Ken Dunton 2015-04-21 23:41:33
By: Ken Dunton, The University of Texas at Austin Marine Science Institute, reporting on behalf of the Hanna Shoal Ecosystem Team Background Although a littleknown feature of the northern Chukchi Shelf, Hanna Shoal captured national attention in late January when President Barack Obama announced the withdrawal 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). Hanna Shoal itself is a shallow topographic feature of the northeastern Chukchi Sea that lies about 100 mi northwest of Barrow, Alaska at latitude 72° N. Water depths on various parts of the Shoal are as shallow as 20 m (60 ft), compared to 55 to 60 m (180 ft) on the surrounding seabed. In contrast to the soft muddy sediments characteristic of the Chukchi Shelf, the shallow areas of Hanna Shoal are heavily ice gouged and scoured, resulting in a seabed that is generally characterized by unsorted course materials, including sand, gravel, small pebbles, and even an occasional boulder. These areas contain a relatively depauperate fauna, in part due to the heavy scouring by ice that effectively removes most long-lived biota. In contrast, the deeper flanks of the Shoal are biologically rich, as reflected in the historically high concentration of walruses there in the summer that actively feed on the abundance of molluscs, crustaceans, polychaete worms, and other benthic fauna. Walrus forage and rest from ice floes trapped on the Shoal that endure long into late summer. But even following ice retreat by late-summer, walrus are known to make the 300-mi round-trip from haul-outs on the northwestern Arctic coast to feed around Hanna Shoal (Jay et al., 2012). Oceanographers attribute the high productivity of Hanna Shoal, and the northeastern Chukchi Sea shelf in general, to the unique physics that steer highly productive water masses into the region, the relatively shallow average depth (42 m on the northeastern Chukchi Shelf), and weak grazing pressure from low zooplankton abundance during spring. These factors facilitate the deposition of a large proportion of pelagic primary production to the seabed, thus providing a major carbon subsidy to the benthic food web. The result is an extraordinary high diversity and biomass of benthic fauna that coincides with high water column chlorophyll a in localized “hotspots” of the Chukchi Sea, first noted nearly three decades ago by Grebmeier et al. (1988). The strong consumptive link between carbon produced in the water column and consumed on the seabed (or pelagic-benthic coupling) has continued to receive strong attention and is now well documented, especially in shallow western arctic shelf ecosystems (Dunton et al., 2005). Baseline Studies of the Northeastern Chukchi Sea Region For decades, arctic oceanographers have been aware of the Hanna Shoal’s unique biological significance and its importance as a feeding ground for marine mammals (Fay, 1982; Jay et al., 2012). In 2008, intensive field studies of the northern Chukchi Sea, including areas bordering Hanna Shoal, were launched following the Chukchi Sea OCS Oil and Gas Lease Sale 193, which produced a record $2.67 billion in revenue for the federal government. In response to the sale, industry-sponsored biological studies on tracts leased by Shell, ConocoPhillips, and Statoil began in 2008 under the Chukchi Sea Environmental Studies Program (CSESP; see Hopcroft and Day, 2013). About the same time, our group (the Hanna Shoal Ecosystem Team) conducted spatially extensive biological and chemical benthic surveys on some 107,000 km2 of seabed on a separate project, the Chukchi Sea Offshore Monitoring in Drilling Area-Chemical and Benthos (COMIDA CAB) study (Dunton et al., 2014). This study was followed by a more interdisciplinary and focused field program on Hanna Shoal in 2012 and 2013 (see: comidacab.org/hannashoal/). Both the COMIDA CAB and Hanna Shoal studies are initiatives funded by the Bureau of Ocean Energy Management (BOEM), although Shell Exploration and Production also complemented these interdisciplinary studies with partial support for ship operations. Our recent work, along with the studies conducted by our CSESP colleagues, has greatly enriched our knowledge of this very productive area in the northern Chukchi Sea that we regard as the Pacific Gateway to the Arctic Ocean. The COMIDA CAB study provided baseline information on the biological, chemical, and physical characteristics of the northern Chukchi Sea, including a description of its trophic structure and identification of key benthic processes during a period of sea-ice loss and climate change. We found that the sediments of the northern Chukchi Sea are essentially pristine with extremely low concentrations of aliphatic hydrocarbons and polycyclic aromatic hydrocarbons (PAHs); 17 trace metals were present in sediments at natural background levels (Trefry et al., 2014; Harvey et al., 2014). The only exception was confined to two previous (1989) exploratory drill sites, but there was no evidence that bioaccumulation of these substances occurred above natural concentrations. Nutrients were found at low concentrations during late summer, but our ship-board experiments revealed that nutrients are recycled extremely rapidly (~1 day), presumably taken up by phytoplankton that are responsible for the region’s high primary productivity. Links to Earlier Research and Marine Megafauna Our biological studies of the northern Chukchi under the COMIDA CAB study confirmed earlier observations that the high primary productivity of the region (as