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Deep-Sea Research II 52 (2005) 3462–3477

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Multi-decadal synthesis of benthic–pelagic coupling in thewestern arctic: Role of cross-shelf advective processes

Kenneth H. Duntona,�, Jonathan L. Goodallb, Susan V. Schonberga,Jacqueline M. Grebmeierc, David R. Maidmentb

aThe University of Texas at Austin Marine Science Institute, 750 Channel View Drive, Port Aransas, TX 78373, USAbDepartment of Civil Engineering, The University of Texas at Austin, JJ Pickle Research Campus Bldg #119,

10100 Burnet Rd. Austin, TX 78712, USAcDepartment of Ecology and Evolutionary Biology, Marine Biogeochemistry and Ecology Group, The University of Tennessee,

10515 Research Dr., Bldg A, Suite 100, Knoxville, TN 37932, USA

Received 5 October 2004; accepted 19 September 2005

Abstract

Using geographic information systems (GIS) software and geostatistical techniques, we utilized three decades of water-

column chlorophyll a data to examine the relative importance of autochthonous versus allochthonous sources of reduced

carbon to benthic communities that occur from the northern Bering to the eastern Beaufort Sea shelf. Spatial trend

analyses revealed areas of high benthic biomass (4300 gm�2) and chlorophyll (4150mgm�2) on both the southern and

northern Chukchi shelf; both areas are known as depositional centers for reduced organic matter that originates on the

Bering Sea shelf and is advected northward in Anadyr and Bering shelf water masses. We found a significant correlation

between biomass and chlorophyll a in the Chukchi Sea, reflective of the strong benthic–pelagic coupling in a system that is

utilized heavily by benthic-feeding marine mammals. In contrast, there was no significant correlation between biomass and

chlorophyll in the Beaufort Sea, which by comparison, is considerably less productive (biomass and chlorophyll,

o75 gm�2 and o50mgm�2, respectively). One notable exception is an area of relatively high biomass (50–100 gm�2) and

chlorophyll (80mgm�2) near Barter Island in the eastern Beaufort Sea. Compared to other adjacent areas in the Beaufort

Sea, the chlorophyll values in the vicinity of Barter Island were considerably higher and likely reflect a long-hypothesized

upwelling in that area and close coupling between the benthos and autochthonous production. In the Bering Sea, a drop in

benthic biomass in 1994 compared with previous measurements (1974–1993) may support earlier observations that

document a decline in biomass that began between the 1980s and 1990s in the Chirikov Basin and south of St. Lawrence

Island. The results of this study indicate that the benthos is an excellent long-term indicator of both local and physical

advective processes. In addition, this work provides further evidence that secondary production on arctic shelves can be

significantly augmented by reduced carbon advected from highly productive adjacent shelves.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Benthic; Fauna; Biomass; Beaufort Sea; Chukchi Sea; Bering Sea

e front matter r 2005 Elsevier Ltd. All rights reserved.

r2.2005.09.007

ng author. Tel.: +361 749 6744; fax: +361 749 6777.

ss: dunton@utmsi.utexas.edu (K.H. Dunton).

ARTICLE IN PRESSK.H. Dunton et al. / Deep-Sea Research II 52 (2005) 3462–3477 3463

1. Introduction

Despite numerous studies that have looked at thecomposition, abundance and biomass of benthicepifaunal and infaunal organisms in the Arctic, onlya few studies have focused on the benthos as long-term integrators of overlying water column pro-cesses (Grebmeier and Cooper, 1994; Grebmeier etal., 1995; Cooper et al., 2002). It is well known thatspatial and temporal patterns in the flux of sinkingmatter are critical to the development of thebenthos, particularly at high latitudes (Kroncke,1994; Grebmeier et al., 1995; Hobson et al., 1995;Reigstad and Wassmann, 1996; Wassmann et al.,1996; Clough et al., 1997). But it is the input of thisenergy, in the form of highly reduced organic matterthat ultimately links production in the water columnwith benthic productivity (Grebmeier and Barry,1991; Wassmann, 1991). Unlike highly dynamicestuarine systems, which are populated by short-lived annual species (Montagna and Kalke, 1992,1995), the benthos of western arctic shelves arerelatively stable communities dominated by largelong-lived perennial organisms that include seaanemones, bryozoans, sea urchins, molluscs, crus-taceans, and large polychaetes (Grebmeier et al.,1989). Evidence of this stability was noted byGrebmeier (1987), who found no major change in

Fig. 1. Station locations in the study area. Measurements of integrate

locations of quantitative benthic grab samples.

the benthic faunal structure of the Bering andChukchi Seas between her study and that of Stoker(1978) made over 10 years earlier.

