The benthic ecosystem of the northeastern Chukchi Sea: An overview of its unique biogeochemical and biological characteristics

The benthic ecosystem of the northeastern Chukchi Sea: An overviewof its unique biogeochemical and biological characteristics

Kenneth H. Dunton a,n, Jacqueline M. Grebmeier b, John H. Trefry c

a Marine Science Institute, The University of Texas at Austin, Port Aransas, TX 78373, United Statesb Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science, Solomons, MD 20688, United Statesc Department of Marine and Environmental Systems, Florida Institute of Technology, Melbourne, FL 32901, United States

a r t i c l e i n f o

Available online 20 January 2014

Keywords:ArcticChukchi SeaFood websBenthosInfaunaEpifaunaMetalsNutrientsPolycyclic aromatic hydrocarbonsMarine mammals

a b s t r a c t

In February 2008, Lease Sale 193 generated renewed interest for oil and gas exploration in thenortheastern Chukchi Sea and prompted a series of studies designed to increase our scientific knowledgeof this biologically rich area. We present in this special issue the results from major field expeditionsduring open-water periods in the summers of 2009 and 2010. Our work focused on the biological andchemical characteristics of the benthos with the goal of establishing a strong baseline for assessing futurechanges that may occur in response to (1) impacts from oil and gas activities, and (2) variations inhydrography, circulation or ice retreat associated with climatic change. We found concentrations ofaliphatic hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), and 17 trace metals in sediments atnatural background levels throughout the study area except at two previous (1989) drilling sites; therewas no evidence that showed bioaccumulation of these substances above natural concentrations.Inorganic-N was recycled within one day throughout the water column, with evidence of substantialremineralization of organic matter in the sediments. Active efflux of sediment NO3

� supports watercolumn primary production that, in turn, sustains a rich benthos dominated by crustaceans andechinoderms that also receive, based on isotopic evidence, a benthic carbon subsidy. Benthic food websare complex, with high trophic redundancy based on the diversity of both infaunal and epifaunalpopulations. The highest trophic levels in the benthos were dominated by predatory gastropods.Comparisons of gray whale and walrus distributions from aerial sightings showed a large differencebetween the two study years relative to the more stable benthic prey base for these animals over thatperiod. A nearly ice-free shelf by early summer 2009 compared to 2010 revealed that walrus distributionswere more closely linked to sea ice rather than to benthic prey items, indicating that rapid retreat of sea-ice could threaten traditional feeding grounds.

& 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Heightened interest for Lease Sale 193 in the northeasternChukchi Sea by the oil and gas industry, as demonstrated by $2.7billion in bids during February 2008 (Dinkelman et al., 2008), ledthe U.S. Bureau of Ocean Energy Management (BOEM) to conductan array of studies to better characterize this biologically richecosystem. One resulting project, entitled Chukchi Sea OffshoreMonitoring in Drilling Area-Chemical and Benthos (COMIDA CAB),focused on benthic resources from a regional perspective.Here, we present the products of that investigation to providea valuable addition to the initial baseline conducted in the 1970sand 1980s by the U.S. Department of Commerce Outer Continental

Shelf Environmental Assessment Program (OCSEAP). The COMIDACAB project was carried out concurrently with the Chukchi SeaEnvironmental Studies Program (CSESP) that was funded by aconsortium of three oil and gas companies including ConocoPhil-lips Alaska, Shell Exploration and Production, and Statoil USA E&P(Hopcroft and Day, 2013). The industry program focused onspecific areas where future drilling may occur. Results from theCSESP were reported in nine papers (summarized by Day et al.,2013) that complement the 12 papers in this volume and greatlyenrich our knowledge about this Pacific gateway to theArctic Ocean.

The COMIDA CAB study provided baseline information on(1) the biological, chemical and physical characteristics of theChukchi Sea prior to renewed oil and gas exploration, and(2) trophic structure and benthic processes during a period ofsea-ice loss and climate change. Biological surveys (reported herein four papers) were carried out to determine the abundance and

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Deep-Sea Research II

0967-0645/$ - see front matter & 2014 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.dsr2.2014.01.001

n Corresponding author. Tel.: þ1 361 749 6744.E-mail address: [email protected] (K.H. Dunton).

