Seasonal and spatial distribution of particulate organic matter (POM) in the Chukchi and Beaufort Seas

Transcription

1 Deep-Sea Research II 52 (2005) Seasonal and spatial distribution of particulate organic matter (POM) in the Chukchi and Beaufort Seas Nicholas R. Bates a,, Dennis A. Hansell b, S. Bradley Moran c, Louis A. Codispoti d a Bermuda Biological Station for Research, Inc., 17 Biological Station Lane, Ferry Reach, Bermuda GE01, USA b Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL 33149, USA c Graduate School of Oceanography, University of Rhode Island, Narragansett, RI , USA d UMCES, Horn Point Laboratory, 2020 Horns Point Road, P.O. Box 775, Cambridge, MD 21613, USA Received 1 March 2005; accepted 9 June 2005 Abstract As part of the Western Arctic Shelf Basin Interactions (SBI) project, the production and fate of organic carbon and nitrogen from the Chukchi and Beaufort Sea shelves were investigated during spring (5 May 15 June) and summer (15 July 25 August) cruises in Seasonal observations of suspended particulate organic carbon (POC) and nitrogen (PON) and large-particle (453 mm) size class suggest that there was a large accumulation of carbon (C) and nitrogen (N) between spring and summer in the surface mixed layer due to high phytoplankton productivity. Considerable organic matter appeared to be transported from the shelf into the Arctic Ocean basin in an elevated POC and PON layer at the top of the upper halocline. Seasonal changes in the molar carbon:nitrogen (C:N) ratio of the suspended particulate organic matter (POM) pool reflect a change in the quality of the organic material that was present and presumably being exported to the sediment and to Arctic Ocean waters adjacent to the Chukchi and Beaufort Sea shelves. In spring, low particulate C:N ratios (o6; i.e., N rich) were observed in nitrate-replete surface waters. By the summer, localized high particulate C:N ratios (49; i.e., N-poor) were observed in nitrate-depleted surface waters. Low POC and inorganic nutrient concentrations observed in the surface layer suggest that rates of primary, new and export production are low in the Canada Basin region of the Arctic Ocean. r 2005 Elsevier Ltd. All rights reserved. Keywords: Ocean carbon cycle; Particulate matter; C:N stoichiometry; Chukchi Sea; Arctic Ocean 1. Introduction The Arctic Ocean marine system has an important influence on global climate via its influence on global ocean thermohaline (overturning) circulation (Aagaard and Carmack, 1989; Walsh and Corresponding author. Tel.: ; fax: address: nick@bbsr.edu (N.R. Bates). Chapman, 1990; Mysak et al., 1990; Ha kkinen, 1993). In addition, the sea-ice-albedo mechanism enhances the influence of the Arctic on global climate, making the Arctic particularly sensitive to global warming and to ecosystem changes associated with warming (Walsh et al., 1990; Moritz and Perovich, 1996; Grebmeier and Whitledge, 1996; Manabe and Stouffer, 2000). The Arctic Ocean continental shelves are important as sites of biological primary productivity that /$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi: /j.dsr

2 N.R. Bates et al. / Deep-Sea Research II 52 (2005) support substantial biomass at higher trophic levels. In portions of the Chukchi and Beaufort Sea shelves, productivity during the short but intense photosynthetic season can support primary production rates of X300 g C m 2 y 1 (Sambrotto et al., 1984; Hansell et al., 1993). Both ice algae and phytoplankton production contribute significantly to primary production, but 90% of the total primary production over the shelves is contributed by the water column phytoplankton with ice algae being quantitatively important early in the growing season (see Hill and Cota, 2005). Although primary production in this region is influenced by a dynamic interplay of light, ice conditions, and coastal inputs, a dominant factor is the northward flow of nutrientrich Pacific waters. After transiting the Bering Strait, these waters are substantially modified over the Chukchi, Beaufort and East Siberian shelves before entering the Canada Basin of the deeper Arctic Ocean (Fig. 1). The waters are known to principally outflow from the Chukchi Sea shelf via Barrow Canyon (BC), the Hope Sea Valley/Herald Canyon depression, and the gap between Herald and Hanna shoals (Woodgate et al., 2005a, b; Maslowski et al., 2005). Nutrient distributions suggest that some of the Chukchi Sea shelf waters also enter the East Siberian Sea (e.g., Codispoti et al., 2005), principally through Long Strait. Because of the importance of this advective input of nutrients and the inhibition of vertical nutrient transport to the oligotrophic surface layer by the Arctic Ocean halocline, the nutrient supply regime in the study region is unusual. The formation and offshore transport of shelf waters into the central basin is hypothesized as an important pathway for transferring terrestrial and marine organic material from shelf seas to the ocean basin (Jones and Anderson, 1990; Walsh et al., 1990, 2005; Grebmeier, 1993; Yunker et al., 2005; Moran et al., 2005). Walsh (1995) and Walsh et al. (1997) have suggested that the high levels of primary production over the Bering and Chukchi shelves result in the export of organic carbon to the halocline of the Arctic Ocean interior. Wheeler et al. (1997) argued against such a transport because of low levels of dissolved organic carbon (DOC) in the nutrient-rich halocline waters over the Arctic slope. Currently, not enough is known about the production and fate of organic carbon from the Chukchi Sea shelf and western reaches of the Beaufort Sea shelf in particular. As part of the Western Arctic Shelf Basin Interactions (SBI) project, the seasonal and spatial distributions of particulate organic matter (POM) were investigated during two cruises in Materials and methods 2.1. Field sampling Physical, biogeochemical and biological measurements were made from the USCGC Healy during two cruises to the Chukchi and Beaufort Seas Fig. 1. Location map: (A) CTD/rosette stations from the HLY spring cruise to the Chukchi Sea (5 May 15 June 2002). Three sections were sampled from Chukchi outer shelf into the Arctic Ocean (Canada Basin) included: (1) West Hanna Shoal (WHS), stations 5 11; (2) East Hanna Shoal (EHS) transect, stations 12 21; and (3) Barrow Canyon (BC) transect, stations (B) CTD/rosette stations from the HLY summer cruise to the Chukchi Sea (17 July 26 August 2002). Four sections were sampled from Chukchi outer shelf into the Arctic basin, included: (1) West Hanna Shoal (WHS), stations 32 39; (2) East Hanna Shoal (EHS) transect, stations 25 31; (3) Barrow Canyon (BC) transect, stations 8 17; (4) East of Barrow (EB), stations 18 24; and (5) Bering Strait section, stations 1 5.