Current evidence suggests that the degree ofbenthic–pelagic coupling is largely a product ofdepth, zooplankton species composition, and phy-sical processes that control the rates of horizontaland vertical advection of zooplankton and particu-late organic matter flux (Wassmann et al., 1996).Hydrodynamic processes are ultimately most im-portant, and at high latitudes the predominance oflarge seasonal variations in physical forcing eventscan result in decoupling between primary andsecondary producers in the water column andincreased sedimentation of ungrazed phytoplank-ton, particularly on relatively shallow continentalshelves (Laws et al., 1988; Wassmann, 1991;Grebmeier et al., 1995). On the Bering-Chukchishelf (Fig. 1), Springer and McRoy (1993) estimatedthat benthic biomass was two to ten times greaterunder plumes of highly productive water advectednorthward from the Anadyr Basin. Tight benthic–pelagic coupling probably has been best documen-ted on the Chukchi Shelf as reflected by stableisotopic measurements of benthic food webs(Dunton et al., 1989), rates of sediment oxygenuptake (Grebmeier, 1993; Naidu et al., 2003), andbenthic biomass concentrations (Zenkevitch, 1963;

d chlorophyll a are depicted in yellow. Points in brown denote

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Golikov et al., 1990; Grebmeier and Barry, 1991). Itis apparent, however, based on both published andunpublished literature, that the character of thebenthos is a time-averaged integration of thedominant physical forcing events that reflect totalsystem productivity (Carey, 1991; Grebmeier, 1993;Ambrose and Renaud, 1995; Wassmann et al.,1996).

Changes in overlying water flow can directlyimpact benthic community structure, carbon de-position, and sediment composition (Feder et al.,1994a; Grebmeier et al., 1995). Roach et al. (1995)noted that interannual variability of Pacific Oceanwater north through Bering Strait in the AnadyrStream can reach nearly 50% of the mean; long-term changes in this flow could therefore have amajor impact on carbon transport and depositionon the Chukchi shelf and slope regions of the ArcticOcean. Springer and McRoy (1993) noted thecritical role of the Anadyr Stream as an advectiveprocess that provides nutrients for nearly contin-uous summer phytoplankton production in north-ern Bering and southern Chukchi Sea shelves.Woodgate and Aagaard (2005) recently documentedsignificant seasonal variability in temperature,salinity, and volume of Bering Strait throughflowinto the Arctic Ocean. Such variability not only hasimportant implications on the ventilation of Pacificwaters in the arctic halocline, which is linked toworld climate (Woodgate et al., 2005 and referencestherein), but directly influences the timing andmagnitude of materials transported onto adjacentarctic shelves. We argue that variability over short-time scales is not easy to detect in a system knownfor extremes in physical processes. Instead signifi-cant long-term decadal changes, however subtle, arelikely to be reflected in the character of the benthiccommunity, which is otherwise resilient to seasonalvariability in physical processes.

Finally, there is strong evidence that advection ofcarbon between arctic shelves (Grebmeier et al.,1988) and between shelves and basins (Grebmeier,2003; Kroncke, 1994; Wassmann et al., 1996) is ofmajor importance in understanding the productivityof arctic shelves. The apparent paucity of publisheddata for the arctic region has limited our under-standing of the biological and physical processesthat influence biological productivity of arcticshelves and basins, yet there exist large andpotentially invaluable historical databases fromthe region that have not been synthesized. Theseinclude tens of thousands of largely unpublished

records on the biomass, density, and composition ofbenthic organisms from the Chukchi and BeaufortSeas. Other unpublished information includes awide variety of physical and geochemical data,including the seasonal extent of ice cover, circula-tion patterns, water column characteristics, andsediment parameters.

The initial objective of this study was to compileand spatially map the distribution of all availablebenthic infaunal biomass data collected in thewestern arctic since the early 1970s. To examinelinkages between benthos and water column, wecompiled water-column chlorophyll a data collectedthroughout the study area during the same period.We also attempted to relate our knowledge of majoradvective processes in this region to explain theunique spatial patterns of benthic biomass andchlorophyll data collected over a 25-year collectionperiod (1970–1995). The knowledge gained from anunderstanding of the linkages between physics andbiology is critical to our ability to assess the effectsof climate change on arctic benthic communitiesand higher trophic level populations (e.g. graywhales and walruses; Moore et al., 2003) that utilizethe benthos as their primary food source.

2. Methods

2.1. Digital base map

The study area extends from 621 to 751N andfrom 1601E to 1281W, encompassing the northernpart of the Bering Sea, Bering Strait, and coastalshelves along the East Siberian, Chukchi andBeaufort Seas (Fig. 1). Since a single digitalbathymetric/topographic base map was not avail-able for the study area, one was constructed usingpoint elevations from three different sources: aNGDC 5-min digital elevation model (http://www.ngdc.noaa.gov/mgg/global/seltopo.html), glo-bal sea-floor topography from satellite altimetryand ship depth (http://topex.ucsd.edu/cgi-bin/get_data.cgi), and for landmasses, a US GeologicalSurvey global 30 arc second elevation data set,GTOPO30 (http://edc.usgs.gov/landdaac/gtopo30/gtopo30.html). Point elevations were incorporatedinto the program Topogrid in ArcInfo (Environ-mental Systems Research Institute (ESRI), 2000)and a grid was built with a cell size of1� 1 km2.Coastline, political boundaries, and riversextracted from the ESRI Digital Chart of the Worldwere added for orientation. The resulting map was

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projected to a Lambert Azimuthal Equal-Areaprojection because it retains the true earth surfacearea at one scale that is critical for the entry ofgeographic information systems (GIS) data that isentered as (x, y) coordinates.

