General description of the Arctic biota

May 7, 2012, 1:25 pm

This is Section 9.3.1 of the Arctic Climate Impact Assessment
Lead Author: Harald Loeng; Contributing Authors: Keith Brander, Eddy Carmack, Stanislav Denisenko, Ken Drinkwater, Bogi Hansen, Kit Kovacs, Pat Livingston, Fiona McLaughlin, Egil Sakshaug;  Consulting Authors: Richard Bellerby, Howard Browman,Tore Furevik, Jacqueline M. Grebmeier, Eystein Jansen, Steingrimur Jónsson, Lis Lindal Jørgensen, Svend-Aage Malmberg, Svein Østerhus, Geir Ottersen, Koji Shimada

 

Biological production in the oceans is based primarily on phytoplankton or planktonic algae. These are microscopic unicellular plants that mostly reside within the water column but in the Arctic are also found in and on the sea ice. Through photosynthesis, they reduce CO2 while releasing oxygen and producing carbohydrates. The carbohydrates are converted, according to the needs of the algae, into essential compounds such as proteins and nucleic acids by incorporating nitrogen, phosphorus, sulfur, and other elements.

The organic matter produced by the algae is primarily consumed by herbivorous (i.e., plant-eating) animals, mainly zooplankton, which in turn may be eaten by fish. The fish are then consumed by seabirds and mammals, including humans. Each segment of the food web within which organisms take in food in the same manner is called a trophic level. Thus phytoplankton are considered the first tropic level, zooplankton the second, etc. The loss of organic matter between one trophic level and the next is about 75 to 80%. The main losses are associated with respiration (i.e., the burning of food) within the organisms themselves, consumption by bacteria (i.e., microbial degradation) of dissolved organic matter, and sinking cellular remains and fecal pellets (i.e., the body’s waste). These processes all result in the release of CO2 or nutrients. Only a small fraction of the organic matter reaches the seabed – the deeper the water column, the smaller this fraction. (Box 9.6)

 Box 9.6. Organisms in the food web

Population abundance, whether for algae, fish, or polar bears (Ursus maritimus), is dependent on the population growth and death rates. Given a growth rate higher than the death rate, the population size will increase, and vice versa. If the two rates are equal, the population is in steady state. Another variable is population migration; stocks may arrive in or leave a given ecosystem. Essentially, a change in any environmental variable, including those affected by climate change, has a direct impact on one or more processes by changing their rate, which in turn causes a change in population biomass.Thus, while the population growth rate is determined by light or nutrient levels (algae), or food availability (animals), the loss rate represents the sum of losses due to natural death, pollution, sedimentation, and being eaten, fished, or hunted.

Populations can be arranged hierarchically within a food web on the basis of what they eat, with the lowest trophic level comprising photosynthetic organisms. Animals can move up the food-web hierarchy as they grow, by becoming able to eat larger prey. Because most of the food intake is spent on maintaining life, reproduction, movements, etc., only 15 to 25% contributes to population growth, which during steady state represents food for the next trophic level. Consequently, marine food chains are short, with a maximum of five trophic levels.

In models, population growth is described by exponential functions in which growth and mortality rates themselves are functions of environmental change, including changes caused by the evolution of the ecosystem itself (feedback). Ideally, ecosystem models should include all trophic levels, including major species as separate entities; however, coupling plankton and fish is difficult, as is the coupling of fish and higher animals.

 

Pelagic ecosystems are those which occur within the water column of the open ocean away from the ocean floor. Arctic pelagic ecosystems, like pelagic ecosystems elsewhere and in contrast to terrestrial ecosystems, are dominated by animal biomass. In the Barents Sea, for example, the mean annual plant biomass is 2 g C/m2 whereas the mean annual animal biomass is at least four times more. Globally, annual marine primary production is about 40 Pg C (i.e., 1,015 grams) or 40% of the global total (marine plus terrestrial) production.

Macroalgal biomass (i.e., large plants such as kelp and sea-tangle) in the Arctic is believed to be small due to habitat restrictions caused by freezing, ice scouring by small icebergs, and local freshwater input. In some areas, however, macroalgal biomass can be large as kelp forests do occur in the Arctic.