noted by Grebmeier et al., 2006), combined with its relatively shallow depths and favorable circulation regimes, sustains a rich epibenthic and infaunal benthos dominated by polychaete worms, molluscs, crustaceans, and echinoderms (Konar et al., 2014; Schonberg et al., 2014) . Benthic food webs are complex, as defined by their trophic redundancy and diversity of both the infauna and epifauna. The high biodiversity and complex trophic relationships are signs of robust benthic communities that likely possess some degree of resiliency to disturbance. Analysis of benthic infaunal biomass through the COMIDA CAB study area revealed areas of potentially very high biomass on the south and southeastern flanks of Hanna Shoal and provide good agreement with earlier (1970s and 1980s) quantitative benthic studies. Taken together, these observations suggest that the high productivity of the region is a persistent feature of the northern Chukchi Shelf that is in part responsible for its importance as a feeding area for marine mammals. For example, Schonberg et al. (2014) found that gray whales, which feed on benthic-dwelling amphipods, were almost exclusively concentrated over an area between Wainwright and Point Barrow, a region shown to have great concentrations of amphipods that were first noted in the 1970s. In addition, although the area south of Hanna Shoal is dominated by the favored prey of walrus, including infaunal bivalves and polychaetes, walrus distribution was observed to be closely associated with remnant sea-ice distributions. Walrus concentrated offshore on ice near Hanna Shoal as long as seaice was available but moved nearer to shore and to new coastal haul-out locations when the ice retreated off the shelf. As noted above, the observed concentration of marine mammals in these areas over decades suggests a temporal stability of available benthic prey items. However, recent decreases in ice extent and persistence during the summer months is likely resulting in a reduction of available time for walrus to forage the rich benthos near Hanna Shoal (Jay et al., 2012). Water Circulation around Hanna Shoal and Implications for Zooplankton The circulation pattern around Hanna Shoal and its corresponding 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 contributions, 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 deployments 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 biomass and zooplankton distribution, abundance, and composition in shelf waters (see Ashjian et al., 2005 and Grebmeier et al., 2006). High chlorophyll a levels were noted on the western, northern, and eastern sections of Hanna Shoal over the period of our study, which is consistent with more nutrient-rich Bering Sea water flowing clockwise around the flanks of Hanna Shoal. Integrated water column chlorophyll a levels approached 200 mg m- 2, among the higher values recorded in the northeastern Chukchi (Grebmeier et al., 2006). In both field years of our study, Bongo net tows produced the greatest biomass of zooplankton along the edges of the Shoal, particularly on the northwest quadrant, which is dominated by Bering Sea Water, compared to on the eastern side, which is dominated by lessproductive Alaskan Coastal Water (see Figure 2). We found the copepod Calanus glacialis/marshallae ubiquitous across Hanna Shoal, which is a key species for the planktivorous bowhead whale. Analysis of data is continuing, with particular emphasis on the large bodied copepod Calanus hyperboreus and euphausiids (krill). Sediment Chemistry Sediment samples were collected using a variety of equipment (grabs, box corers, gravity cores, etc.) from the Healy. Analyses revealed that total organic carbon was highest in fine-grained sediments from stations on the flanks of the Shoal and that, interestingly, up to 35% of the organic matter is from terrigenous sources, likely from coastal erosion and inputs from arctic rivers (e.g., the Yukon). No Hanna Shoal stations exhibited trace metal contamination, and evidence from gravity cores, which record decades to centuries of deposition, indicate there has been no detectable anthropogenic contributions. Similarly, concentrations of a suite of 52 targeted PAHs were very low and were present at background levels in surface sediments surrounding Hanna Shoal with few exceptions. As part of a separate study, chemical analysis of muscle tissues in the whelk Neptunea revealed that this omnivorous species is a valuable indicator of anthropogenic inputs because, as a long-lived resident of the benthic community, ingestion of sediments leads to long-term accumulation of trace metals (e.g., mercury) and PAHs. Sedimentation studies using natural and bomb fallout radionuclides indicate little deposition on Hanna Shoal, with higher sedimentation rates on the periphery of the Shoal, where there is considerable bioturbation by benthic animals. Benthic Biology As mentioned above, a shallow shelf and weak grazing pressure allows a large proportion of pelagic production to reach the shallow benthos, providing a major carbon subsidy to the benthic food web of the Chukchi. Consequently, it has been hypothesized that Arctic shelf sediments can act as repositories for the various pelagic microalgae that sink to the bottom, essentially creating “food banks” (Pirtle-Levy et al., 2009) for benthic grazers. These areas of seabed can be identified by the high chlorophyll a concentrations in the sediments that have been deposited since ice retreat that include contributions from both phy-toplankton and