2.2. Benthic biomass and chlorophyll data

Our knowledge of productivity along arctic shelvesis based largely on a series of intensive surveysprompted by the discovery of oil in Prudhoe Bay andcompleted between the 1970s and early 1990s in theBering, Chukchi and Beaufort Seas under thefederally sponsored Outer Continental Shelf Envir-onmental Assessment Program (OCSEAP). TheOCSEAP program was the first comprehensive studyof America’s arctic coastline, ultimately involvingthousands of scientists and the expenditure of over 50million dollars for studies of the benthic environmentalone (Houghton et al., 1984; Capuzzo et al., 1992).Nearly all of the data collected under the AlaskanOCSEAP program in the 1970s and 1980s weredeposited with the National Oceanographic DataCenter (NODC) for archival storage.

Benthic biomass was compiled from numeroussources (Table 1), organized as a relational database

Table 1

Data sources used in the compilation of benthic biomass

Principal

investigator

References Sampling dates Re

Sta

Broad Broad (1980) 1975–1980 3

Carey Carey (1971, 1976,

unpublished), Carey et al.

(1984a)

1971, 1975–1978 5

Feder Feder (1986,

unpublished), Feder et al.

(1985), Feder et al.

(1994a)

1979–1980, 1986 5

Grebmeier Grebmeier (1987,

1992a, b), Grebmeier and

McRoy (1989),

Grebmeier et al. (1988,

1989), Grebmeier and

Cooper (1995), Reed

(1998), Grebmeier

(unpublished data)

1984–1986, 1988,

1990, 1993–1995

4

Stoker Stoker (1978, 1981) 1970–1974 5

Wacasey Wacasey (1975) 1971–1975 3–1

The majority of samples were collected during the summer open-water

in Microsoft Access, and then mapped onto a digitalbase map using GIS. The majority of samples werecollected during summer months with the largestnumber from August. Several of the data sets wereentered from handwritten datasheets, others fromold mainframe printouts, and others laboriouslyextracted from an antiquated NODC database.Only a fraction of the OCSEAP data is publishedin the scientific literature although it represents asubstantial proportion of our knowledge for thisgeographic region. Since the data were stored inmany different formats, a considerable effort wasrequired to convert and transfer the data intoMicrosoft Access files. Investigators used threetypes of quantitative grabs to collect the benthicsamples: van Veen, Smith-McIntyre (42m depths),and Ekman grabs (o2m). The actual grab area wasextrapolated mathematically to a square meter topresent biomass in gm�2. Station means reportedhere normally represent the average of three to fivegrab samples. Previous work in the Bering Sea(Feder et al., 1985; Grebmeier, 1987) demonstratedthat four grab samples per station were adequate toprovide statistically representative samples.

Samples were preserved in 10% buffered formalinand with the exception of Stoker (1978, 1981),

ps/ Sampling gear Sieve mesh

(mm)

Sampling area

0.1m2

Smith–McIntyre

0.5 Bering &

Beaufort

0.02m2 Eckman Nearshore

0.1m2

Smith–McIntyre

0.42 Beaufort Sea

shelf

0.1m2 van Veen 1 Bering &

Chukchi Seas

0.1m2 van Veen 1 Bering, Chukchi,

& East Siberian

Seas

m2 van Veen 3 Bering &

Chukchi

0 Grab 1? Canadian

Beaufort

period (late June–September).

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sieved through a 1-mm sieve. Stoker reported resultsfrom 3-mm screen fractions based on his analysisthat there was no significant difference between 1and 3-mm fractions. All biomass values reflectformalin-preserved wet weights (molluscs in theshells and polychaetes without tubes). Animals wereeither blotted dry before weighing or alternatively,animals were placed on filters and excess surfacewater was removed with a vacuum.

Chlorophyll a data were compiled from numer-ous sources (Table 2) and arranged into an Accessdatabase and mapped using ArcInfo’s GIS. As withbiomass, the majority of samples were collectedduring summer months with the largest numberfrom August. A trapezoidal integration macro,written in Visual Basic, was used to integratechlorophyll a data (mgm�2) collected from a seriesof water depths. Shallow-water stations (20–50m)with only one measurement were excluded on thebasis that they did not accurately represent inte-grated chlorophyll a at the measurement location.

Following examination of the raw data, two typesof geostatistical analyses were performed on the dataset. Firstly, spatial trend analyses of biomass andchlorophyll a data were used to create maps ofpredicted continuous spatial distribution over thedefined study area. The resulting maps providedvisual images of abnormally high or low biomass andchlorophyll concentrations. Secondly, the entirebenthic biomass data set was analyzed by location,year, and month of collection in an attempt toidentify stations that had been revisited over time.The only location within the study area with repeatedsampling over a span of several years was in thevicinity of St. Lawrence Island in the Bering Sea.

