silicon between August and November. The high numbers of zooplankton fecal pellets in settled material during these same months indicates a close coupling between the timing of phytoplankton production and its consumption by herbivorous zooplankton populations. The abundance a single species of the copepod (Oncaea borealis) in trap samples from our study contrasts previous observations from the ice island (HAR6RAVE et al., 1989a) where other taxa such as Calanus glacialis, C. hyperboreus and Metridia longa were the dominant species. A recent study reported that these same zooplankton species were abundant in sediment traps placed in the eastern Beaufort Sea during winter months (FORBES et al., 1992). O. borealis is characteristic of deep water fast ice communities in the Arctic Ocean (CAREV, 1985) and along with the larger calanoid species it is one of the predominant zooplankton species in Arctic surface waters (GRAIN6ER, 1965). Other species of this genus have been observed to be associated with abandoned larvacean houses (ALLI)REDGE, 1972) and with settling flocculated organic matter (SILVER and GowIN6, 1991). It was not possible in the present study, however, to determine if individuals entered traps by active swimming or passive settling in association with flocculated material. Mineral particles containing high amounts of chlorite, mica, illite and quartz collected in August, October and December were similar to particles deposited in cryoconites of pack ice in both the western and eastern Arctic Ocean (Mol)m et al., 1985; PFm~AN et al., 1989) and melted from ice cores on the Ice Island (MUDIE et al., 1985; JEFVVaESet al., 1988; HEINOAL, 1989). By contrast, surficial sediments from the coastal margin under and proximal to the ice island in 1989 contained smectite and kaolinite in addition to the minerals listed above (P. Mudie, unpublished data). The absence of smectite and kaolinite

Fig. 3. SEM and light microscope photographs of the most common particle types found in the traps. (A) Aggregates of mineral particles with no visible binding agent surrounded by loose individual organic and inorganicparticles. (B) Aggregateof biogenicand inorganicparticlesbound by organicmatter; note that no large mineral particles are present in contrast to (A). (C) Ellipsoid fecal pellet. Inset shows pennate and centric diatoms covered by an organic membrane. (D) Nematocystfrom the settled material collectedin traps (right, SEM) as comparedto a nematocystfrom the NarcomedusaeAeginopis laurentiicollectedby a planktonnet from the ice island in June 1987(HA~V,AV~et al., 1989b) (inset, lightmicroscope). All scale bars = 30~m.

Seasonal variability in particle sedimentation in the Arctic Ocean

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in trap samples shows that local sediment resuspension was not a source of particles settled in the traps. The bulk of individual mineral particles in settled material throughout the year was in the silt and clay size range (