3 3326 ARTICLE IN PRESS N.R. Bates et al. / Deep-Sea Research II 52 (2005) as part of the 2002 field phase of the SBI project. CTD/rosette stations were sampled for salinity, dissolved oxygen, primary production, chlorophyll content, inorganic nutrients (ammonium, nitrate, nitrite; phosphate, reactive silicon, and urea), suspended particulate organic matter (POM), dissolved inorganic carbon (DIC), biomarkers, microzooplankton, radioisotopes, and several other variables, for example. Methods used to collect and analyze water samples for salinity, dissolved oxygen, and inorganic nutrients were similar to those used in the World Ocean Circulation Experiment (WOCE) and Joint Global Ocean Flux Study (JGOFS) programs. Analytical details are maintained by the WOCE project office ( whpo.ucsd.edu) and on the SBI web site ( maintained by the Joint Office for Science Support, JOSS). Continuous vertical profiles of temperature, salinity, dissolved oxygen, fluorometric chlorophyll, light transmission, Haardt fluorescence (an index of colored organic matter), and photosynthetically available radiation (PAR) also were collected at each station. CTD, bottle and rate measurement data are available at the SBI website, and archived at the National Snow and Ice Data Center (NSIDC; During the spring cruise (5 May 15 June 2002), a total of 40 stations were occupied on the Bering, Chukchi and Beaufort Sea shelves and into the Arctic Ocean basin (Fig. 1). Three sections were sampled from the Chukchi and Beaufort Sea shelves into the Arctic Ocean basin, including: (1) West Hanna Shoal (WHS); (2) East Hanna Shoal (EHS); and (3) Barrow Canyon (BC). In addition, stations were occupied near Pt. Barrow, over the inner shelf of the Chukchi Sea and just south of the Bering Strait. With the exception of the Bering Strait, ice cover in spring was generally greater than 75%. Heavy sea ice limited sampling in the northwest region of the Chukchi Sea and along the easternmost (BC) line in the Beaufort Sea (Fig. 2). During the summer cruise (17 July 26 August 2002), a total of 45 CTD/rosette stations were occupied in the region (Fig. 1). The three sections sampled in spring were re-occupied (i.e., WHS, EHS, and BC). In addition, a fourth section, East of Barrow (EB), was conducted. A five CTD/rosette station section extending from the Alaskan side of the Bering Strait to the Diomede Islands also was taken. As on the spring cruise, a few more Fig. 2. Sea-ice distributions in the Chukchi Sea: (A) spring cruise (HLY 02-01; 5 May 15 June 2002) and (B) summer cruise (HLY 02-03; 17 July 26 August 2002). Sea-ice cover was determined from individual station logs, as well as remotely sensed sea-ice products ( Although the contours are arbitrary, they are provided to give a general sense of spatial distributions of sea ice. stations were occupied over the inner shelf of the Chukchi Sea and near Pt. Barrow. During the summer cruise, ice-free conditions were observed from Bering Strait to 701N in the Chukchi Sea (Fig. 2B). Minimal ice conditions (i.e., o25%) were observed in the BC region and west to Hanna Shoals. Heavy ice conditions (i.e., 475%) were present offshore and eastward of Point Barrow into the Beaufort Sea Particulate organic carbon and nitrogen analysis For determination of particulate organic carbon (POC) and nitrogen (PON), seawater was drawn from 30 l Niskin samplers at pre-determined depths into Nalgene bottles. Known volumes (1 4 l) of

