HANNA SHOAL: An Integrative Study of a High

EDITORIAL FOCUS EDITORIAL Arctic Marine Ecosystem

By: Ken Dunton, The University of Texas at Austin Marine Science Institute, reporting on behalf of the Hanna Shoal Ecosystem Team

24 eco MAY 2015 Groups of walrus rest on ice floes in the northern Chukchi Sea. Photo credit: Ken Dunton . Background Although a little­known feature of the northern Chukchi Shelf, Hanna Shoal captured national attention in late January when President Barack Obama announced the with­ drawal of 9.8 million acres of Beaufort and Chukchi Sea seabed from future oil and gas lease sales. The announcement caught many by surprise since the withdrawals, including small areas of the Beaufort and a 25­mi buffer along the Chukchi Coast, were already deferred from oil and gas leasing in the Department of Interior’s draft 5­year plan, and this action added Hanna Shoal. The Hanna Shoal withdrawal is about 1.6 million acres, or some 300 lease blocks, located just north of Chukchi Sea Outer Continental Shelf Oil and Gas Lease Sale 193 (Figure 1).

Figure 1: The location of Hanna Shoal and stations occupied in the northeast Chukchi Sea during summers 2009, 2010, 2012, and 2013 for the COMIDA program in relation to oil and gas tracts from Lease Sale 193. Water depths are in meters. Graphic credit: Susan Schonberg and Tim Whiteaker. eco MAY 2015 25 Hanna Shoal itself is a shallow topographic feature of general, to the unique physics that steer highly produc- the northeastern Chukchi Sea that lies about 100 mi tive water masses into the region, the relatively shal- northwest of Barrow, Alaska at latitude 72° N. Water low average depth (42 m on the northeastern Chukchi depths on various parts of the Shoal are as shallow as 20 Shelf), and weak grazing pressure from low zooplank- m (60 ft), compared to 55 to 60 m (180 ft) on the sur- ton abundance during spring. These factors facilitate rounding seabed. In contrast to the soft muddy sedi- the deposition of a large proportion of pelagic primary ments characteristic of the Chukchi Shelf, the shallow production to the seabed, thus providing a major car- areas of Hanna Shoal are heavily ice gouged and bon subsidy to the benthic food web. The result is an scoured, resulting in a seabed that is generally charac- extraordinary high diversity and biomass of benthic terized by unsorted course materials, including sand, fauna that coincides with high water column chloro- gravel, small pebbles, and even an occasional boulder. phyll a in localized “hotspots” of the Chukchi Sea, These areas contain a relatively depauperate fauna, in first noted nearly three decades ago by Grebmeier et part due to the heavy scouring by ice that effectively al. (1988). The strong consumptive link between car- removes most long-lived biota. In contrast, the deeper bon produced in the water column and consumed on flanks of the Shoal are biologically rich, as reflected in the seabed (or pelagic-benthic coupling) has continued the historically high concentration of walruses there in to receive strong attention and is now well document- the summer that actively feed on the abundance of mol- ed, especially in shallow western arctic shelf ecosys- EDITORIAL FOCUS EDITORIAL luscs, crustaceans, polychaete worms, and other benthic tems (Dunton et al., 2005). fauna. Walrus forage and rest from ice floes trapped on the Shoal that endure long into late summer. But even Baseline Studies of the Northeastern Chukchi Sea following ice retreat by late-summer, walrus are known Region to make the 300-mi round-trip from haul-outs on the For decades, arctic oceanographers have been aware of northwestern Arctic coast to feed around Hanna Shoal the Hanna Shoal’s unique biological significance and its (Jay et al., 2012). importance as a feeding ground for marine mammals (Fay, 1982; Jay et al., 2012). In 2008, intensive field Oceanographers attribute the high productivity of studies of the northern Chukchi Sea, including areas Hanna Shoal, and the northeastern Chukchi Sea shelf in bordering Hanna Shoal, were launched following the

Photo credit: Lee Cooper.

