Effects on Bivalves and Associated Ecology in the Chukchi Sea Due to Changing Sea Ice Conditions

The Arctic Ocean sea ice extent has been rapidly declining since 1979. Positive feedback loops are expected to increase the rate of melting, posing a serious threat to the entire Arctic marine ecosystem. The Arctic Ocean ecosystem, including the Chukchi Sea, is currently benthic dominated, but is expected to become pelagic dominated due to changes in algal and zooplankton dominance.

The Arctic Ocean sea ice extent has been rapidly declining since 1979. Positive feedback loops are expected to increase the rate of melting, posing a serious threat to the entire Arctic marine ecosystem. The Arctic Ocean ecosystem, including the Chukchi Sea, is currently benthic dominated, but is expected to become pelagic dominated due to changes in algal and zooplankton dominance. As bivalves are benthic organisms, they will be negatively affected by this change. Bivalves are integral to the benthic environment, serving as bioturbators and filtering the water. Animals that feed on bivalves, such as walruses (Odobenus rosmarus), tanner crabs (Chionoecetes bairdi), snow crabs (Chionoecetes opilio), and spectacled eiders (Somataria fischeri) will be affected by a changing bivalve population. There will also be an indirect cultural and economic impact in the region. Native Alaskans hunt walruses for subsistence purposes.

Expansion of species range and commercial fisheries into the Arctic Ocean as ice retreats will also affect the ecosystem. The success of the bivalves in the Chukchi Sea is thus essential for sustaining a healthy ecosystem. We propose monitoring bivalve populations, along with the populations of certain key species, to establish exact changes. As reversing sea ice melt is not a viable option to protect these animals, we also explore the idea of a bivalve hatchery for N. radiata to augment the existing population. Although this would be an expensive mitigation measure, a strong bivalve population is necessary to support the ecosystem.

I. Introduction

From an ecosystem standpoint, sea ice is vital to a wide variety of organisms in the Arctic. The Arctic Ocean is currently benthic-dominated due to the large amounts of sea ice algae and phytoplankton that sink down to the bottom and provide food resources for benthic organisms. However, in ice free waters zooplankton are more abundant, as they are no longer limited by temperature, and they consume more of the food at the surface. This creates a more pelagic ecosystem, as the majority of the food is not reaching the bottom. Benthic organisms will be adversely affected by the shift to a pelagic ecosystem. Thus, the significant changes in sea ice extent that are predicted to take place in the Arctic over the next fifty years will have a substantial impact on Arctic bivalve species.

Since 1979, the extent of Arctic sea ice has been rapidly declining. The sea ice extent at summer minimum declined from 7.9 x 106 km2 in 1980 to 3.5 x 106 km2 in 2012. The quantity of older and thicker sea ice is also decreasing rapidly (Vaughan et. al., 2013). Positive feedback loops, are increasing the rate at which sea ice is being lost.

In this paper, we examine how changes in seasonal ice cover will impact marine ecosystems in the Arctic, especially bivalves. Since there is no commercial fishery for bivalves in the Chukchi Sea, and subsistence use is minimal, there is little management from Alaska Department of Fish and Game. We thus focus most of the paper on ecosystem consequences, rather that socioeconomic concerns. However, it is important to note that indirectly, bivalves are of socioeconomic concern.

A decline of bivalve species would have negative impacts on various other animals, including walruses (Odobenus rosmarus), tanner crabs (Chionoecetes bairdi), snow crabs (Chionoecetes opilio), and spectacled eiders (Somataria fischeri), all of which are important to sustain. Native Alaskans near the Chukchi Sea depend on walruses for subsistence, spectacled eiders are threatened, and both tanner and snow crabs are commercially harvested in the Bering Sea, a fishery that could expand into the Chukchi Sea as sea ice retreats.

II. Bivalves

The class Bivalvia, in phylum Mollusca, includes many different organisms, such as clams, oysters, mussels, and scallops. The general biology of bivalves is presented in multiple texts, such as Southeast Alaska’s Rocky Shores (O’Clair & O’Clair, 1998). Bivalves have two calcium carbonate valves, or shells, attached by a hinge that encloses the soft part of the organism. Most species of bivalves are benthic filter or deposit feeders as adults. The life cycle of bivalves includes the planktonic larval stage, and the post-planktonic phase after settlement in the benthos. The pre- settlement phase has a very high mortality rate, but the dispersal of organisms is essential to reduce competition for resources and to colonize new areas (O’Clair & O’Clair, 1998).