Generally, the higher the trophic level, the smaller the production. In the Barents Sea, major pelagic fish species represent a few hundred milligrams of carbon biomass per square meter and seabirds and polar bears, only 2.5 and 0.25 mg C/m2, respectively (Table 9.5). The table shows the inverted biomass pyramid which is typical for phytoplankton and zooplankton in the marine pelagic food web. On the Bering Shelf, the annual primary production is higher than in the Barents Sea. In the shallow areas of the Bering Sea (40 to 100 m depth), the "rain" of organic particles from the upper layers to the benthic (bottom-dwelling) animals can be higher than the fraction grazed by pelagic animals[5] . This input is also much higher than in the Barents Sea, which has an average depth of 230 m. Benthic biomass and production are lowest in the deep Arctic Ocean.

Table 9.5. Average carbon biomass and annual carbon productivity for different trophic levels within the Barents Sea, compared with that for human populations in Norway and Japan. Data recalculated from Sakshaug et al.[6].

 

Biomass
(mg C/m2)

Productivity
(mg C/m2/yr)

Bacteria

400

60,000

Phytoplankton

2,000

90,000

Zooplankton (copepods and krill)

>3,000

9,500

Zoobenthosa

5,160

1,550

Capelinb

600

300

Codc

300

100

Minke whales

110

2.6

Seals

30

0.5

Seabirds

2.5

0.4

Polar bears

0.25

0.027

People, Norway

107

1.5

People, Japan

2,200

22

aInterannual biomass variation, 3,000–7,350 mg C/m2 [7]; bInterannual biomass variation, 30–700 mg C/m2; cInterannual biomass variation, 50–700 mg C/m2.

 

Box 9.7 reviews the highly specialized communities associated with seasonal and multi-year sea ice.

Although phytoplankton generally grow more slowly in the Arctic than in warmer areas, near-freezing temperatures would not delay the onset of the initial phytoplankton bloom (i.e., period of very high production) by more than two to three days compared to that at 5 to 10°C. Light- and nutrient-limitation is more important than temperature. Arctic zooplankton, certainly the predominant copepods, have adapted to cold conditions by having life cycles that are two to ten times longer than corresponding species in temperate conditions.

The Arctic Ocean as a whole is not particularly productive yet seasonal productivity in patches of the Barents and Chukchi Seas, and on the Bering Shelf, is among the highest of anywhere in the world[8]. In these areas, the primary production supports large populations of migratory seabirds, a large community of various mammals, and some of the world’s richest fisheries.

Many seabirds and some marine mammal species either migrate into the Arctic during the summer pulse of productivity or can cope with the long periods when food supplies are limited. Many of the permanent residents store large quantities of reserve energy in the form of lipids (oils) during periods of abundant food supply while others survive winter in a dormant stage.

Box 9.7. Sea-ice communities

Sympagic organisms are those that live in close association with sea ice, either within channels in the ice itself, on the underside of the ice, or at the interface with the water immediately below the ice.The organisms that inhabit this environment are highly specialized, but cover a wide taxonomic range, from bacteria and simple algae, to vertebrate fauna. Some species, particularly microorganisms, become incorporated into the sea ice as the ice crystals grow. While it may seem an inhospitable environment, the sea ice is actually quite a stable and organically-enriched environment for those organisms that can tolerate its extreme conditions. While some organisms occupy the sea ice as it forms, others actively or passively migrate into the ice ecosystems. Organisms that live within the interstitial spaces of sea ice include microfauna such as protists, and larger organisms such as ciliates, nematodes, rotatorians, turbellarians, and copepods. Multi-year sea ice has the most complex communities and often serves as a platform for colonizers to young ice. In addition, the abundance and biomass of the multi-year sea ice organisms can be very high. For example, copepods may easily exceed densities of 150 individuals per square meter. Given the correct conditions vast algal mats can form on the under-surface of sea ice, including both microalgae and macroalgae such as sea-tangle (Fucus distichus), and their associated epiphytic organisms such as Pylaiella littoralis. In comparison, seasonal sea ice normally has lower densities and lower biomasses and tends to support more simple communities.