Table 2

Data sources used in the compilation of integrated chlorophyll a

Principal

investigator

References

Cota & Pomeroy Subset of data compiled for SBI I, Cota and

Wang et al. (2005, this volume)

Macdonald et al. NOGAP B.6 Dept. Fisheries & Oceans, Mac

(1988a–c)

ISHTAR Data Reports UAF, IMS, 1985–19

Grebmeier Grebmeier (1987), Grebmeier and McRoy (19

al. (1988, 1989), Grebmeier and Cooper (199

(2002), Grebmeier and Cooper (unpublished

Parrish Parrish (1987)

The majority of samples were collected during the ice-free period from

Temporal analyses of two areas located north andsouth St. Lawrence Island in the Bering Sea wereperformed to determine if there was a significantchange in benthic biomass over a 30-year time span.Although our temporal analysis does not addressdifferences in specific benthic community compo-nents and is limited by the frequency of sampling,overall trends are apparent and can be useful indocumenting ecosystem-wide changes.

In the Chirikov Basin north of St. LawrenceIsland, and in the northern Bering Sea south of St.Lawrence Island, we identified two 200� 200-km2

regions that each contained over 20 stations ofbenthic measurements collected between 1973 and1994. In Chirikov Basin, samples were collected at35 stations in the summers of 1973 and 1974. Thesedata were compared with biomass measurementscollected during the period 1984–1988 (28 stationstotal). In the region south of St. Lawrence Island,summer biomass samples were only available in1974 (six stations) and from 1988 to 1994 (46stations). Because previous observations in this areasuggest a significant decline in biomass that beganby the early 1990s (see Grebmeier et al., 1995,Grebmeier and Dunton, 2000, Cooper et al., 2002,Lovvorn et al., 2003, Simpkins et al., 2003), aninterannual comparison was considered a moreappropriate statistical analysis.

2.3. GIS and geostatistical analysis

The spatial and temporal distribution in the benthiccommunity of the western Arctic Ocean was analyzedfor trends through geostatistical interpolation of aretrospective (1970–1995) database of 1093 point

Sampling dates Sampling area

Pomeroy (2001), 1974–1979 Bering, Beaufort

Seas

donald et al. 1986–1987 Canadian Beaufort

Sea

88 1985–1988 Bering, Chukchi

Seas

89), Grebmeier et

5), Cooper et al.

data)

1984–1986,

1990,

1994–1995

Bering, Chukchi,

East Siberian Seas

1980–1983 Bering, Chukchi,

Beaufort Seas

late May to September.

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measurements that represented station averages ofthree to five replicate grab samples. For chlorophyll a,the final data set included 2514 integrated stationvalues. We used GIS software to manage, analyze,and display spatially referenced point samples, andinterpolate surfaces of benthic biomass and integratedchlorophyll a. ArcGIS 8.2 was chosen for the analysissince it is appropriate for spatially referenced datameasurements and related attributes (Minami et al.,1999). The geostatistical interpolation of benthicbiomass and chlorophyll a values produced bothmean predictions and prediction standard errors on acontinuous scale within the study region that includedthe Bering, Chukchi, East Siberian, and Beaufort Seas(Fig. 1). Natural variability of benthic biomass wasevident in the standard deviations, which oftenmatched or slightly exceeded the calculated meanbiomass in each of the four regions (Goodall, 2003).

Geostatistical modeling of benthic biomass andintegrated chlorophyll a was accomplished in foursteps (for details see Goodall, 2003). Firstly,exploratory data analyses were performed on rawsamples to understand their basic statistical dis-tributions in time and space. These initial analysesidentified the presence of outliers, large-scale trends,and small or micro-scale variability which deter-mined the selection of a geostatistical model.Secondly, a geostatistical model was developed tointerpolate data and create continuous predictedsurfaces of benthic biomass and chlorophyll a in thestudy area. Thirdly, a temporal analysis wasconducted to determine where repeated samplinghad occurred over time and whether any long-termtrends were evident in the data set. Finally, spatialand temporal interpolated surfaces were subjectedto uncertainty analysis that displayed predictionerrors that indicated how well the model estimatedthe unmeasured locations.

Data interpolation followed geostatistical techni-ques that accounted for the spatial correlation indata when predicting values at unmeasured loca-tions (Isaaks and Srivastava, 1989). The attributevalue at an unmeasured location ðxÞ is calculated asthe sum of three factors: (1) the deterministic trendmðxÞ, (2) the autocorrelated variation e0ðxÞ, and (3) aconstant correction factor for statistical noise, e00

(Burrough and McDonnell, 1998). The generalequation for kriging interpolation is shown as.

ZðxÞ ¼ mðxÞ þ e0ðxÞ þ e00. (1)

Ordinary kriging, the geostatistical techniqueused in this study, assumes that mðxÞ is constant

over space and equal to the mean of the samplespace (Johnston et al., 2001). Thus, the attributevalue at unmeasured location x, ZðxÞ, is estimatedas a spatially correlated term ðe0ðxÞÞ and randomnoise term ðe00ðxÞÞ around a constant mean value(m). We used the Geostatistical Analyst extension toArcGIS 8.2 to describe the spatial and temporaldistribution of benthos from within a GIS environ-ment. The geostatistical interpolation producedboth mean predictions and prediction standarderrors on a continuous scale within the studyregion. To reduce the variability of the predictions,samples were binned and averaged over 100-km2

grid cells to smooth small-scale variabiliy (‘‘patchi-ness’’) prior to geostatistical interpolation (Goodall,2003). To examine the relationship between chlor-ophyll a and benthic biomass using simple regres-sion analysis, samples were binned and averagedwithin 10-km2 grid cells.