Bivalves release their gametes either in a mass spawning event or continuously over a period of several weeks. A few species residing in warm water areas release gametes throughout the year. The timing of the gamete release is partially associated with the lunar calendar (M. Ridgeway, pers. comm., November 10, 2014). In many species, the presence of sperm from the same species in the water will induce spawning. Larvae rely on plankton as a food source (M. Ridgeway, pers. comm., November 10, 2014), and pass through several developmental stages before settling as juveniles (spat).

Until 2003, Macoma calcarea was the dominant species in the Chukchi Sea (Richman & Lovvorn, 2003). Currently, the dominant species is Nuculana radiata (Grebmeier, 2012), of which there are multiple populations (Figure 1). N. radiata is a mollusk that typically grows to 20-25 mm long. N. radiata are mostly infaunal; they live in silty sediments in the sublittoral zone (Voronkov, 2010). There is a lack of published information regarding the specific biology of these species. Bivalves of the genus Nuculana are generally not very mobile and are easily disrupted by dredging and sediment deposition. They typically breed throughout the year and the larvae settle within 12 hours, usually 10 m or less from their origin (Marine Macrofauna Genus Trait Handbook). They feed mostly on detritus that has sunk to the bottom (N. Foster, pers. comm., November 17, 2014).

III. Chukchi Sea

The Chukchi Sea comprises one of eight large marine ecosystems in the Arctic Ocean (Zeller et. al., 2011). It is located north of the Bering Sea, off of the coast of Northwest Alaska (Figure 1). The Chukchi Sea shelf is unique among Arctic shelves due to the organisms, nutrients, heat, and carbon that flow in through the Bering Strait. The shelf is roughly 50 m deep and extends 200 km offshore (Hopcroft, et. al., 2008). The shallow depth facilitates the quick dispersion of nutrients to the benthic environment.

Figure 1. Currents and Nuculana radiata population distribution in the Chukchi Sea. Background image from https://maps.google.com

Pacific water flows north into the Chukchi Sea from the Bering Sea through the Bering Strait in three currents: the Anadyr Current, carrying Anadyr Water, the Alaska Coastal Current, carrying Alaska Coastal Water (ACW), and the Bering Shelf Current, carrying Bering Shelf Water (BSW) (Figure 1). The pressure differences that build here are strong enough to counteract the prevailing northeast winds of around 4 m/s (Weingartner, 2008). Northward flow is larger in the summer and early fall, and slower in fall and winter (Weingartner, 2008).

The cold, salty (32.5 ppt), nitrate-rich, Anadyr Current contributes 60% of the water going through the Bering Strait. It is -1-0˚C southwest of St. Lawrence Island, but warms to 1-2˚C as it moves northward and mixes with BSW. The Anadyr Current also mixes with the cold, fresh, nutrient-poor ice melt of the Siberian Coastal Current, which comes from the west. It then continues northward and exits the basin at Herald Valley (Walsh et. al., 1989).

The ACW is the warmest (2-4˚C) and freshest (31.5 ppt) of the three water masses. The ACW also has low nitrate concentrations (<1 µg/L) throughout the water column. Silicate concentrations in ACW range from 5 µg/L at the bottom up to 5-10 µg/L at the surface. ACW hardly mixes and therefore retains these properties as it flows northeast along Alaska’s coast and exits at barrow canyon (Walsh et. al., 1989).

BSW separates Anadyr Water from ACW as it moves north. It partially mixes with the Anadyr Current through the Bering Strait on its west side, then eventually splits around Hanna Shoal, where it turns east, rejoins, and exits the Chukchi Shelf at Barrow Canyon. Its temperature, salinity, deep and surface nitrate concentrations, and deep silicate concentrations all have intermediate values between the extremes of Anadyr Water and ACW (Walsh et. al., 1989).

Biological activity plays a significant role in the variations of these nutrients as well as other physical and chemical parameters. For example, while biological activity is high, pCO2 values fall to a minimum and organic carbon concentrations rise. Following the summer’s phytoplankton bloom, nitrate and silicate concentrations and pCO2 return to higher values and remain there until the following spring’s bloom. Organic carbon concentrations then fall to a steady low during this same time period. As these nutrients settle, the benthic realm experiences a similar trend in their concentrations. However current variability and storm-facilitated mixing complicate these trends in the shallow Chukchi Sea (Hopcroft et. al., 2008).

The area under the sea ice in the Chukchi Sea is known as the sympagic zone. As the sea ice ages, the salts start to melt out of the ice, forming brine channels in which algae live. Sea ice algae, which are high in polysaccharides, also live on the bottom of the ice. Spring ice-melt results in a stratified system and affords a secondary plankton bloom. Eventually, sinking organic matter is consumed by benthic organisms (M. Ridgeway, pers. comm., November 10, 2014).