4 N.R. Bates et al. / Deep-Sea Research II 52 (2005) seawater were vacuum filtered through a funnel array onto pre-combusted GF/F filters (25-mm Whatman, 0.7-mm pore size). Filters were then folded, placed into acid-washed pre-combusted scintillation vials, and stored until analysis. After acidification with HCl to remove inorganic carbon, filters were dried and samples analyzed for carbon and nitrogen using a Control Equipment Corporation (CEC) 240-XA Elemental Analyzer at the Bermuda Biological Station For Research, Inc. (Knap et al., 1997). Filter blanks represent the total blank associated with the precombusted GF/F filter and any DOC adsorption from ambient seawater. Filter blanks for suspended POM samples from SBI were mmoles kg 1 for POC and mmoles kg 1 for PON. Suspended POM samples drained from the spigot of the Niskin sampler can underestimate total suspended POM due to sinking of larger particles within the sampler (Gardner, 1977). Gundersen et al. (2001) observed, over a couple of years at the Bermuda Atlantic Time-series Study (BATS) site in the North Atlantic Ocean, a mean 26% underestimate when comparing suspended POC drained from a Niskin spigot compared to an entire sampler. Here, the suspended POM data are not corrected since similar studies were not conducted during the SBI program Large POC and PON sampling and analysis Large-volume water samples ( l) for determination of POC and PON were collected separately from the suspended POC and PON samples using battery powered in situ pumps (Challenger Oceanic Systems and Services, UK). Seawater was pumped at a flow rate of 2 3 l min 1 through a 142-mm diameter, 53-mm pore-size Nitex screen. The Nitex screens were then immersed onboard in GF/F filtered seawater in an ultrasonicator for 2 5 min to resuspend particulate matter. The large-particle seawater suspensions were immediately filtered through precombusted 25-mm GF/F filters and stored frozen in petri dishes for subsequent POC and PON analysis. In the shore-based lab at University of Rhode Island, the 25 mm GF/F filters containing particulate matter from the 53-mm Nitex screens were thawed, acidified in a desiccator using fuming concentrated HCl for 24 h to remove particulate inorganic carbon, and then dried in a 60 1C oven. Subsamples were cut from the 25-mm filters using nickel-plated scissors to a size appropriate for CHN analysis, approximately 15 20% by weight of the total filter. Organic carbon and nitrogen on the filters was quantified using a Carlo-Erba CHN Analyzer (Pike and Moran, 1997). The total mass of carbon and nitrogen per filter was corrected for the filter blank, converted to molar units, and divided by the sample volume filtered. Filter blanks for large-particle samples were prepared by filtering ml of GF/F prefiltered seawater and therefore represent the total blank associated with the precombusted GF/F filter and any DOC adsorption from ambient seawater (Moran et al., 1999). For the spring cruise, GF/F filter blanks (N ¼ 5) were mmoles C and mmoles N/25-mm filter. For the summer cruise, filter blanks (N ¼ 6) were mmoles C and mmoles N/25- mm filter. The 4 6 mmol C/25-mm GF/F filter is consistent with previously reported filter blanks that include the contribution from DOC adsorption, which was determined empirically by successive filtration of varying volumes of seawater and extrapolation to zero water filtered (Moran et al., 1999). Density (e.g., salinity and temperature) was not monitored during the pumping for large-fraction POM samples, and the data cannot be appropriately converted to mmoles kg 1 from mmoles l 1. Thus both suspended POM and large fraction POM data are reported here as mmoles l 1, rather than mmoles kg Identification of water masses The upper several hundred meters of the Arctic Ocean and adjacent seas are strongly stratified (e.g., Kinney et al., 1970; Aagaard et al., 1985; Jones and Anderson, 1986; Anderson et al., 1988, 1989, 1994a, b; Aagaard and Carmack, 1994; Schlosser et al., 1995; Codispoti et al., 2005). Surface layer, halocline, and deeper water masses on the Chukchi and Beaufort Sea shelves and adjacent Arctic Ocean basin can be identified by characteristic temperature, salinity (Aagaard et al., 1981), nutrient, oxygen distributions and stoichiometry (Wallace et al., 1987; Salmon and McRoy, 1994; Jones et al., 1991), and alkalinity (Anderson, 1995) properties. Surface waters (or Polar Mixed Layer (PML); upper m) in the Chukchi Sea are derived from local ice melt, fresh-water inputs into the Arctic Ocean (Aagaard, 1989; Schlosser et al., 1995), and

5 3328 ARTICLE IN PRESS N.R. Bates et al. / Deep-Sea Research II 52 (2005) inflow of Pacific Ocean water through the Bering Strait (Coachman and Barnes, 1961; Aagaard et al., 1981; Bjork, 1989). Water transiting Bering Strait is largely composed of Alaskan Coastal Water (ACW) in the east, and Anadyr Current water in the west (Coachman et al., 1975). Anadyr Water is colder, saltier and more nutrient-rich than ACW. An additional water mass, Bering Shelf water, which transits through Bering Strait, is thought to be a blend of Anadyr Water and ACW (Codispoti et al., 2005). A strong permanent halocline underlies the PML, particularly in the Arctic Ocean basin. Halocline waters can be subdivided into Upper Halocline Layer (UHL) and Lower Halocline Layer (LHL) based on different physical and chemical characteristics (Jones and Anderson, 1986; Codispoti et al., 2005). The core of the UHL (at m deep) has a salinity of , nitrate concentration of 1472 mmoles kg 1, and phosphate concentration of mmoles kg 1. This water mass has a predominantly Pacific Ocean origin. There can be a temperature maximum at m deep in the upper part of the UHL reflecting the seasonal inflow of relatively warmer Pacific Ocean water through the Bering Strait (Coachman and Barnes, 1961). In the Arctic Ocean basin, LHL and deeper water were present beneath the UHL. The LHL has a salinity range of ( m deep; Kinney et al., 1970; Jones and Anderson, 1986) and typical nitrate concentration of 1271 mmoles kg 1 and phosphate concentration of mmoles kg 1. The LHL has a predominantly Atlantic Ocean origin. At deeper depths, the Atlantic Water layer (AWL; m deep) has a core salinity of and often is referred to as the Warm Atlantic Layer. Beneath the AWL is the Arctic Ocean Deep Water (AODW) that fills the Canada and Eurasian basins of the Arctic Ocean. sampled offshore during the shelf to Arctic Ocean basin sections (i.e., WHS, EHS, BC, and EB) Polar mixed layer (PML); hydrographic and biogeochemical properties During the spring cruise, surface-layer temperature ranged from 1.5 to 1.8 1C (Fig. 3A). Surface-layer salinities close to the Bering Strait inflow and across much of the Chukchi Shelf ranged from o32 to 33.5 (Fig. 4A). High nitrate (10 15 mmoles kg 1 ) and phosphate contents also were observed in this region (Fig. 5A). In the Chukchi Sea outer shelf (north of 701N) to Arctic Ocean basin region, surface-layer salinities freshened, decreasing to below 30 at the northernmost stations. Nitrate concentrations also decreased to below 0.2 mmoles kg 1 at the offshore stations 3. Results and discussion 3.1. General hydrography and biogeochemical patterns The general temperature, salinity and nutrients regimes in the Chukchi Sea are discussed in detail elsewhere (e.g., Codispoti et al., 2005). Surface-layer (PML) and UHL waters were present on the Chukchi Sea shelf during both cruises. Surfacelayer, UHL, LHL and Atlantic layer waters were Fig. 3. Surface-layer temperature (1C) distributions in the Chukchi Sea: (A) spring cruise (5 May 15 June 2002) and (B) summer cruise (17 July 26 August 2002). Although the contours are arbitrary (given the spatial heterogeneity of the region), they are provided to give a general sense of the spatial distributions of temperature.