26 eco MAY 2015 Chukchi Sea OCS Oil and Gas Lease Sale 193, which The COMIDA CAB study provided baseline informa- produced a record $2.67 billion in revenue for the feder- tion on the biological, chemical, and physical char- al government. In response to the sale, industry-spon- acteristics of the northern Chukchi Sea, including a sored biological studies on tracts leased by Shell, description of its trophic structure and identification of ConocoPhillips, and Statoil began in 2008 under the key benthic processes during a period of sea-ice loss and Chukchi Sea Environmental Studies Program (CSESP; climate change. We found that the sediments of the see Hopcroft and Day, 2013). northern Chukchi Sea are essentially pristine with extremely low concentrations of aliphatic hydrocarbons About the same time, our group (the Hanna Shoal and polycyclic aromatic hydrocarbons (PAHs); 17 trace Ecosystem Team) conducted spatially extensive bio- metals were present in sediments at natural background logical and chemical benthic surveys on some 107,000 levels (Trefry et al., 2014; Harvey et al., 2014). The km2 of seabed on a separate project, the Chukchi Sea only exception was confined to two previous (1989) Offshore Monitoring in Drilling Area-Chemical and exploratory drill sites, but there was no evidence that Benthos (COMIDA CAB) study (Dunton et al., 2014). bioaccumulation of these substances occurred above This study was followed by a more interdisciplinary natural concentrations. Nutrients were found at low con- and focused field program on Hanna Shoal in 2012 centrations during late summer, but our ship-board and 2013 (see: comidacab.org/hannashoal/). Both the experiments revealed that nutrients are recycled COMIDA CAB and Hanna Shoal studies are initia- extremely rapidly (~1 day), presumably taken up by tives funded by the Bureau of Ocean Energy phytoplankton that are responsible for the region’s high Management (BOEM), although Shell Exploration primary productivity. and Production also complemented these interdiscipli- nary studies with partial support for ship operations. Links to Earlier Research and Marine Megafauna Our recent work, along with the studies conducted by Our biological studies of the northern Chukchi under the our CSESP colleagues, has greatly enriched our COMIDA CAB study confirmed earlier observations knowledge of this very productive area in the northern that the high primary productivity of the region (as Chukchi Sea that we regard as the Pacific Gateway to noted by Grebmeier et al., 2006), combined with its rel- the Arctic Ocean. atively shallow depths and favorable circulation

A sediment core collected with the benthic lander is brought aboard by graduate student Nathan McTigue. Photo credit: Ken Dunton

eco MAY 2015 27 regimes, sustains a rich epibenthic and infaunal benthos For example, Schonberg et al. (2014) found that gray dominated by polychaete worms, molluscs, crustaceans, whales, which feed on benthic-dwelling amphipods, and echinoderms (Konar et al., 2014; Schonberg et al., were almost exclusively concentrated over an area 2014). Benthic food webs are complex, as defined by between Wainwright and Point Barrow, a region shown their trophic redundancy and diversity of both the infauna to have great concentrations of amphipods that were and epifauna. The high biodiversity and complex trophic first noted in the 1970s. In addition, although the area relationships are signs of robust benthic communities that south of Hanna Shoal is dominated by the favored prey likely possess some degree of resiliency to disturbance. of walrus, including infaunal bivalves and polychaetes, walrus distribution was observed to be closely associat- Analysis of benthic infaunal biomass through the ed with remnant sea-ice distributions. Walrus concen- COMIDA CAB study area revealed areas of potentially trated offshore on ice near Hanna Shoal as long as sea- very high biomass on the south and southeastern flanks ice was available but moved nearer to shore and to new of Hanna Shoal and provide good agreement with earli- coastal haul-out locations when the ice retreated off the er (1970s and 1980s) quantitative benthic studies. Taken shelf. As noted above, the observed concentration of together, these observations suggest that the high pro- marine mammals in these areas over decades suggests a ductivity of the region is a persistent feature of the temporal stability of available benthic prey items. northern Chukchi Shelf that is in part responsible for its However, recent decreases in ice extent and persistence EDITORIAL FOCUS EDITORIAL importance as a feeding area for marine mammals. during the summer months is likely resulting in a reduc-