The spatial distribution of sea-ice fauna is generally patchy, even within single ice fields, because the origin, history, size, snow thickness, and the thickness of the sea ice itself can vary dramatically. Interannual variability within a community is also high. In areas such as the Barents Sea, the size of the Atlantic Water inflow varies from year to year, causing dramatic changes in the sympagic community. Nevertheless, sympagic communities are characterized by fauna that can withstand high levels of variation in food availability and low temperatures. Generally, the older the sea ice, the more complex and established the sympagic community. Ice-living invertebrates tend to have low basal metabolic rates, concomitant slow growth, and long life cycles. For example, some arctic ice amphipods live for five or six years, while their more temperate counterparts, or amphipods in other arctic habitats, have average life spans of two to three years. Large, lipid-rich ice-dwelling amphipods are prime prey for the circumpolar polar cod (Boreogadus saida).This small, arctic fish is an opportunistic feeder that can live pelagically, or in association with sea-ice communities. In ice-filled waters its diet largely comprises Themisto libellula and Apherusa glacialis.The fish capture these small “lipid packages” and convert them into prey that is substantial enough to support higher vertebrates such as seabirds and marine mammals. Some seabirds and marine mammals also eat large invertebrate ice-dwellers directly. Black guillemots (Cepphus grylle) and thickbilled murres (Uria lomvia) feed on the amphipod Gammarus wilkitzkii. Little auks (Alle alle) and ivory gulls (Pagophila eburnea) also eat sympagic amphipods. First-year harp seal (Phoca groenlandica) feed extensively on sympagic amphipods when they start to self-feed. However, the preferred prey within the sympagic community is polar cod for most marine mammals.

Thinning, and reduced coverage of arctic sea ice will have dramatic impacts on the entire sympagic ecosystem, particularly on interstitial organisms as these do not have alternate habitats in which to live. Also, given that the sympagic community is important in providing pelagic and benthic communities with food, particularly during the summer when the sea ice melts, changes in this highly specialized environment are likely to have repercussions throughout the arctic marine community as a whole.

 

Phytoplankton, microalgae, and macroalgae (9.3.1.1)

Phytoplankton are often classified according to size. Nanoplankton (2–20 ?m) are the most abundant yet several microplankton species (>20 ?m; some reaching 500–750 ?m) can produce intense blooms given sufficient light, nutrients, and stratification. Microalgae can join together and then sink to form thick mats on the bottom in shallow coastal waters[9]. Among the approximately 300 species of marine phytoplankton known in high northern latitudes, diatoms and dinoflagellates comprise around 160 and 35 species, respectively[10]. Diatoms have non-growing siliceous shells and thus need silicate for growth while dinoflagellates move by the action of tail-like projections called flagella. Diatoms are responsible for most of the primary production in arctic pelagic ecosystems. Within the Arctic, the Arctic Ocean has the lowest number of different species and the western Barents Sea the most[11].

Prymnesiophytes (another group of swimming flagellates) include the two bloom-forming species; Phaeocystis pouchetii and Emiliania huxleyi (the latter being an exception among prymnesiophytes by lacking flagella and having a cover of calcite platelets, and as such are highly relevant to the carbon cycle). Phaeocystis pouchetii is common throughout the Arctic except in the deep Arctic Ocean[12]. Emiliania blooms have been observed south of Iceland [13], in the Norwegian and Bering seas[14], and in Norwegian fjords [15]. Emiliania huxleyi blooms were first recorded on the southeastern Bering Shelf in 1997 during an extremely bright summer [16] and in the Barents Sea in 2000[17]. Emiliania huxleyi continues to bloom in both areas.

Dinoflagellates, chrysophytes, cryptophytes, and green flagellates are common in arctic waters. Cyanobacteria (formerly called blue-green algae), common in temperate and tropical waters, are abundant in the deep reaches of the Bering Sea[18]. They are also transported into the Barents Sea by the Atlantic inflow. Dinoflagellates are particularly important in multi-year ice, and a variety of flagellates thrive in melt ponds on top of the sea ice in summer[19].

The major species of diatom and prymnesiophyte possess the water-soluble reserve carbohydrate ß-1,3 glucan (chrysolaminarin), which is by far the most important carbon source for marine bacteria. Although in most phytoplankton species lipids comprise <10% of dry weight, a large proportion comprises essential polyunsaturated fatty acids that are distributed throughout the ecosystem[20]. Healthy phytoplankton cells are protein-rich, with proteins comprising up to 50% of dry weight[21].

Locally, the hard-bottom intertidal zone in the Arctic Ocean supports beds of sea-tangle (Fucus distichus) and in the littoral and sublittoral regions (down to about 40 m in clear water) are kelp forests of Alaria esculenta, Laminaria saccharina, L. digitata, and L. solidungula[22]. Laminaria saccharina, L. digitata, and the red alga Ahnfeltia plicata are commercially important in the northern coastal areas of Russia[23].

Microheterotrophs (9.3.1.2)

Microheterotrophs are non-photosynthetic microorganisms. Their role is not well documented in the Arctic, but bacterial production is generally thought to be high, albeit somewhat reduced due to the low temperatures[24]. Rates of bacterial production are mainly determined by the amount of decaying organic matter available, although limitation by mineral nutrients cannot be excluded in some cases[25].