Deep-Sea Research II 102 (2014) 1–8

distribution patterns of benthic infauna and epifauna in relation tocirculation, hydrography, and sediment characteristics. Two paperspresent and explain patterns for sediment grain size, accumulationrates, organic carbon, trace metals, aliphatic n-alkanes and poly-cyclic aromatic hydrocarbons (PAHs). Organic carbon dynamicsand nitrogen cycling were the focus of three papers. Several papersaddress food web structure and linkages between lower and upperlevel trophic consumers. Finally, one paper evaluates the caloricvalue of potential benthic prey species for marine mammals andanother paper investigates the distribution patterns of mercury inseawater, sediments and benthic biota. Collectively, the results ofthe study have provided several new perspectives on the biogeo-chemical and biological characteristics as described below.

2. Stations, data, and setting

2.1. Sampling approach

Station locations were initially determined using a generalrandomized tessellation stratified (GRTS) design in the coreCOMIDA CAB area (30 stations) as described in Hersh andMaidment (2014). We then added additional stations over theGRTS grid (18 in 2009 and 15 in 2010) to fill spatial gaps, sampleareas of known historical or biological significance and to ensure arandom but even distribution in a region that spans over107,000 km2. This arrangement resulted in the placement of 30GRTS core stations in a spatial grid (Fig. 1). Of the 30 GRTS stations,10 were chosen as overlapping stations for cross-calibration ofvarious benthic measurements. We also added stations in the“upstream” Bering Strait/SE Chukchi region (station 103) and inthe core grid (station 105; Fig. 1) in 2010. A subset of the 2009

stations (n¼25) was reoccupied in 2010 and scientific sampling wasexpanded to enable a systems’ approach to our broad spatialanalysis of the Chukchi Sea using ArcGIS.

2.2. Data management and GIS

In support of the extensive datasets generated during theCOMIDA CAB study, this project included a large data managementcomponent as outlined by Hersh and Maidment (2014). Datamanagement was carried out using an observations data model(ODM) that incorporated a significant ArcGIS component. Thegraphical displays from this effort show spatial patterns of broadsignificance with respect to the complex nature of the ChukchiShelf. The ODM developed for this project proved highly effective.Field products, reports, data, and shapefiles are available on theCOMIDA CAB website maintained at The University of Texas atAustin (http://www.comidacab.org).

2.3. Ice conditions

Sea-ice conditions varied considerably between the 2009 and2010 field seasons (Fig. 2). In 2009, sea-ice remained in the HannaShoal area until late July, after which it completely retreated. Incontrast, sea-ice remained over both Herald and Hanna Shoals intolate July 2010 (Weingartner et al., 2013) and over Hanna Shoal andother areas of the northeastern Chukchi Sea well into August.Interestingly, Schonberg et al. (2014) observed that walrus con-centrated offshore near Hanna Shoal as long as sea-ice wasavailable but moved nearer to shore and coastal haul-out locationswhen the ice disappeared. Ice retreat also was linked to the timingand duration of ice algal deposition on the seabed, as addressed byMcTigue and Dunton (2014) in their work to establish trophic

Bering Sea WaterAlaskan Coastal WaterWinter waterWinter water

Fig. 1. Stations in the northeastern Chukchi Sea during summer 2009 and 2010 for the COMIDA CAB project in relation to oil and gas tracts for Lease Sale 193, historic wellsites (see Day et al., 2013; Blanchard and Feder, 2014), and mean circulation (adapted from Spall, 2007 and Weingartner et al., 2013).