Figure 2: A synopsis of major water circulation patterns in the northern Chukchi Sea. Nutrient-rich Bering Sea water flows north through the Central Channel before turning east along the Chukchi Sea shelf break and circumventing Hanna Shoal and exiting through Barrow Canyon to the Beaufort Sea. The location of hydrographic and current moorings deployed around Hanna Shoal Ecosystem are denoted by yellow circles. Graphic credit: Robert Pickart. Adapted From Brugler et al., 2014 (with permission). 28 eco MAY 2015 A University of Alaska (UAF) oceanographic mooring is prepared for deployment off Hanna Shoal from the USCGC Healy in 2012. Photo credit: Ken Dunton. tion of available time for walrus to forage the rich ben- thos near Hanna Shoal (Jay et al., 2012).

Water Circulation around Hanna Shoal and Implications for Zooplankton The circulation pattern around Hanna Shoal and its cor- responding water mass properties have been studied extensively over the past couple of years by a number of physical oceanographers. Thanks to their cooperative and coordinated efforts, a much clearer picture of the complex interplay of bathymetry, water mass contribu- tions, and formation of dense winter water is emerging as outlined recently by Weingartner et al. (2013). Circulation patterns (see Figure 2 and Brugler, 2014) show a general clockwise flow around the north and east sides of the Shoal as well as on the west flank based on moorings deployed from both the USCGC Healy in 2012 (retrieved in September 2014) and other mooring deploy- ments and CTD (conductivity, temperature, depth) data.

However, the generation of both very cold and salty waters during winter sea ice formation, as well as and fresher ice meltwaters in summer complicate this overall pattern. In addition, eddies form with the infiltration of northward-flowing, nutrient-rich Bering Sea water.

The currents and water mass movements around Hanna Shoal have profound impacts on water column chlorophyll a

eco MAY 2015 29 biomass and zooplankton distribution, abundance, and ingly, up to 35% of the organic matter is from ter- composition in shelf waters (see Ashjian et al., 2005 and rigenous sources, likely from coastal erosion and Grebmeier et al., 2006). High chlorophyll a levels were inputs from arctic rivers (e.g., the Yukon). No Hanna noted on the western, northern, and eastern sections of Shoal stations exhibited trace metal contamination, Hanna Shoal over the period of our study, which is con- and evidence from gravity cores, which record sistent with more nutrient-rich Bering Sea water flowing decades to centuries of deposition, indicate there has clockwise around the flanks of Hanna Shoal. Integrated been no detectable anthropogenic contributions. water column chlorophyll a levels approached 200 mg m- Similarly, concentrations of a suite of 52 targeted 2, among the higher values recorded in the northeastern PAHs were very low and were present at background Chukchi (Grebmeier et al., 2006). levels in surface sediments surrounding Hanna Shoal with few exceptions. In both field years of our study, Bongo net tows pro- duced the greatest biomass of zooplankton along the As part of a separate study, chemical analysis of mus- edges of the Shoal, particularly on the northwest quad- cle tissues in the whelk Neptunea revealed that this rant, which is dominated by Bering Sea Water, com- omnivorous species is a valuable indicator of anthro- pared to on the eastern side, which is dominated by less- pogenic inputs because, as a long-lived resident of the productive Alaskan Coastal Water (see Figure 2). We benthic community, ingestion of sediments leads to EDITORIAL FOCUS EDITORIAL found the copepod Calanus glacialis/marshallae ubiqui- long-term accumulation of trace metals (e.g., mercury) tous across Hanna Shoal, which is a key species for the and PAHs. Sedimentation studies using natural and planktivorous bowhead whale. Analysis of data is con- bomb fallout radionuclides indicate little deposition on tinuing, with particular emphasis on the large bodied Hanna Shoal, with higher sedimentation rates on the copepod Calanus hyperboreus and euphausiids (krill). periphery of the Shoal, where there is considerable bioturbation by benthic animals. Sediment Chemistry Sediment samples were collected using a variety of Benthic Biology equipment (grabs, box corers, gravity cores, etc.) As mentioned above, a shallow shelf and weak grazing from the Healy. Analyses revealed that total organic pressure allows a large proportion of pelagic produc- carbon was highest in fine-grained sediments from tion to reach the shallow benthos, providing a major stations on the flanks of the Shoal and that, interest- carbon subsidy to the benthic food web of the Chukchi.