6 N.R. Bates et al. / Deep-Sea Research II 52 (2005) Fig. 4. Surface-layer salinity distributions in the Chukchi Sea: (A) spring cruise (5 May 15 June 2002) and (B) summer cruise (17 July 26 August 2002). Although the contours are arbitrary (given the spatial heterogeneity of the region), they give a general sense of the spatial distributions of salinity. Fig. 5. Surface-layer nitrate (mmoles kg 1 ) distributions in the Chukchi Sea: (A) spring cruise (5 May 15 June 2002) and (B) summer cruise (17 July 26 August 2002). Although the contours are arbitrary (given the spatial heterogeneity of the region), they give a general sense of the spatial distributions of nitrate. (particularly on the WHS and EHS transects). With the exception of Bering Strait (station 0; 1 2 mgl 1 ), chlorophyll biomass across the Chukchi Sea shelf, slope and Arctic Ocean basin region was generally low (o0.8 mgl 1 ; Hill and Cota, 2005) and restricted to the PML. The slope and basin stations located along the WHS and EHS transects had the lowest chlorophyll biomass (o0.2 mgl 1 ; Hill and Cota, 2005). Two months later, seasonal warming of the surface layer was evident from the Bering Strait and across the Chukchi Sea shelf (to 711N) (Fig. 3B). Surfacelayer salinities ranged from o10 to 32, with lower salinity water evident in the ACW at Bering Strait (Fig. 4B). North of this region, surface-layer temperatures remained near freezing. A freshened mixed layer was also evident (salinities of 27 28; Fig. 4B) in the outer Chukchi Sea shelf Arctic Ocean basin transition. Inorganic nutrient contents were low across the regions with surface nitrate less than 0.2 mmoles kg 1 at all stations (Fig. 5B). During the summer, relatively low rates of primary productivity were observed at the shelf (340mgCm 2 d 1 ) and slope stations (404mgCm 2 d 1 ; Hill and Cota, 2005). Rates of net community production (NCP) determined by DIC changes between the spring and summer period were highly variable, ranging from 15 over the Canada Basin of the Arctic Ocean to mg C m 2 d 1 over the shelf (Bates et al., 2005). The highest NCP rates ( mg C m 2 d 1 ) were observed in the shelf region of Barrow Canyon and east of Point Barrow (Bates et al., 2005; Walsh et al., 2005). The highest concentrations of particles (e.g., Ashjian et al., 2005) and chlorophyll biomass (42 25 mgl 1 ; Hill and Cota, 2005) also was observed in the Barrow Canyon region, with a pronounced subsurface maximum at

7 3330 ARTICLE IN PRESS N.R. Bates et al. / Deep-Sea Research II 52 (2005) m. The highest vertical export of organic carbon also occurred in this region (Moran et al., 2005; Grebmeier et al., 2004). Lower rates of NCP (Bates et al., 2005; Walsh et al., 2005), particle concentration (Ashjian et al., 2005), chlorophyll a biomass (HillandCota,2005), and export production (Moran et al., 2005) was observed in the slope and basin regions, particularly at WHS and EHS on the Chukchi Sea shelf Halocline and deeper waters: hydrographic and biogeochemical properties UHL waters were present on the Chukchi Sea shelf, extending offshore during both cruises (at depths of m deep). The core layer of the UHL is well characterized with a salinity range of , and nitrate and phosphate contents of 14 and 2 mmoles kg 1, respectively (Fig. 6). At the offshore stations, LHL and AWL water were typically observed at depths of , and 4500 m, respectively. The LHL is characterized by a salinity range of , and nitrate and phosphate contents of 12 and 0.9 mmoles kg 1, respectively. The deep Atlantic water layer is warmer, saltier (34.9) with mean nitrate and phosphate contents of 14 and 1.0 mmoles kg 1, respectively Spatial and seasonal distribution of suspended POC and PON Springtime suspended POC and PON variability During the spring, suspended POC and PON concentrations in the surface layer were generally low (o3, and 0.5 mmoles kg 1, respectively; Fig. 7). The highest POC concentrations (45 mmoles l 1 were observed at the Bering Strait station. The lowest POC concentrations (o1.2 mmoles l 1 ) in the surface layer were observed offshore in the northernmost stations along the WHS and EHS sections. A plume of slightly elevated POC (and PON) Fig. 6. Hydrographic relationships observed in the Chukchi Sea: (A) temperature (1C) versus salinity on the spring cruise (5 May 15 June 2002), (B) temperature (1C) versus salinity on the summer cruise (17 July 26 August 2002), (C) nitrate (mmoles kg 1 ) versus phosphate (mmoles kg 1 ) on the spring cruise (5 May 15 June 2002), and (D) nitrate (mmoles kg 1 ) versus phosphate (mmoles kg 1 ) on the summer cruise (17 July 26 August 2002). PML denotes the surface-layer water, UHL the Upper Halocline waters, LHL the Lower Halocline Layer waters, AWL the Atlantic Layer waters, and AODW the Arctic Ocean Deep Water.