There are upward of 1011 to 1012 bacteria cells per cubic meter in the water column[26]. Phages, a group of highly species-specific viruses, which are even more abundant than bacteria, attack and kill bacteria and phytoplankton, thus regulating their abundance[27]. A well-developed community of heterotrophic flagellates grazes on the bacteria. These in turn are eaten by a variety of protozoans such as ciliates, which are in turn eaten by copepods. Thus the ciliates form an important link between the microbial (i.e., bacteria-based) and grazing food webs.

Excluding bacteria, the microheterotrophs in sea ice and ice-filled waters comprise 60 to 80 species of flagellate and about 30 species of protozoan, especially ciliates[28]. In contrast to first-year ice, multi-year ice has a well-developed microbial community. The abundance of microheterotrophs is particularly high during and immediately after phytoplankton maxima[29].

Zooplankton (9.3.1.3)

Mesozooplankton play a major role in pelagic ecosystems including those of the Arctic, where a diverse array of planktonic animals comprise, on average more than 50% of the total pelagic biomass[30]. Marine mesozooplankton comprises ~260 species in the Arctic, ranging from less than 40 species in the East Siberian Sea to more than 130 species in the Barents Sea[31].

Herbivorous mesozooplankton belonging to the family Calanoidae in the crustacean order Copepoda are predominant in terms of species richness, abundance, and biomass. Large herbivorous copepods (2–5 mm adult size) can make up 70 to 90% of the mesozooplankton biomass in the arctic seas. The most important are Calanus finmarchicus, C. hyperboreus, and C. glacialis in Atlantic and Arctic Water, and C. marshallae, Eucalanus bungii, Neocalanus spp., Metridia longa, and M. pacifica in the North Pacific and the Bering Sea. Calanus finmarchicus predominates in Atlantic Water, C. hyperboreus is found in both Atlantic and Arctic Water, and C. glacialis is foun almost exclusively in Arctic Water. Variations in the distribution and abundance of Calanus species are considered early indicators of climate-induced change in the North Atlantic system[32] with major consequences for the recruitment of fish species such as cod, which depend on them[33].

The large copepods in the Arctic represent, as elsewhere, important links between primary production and the upper levels of the food web because they store large amounts of lipid for overwintering and reproduction[34]. Calanoid copepods overwinter at depths of several hundred meters and then ascend to surface waters in spring to reproduce. Adults and the late copepodite stage V feed on phytoplankton in the surface waters storing lipids through the spring and summer[35]. Daily vertical migrations, common in most seas, have not been observed in the Arctic, not even under sea ice [36]. Many small copepods, <2 mm adult size, are known to be herbivorous while some are carnivorous[37].

Krill (euphausiids) are swarming shrimp-like crustaceans that are common on the Atlantic side of the Arctic Ocean and in the Bering Sea but are not common in the central Arctic Ocean. They can make up to 45% of mesozooplankton catches by weight[38] but are generally less abundant in the Arctic than in some areas of the Southern Ocean[39]. Some species, for example Thysanoessa inermis, are herbivorous whereas others are omnivorous or even carnivorous, for example T. raschii, T. longipes, T. longicauda, and Euphausia pacifica. Most graze diatoms and Phaeocystis pouchetii efficiently[40].

Amphipods, another crustacean group, are represented in the Arctic by Apherusa glacialis, Onisimus spp., Gammarus wilkitzkii, and Themisto libellula, all of which are associated with sea ice or ice-influenced waters. Except for the latter, they live in the interstitial cavities (brine channels) in the ice and on the underside of the pack ice, where G. wilkitzkii constitutes >90% of the amphipod biomass at times. Apherusa is common in first-year ice, and Onisimus in fast ice[41].

Themisto libellula lives in ice-filled waters but is not dependent on sea ice. It is an important food source for the upper trophic levels and is itself carnivorous, feeding on herbivorous copepods and other ice-associated zooplankton. It appears to fill the same niche as krill where these are absent [42]. The largest of the ice amphipods, Gammarus wilkitzkii, can reach 3 to 4 cm in length. Apherusa glacialis and G. wilkitzkii, which are closely associated with multi-year ice, have a high fecundity[43]. Although copepods, amphipods, and euphausiids are predominant in terms of mesozooplankton biomass in the arctic seas, virtually all major marine zooplankton groups are represented, namely, hydrozoans, ctenophores, polychaetes, decapods, mysids, cumaceans, appendicularians, chaetognaths, and gastropods[44]. Pteropods (planktonic snails) such as Limacina helicina occur in vast swarms some years[45].