3. Results

3.1. Spatial trend analyses

Several interesting features were visually distin-guished using geostatistical techniques. Firstly,contour lines showed very distinct trends ofdecreasing benthic biomass along the Beaufort Seacoast from west to east and with increasing distanceoffshore (Fig. 2). The decrease in biomass westwardinto the East Siberian Sea was much less dramatic.Secondly, areas of highest biomass occurred in thenorthern Bering and Chukchi Seas. Hanna Shoal, inthe northeastern Chukchi Sea, was characterized bya predicted mean benthic biomass of over 360 gm�2.A large area including the western portion of thenorthern Bering Sea, Bering Strait, and the south-western Chukchi Sea were also characterized byextremely high biomass values (up to 1500 gm�2).In contrast, an area of very low benthic biomass(o1 gm�2) occurred at the mouth of the MackenzieRiver. The mapped benthic biomass clearly showsthat values in the Beaufort Sea were distinct fromthat of the other seas in the study area. The meanbenthic biomass for the Beaufort Sea was 33 gm�2,which is significantly lower than the means for theBering, East Siberian, and Chukchi Seas (370, 225,and 167 gm�2, respectively). Over the entire studyarea, biomass ranged from a minimum of 0.01 gm�2

to a maximum value of 3222 gm�2. Analysis ofunpublished data (Table 3) revealed that across theAlaskan Beaufort Sea coast, from Pt. Barrow to

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Fig. 2. The spatial variation in benthic biomass (gm�2) recorded in numerous studies for the period from 1971 to 1995 (see Table 1). Black

points denote locations of quantitative benthic grab samples.

Table 3

Variation in species composition (percentage of total biomass) of benthic organisms collected in summer 1976 by A.G. Carey from Barrow

(1571W) to Barter Island (1441W) on the Beaufort Sea coast at depths varying from 5 to 25m

Location

Group Pt. Barrow (5) Pingok Island (3) Barter Island (5)

Anthozoa 5.976.7 — 1.772.4

Annelida 32.6728.5 48.9735.2 41.6715.7

Arthropoda 28.3727.0 6.377.0 14.0723.1

Mollusca 27.4728.4 35.6731.3 25.0714.8

Echinodermata 0.470.1 0.170.2 1.872.4

Misc. Phyla 5.777.5 9.1712.8 15.8714.6

Values are station mean percentages 7SD. The number of stations analyzed to calculate a mean percentage is denoted in parentheses

(n ¼ 325).

K.H. Dunton et al. / Deep-Sea Research II 52 (2005) 3462–34773468

Barter Island, annelids, arthropods, and molluscsdominated the infaunal species composition.

After initial development of a geostatistical modelbased on the complete data set, we determined thatthe small variability in Beaufort Sea biomass valueswas overwhelmed by higher values in other parts ofthe study area. The Beaufort Sea benthos data wereremoved and modeled separately to reveal finerspatial distribution patterns within this area (Fig. 3).Analysis of these data showed that in stark contrastto the relatively barren nearshore shelf at theMackenzie River, an area of relatively high biomass(4100 gm�2) occurred directly offshore of theColville River. A tongue of elevated biomass(450 gm�2) also extended eastward on the shelfto Barter Island.

Integrated chlorophyll a values showed similar,but not identical spatial trends. Across the entirestudy area, chlorophyll a ranged from 0.15 to1831.55mgm�2, but was generally highest (above150mgm�2) along the western portions of thenorthern Bering Sea, Bering Strait, and ChukchiSea (Fig. 4). Peak values (4300mgm�2) occurredin a defined area southeast of Wrangell Island in thenorthern Chukchi Sea. Higher values of chlorophylla were coincident with increased benthic biomass; inthe Chukchi Sea, the correlation between chloro-phyll a and biomass was significant (r2 ¼ 0:21,p ¼ 0:0015, n ¼ 44). Chlorophyll a values wereclearly lowest along the Alaskan Bering andChukchi coasts and with increasing distance off-shore in Beaufort Sea. The Beaufort Sea was

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generally characterized by comparably lower chlor-ophyll values (o50mgm�2) with the exception of adistinct region at Barter Island. Here, integratedchlorophyll a measurements rise to 80mgm�2.However, in contrast to the Chukchi, there was noobvious correlation between chlorophyll andbenthic biomass in the Beaufort (r2 ¼ 0:001,p ¼ 0:88, n ¼ 20).

Fig. 3. Benthic biomass (gm�2) in the Beaufort Sea (1571–1301W). Yel

Fig. 4. Contours of integrated chlorophyll a concentrations (mgm�2)

locations) collected during the period 1974–1995 (see Table 2).