8 N.R. Bates et al. / Deep-Sea Research II 52 (2005) Fig. 7. Surface-layer particulate organic carbon (POC) and nitrogen (PON) (mmoles l 1 ) distributions in the Chukchi Sea: (A) spring cruise (5 May 15 June 2002), and (B) summer cruise (17 July 26 August 2002); and (C) Spring cruise (5 May 15 June 2002), and (D) summer cruise (17 July 26 August 2002). Although the contours are arbitrary (given the spatial heterogeneity of the region), they are provided to give a general sense of the spatial distributions of PON. extended out from the shelf and shelf break into the Canada Basin at a depth of 20 m at both the WHS and EHS sections (Figs. 8 and 9). A similar feature was observed in the BC section (Fig. 10). Chlorophyll biomass ( mgl 1 ) was typically restricted to shallower depths (0 20 m) than the subsurface suspended POM maxima. On the shelf, in halocline waters, relatively high POC concentrations (43 mmoles l 1 ), similar in magnitude to the near surface maxima, were observed in bottom waters (40 50 m depth) at the shelf break (stations 5, 22 and 37). Offshore, beyond the shelf break, the near-surface suspended POC maxima decreased horizontally. At depth, suspended POC concentrations were less than 1 mmoles l 1 in UHL and LHL waters. Distributions of large-particle (453 mm) size class POC and PON were generally similar to that observed for the suspended particulate size class, although the concentrations were approximately 10-fold lower, ranging from mmoles l 1 for 453-mm POC and mmoles l 1 for 453 mm PON (Figs. 8 10). A subsurface maxima in large-particle (453 mm) size class was observed at a depth of m deep (at each section WHS, EHS, and BC) extending out from the shelf break into the basin. Due to limited sampling, it could not be ascertained whether there was a higher concentrations of large-particle (453 mm) size class POC and PON associated with the near-bottom suspended POM observed over the shelf Summertime suspended POC and PON variability Six weeks later, suspended POC and PON concentrations were generally much higher across

9 3332 ARTICLE IN PRESS N.R. Bates et al. / Deep-Sea Research II 52 (2005) Fig. 8. Section of POM along the West Hanna Shoals (WHS) section during the spring cruise (5 May 15 June 2002). (A) POC (mmoles l 1 ) and (B) POC (mmoles l 1 ) in the large-particle (453 mm) size class; and (C) PON (mmoles l 1 ) and (D) PON (mmoles l 1 ) in the largeparticle (453 mm) size class. the region (Fig. 7). In the Bering Strait, elevated POC (415 mmoles l 1 ) and PON was observed at the westernmost station (station 2) where the influence of Anadyr and Bering Shelf water is likely to be the strongest. To the east, lower POC (5 mmoles l 1 ) was observed in the ACW, perhaps extending into the Chukchi Sea (stations 7 and 8). In the surface layer of the Chukchi Sea shelf, the highest POC concentrations (420 mmoles l 1 ) were observed near BC and EB into the Beaufort Sea. POC concentrations higher than 4 mmoles l 1 were generally restricted to the Chukchi Shelf except east of Pt. Barrow where elevated POC extended offshore in the surface layer. To the west, POC concentrations decreased offshore to o2 mmoles l 1 at the northernmost stations in the Arctic Ocean basin. There were considerable vertical and spatial differences in the suspended POC and PON concentrations within halocline waters. To the west, at the WHS and EHS sections, high POC concentrations (410 mmoles l 1 ) were observed at the shelf break (Figs. 11 and 12) at depths of m. Although POC concentrations decreased northwards into the Arctic Ocean basin, elevated POC concentrations (46 mmoles l 1 ) extended offshore at a subsurface depth range of m (in transitional waters between the surface layer and UHL). Similar to spring conditions, POC concentrations in offshore waters below 40 m decreased vertically. Within the LHL, suspended POC concentrations were typically o1 mmoles 1. Summertime distributions of large-particle suspended POC and PON are generally similar to that observed for the suspended particulate size class, although the concentrations were approximately 10-fold lower, ranging from o mmoles l 1 for 453 mm POC and mmoles l 1 for 453 mm PON. Similar to springtime conditions, a subsurface maxima in large-particle (453 mm) size class was observed at a depth of m deep extending out from the shelf break into the basin.