K.H. Dunton et al. / Deep-Sea Research II 102 (2014) 1–82

Fig. 2. Sea-ice distribution for the same periods in the Chukchi Sea at the start (28 July) and end (16 August) of our 2009 and 2010 cruises aboard the R/V Alpha Helix and R/VMoana Wave, respectively. Overall, ice conditions were more variable in 2009, with large ice fields common in the northern part of the study area. In 2010, the area waspredominantly ice-free. Maps were compiled from ftp://sidads.colorado.edu/DATASETS/NOAA/G02135/ at the U.S. National Ice Center, http://www.natice.noaa.gov/.

K.H. Dunton et al. / Deep-Sea Research II 102 (2014) 1–8 3

relationships between producers and consumers. Ice algae areimportant carbon sources to benthic consumers, and the timing ofice retreat is an important factor in delineating the role of ice algalcarbon from other sources of carbon when constructing food webs.The timing of ice retreat also appears to be influenced by theinvasion of Bering Shelf Water from the south, which displacedcold, saline winter waters that formed during the ice coveredperiod (Weingartner et al., 2013), and influenced benthic fluxes ofinorganic-N (Souza et al., 2014b).

3. Organic compounds, trace metals and nitrogen cycling

Harvey et al. (2014) determined concentrations of aliphatichydrocarbons and PAHs in surface sediments throughout the studyarea. With one exception near a 1980s drilling site, PAH concentra-tions in surface sediments on the Chukchi shelf were at backgroundvalues (o1600 ng/g) based on a mixture of compounds thatrepresent pyrogenic, petrogenic and biogenic sources. The distribu-tion of aliphatic n-alkanes in surface sediments was linked to naturalbackground and petroleum hydrocarbon sources with significantinputs of vascular plant debris from the Alaskan shoreline. Harveyet al. (2014) also found that concentrations of PAHs in muscle tissuefrom the gastropod Neptunea decreased in larger size specimens perunit weight whereas aliphatic n-alkanes in Neptunea gastropodmuscle increased in larger organisms (Fig. 3). Based on this study,Neptunea represents a potential indicator species for monitoringchanges in the loadings of organic contaminants to the system.

Trefry et al. (2014) reported that concentrations of 17 tracemetals varied considerably across the study area as a function ofsediment grain size; however, essentially all values were at naturalbackground. They used ratios of metals/Al as a model for deter-mining background metal concentrations and identifying anthro-pogenic inputs. Anomalies were linked to old drilling sites in theChukchi Sea (Fig. 1) and the Red Dog mine (near station 104). Datafor 137Cs and excess 210Pb in sediment cores from the northeasternChukchi Sea were used to determine sedimentation rates thatranged from 0.03 to 0.14 cm year�1, not including the impacts ofbioturbation. Corresponding vertical distributions for most metalsin age-dated sediment cores showed4100-year records of uni-form metal/Al ratios, including Pb. Total organic carbon concen-trations in sediments ranged from 0.06 to 1.56% with a marineorigin for at least two-thirds of the organic matter at each stationbased on δ13C values and C/N ratios.

In a related study, Fox et al. (2014) presented results for mercury inseawater, sediments and biota. They showed that concentrations ofdissolved mercury were lower at water depths with markedly higherconcentrations of chlorophyll a. In addition, Fox et al. (2014) alsofound a long-term record of natural, background mercury values insediment cores, except near two old drilling sites. Concentrations ofmethylmercury in benthic biota increased with increasing trophiclevel following a predicted relationship with highest concentrations ofmethylmercury in the whelks.

In two related papers, Souza et al. (2014a,b) explored nutrientand nitrogen gas fluxes in the water column and at the sediment–water interface. They found that the sediments in the Chukchi Seaare sites of intense denitrification. Despite this uptake, the ChukchiSea shelf is not a sink for N in late summer because losses of fixed Nare compensated by benthic NO3

- regeneration and an efflux of NO3�

from sediments into the overlying water that exceeds that of N2.Regeneration of both N and P was active, with excess phosphorusresulting in nutrient fluxes that produced low (o4) N:P ratios. In thewater column, nitrification accounted for most NH4

þ uptake, and wasa likely a significant source of regenerated NO3

- during the latesummer.