Jackie Grebmeier (left) retrieves HAPS Multi-Corer with assistance from Christian Johnson and Philip Bucolo (right) aboard the USCGC Healy. Photo credit: Ken Dunton.

30 eco MAY 2015 Consequently, it has been hypothesized that Arctic that indicate active consumption of chlorophyll a by shelf sediments can act as repositories for the various benthic fauna. In addition, despite the active assimila- pelagic microalgae that sink to the bottom, essentially tion of sediment chlorophyll a by scavenging fauna and creating “food banks” (Pirtle-Levy et al., 2009) for natural degradation, the ratio of chlorophyll a to total benthic grazers. These areas of seabed can be identi- pheopigments was generally >1, suggesting that viable fied by the high chlorophyll a concentrations in the cells in the sediments may be continuing to produce sediments that have been deposited since ice retreat under low light levels. These observations further cor- that include contributions from both phy-toplankton roborate stable isotopic measurements that “food banks” and ice algae (Cooper et al., 2009). of chlorophyll a and other deposited organic matter pro- vide a critical source of carbon to a rich and diverse To assess the importance of such chlorophyll a-rich benthic food web that includes representatives from vir- sediment “food banks,” our team performed hundreds tually every major invertebrate taxonomic group, of extractions on benthic grab samples. Using both flu- including those actively consumed by fish, diving birds, orescence and high-performance liquid chromatogra- and marine mammals. phy (HPLC), we found chlorophyll a concentrations among the highest ever reported in marine sediments The Epibenthic and Infaunal Community (up to 665 mg m-2). Levels varied depending on the Congruent with the benthos functioning as a “food overlying water mass type (rich offshore Bering Sea- bank” for the benthic consumers, estimates of epiben- Anadyr water compared to Alaskan coastal water), thic and infaunal organisms around Hanna Shoal, col- again revealing the link between productivity and the lected using plumb staff beam trawls and van Veen physical dynamics of the system. grabs (respectively), are enormous. Epibenthic assemblages range to 500 g m-2 (and thousands of Our HPLC measurements revealed an abundance of individuals m-2); infaunal biomass and abundances fucoxanthin, which confirmed that most of the chlo- approach 820 g m-2 and 5,500 individuals m-2, rophyll a was derived from diatoms, which are highly respectively. In both sampling years, the greatest bio- prevalent in melting sea ice. Yet even more interesting mass was not on the Shoal itself, but on its northwest were the concentrations of chlorophyll a degradation and southeast flanks (or both), which receive Bering products (pheopigments) in the sediments. McTigue et Sea water that originates in the North Pacific (see al. (2015) found an abundance of various pheopigments Figure 2). Brittle stars and shrimp dominated the

Taxonomist Susan Schonberg (standing left), with graduate students Christina Bonsell and Jordann Young sort infaunal biota on the 'sluice box'. Photo credit: Carolyn Blackwood.