10 N.R. Bates et al. / Deep-Sea Research II 52 (2005) Fig. 9. Section of POM along the East Hanna Shoals (EHS) section during the spring cruise (5 May 15 June 2002). (A) POC (mmoles l 1 ), (B) POC (mmoles l 1 ) in the large-particle (453 mm) size class; and (C) PON (mmoles l 1 ) and (D) PON (mmoles l 1 ) in the large-particle (453 mm) size class. At BC, similarities and differences to the WHS and EHS sections in POC and PON distributions were observed (Fig. 13). High POC concentrations (430 mmoles l 1 ) were observed at depths of m. Elevated POC concentrations (410 mmoles l 1 ) were observed to extend offshore into the Arctic Ocean basin in a subsurface maxima at a depth of m. A subsurface maxima in largeparticle (453 mm) size class POC (1 5 mmoles l 1 ) and PON ( mmoles l 1 ) was also observed to extend offshore at a depth of m (Fig. 13). Beneath this layer, suspended particle and large size class particle concentrations decreased in the upper part of UHL (40 90 m deep). At deeper depths in the UHL and LHL, suspended particle and large-size class particle concentrations were low (for example, suspended POC was than 1 mmoles l 1 ). Close to the shelf break, elevated POC concentrations (4 8 mmoles l 1 ) were observed in waters of the upper halocline at m deep near to the shelf bottom (perhaps indicative of sediment resuspension). At the EB section, high POC concentrations (410 mmoles l 1 ) were restricted to the upper 10 m in offshore stations (Fig. 14). Beneath the surface layer, POC and PON concentrations decreased, although elevated suspended POC (412 mmoles l 1 ) was evident in a subsurface layer at m deep. The highest concentration of large-particle (453 mm) size class POC (3 mmoles l 1 ) and PON (0.3 mmoles l 1 ) was observed offshore rather than the shelf break region (Fig. 14). Unlike the BC section, elevated POC concentrations were not observed in the m deep layer. Within the deeper LHL waters located at the BC and EB sections, suspended POC concentrations were typically o1 mmoles l Seasonal production of POM Seasonal observations of the suspended POM pool indicate that there was a large subsurface accumulation of POM between spring and summer from phytoplankton production in the surface layer.

11 3334 ARTICLE IN PRESS N.R. Bates et al. / Deep-Sea Research II 52 (2005) Fig. 10. Section of POM along the Barrow Canyon (BC) section during the spring cruise (5 May 15 June 2002): (A) POC (mmoles l 1 ) and (B) POC (mmoles l 1 ) in the large-particle (453 mm) size class; and (C) PON (mmoles l 1 ) and (D) PON (mmoles l 1 ) in the large-particle (453 mm) size class. Similar seasonal POC accumulations have been observed in other Arctic Ocean regions (Huston and Deming, 2002; Olli et al., 2002). These were the first measurements of 453-mm POC and PON in the Arctic Ocean, and hence there are no prior observations that may be directly compared to these data. The large-particle POC and PON concentrations from the Chukchi Sea were, however, similar to concentrations reported in the North Water Polynya study conducted off eastern Greenland (Amiel et al., 2002). The highest concentrations of suspended POM and large-particle (453 mm) size class POM was generally observed subsurface (15 35 m) on the shelf in the vicinity of BC and east of Pt. Barrow. On the shelf, this feature was co-located with the region of large nitrate depletion (Grebmeier et al., 2004), highest rates of primary (Hill and Cota, 2005) and net community productivity (NCP; Bates et al., 2005; Walsh et al., 2005), particle concentration (Ashjian et al., 2005) and chlorophyll biomass (Hill and Cota, 2005). A large seasonal flux of POM to the sea floor has been observed previously on the Chukchi Sea (Moran et al., 1997). During the SBI 2002 field program, the highest rates of export productivity (measured by 234 Th/ 238 U activities) were observed close to the shelf break in the vicinity of BC (Moran et al., 2005). The subsurface suspended POM maximum typically occurs at the depth of the strong density boundary between the fresher PML and underlying UHL. This feature presumably results primarily from surface production, vertical sinking and temporal accumulation of organic matter at the density discontinuity, rather than in situ subsurface production. Primary productivity was typically restricted to the upper m (Hill and Cota, 2005), with a similarly shallow 1% light level. There is little evidence for growth in the subsurface although a pronounced aphotic chlorophyll subsurface maximum

12 N.R. Bates et al. / Deep-Sea Research II 52 (2005) Fig. 11. Section of POM along the West Hanna Shoals (WHS) section during the summer cruise (17 July 26 August 2002): (A) POC (mmoles l 1 ), (B) POC (mmoles l 1 ) in the large-particle (453 mm) size class; and (C) PON (mmoles l 1 ) and (D) PON (mmoles l 1 ) in the large-particle (453 mm) size class. was observed at m deep along the summertime WHS, EHS, and BC sections. Although the highest POM concentrations were observed on the Chukchi Sea shelf, subsurface tongues of elevated POM extended northwards from the shelf break into the Arctic Ocean basin at a depth of m (just beneath the mixed layer within the upper part of the UHL). Tongues of elevated inorganic nutrient concentrations (e.g., nitrate, phosphate, silicate, ammonium, and urea) were also present at these depths extending into the Arctic Ocean basin (Grebmeier et al., 2004). Rates of primary and NCP in the overlying PML at the Arctic Ocean basin stations were very low mg C m 2 d 1 (Hill and Cota, 2005; Bates et al., 2005). Surface-layer chlorophyll biomass concentrations were also low (o0.5 mgl 1 )at the Arctic Ocean basin stations (Hill and Cota, 2005). This suggests that productivity and subsequent vertical sinking of particles did not contribute much to the observed subsurface suspended POM tongues extending into the Arctic basin. The mean transport of shelf water is from the Chukchi Sea shelf out into the Arctic Ocean basin primarily through the Herald Valley and Barrow Canyon (Woodgate et al., 2005a,b). It seems likely that the tongues of elevated POM reflect transport of allochthonous POM produced on the shelf into the basin. Indeed, algal biomarkers of primary productivity on the Chukchi Sea have been observed offshore in sediments of the Arctic Ocean basin (Belicka et al., 2002; Yunker et al., 2005). However, without appropriate tracers, it is difficult to quantify the contribution of shelf POM relative to a potential in situ contribution from ice algae, for example. Thus, it remains difficult to quantify the transport of carbon and nitrogen from the shelf to the basin. The tongues of elevated POM were strongly developed at the WHS, EHS and BC sections, but not at the East of Barrow (EB) section. Offshore at this section, elevated POM concentrations are