Collectively, results from Souza et al. (2014a,b) indicate thatnitrifiers have a quantifiable ability to convert regenerated NH4

þ

into the NO3� pool, which supports the argument that “new”

production may not be equated indiscriminately to export pro-duction in this arctic ecosystem. These measurements are amongthe first to describe N-cycling on the Chukchi shelf, and areimportant to understanding nitrogen dynamics, including regu-lation of both primary and secondary production (Fig. 4). Com-munity NH4

þ regeneration rates measured by Souza et al. (2014a)reflect mainly heterotrophic (bacteria, microbial grazers) pro-cesses, but there is a competition for NH4

þ uptake by bothbacteria (which prefer NH4

þ over NO3�) and phytoplankton,

depending on light, nutrient, and organic-substrate availability(Gardner et al. 2004). With the steady retreat of ice over theChukchi Shelf, two questions arise: how will the dynamics of N-cycling change, and how will these changes influence rates ofprimary productivity, both in the water column and in thebenthos? The availability of inorganic-N is closely coupled toprimary and thus secondary production (Fig. 4), and futurestudies that clarify the magnitude of N-flux between the sedi-ments and water column, under both open-water and ice coveredperiods, thus become increasingly important.

4. The benthic and epibenthic community

4.1. Distributional patterns in biomass and density

The northern Chukchi Sea is universally recognized as a regionwith an abundant and diverse benthic fauna that exhibits highspatial and temporal variability in carbon production (Grebmeier,et al. 2006). As noted by Blanchard and Feder (2014), mesoscaleenvironmental gradients can be important in structuring thebiological community. Schonberg et al. (2014) noted that low C:N ratios, particularly in the vicinity of Hanna Shoal and BarrowCanyon, were correlated with high abundances of infaunal andepifaunal (Ravelo et al., 2014) biota collected in grabs and trawls,respectively (Fig. 5). Bivalves dominated the infaunal abundanceclosest to the southern edge of Hanna Shoal and polychaetes werecollected in great numbers slightly to the south, overlapping withelevated epifaunal abundances driven by numerous Ophiuroids(brittle stars). These observations suggest that the presence ofmore labile carbon sources may support the higher secondaryproductivity of this region.

0.00

0.01

0.02

2.00

4.00

6.00Parent PAHsAlkyl-PAHs

< 5 cm 5-8 cm > 8 cmNeptunea shell size

Long-chain n-alkanesShort-chain n-alkanes

Hyd

roca

rbon

(µg

g-1 m

uscl

e w

et w

t)

Fig. 3. Concentrations of n-alkanes and PAHs in the foot muscle of the whelkNeptunea heros. Accumulation as a function of size was only observed for somecompound classes. From Harvey et al. (2014).

K.H. Dunton et al. / Deep-Sea Research II 102 (2014) 1–84

Konar et al. (2014) collected baseline data on size frequencydistributions for a number of trophically important epibenthicspecies including Chionoecetes and Hyas crabs, the gastropodsPlicifusus, Colus, Cryptonatica, and Neptunea, and the echino-derms Gorgonocephalus, Leptasterias, and Echinarachnius.Because variations in food supply or predation, both possibleconsequences of climatic change, could alter population size fre-quencies, benchmark data are critical for detecting large scale anddownstream effects on benthic community structure. Many of thesespecies include representatives in the crustacean or echinodermgroups, which dominate the epibenthic fauna (Fig. 6). Communitiesdominated by crustaceans exhibited higher diversity and evennessindex values compared to communities dominated by echinoderms.As noted earlier, stations with the highest biomass seemed tocoincide with water masses that merge and then flow eastward tothe south of Hanna Shoal in the vicinity of Barrow Canyon(Weingartner et al., 2005).