IV. Sea Ice

The current decline of Arctic sea ice began in the late 19th century and has continued to the present day (Table 1). The rate of ice loss appears to be increasing due to polar amplification as well as continuing CO2 emissions and the resulting increase in global temperatures. According to the Intergovernmental Panel on Climate Change (IPCC), the sea ice extent at the summer minimum in 2012 fell to a record low of 3.44 x 106 km2 (Vaughan et. al., 2013).

Year

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

2013

Minimum

Sea Ice

5.62

5.97

5.78

5.32

5.75

4.16

4.55

5.05

4.60

4.33

3.37

5.08

Extent

                       

Maximum Sea Ice Extent

15.6

15.6

15.3

14.9

14.7

14.8

15.3

15.2

15.3

14.7

15.3

15.1

Table 1. Sea ice extent (in 106 km2) in the Arctic at the annual minimum and maximum, from 2002 to 2013 (National Snow and Ice Data Center, November 20, 2014).

Compiled data from multiple sources clearly shows that the current rates of ice loss are unprecedented and can be attributed to the disproportionate temperature increase in the Arctic region as the planet warms. A positive feedback loop, associated with a reduced albedo of open water compared to that of sea ice facilitates further warming and melting (Curry et. al., 1994).

First year ice accounted for only 38% of total ice coverage in the 1980’s but 52% of the total ice in 1996. This decrease is alarming, as older ice is generally thicker and takes longer to melt. This demonstrates that the rate of melting has increased (Stroeve et. al., 2007).

Sea ice is currently decreasing at a rate of 12.4% per decade and the Arctic Ocean could be ice-free in the summers as early as 2030 (Stroeve et al., 2007). The newest version of the Community Climate System Model (CCSM3), a model that uses simulations for the atmosphere, land surface, sea ice, and ocean, and a coupler to connect them all, predicts that over the next 50 years the extent of sea ice in the Arctic will continue to decline as global warming continues. The rate at which the ice is melting is predicted to slow until 2024. After that, the extent of the ice in September is projected to decrease rapidly. Sea ice is expected to cover 6 million square kilometers in 2024, and by 2034 that area will decrease to 2 million square kilometers. The sea ice is also expected to thin out, which will allow it to melt at faster rates. By 2040, only a small portion of perennial ice will remain, near the northern parts of Greenland and Canada, and the Arctic basin will be essentially ice-free every September (Holland et. al., 2006).

VI. Potential Ecosystem Changes

It is predicted that by 2050 the shelf waters of the Chukchi Sea will be ice free from August through October. By the end of the century, the ice-free period could increase to 4-5 months out of the year. Different models predict different freezing times in the fall; however, for the rest of this century, the Chukchi Sea is expected to be completely ice-covered from February through April (Douglas, 2010). Many parts of the Chukchi Sea ecosystem are dependent on sea ice, and the decline in its extent will create changes to the whole system.

An additional consequence of climate change will be an increase in storm intensity and frequency (T. Weingartner, pers. comm., October 26, 2014). As the currents flowing north through the Chukchi Sea are flowing in opposition to the prevailing winds, large increases in winds from storms could intermittently slow down or reverse these currents. However, ultimately, ice melt will decrease pressure in the Arctic causing the Alaska Coastal Current to increase in speed (T. Weingartner, pers. comm., October 26, 2014). Additionally, increasing storms will exacerbate coastal erosion and increase the terrestrial content of coastal sediments.

With more open water extending farther north into the Chukchi Sea during summer months, the northward extent of the pelagic phytoplankton bloom will increase. Compared to ice- associated blooms, these pelagic blooms are much smaller. Thus, certain latitudes that become devoid of ice during the spring may experience less primary productivity (M. Ridgeway, pers. comm., November 10, 2014).

When there is more sea ice cover, the amount of CO2 transferred from the air to the sea is relatively low. As ice cover decreases, more CO2 will be added to the ocean. This, coupled with increased anthropogenic releases of CO2 to the atmosphere, is acidifying the Chukchi Sea, lowering the saturation states of calcite and aragonite, and harming calcifying benthic fauna, particularly bivalves and echinoderms (Hopcroft, et. al., 2008).

In years with greater sea ice extent there is more ice algae, and the quantity of zooplankton is not large enough to consume the majority of the phytoplankton bloom. In warmer years, however, zooplankton are not limited by temperature and consume more primary production at the surface, which prevents the nutrients from reaching the benthic environment. As sea ice continues to retreat, the timing of plankton blooms will vary, and more warm water blooms will occur. The loss of nutrients in the benthic environment will change the area into a more pelagic- driven ecosystem. Pelagic fish and sea birds will obtain more nutrients than the demersal fish and walrus (Hopcroft et. al., 2008). A loss of sea ice will also reduce ice edge plankton blooms, which will result in less phytoplankton and detritus sinking to the benthic environment (N. Foster, pers. comm., November 17, 2014).