ice algae (Cooper et al., 2009). To assess the importance of such chlorophyll a-rich sediment “food banks,” our team performed hundreds of extractions on benthic grab samples. Using both fluorescence and high-performance liquid chromatography (HPLC), we found chlorophyll a concentrations among the highest ever reported in marine sediments (up to 665 mg m-2). Levels varied depending on the overlying water mass type (rich offshore Bering Sea- Anadyr water compared to Alaskan coastal water), again revealing the link between productivity and the physical dynamics of the system. Our HPLC measurements revealed an abundance of fucoxanthin, which confirmed that most of the chlorophyll a was derived from diatoms, which are highly prevalent in melting sea ice. Yet even more interesting were the concentrations of chlorophyll a degradation products (pheopigments) in the sediments. McTigue et al. (2015) found an abundance of various pheopigments that indicate active consumption of chlorophyll a by benthic fauna. In addition, despite the active assimilation of sediment chlorophyll a by scavenging fauna and natural degradation, the ratio of chlorophyll a to total pheopigments was generally >1, suggesting that viable cells in the sediments may be continuing to produce under low light levels. These observations further corroborate stable isotopic measurements that “food banks” of chlorophyll a and other deposited organic matter provide a critical source of carbon to a rich and diverse benthic food web that includes representatives from virtually every major invertebrate taxonomic group, including those actively consumed by fish, diving birds, and marine mammals. The Epibenthic and Infaunal Community Congruent with the benthos functioning as a “food bank” for the benthic consumers, estimates of epibenthic and infaunal organisms around Hanna Shoal, collected using plumb staff beam trawls and van Veen grabs (respectively), are enormous. Epibenthic assemblages range to 500 g m-2 (and thousands of individuals m-2); infaunal biomass and abundances approach 820 g m-2 and 5,500 individuals m-2, respectively. In both sampling years, the greatest biomass was not on the Shoal itself, but on its northwest and southeast flanks (or both), which receive Bering Sea water that originates in the North Pacific (see Figure 2). Brittle stars and shrimp dominated the epibenthos of the study area, although many other invertebrates are often very common at particular locations (e.g., the gastropod Neptunea, hermit crabs, snow crabs, and sea cucumbers). Extensive statistical analysis of the observed patterns in species biomass, abundance, and distribution are ongoing, but depth, temperature, and sediment grain size appear to be among the most important environmental drivers of benthic community structure. Bivalves, sipunculids, and polychaete worms generally dominate the infaunal assemblages. Some of our more recent analyses reveal that for bivalves, areas of highest abundance, biomass, and caloric value are centered on the southeast side of Hanna Shoal, which corresponds to feeding areas for Pacific walrus, based on satellite telemetry (Jay et al., 2012). In addition, taxonomic specialists have found a plethora of undescribed polychaete species in our samples. Some of these worm species are simply mis-identified, but many others are new species to science. Summary The location of Hanna Shoal on the northern shelf edge of the Chukchi Sea is associated with physical oceanographic conditions that have led to the development of rich biological assemblages on the flanks of the Shoal. The bifurcation of nutrient-rich Bering Sea waters around Hanna Shoal, the formation of ice melt waters and cold salty winter waters, and the entrapment of summer ice on the ice on the Shoal all contribute to a highly dynamic and changing hydrography. In particular, the relatively slow retreat of sea ice from Hanna Shoal in summer makes it a productive feeding ground for the large numbers of walrus that use the ice as a platform to access the abundant populations of bivalves, crustaceans, and polychaete worms on the seabed. The Hanna Shoal region will continue to draw attention as long as the area holds promise for significant oil and gas reserves and polar amplification of a warming climate continues to produce biological changes in response to decreases in ice extent and duration. Acknowledgements I am very grateful to an extremely talented group of oceanographers who compose the Hanna Shoal Ecosystem Team for their contributions to this article. They include Carin Ashjian (Woods Hole Oceanographic Institution); Bob Campbell (University of Rhode Island); Lee Cooper and Jackie Grebmeier (University of Maryland Center for Environmental Science); Rodger Harvey (Old Dominion); Brenda Konar and Tom Weingartner (University of Alaska Fairbanks); John Trefry (Florida Institute of Technology); and David Maidment, Susan Schonberg, and Tim Whiteaker (University of Texas at Austin). The COMIDA CAB and Hanna Shoal Ecosystem Study are funded by the U.S. Department of Interior, Bureau of Ocean Energy and Management (BOEM), Alaska Outer Continental Shelf Region, Anchorage, Alaska as part of the Chukchi Sea Offshore Monitoring in Drilling Area (COMIDA) Project and the BOEM Alaska Environmental Studies Program. We are deeply appreciative to Dick Prentki and Heather Crowley of BOEM for their participation on the research cruises, unqualified support of our research, and active role in project planning. 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