5. Sources and fates of organic carbon: food web implications

5.1. The case of the missing 13C-enriched carbon

McTigue and Dunton (2014) investigated the trophic structureof the northeastern Chukchi Sea using stable carbon and nitrogenisotopic measurements for hundreds of specimens collected from theseabed and water column. Based on their work and other isotopicdata collected in the COMIDA CAB study area (Dunton et al., 2012), aworking food web model of the region incorporates two likely, butdistinct carbon source end-members preferred by benthic consumers(Fig. 7). Energy moves from pelagic carbon, one of the two end-members, to zooplankton, amphipods, holothurians, echinarachniidsand dollars, isopods, and forams. These groups rely little on 13C-enriched benthic microalgal carbon, and therefore, assimilate almostexclusively pelagic carbon. Fish obtain most of their carbon fromthese groups, although some carbon also is acquired from othergroups that rely on benthic sources, including ice algal carbon thatsettles to the benthos earlier in the season.

In contrast, bivalves, bryozoans, hydrozoans, sponges, andascidians appear to derive a major portion of their carbon froman isotopically enriched 13C benthic carbon source whoseidentity remains largely unconfirmed. McTigue and Dunton(2014) postulate on the sources for this carbon, which includebenthic microalgae, ice algae, or microbially degraded organicmatter. Primary carbon sources are notoriously difficult toextract and isolate from sediments, but it is apparent that13C-enriched carbon is transferred to benthic predators, includ-ing gastropods, cephalopoda, priapulids, decapods, and poly-chaetes. Third trophic level omnivores, including sipunculids,ophiuroids, and polychaetes, obtain carbon from both primaryproducers and primary consumers. As indiscriminant feeders,they assimilate both benthic microalgae and pelagic carbon.Both pelagic carbon and benthic algal carbon seem to play animportant role in the benthic food web of the Chukchi Sea.These data, along with new samples collected by BOEM overHanna Shoal in 2012 and 2013 (http://www.boem.gov/BOEM-Newsroom/Press-Releases/2012/Press08062012%282%29.aspx),will provide an invaluable opportunity to assess the potentialexposure effects and uptake of anthropogenic contaminants tokey prey species in Chukchi Sea food webs.

5.2. Linking benthic fauna with marine mammals

Schonberg et al. (2014) examined the relationships betweeninfaunal distributions of preferred prey with walrus and graywhale locations during July and August within the COMIDA studyarea. They found gray whales were almost exclusively concen-trated over an area between Wainwright and Pt. Barrow, a regionshown to have great concentrations of benthic amphipods thatthrive on a high carbon flux to the surface sediments. Although thearea south of Hanna Shoal is dominated by the favored prey ofwalrus, including infaunal bivalves and polychaetes, walrus dis-tribution was observed to be closely associated with sea-icedispersal (Fig. 2). Walrus concentrated offshore near Hanna Shoalas long as sea-ice was available but moved nearer to shore andcoastal haul-out locations when the ice disappeared. The observedpersistence of marine mammals in these areas over decades of

anoxic sediments

organic matter(phytoplankton,

ice algae)

organic matter(including microalgae)

NH4+

NH4+

diffusion

NO3- , NO2

- regeneration

settling

N2

denitrification

assimilationdiffusion

DNRA

water column

oxicsediments

regeneration

nitrification

nitrificationassimilation

higher trophic levels

NO3- , NO2

-

Fig. 4. The dynamic processes of nitrogen cycling in Chukchi shelf waters based on measurements of uptake and regeneration made in the water column, near-bottomwaters and at the sediment surface by Souza et al. (2014a,b). Graphic modified from Gardner et al. (2004). DNRA, dissimilatory nitrate reduction to ammonium.

K.H. Dunton et al. / Deep-Sea Research II 102 (2014) 1–8 5

time suggests a temporal stability of available benthic prey itemsalthough the change in ice extent in recent years is forcing areduction in available time for walrus to forage the rich benthosnear Hanna Shoal.

Wilt et al. (2014) confirmed the link between caloric contentand taxon for potential walrus prey items. Of the 11 classes ofinvertebrates surveyed, mean caloric content was highest in bivalves,

gastropods, and polychaetes (over 20 MJ kg�1) and lowest in echi-noids (15 MJ kg�1). They noted a clear latitudinal gradient ofincreasing caloric value from south to north, presumably related tohigher food availability in the northern reaches of the study area.These observations further validate concerns for the disruption ofwalrus feeding in historical feeding grounds by sea-ice decline andearly retreat.