13 3336 ARTICLE IN PRESS N.R. Bates et al. / Deep-Sea Research II 52 (2005) Fig. 12. Section of POM along the East Hanna Shoals (EHS) section during the summer cruise (17 July 26 August 2002): (A) POC (mmoles l 1 ) and (B) POC (mmoles l 1 ) in the large-particle (453 mm) size class; and (C) PON (mmoles l 1 ) and (D) PON (mmoles l 1 ) in the large-particle (453 mm) size class. limited to the upper m. Further to the east in the Beaufort Sea, Mackenzie River runoff can contribute as much as an 8-m inventory of meteoric water to the 40-m mixed layer (Macdonald et al., 2002). Although terrestrial sources of POM from the Mackenzie River could contribute to the suspended POM pool in EB region of the Beaufort Sea shelf, this region is 200 km away from the Mackenzie River outflow. Furthermore, the carbon:nitrogen (C:N) ratio of the suspended POM observed in both spring and summer was low (o6) compared to typical C:N ratios of POM in arctic rivers (e.g., ; Lobbes et al., 2000) implying a marine source for POM rather than a terrestrial source. In the Arctic Ocean basin stations, low (o1 2 mmoles l 1 ) POC concentrations were observed in waters of the surface layer (PML) and LHL during both spring and summer cruises. In previous studies, Wheeler et al. (1997) reported relatively high suspended POC values for the surface layer across the Canada Basin, with no POC values lower than 2 mmoles l 1. Such biomass accumulation requires significant rates of primary and importantly, new and export production in the Arctic Ocean basin, a region believed to be oligotrophic. However, the low suspended POC and inorganic nutrient concentrations observed during the SBI cruises, suggest that rates of primary, new and export production are indeed low in the Canada Basin region of the Arctic Ocean. Rates of net community production estimated from changes in DIC distributions during SBI cruises were low (o15 25 mg C m 2 d 1 (Bates et al., 2005) Seasonal changes in the C:N of suspended and large-particle POM Springtime C:N variability of the suspended POM pool The C:N ratio of suspended particulate matter varied seasonally, geographically and between

14 N.R. Bates et al. / Deep-Sea Research II 52 (2005) Fig. 13. Section of POM along the Barrow Canyon (BC) section during the summer cruise (17 July 26 August 2002): (A) POC (mmoles l 1 ) and (B) POC (mmoles l 1 ) in the large-particle (453 mm) size class; and (C) PON (mmoles l 1 ) and (D) PON (mmoles l 1 ) in the largeparticle (453 mm) size class. different water masses. During the spring cruise, the surface-layer suspended POC:PON ratio was (Fig. 15), lower than the canonical dissolved elemental stoichiometric C:N (Redfield et al., 1963; Takahashi et al., 1985; Anderson and Sarmiento, 1984). In halocline waters, the POC:PON ratios were higher, and , in waters of the UHL and LHL, respectively (Table 1). Similar suspended POC:PON ratios have been observed on the Chukchi Sea shelf (i.e., C:N of 6.77; Wheeler et al., 1997) and in other polar waters (e.g., C:N of 6.7; Daly, 1997). The relatively low C:N of the suspended particulates in the surface layer implies that the phytoplankton community was producing relatively nitrogen-rich organic matter in the early part of the season. Inorganic nutrient concentrations were relatively high, with nitrate contents 45 mmoles l 1 over most of the Chukchi Sea shelf region (Fig. 5A). There was considerable geographic variability in the C:N of suspended particulates in surface water (0 30 m deep). In the region near Pt. Barrow (i.e., stations 24, 29, 30, 37, 39), relatively low POC:PON ratios of were observed compared to other surface waters of the Chukchi Sea and into the Arctic Ocean basin (Fig. 16). The C:N ratio of the 453-mm-size particulate matter was higher than observed for suspended particulate matter, ranging from 5 to 30 in the surface waters (Fig. 16). The high C:N ratios of 30 might reflect the production of carbon-rich mucous/ polysaccharides in the surface layer. In deeper waters, the C:N ratio of the 453-mm-size particulate matter had invariant values of Summertime C:N variability of the suspended POM pool By the summer cruise, the surface-layer suspended POC:PON ratios were slightly higher with a mean of compared to earlier in the

15 3338 ARTICLE IN PRESS N.R. Bates et al. / Deep-Sea Research II 52 (2005) Fig. 14. Section of POM along the East of Barrow (EB) section during the summer cruise (17 July 26 August 2002): (A) POC (mmoles l 1 ) and (B) POC (mmoles l 1 ) in the large-particle (453 mm) size class; and (C) PON (mmoles l 1 ) and (D) PON (mmoles l 1 ) in the largeparticle (453 mm) size class. season. In halocline waters, the POC:PON ratios were slightly lower than spring values at and in UHL and LHL, respectively (Fig. 15, Table 1). There was much greater variability of surface-layer C:N ratios (Fig. 16) in the summer compared to the spring cruise. In the Hanna Shoals region, the highest C:N ratios ( ) were observed close to the ice margin and on the shelf but immediately offshore was a region of relatively low C:N (o6; Fig. 16). The largest variability was observed in the vicinity of BC and off Pt. Barrow. High C:N ratios (mean of ) were observed at stations 10, 12, 13, and 31. The C:N ratio of the 453-mm-size particulate matter was higher than observed for suspended particulate matter, ranging from 8 to 60 in the surface waters (Fig. 16). Similar seasonal changes in C:N ratios of the suspended particulate pool have been observed elsewhere in subpolar regions. In the North Water Polynya, Huston and Deming (2002) observed that mean suspended particulate C:N increased from 5.47 (May) to 7.04 (July). The seasonal changes in the C:N of the suspended POM pool observed on the Chukchi Sea shelf presumably reflects a significant change in the quality of material being produced and/or changes in the relative contributions of autotrophs, heterotrophs, bacteria and detritus. Thomas et al. (1999) have proposed that seasonal changes in C:N ratios of POM also may reflect changing physiological conditions of phytoplankton in the presence of decreasing nitrate availability. In the Chukchi Sea, it is evident that in localized areas, there is a shift from N-rich POM production in nitrate-replete waters to N-poor POM production in nitratedeplete waters. This might result from seasonal shifts to a diatom-dominated phytoplankton community since centric diatoms have been observed to dominate the phytoplankton community of the