3.2. Temporal trends

Using ordinary kriging, benthic biomass wasinterpolated for each of the two time periods forthe Chirikov Basin north of St. Lawrence Island(Fig. 5). This region did not show a significant(p ¼ 0:22) change in benthic biomass when the1973–1974 sample data (mean annual range

low points denote locations of quantitative benthic grab samples.

based on discreet measurements (black points denote sampling

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F

1

b

p

p

n

w

K.H. Dunton et al. / Deep-Sea Research II 52 (2005) 3462–34773470

490–650 gm�2) were compared with the period1984–1988 (mean annual range 374–878 gm�2).Similarly, no temporal trend was observed in theregion south of St. Lawrence Island as reflected inmean annual biomass values that ranged from 302to 405 gm�2 over the period 1974–1993. However,the mean biomass at 12 stations in 1994 (229 gm�2)was 35% lower than the average biomass(350 gm�2) recorded in this area in all previousyears. Because of the high variances and modestsamples that are inherent with benthic biomassmeasurements from this region (see Grebmeier andDunton, 2000), this difference was not significant(p ¼ 0:16; one-way ANOVA).

4. Discussion

4.1. Benthic– pelagic coupling and role of

environmental factors

Large spatial variations in benthic faunal biomassand abundance have been noted previously onwestern arctic shelves by Carey et al. (1974),Thomson (1982), Feder et al. (1994b), and Greb-meier and Cooper (1994). In the northeasternChukchi Sea, Feder et al. (1994b) noted highbenthic biomass north of Icy Cape and, based ona multiple discriminate analysis, concluded thatvariations in benthic faunal biomass were notrelated to differences in sediment sorting, but toenhanced local depositional flux of particulateorganic carbon (POC). In the Canadian Arctic,Thomson (1982) also attributed the high standingcrop of benthos in Lancaster Sound to greater foodavailability. By comparison, hydrography, water-column chlorophyll a content, and sediment grainsize were key factors in benthic biomass andcommunity structure in the northern Bering andChukchi Seas (Grebmeier et al., 1988, 1989, 1995).

Other factors, including ice gouging, wave-cur-rent action, and marine mammal feeding activitieshave explained differences in faunal diversitybetween inshore and offshore regions (Feder et al.,1994b), but are probably not responsible for thelarge geographic-wide changes in benthic biomass

ig. 5. Top panel: temporal distribution of benthic collection efforts th

990’s: red). Long-term comparisons of benthic biomass in 200� 200 km

oxes, inset). For Chirikov Basin, north of St. Lawrence Island, sample

eriod 1984–1988 (n ¼ 28). For the region south of St. Lawrence Island

eriod 1974–1994 (bottom panel). No significant (p ¼ 0:22) temporal d

orth of St. Lawrence Island (middle panel). A 35% drop in biomass wa

as not significant (p ¼ 0:16) because of the high sample variance, espe

observed here. Wave-generated bottom turbulencein the ice-dominated Beaufort Sea ecosystem isminimal and therefore not a reasonable explanationfor the decreased abundance of infauna (Carey etal., 1984b); instead, run-off and ice-related stressesare probably more important, especially on regionalscales. Feeding by gray whales and walrus cancreate large excavations (2–20m2) on the seabed(Oliver and Slattery, 1985; Moore et al., 2000), butthese disturbances occur on relatively fine spatialscales relative to variations in benthic biomass,which were averaged over 100 km2 grid cells in thisstudy.

Analysis of the NODC database revealed thatannelids, arthropods, and molluscs generally char-acterize benthic fauna across the entire study area.Carey et al. (1974) found that these groupsdominated the benthos in the western BeaufortSea, but noted no transitional trends in speciescomposition with increasing depth or distance fromshore. They concluded that broad geographicreductions in density and biomass of the benthicmacroinfauna occurred only at the shallowest shelfstations due to grounded ice or pressure ridge keels(Carey et al., 1974). Such groundings can producesevere perturbations of the substrate that can varyconsiderably in size and orientation (Reimnitz et al.,1972). The nearshore infauna at the mouth of theMackenzie River is also characterized by low speciesdiversity (Wacasey, 1975) and biomass (Fig. 3).However, the impoverished nature of the benthicfauna here can be attributed to the area’s highlyestuarine conditions and low productivity (Waca-sey, 1975; Carmack et al., 2004).

In contrast, the large patch of relatively highbiomass in the vicinity of Barter Island correspondswith a well-documented upwelling of deeper shelf-basin waters (Hufford, 1974; Mountain, 1974). Theupwelling appears to be related to the east–westorientation of the coast, and the predominance ofeasterly winds, which transport surface water off-shore (Hufford, 1974). Integrated values of chlor-ophyll a are 5 times higher in the vicinity of BarterIsland than on the adjacent shelf. Consequently, thisupwelling may be a consistent feature of the eastern

roughout the study area by decade (1970’s: blue; 1980’s: green;2 areas were possible north and south of St. Lawrence Island (see

s collected from 1973 to 1974 (n ¼ 35) were compared with the

, interannual variations in biomass (x7SE) were plotted for the

ifference in the biomass among sites was detected in the region

s observed south of St. Lawrence Island in 1994, but this change

cially in 1988 (see text for details). nd: no data available.