Fig. 5. Comparative measurements of epifauna and infauna abundance by station. Panels: (A) epifauna abundance (number of individuals m�2) and (B) infaunal abundance(number of individuals m�2). Equivalent areas of biota concentration are circled in blue. Graphics are from Ravelo et al. (2014) and Schonberg et al. (2014).

K.H. Dunton et al. / Deep-Sea Research II 102 (2014) 1–86

6. Summary

Findings from our studies on the northeastern Chukchi Seashelf clearly reveal a benthic system of remarkable heterogeneitywith respect to its biology and chemistry. Patterns in the

distribution of the benthic infauna and epifauna, variations insediment organic matter content and composition, and pristinecondition with respect to trace metals and PAHs are likelyproducts of the dynamic nature of water mass movementsthrough the region as described by Weingartner et al. (2013).

Fig. 6. Relative abundance of epibenthic taxa collected in trawls throughout the northeastern Chukchi Sea (from Ravelo et al., 2012).

HolothuroideaAmphipodaEchinarachniidaeIsopodaForaminifera

Pisces

BivalviaPoriferaHydrozoaBryozoaAscidiacea

GastropodaCephalopodaPriapulidaDecapodaPolychaeta (predatory)

Polychaeta (omnivorous)SipunculidaOphiuroidea

Zooplankton

13C-depleted carbon (phytoplankton)

13C-enriched carbon (ice algae, benthic

microalgae, microbiallydegraded org C)

TL=2

TL=2-3

TL=3-4

TL=2

TL=4

TL=3

Fig. 7. A conceptual food web depicting the connections between hypothesized carbon sources (gray boxes) and upper trophic level consumers from epibenthic (Raveloet al., 2014) and infaunal (Schonberg et al., 2014) communities. Trophic pathways and levels (TL) inferred from McTigue et al. (2012) with dashed arrows representing weakerconnections of energy flow.

K.H. Dunton et al. / Deep-Sea Research II 102 (2014) 1–8 7

Annual variations in the rate of sea ice retreat provide yetanother response variable, resulting in a system that Day et al.(2013) describe as “unexpectedly complex”. We believe thatresults from the CSESP (Day et al., 2013) and the COMIDA CABProject (2014) provide an important step forward in under-standing basic processes and tracking future changes in thenortheastern Chukchi Sea.

Acknowledgments

The U.S. Department of Interior, Bureau of Ocean EnergyManagement (BOEM), Alaska Outer Continental Shelf Region,Anchorage, Alaska provided funding under Contract no.M08PC20056 as a part of the Chukchi Sea Offshore Monitoringin Drilling Area (COMIDA) Project and the BOEM Alaska Environ-mental Studies Program. We are deeply appreciative to DickPrentki of BOEM for his participation on the research cruises,unqualified support of our research, and active role in projectplanning throughout the COMIDA CAB project period. His enthu-siasm for our science stimulated great work and dedication to theproject by us all. We salute Captain John Seville and crews of bothR/V Alpha Helix and R/V Moana Wave for their dedication and workto make collection of our diverse datasets possible. We areextremely grateful to Jackie Grebmeier who provided leadership,humor, and vision as Chief Scientist on both cruises. Our thanksalso to Heather Crowley at BOEM for facilitating the publicationefforts of this special issue. Funds for partial support of shipoperations were provided by Shell Exploration and Productionthrough the dedicated efforts of Michael Macrander to enhanceour scientific knowledge of this productive system. Editorialsupport for this special issue was provided by Zack Darnell. Finally,as COMIDA CAB science team lead, Ken Dunton would like toexpress his sincere thanks to the COMIDA CAB PIs, their studentsand associates for their teamwork and friendship that resulted incommendable science over the past 4 years.

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