eco MAY 2015 31 epibenthos of the study area, although many other The bifurcation of nutrient-rich Bering Sea waters invertebrates are often very common at particular around Hanna Shoal, the formation of ice melt waters locations (e.g., the gastropod Neptunea, hermit crabs, and cold salty winter waters, and the entrapment of snow crabs, and sea cucumbers). Extensive statistical summer ice on the ice on the Shoal all contribute to a analysis of the observed patterns in species biomass, highly dynamic and changing hydrography. In particu- abundance, and distribution are ongoing, but depth, lar, the relatively slow retreat of sea ice from Hanna temperature, and sediment grain size appear to be Shoal in summer makes it a productive feeding ground among the most important environmental drivers of for the large numbers of walrus that use the ice as a benthic community structure. platform to access the abundant populations of bivalves, crustaceans, and polychaete worms on the seabed. The Bivalves, sipunculids, and polychaete worms generally Hanna Shoal region will continue to draw attention as dominate the infaunal assemblages. Some of our more long as the area holds promise for significant oil and gas recent analyses reveal that for bivalves, areas of highest reserves and polar amplification of a warming climate abundance, biomass, and caloric value are centered on continues to produce biological changes in response to the southeast side of Hanna Shoal, which corresponds to decreases in ice extent and duration. feeding areas for Pacific walrus, based on satellite telemetry (Jay et al., 2012). In addition, taxonomic spe- Acknowledgements EDITORIAL FOCUS EDITORIAL cialists have found a plethora of undescribed polychaete I am very grateful to an extremely talented group of species in our samples. Some of these worm species are oceanographers who compose the Hanna Shoal Eco- simply mis-identified, but many others are new species system Team for their contributions to this article. They to science. include Carin Ashjian (Woods Hole Oceanographic Institution); Bob Campbell (University of Rhode Summary Island); Lee Cooper and Jackie Grebmeier (University The location of Hanna Shoal on the northern shelf edge of Maryland Center for Environmental Science); of the Chukchi Sea is associated with physical oceano- Rodger Harvey (Old Dominion); Brenda Konar and graphic conditions that have led to the development of Tom Weingartner (University of Alaska Fairbanks); rich biological assemblages on the flanks of the Shoal. John Trefry (Florida Institute of Technology); and

A common assortment of epibenthic fauna from the northern Chukchi Sea shelf includes sea anemones, isopods, shrimp, brittle stars, sea cucumbers, whelks, clams, and crabs. Photo credit: Steve Crawford. 32 eco MAY 2015 The UAF all women, plumb-staff beam trawl team. From left to right, Brenda Konar with graduate students Kim Powell, Tanja Schollmeier, and Alex Ravello. Photo credit: Ken Dunton.

David Maidment, Susan Schonberg, and Tim Whiteaker Dunton, K.H., J.M. Grebmeier, and J.H. Trefry. 2014. The benthic ecosystem of the northeastern Chukchi Sea: An overview of its unique biogeochemical and (University of Texas at Austin). The COMIDA CAB biological characteristics. Deep-Sea Research II 102:1-8. and Hanna Shoal Ecosystem Study are funded by the Fay, F.H. 1982. Ecology and biology of the Pacific walrus, Odobenus rosmarus U.S. Department of Interior, Bureau of Ocean Energy divergens. North American Fauna 74:1-279. and Management (BOEM), Alaska Outer Continental Shelf Region, Anchorage, Alaska as part of the Chukchi Grebmeier, J.M., C.P. McRoy, and H.M. Feder. 1988. Pelagic-benthic coupling on the shelf of the Northern Bering and Chukchi Seas .I. Food-supply source Sea Offshore Monitoring in Drilling Area (COMIDA) and benthic biomass. Marine Ecology Progress Series 48:57-67. Project and the BOEM Alaska Environmental Studies Program. We are deeply appreciative to Dick Prentki Grebmeier, J.M., L.W. Cooper, H.M. Feder, and B.I. Sirenko. 2006. Ecosystem dynamics of the Pacific-influenced northern Bering and Chukchi Seas in the and Heather Crowley of BOEM for their participation Amerasian Arctic. Progress in Oceanography 71:331-361. on the research cruises, unqualified support of our Harvey, H.R., K.A. Taylor, H.V. Pie, and C.L. Mitchelmore. 2014. Polycyclic research, and active role in project planning. Funds for aromatic and aliphatic hydrocarbons in Chukchi Sea biota and sediments and partial support of ship operations were provided by their toxicological response in the Arctic cod, Boreogadus saida. Deep-Sea Shell Exploration and Production through the dedicated Research II 102:32-55. efforts of Michael Macrander to enhance our scientific Jay, C.V., A.S. Fischbach, and A.A. Kochnev. 2012. Walrus areas of use in the knowledge of this productive system. Chukchi Sea during sparse sea ice cover. Marine Ecology Progress Series 468:1-13.

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