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Beaufort Sea, and could explain the high summerconcentration of bowhead whales that are recordedbetween Barter Island and east to Demarcation Bay.Benthic biomass near Barter Island (50–100 gm�2)was two to three times greater than at similar depthsin adjacent areas of the eastern Beaufort Sea andrepresents some of the greatest values for high arcticcontinental shelves (Golikov et al., 1990; Scarlato,1994a, b, c), excluding Barrow Canyon and thesoutheast Chukchi Sea (Grebmeier and Dunton,2000; Grebmeier unpubl. data). For comparativepurposes, it should be noted that the ‘‘hot spot’’region in the southern Chukchi Sea (Hope Basin;Fig. 2) had wet weight values of �1500 gwet wtm�2

(�50–68 gCm�2; Grebmeier, 1987; Grebmeieret al., 1988). The average benthic biomass valuesin Bering Shelf-Anadyr water in the north centralportion of the northern Bering Sea were about300–500 gwetwtm�2 (20–30 gCm�2); in Alaskancoastal water, benthic biomass values were less than100 gwetwtm�2 (o10 gCm�2). Therefore, eventhough the Barter Island biomass is high for theBeaufort Sea and indicates hydrographic focusing,the values are still on the low end in comparisonwith Bering/Chukchi communities, indicative of theoverall lower primary production observed in theBeaufort Sea (Parrish, 1987).

4.2. Role of physical advective processes

High concentrations of benthic infaunal organ-isms (as reflected by elevated biomass), in areascharacterized by lower water-column chlorophyll,suggest that external contributions of POC mayplay an important role in this linked shelf ecosystem(Hansell et al., 1989; Feder et al., 1994b). Forexample, a region of high biomass occurs near themouth of Barrow Canyon (Hanna Shoal) in thenortheastern Chukchi Sea (also noted by Feder etal., 1994b; Grebmeier unpubl. SBI data). Faunalbiomass was greatest in the trough (not its sides),suggesting a shunting of organic-rich material fromthe shelf downslope (Grebmeier and Cooper, 1994;Grebmeier unpubl. SBI data). This physical trans-port of carbon-rich waters from the productivesouthern Chukchi to the northern outer shelf (Federet al., 1994b; Weingartner et al., 1998), thendownslope via canyons into the Arctic Basin mayexplain the high benthic biomass west of PointBarrow (1571W) on the northeastern Chukchi coast.This tongue of high biomass extends eastward intothe Beaufort Sea to at least Harrison Bay (1501W),

north of the Colville River Delta. Biomass inHarrison Bay averages 100 gm�2, with somepatches reaching 200 gm�2 (Fig. 3). Interestingly,the carbon isotopic signatures of fauna in HarrisonBay closely match those of the same organisms inthe Chukchi Sea, indicating a common source offixed carbon that is derived from the northernBering (Dunton et al., 1989). The high biomass ofpolychaetous annelids in Harrison Bay in conjunc-tion with the presence of Bering shelf water ledCarey and Ruff (1977) and Bilyard and Carey(1979) to hypothesize that organic contributionsfrom these waters enrich the benthos west of 1481W.

The integrated chlorophyll a data presented hereare in agreement with those published previouslyfor areas of the Chukchi (Hansell et al., 1989;Grebmeier, 1993; Springer and McRoy, 1993). Thehigh biomass of infaunal organisms in the northernBering Sea (Chirikov Basin), in the southernChukchi, and near Hanna Shoal likely reflect anintegrated and long-term response by the benthiccommunity to a continuous and abundant supply ofadvected nutrients and fixed carbon that originatesin the Gulf of Anadyr (Springer and McRoy, 1993;Grebmeier, 1993). The plume of nutrient-richAnadyr water flows eastward through the ChirikovBasin (Fig. 6) then northward through the southernChukchi Sea and Herald Valley between WrangellIsland and Herald Shoal (Coachman et al., 1975).Anadyr water is joined by water flowing south-eastward from the East Siberian Sea, but it is clearthat northward flowing Anadyr water is associatedwith high benthic biomass along its entire routeonto the Chukchi Shelf.

The effects of northward flowing Anadyr wateron phytoplankton production can clearly be seen indistinctly elevated chlorophyll a concentrations inthe southern Chukchi and southeast of WrangellIsland (Fig. 4). In contrast, low phytoplanktonbiomass along the coast reflects the characteristiclow-salinity and nutrient-poor Alaskan coastalwater (ACW), which originates in the southeasternBering Sea and flows northward across the easternChukchi shelf into the Beaufort Sea as a swiftcoastal current (Coachman et al., 1975; Walsh et al.,1989). The third water mass, Bering shelf water(BSW), originates on the middle shelf of the BeringSea (Fig. 6) and becomes entrained with high levelsof POC as it transits the Chirikov Basin andsouthern Chukchi into the northern Chukchi, eastof Herald Shoal (Whitledge et al., 1986; Walshet al., 1989; Hansell et al., 1989). Feder et al. (1994b)

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Fig. 6. Major water currents in relation to benthic biomass (gm�2) in the SBI study area. Numbers in parentheses denote specific

northward flowing water masses: (1) Anadyr water, (2) Bering Sea water (BSW), and (3) Alaskan coastal water (ACW). See corresponding

text for details. The Siberian Coastal Current (SCC) is an unsteady southward flowing current in the western Chukchi Sea. Black points

denote locations of quantitative benthic grab samples.