As previously mentioned, an increase in storms will result in more eroded sediments. Detrimental effects to benthic organisms, including bivalves, will ensue as a result (N. Foster, pers. comm., November 17, 2014). Some species, such as N. radiata, cannot move easily through the sediment. This may result in bivalves trapped under the additional layers (N. Foster, pers. comm., November 17, 2014).

In 2004, blue mussels (Mytilus edulis) were found in the Arctic Archipelago of Svalbard, a location in which they had not lived since the Viking age. The larvae were most likely transported to this northern location in the West Spitsbergen Current. As a result of sea surface temperatures changing, there was an increase in warm water flowing northward and the larvae were no longer temperature limited (Berge et. al., 2005). A similar situation could occur in the Chukchi Sea with Pacific organisms entering the Arctic ecosystem.

Nuculana radiata is currently a dominant species in the region; however, since 2003 their population size has been decreasing and there has been a trend of increasing amounts of Ennucula tenuis, which is a smaller mollusk than N. radiata that typically reaches only 10-13 mm (Grebmeier, 2012). This trend has continued as climate change progresses (N. Foster, pers. comm., November 17, 2014). Decreasing bivalve populations will have many effects on the Chukchi Sea ecosystem. Many animals eat bivalves such as walruses, seals, and crabs.

Specifically, spectacled eiders (Somateria fischeri), snow crabs (Chionoecetes opilio), tanner crabs (Chionoecetes bairdi) and others prey on N. radiata (Figure 2) (N. Foster, pers. comm., November 17, 2014). A decrease in the populations of bivalves will thus hinder the populations of these animals.

Figure 2. Chukchi Sea food web based on bivalve populations.

Individual images taken from:

http://en.wikipedia.org/wiki/Karenia_brevis,

http://tolweb.org/’radial_centric_diatoms’/125295, http://www.nhm.ac.uk/research-curation/life- sciences/genomics-microbial-diversity/research/diatom-research/species-rich-taxa/index.html, http://commons.wikimedia.org/wiki/File:Somateria_fischeri_%28Spectacled_Eider_-_Plueschkopfente%29_-_Weltvogelpark_Walsrode_2012-33.jpg, http://animals.nationalgeographic.com/animals/mammals/walrus/, http://www.mbari.org/mars/science/biology_photo_gallery/images/pages/E-tanner_crab_jpg.htm, http://www.arcticvoice.org/inuit.html, and http://commons.m.wikimedia.org/wiki/File:Bivalvia.jpeg

Walruses have both a cultural and nutritional importance to Native Alaskans living near the Chukchi Sea. From 1878 to 1880 there was a “Great Famine” on St. Lawrence Island when 1000- 2000 islanders died of starvation because the walruses had been over hunted by outsiders. This is one example of these communities’ dependence on walrus, which continues to the present. The Eskimo Walrus Commission (EWC) monitors walrus populations. Walrus migration patterns are changing and populations are shrinking because animals are being affected by a lack of sea ice, which makes subsistence hunting more difficult (Metcalf and Robards, 2008). A smaller bivalve population will further strain the walrus population which will, in turn, adversely affect native communities dependent on subsistence hunting.

Spectacled eiders are listed as threatened by the U.S. Fish and Wildlife Service (Environmental Conservation Online System). These ducks dive 40 to 70 m in order to eat benthic invertebrates. Although these dives require a considerable amount of energy, the benthic environment has had enough biomass to sustain them. In 2003, it was observed that the dominant species was changing from Macoma calcarea to Nuculana radiata. Richman and Lovvorn (2003) conducted a study to find how this change would affect the spectacled eiders. They tested the relative digestibility, differences in nutrient and energy content, areal density, crushing resistance of shells, shell length, gut retention time, and sedimentary depth of M. calcarea and N. radiata, and how these factors affected the relative foraging value. It was found that the energy intake was 14 to 19% higher for N. radiata than M. calcarea of the same length. However, when the M. calcarea specimens collected were larger than the N. radiata, 58% more energy was obtained from the M. calcarea than from the N. radiata (Richman & Lovvorn, 2003). This shows that a major factor in the energy obtained is the size of the prey. Based on this information, if the dominant bivalve species in the Chukchi Sea changes from N. radiata to the smaller E. tenuis, this could present energy challenges for the already threatened spectacled eider population.