16 N.R. Bates et al. / Deep-Sea Research II 52 (2005) Table 1 Observed C:N ratios of suspended particulate organic matter in the Chukchi and Beaufort Seas Cruise Water mass C:N ratio of suspended POM Spring Cruise (HLY 02-01) Summer Cruise (HLY 02-03) PML a UHL LHL PML PML at BC b UHL LHL The C:N ratio is averaged for the 0 30 m surface layer (i.e., PML). a A mean C:N ratio of was observed in the upper 10 m of the PML in the vicinity of Barrow Canyon and Pt. Barrow. b C:N ratio of POM observed in the upper 10 m of the PML in the vicinity of Barrow Canyon. Fig. 15. Relationships of POC (mmoles l 1 ) versus PON (mmoles l 1 ) observed in the Chukchi Sea: (A) spring cruise (5 May 15 June 2002) and (B) summer cruise (17 July 26 August 2002). PSL denotes the surface-layer water (cross and solid regression line); UHL the Upper Halocline Layer waters (open diamond and dashed regression line); LHL the Lower Halocline Layer waters (open circle and dotted regression line). High C:N ratio surface waters in the vicinity of Barrow Canyon are denoted by star symbol and dash/dotted regression line. Chukchi Sea shelf during summer conditions (i.e., Thalassiosira spp. in July August; Gosselin et al., 1997). However, phytoplankton community structure data are lacking to confirm this. It is not clear whether the changes in C:N composition of the suspended POM pool led to export of higher C:N sinking POM on the Chukchi Sea shelf. POC fluxes were quantified by determination of 234 Th/ 238 U disequilibrium and POC/ 234 Th ratios in large (453 mm) aggregates collected using in situ pumps (Moran et al., 2005), rather than sediment trap collections of sinking particles. In the North Water Polynya, high C:N ratios of the sinking particulate flux (9.7) observed in sediment traps also were reported (Daly, 1997; Daly et al., 1999; Michel et al., 2002) in addition to changes in the C:N ratio of the suspended POM pool. The high C:N ratio of sinking POM has been to attributed nutrient-deficient diatoms in low nitrate conditions (Daly et al., 1999; Tremblay et al., 2002), with intact algae dominating early season particulate fluxes, and then shifting to a flux dominated by high C:N resting spores and fecal material later in the season (Huston and Deming, 2002). In other Arctic Ocean regions, high C:N ratios of the sinking particulate flux (9.7) also have been reported (e.g., Olli et al., 2002). Although sinking particle composition is typically different (e.g., due to coagulation and aggregation processes) to suspended POM composition, we suggest there may be seasonal changes in the quality of POM reaching the Chukchi Sea shelf benthos or being horizontally exported off the shelf into the Arctic Ocean basin.

17 3340 ARTICLE IN PRESS N.R. Bates et al. / Deep-Sea Research II 52 (2005) Fig. 16. Average C:N ratio of suspended particulate organic matter (total) and large-particle (453 mm) size class POM in the upper 0 30 m at each station in the Chukchi Sea: (A) spring cruise (5 May 15 June 2002), (B) summer cruise (17 July 26 August 2002), (C) largeparticle (453 mm) size class POM on the spring cruise (5 May 15 June 2002), and (D) summer cruise (17 July 26 August 2002). Acknowledgements Jackie Grebmeier and Lee Cooper are thanked for their overarching contributions to the SBI program. We also thank the captain, crew and all scientific participants on the two 2002 Healy cruises. Jim Swift and the service team are thanked for their dedicated efforts in providing CTD and hydrographic data to the project. Our thanks to Charlie Farmer (RSMAS), Cindy Moore (RSMAS), Christine Pequignet (BBSR) and Paul Lethaby (BBSR) for their participation in the field activities of SBI. Christine Pequignet and Julian Mitchell (BBSR) are thanked for their organization of pre-cruise and post-cruise logistics. Keven Neely is thanked for conducting CHN analyses at BBSR. Pat Kelly and Rick Nelson are thanked for sample collection and analysis. Wolfgang Koeve and an anonymous reviewer are thanked for their detailed and helpful reviews. NSF supported this research through grant OPP References Aagaard, K., A synthesis of the Arctic Ocean circulation. Rapporteur Par-Volume Re un. Conseilles Internationale Exploration de la Mer 188, Aagaard, K., Carmack, E.C., The role of sea ice and other fresh water in the Arctic circulation. Journal of Geophysical Research 94, Aagaard, K., Carmack, E.C., The Arctic Ocean and Climate: A perspective. In: Johannessen, O.M., Muench, R.D., Overland, J.E. (Eds.), The Polar Oceans and Their Role

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