K.H. Dunton et al. / Deep-Sea Research II 52 (2005) 3462–3477 3473

argued that the high benthic biomass in the north-eastern Chukchi Sea (near the mouth of the BarrowCanyon) was a product of POC transported byBSW that provided an allochthonous supplement tolocal food production. It appears, based on our dataand Feder et al. (1994b), that an eastward branch ofthe BSW probably mixes with or closely parallelsthe northward flow of the ACW. Consequently, webelieve that the high benthic biomass on the easternChukchi shelf, especially along the coast, is also aresult of advective processes that provide a POCsubsidy from more productive southern shelfsystems.

4.3. Long-term decadal changes

Recent retrospective studies of benthic commu-nities indicate a changing marine system in theBering and Chukchi Seas (Sirenko and Koltun,1992; Grebmeier and Cooper, 1995, 2004; Francis etal., 1996; Grebmeier and Dunton, 2000). Inparticular, the region just north of Bering Straithas historically been a settling basin for organiccarbon, resulting in extremely high benthic standingstock and oxygen uptake rates (Grebmeier andMcRoy, 1989; Grebmeier et al., 1988, 1989;Grebmeier, 1993). Studies south of St. LawrenceIsland also indicate a region of high benthic biomassand carbon deposition, but recent time-series studies

suggest that changes in bivalve populations over thelast few decades may be associated with changes inthe northward transport of water across the shelf(Walsh et al., 1989; Grebmeier and Cooper, 1995,2004; Grebmeier and Dunton, 2000; Lovvorn et al.,2005). Benthic productivity in the northern Beringand Chukchi Seas is directly linked to higher trophiclevels, including marine mammals and seabirds,since the regional food web is dominated bypredation on bivalves and amphipods (Oliver andSlattery, 1985; Grebmeier and Harrison, 1992;Grebmeier and Dunton, 2000; Lovvorn et al.,2003, 2005; Moore et al., 2003; Richman andLovvorn, 2003; Simpkins et al., 2003). The preybase for both gray whales and walruses is appar-ently declining as these apex predators approach orexceed carrying capacity (Lowry et al., 1980; High-smith and Coyle, 1992; Moore et al., 2003) so thatenvironmental changes affecting prey communitiesare likely to have significant effects. Potentialwidespread changes in a variety of marine habitatson the Chukchi and Beaufort shelves could havecascading effects from lower to higher trophicpopulations, emphasizing the importance of under-standing links between climate change, benthiccommunities, and top benthic predators.

Our temporal analysis of benthic biomass in thearea south of St. Lawrence Island showed a decreasein biomass in 1994 relative to all previous years for

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which we have data. A decrease in biomass for thisregion was first noted by Grebmeier and Cooper(1995; unpublished data) and reported later byGrebmeier and Dunton (2000). These observationsare coincident with indications that benthic com-munity structure in this region also has beenchanging (Sirenko and Koltun, 1992) and may berelated to hydrographic changes that have led todecreased water column productivity that is influen-cing spectacled eider populations (Grebmeierand Cooper, 1995; Grebmeier and Dunton, 2000;Richman and Lovvorn, 2003). Other, more recentstudies in the northern Bering Sea show thatdeclines in carbon flux and benthic biomass havecontinued into the late 1990s and early 2000s(Grebmeier and Cooper, 2004; unpublished data)along with declines in benthic amphipod biomass,the main food for gray whales, starting in the late1990s in Chirikov Basin, north of St. LawrenceIsland (Moore et al., 2003).

Consequently, evaluation of potential relation-ships between physical processes and biologicalproductivity, as reflected by the benthos, wouldprovide an unprecedented opportunity to increaseour understanding of the Arctic Ocean system. Inparticular, it is clear that there are undeniablelinkages between advective processes and ecosystemproductivity. Long-term measurements of watercolumn chlorophyll a and benthic standing stocksrepresent key ecosystem processes that provide apowerful tool to assess change, both on largetemporal and spatial scales.

Acknowledgments

We dedicate this paper to the late A.C. Broad whoplaced his trust and confidence in a young graduatestudent (K.H.D.) to implement a coast-wide benthicsampling program in the Beaufort Sea in 1977 andfor providing the data that made this paper possible.We also thank A.G. Carey and H.M. Feder for theirsupport and sharing invaluable unpublished datathroughout the duration of this project, which begannearly three decades ago. We are grateful to JonaFinndıs Jonsdottir (The University of Texas) for herearly contributions to the GIS component of thisproject and to Holly Kelly, Becky Brown andAndrew Reed (The University of Tennessee) forproviding valuable technical assistance for data setpreparations. This research was supported by NSFGrant OPP-9815808 to K.H.D. and OPP-9815497to J.M.G.

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