Bivalves also serve as bioturbators. As they move around in the sediment, their movements bring in oxygen and water that other infaunal organisms can use, and allow nutrients from the sediment to go back into the water. This is a key role in the benthic ecosystem. As previously mentioned, bivalves are filter feeders. This feeding mechanism filters the water by removing bacteria, particulate organic matter, and phytoplankton (Vaughn & Hakenkamp, 2001). A relevant example of this is that reductions in the oyster population in Chesapeake Bay resulted in worsened water quality because the benthic filter feeder role was missing from the ecosystem (Newell, 1988). Reductions in bivalve populations in the Chukchi Sea may also have adverse effects on water quality. Furthermore, a shift to smaller species of bivalves dominating the Chukchi Sea may reduce the effectiveness of bivalves as bioturbators.

VII. Management Recommendations

Bivalve play several key roles in the Chukchi ecosystem, including providing a food source for walruses, crabs, and spectacled eiders, oxygenating sediments through bioturbation, and improving water quality through filtration, that would be jeopardized if their populations decrease.

Bivalves are an integral part of the Arctic food web (Figure 2), and thus, their success indirectly plays important cultural and economic roles. Walruses, which predate bivalves, are hugely important to subsistence hunters in Western Alaska and on the North Slope, who rely on them for food as a way of life. Spectacled eiders are a threatened species that could become extinct as smaller bivalves gain dominance. Tanner and snow crabs currently have considerably important fisheries in the Bering Sea, and as sea ice retreats and commercial opportunity in the Chukchi Sea increases, it would not be unrealistic to see these fisheries expand northward into the Chukchi region. In this case, healthy Arctic bivalve populations would be imperative to sustaining tanner and snow crab fisheries.

Several bivalve species, including N. radiata, have already begun showing a decline in population. As zooplankton populations grow, lowering the settlement of phytoplankton detritus to the benthos, and as erosion increases and alters sediment composition, we expect that other bivalve species are at risk of following this trend. Currently, bivalve population dynamics in the Chukchi Sea are not well understood, but we believe they are essential to ecosystem sustainability and should not be ignored. Therefore, we recommend that the populations of several important bivalves, including N. radiata, be monitored to collect baseline data and help determine the extent that bivalves are being affected. These monitoring operations should also be extended to walruses, spectacled eiders, and tanner and snow crabs to investigate the connections between these species and bivalves.

Should the monitoring projects verify a declining bivalve population, we feel this trend should be combated. Although expensive, one possible method for combating this decline is creating a bivalve hatchery to augment natural populations. Simply adding bivalves to an environment will not increase populations in a sustainable fashion, but controlling the location and timing of out-planting can reduce the risk of bivalves being affected by erosion and can better optimize the timing of bivalve abundance with the phytoplankton bloom. Much of the mariculture specifics for this operation must be researched, but many can be structured analogous to other shellfish operations in the Pacific Northwest. The timing, placement, and methodology of the out- planting process, however, must be specific to this project and further studied.

Currently, there is no Alaska statute that allows for this type of project. Bivalves can legally be cultured, but they are not out-planted on a commercial level (C. Pring-Ham, pers.comm., November 14, 2014). However, in 1974, the Alaska State Legislature passed the Private Non-Profit Hatchery Act, allowing the Alaska Department of Fish and Game to give permits to private non- profit (PNP) salmon hatcheries aimed at restoring a thriving salmon population. A new statute, modeled off of this act, should be pursued if monitoring operations confirm that bivalve success is declining and the ecosystem is being adversely impacted.

The rapid reduction of the polar ice cap, as a result of global warming, has already begun to bring great ecological changes to the Chukchi Sea which will further proliferate in the coming years. Bivalves, in particular, have already shown population declination resulting from the northward movement of zooplankton that consume phytoplankton before they settle to the benthos. Bivalves will also be threatened by habitat loss as increased storms increase coastal erosion. If bivalve populations decline, walrus, spectacled eider, and tanner and snow crab populations could

also decline, which would bring about significant cultural and fiscal harms. Given the important uses of the Chukchi Sea and the ecosystem’s reliance on bivalves, we feel that understanding bivalve population dynamics and mitigating any declines is crucial and monitoring operations and potential remedies should be pursued.

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Effects on Bivalves and Associated Ecology in the Chukchi Sea Due to Changing Sea Ice Conditions

The Arctic Ocean sea ice extent has been rapidly declining since 1979. Positive feedback loops are expected to increase the rate of melting, posing a serious threat to the entire Arctic marine ecosystem. The Arctic Ocean ecosystem, including the Chukchi Sea, is currently benthic dominated, but is expected to become pelagic dominated due to changes in algal and zooplankton dominance.

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