Benthos (9.3.1.4)

The benthic fauna differs substantially between the continental shelves and the abyssal areas of the Arctic due to differences in hydrography, with warmer and more saline water in the deeper areas[46]. The benthos of the Bering Sea and the Canadian Archipelago between the New Siberian Islands and Bathurst Island is primarily Pacific[47]. The Atlantic fauna are carried into the Barents Sea by the Atlantic inflow and into the central Arctic by strong boundary currents. The fauna of the shallow Kara, Laptev, and Pechora Seas has to contend with large seasonally fluctuating physical conditions and massive amounts of freshwater from the Russian rivers. The littoral (i.e., near-coastal) zone varies from the rocky shore of exposed coasts, to sand and mud in sheltered areas of fjords and bays, and is influenced to varying degrees by ice cover and scouring. Despite the formative studies by Russian workers in the first decades of the twentieth century[48], detailed quantitative information on the distribution of the benthos and the structure of benthic communities in the Eurasian Arctic (especially in coastal and estuarine areas) is limited. Since around 1980, extensive regions of the North American arctic shelf and fjord areas have been sampled and their communities described and related to environmental influences[49]. The greatest numbers of benthic species are found in areas of mixing between cold polar waters and temperate waters, for example between the Barents Sea and the Bering Sea, and off West Greenland and Iceland. The total number of benthic invertebrate species in the Barents Sea has been estimated at around 1,600, but in the western parts of the Bering Sea alone the total number may exceed 2,000[50]. In the shallow waters of the Laptev Sea there are 365 benthic species[51] and even fewer in the Beaufort Sea owing to the cold, unproductive arctic water masses, and to the brackish conditions[52]. In the deep Arctic Ocean, the number of benthic macrofauna species varies from 0 to 11[53]. The number of species in the intertidal zone of Svalbard[54], Bjørnøya[55], Baffin Island[56], and Greenland[57] varies between 30 and 50. The low number of benthic macrofauna species in the arctic intertidal zone is usually attributed to ice scouring[58], a combination of tidal height and ice thickness[59], or heavy wave action[60].

 

caption Fig. 9.17. Levels of benthic faunal biomass n the northern regions of the Bering, Chukchi, East Siberian, and Beaufort Seas [1].

 

Most recent benthic research has focused on specific patterns and processes resulting in biological hot spots such as below predictable leads in the sea ice, polynyas, oceanographic fronts, areas of intense mixing, and the marginal ice zone[61].

Because a relatively large proportion of the primary production in highly productive water columns can potentially reach the bottom, primary and benthic production tends to be coupled. The fraction of sinking matter that reaches the bottom is related to bottom depth; the shallower the water body, the greater the amount of material reaching the bottom. In shallow arctic waters, the benthic food web plays a greater role than in the deep seas or at lower latitudes[62]. The Bering Shelf and the southern Chukchi Sea exhibit some of the highest levels of faunal biomass in the world’s oceans (Fig. 9.17), supporting a rich fauna of bottom-feeding fish, whales, seals, walruses, and sea ducks[63]. Other rich benthic communities in the Arctic occur in Lancaster Sound and the shallow parts of the Barents Sea.

The benthic fauna varies with depth and habitat. For example, off Svalbard the most common species in the steep rocky littoral zone include the macroalgae Fucus spp., sessile (i.e., non-mobile) barnacles (Balanus balanoides), and motile (i.e., mobile) gastropods (Littorina saxatilis) and amphipods (Gammarus setosus and G. oceanicus). The tidal flats are inhabited by a rich and diverse non-permanent fauna due to sediment freezing for six to eight months each year[64]. The sediment fauna is dominated by small polychaetes (Scoloplos armiger, Spio filicornis, Chaetozone setosa) and oligochaetes[65]. Sublittoral organisms include the barnacle Balanus balanus that contributes a large proportion of the biomass of sessile species[66]. Other conspicuous, sessile species are the bivalve Hiatella arctica, actinarians Urticina eques and Hormathia nodosa, bryozoans,and Ophiopholis aculeata. Many, small, motile amphipods (Calliopidae sp.), isopods (Munna sp. And Janira maculosa), snails (Alvania sp.) and barnacles (Tonicella sp.), are observed together with infaunal polychaetes, nematodes, bivalves (Thyasira sp.), and amphipods (Harpinia spp.). The infauna occur in pockets of sediment on the rocky wall. At depths between 100 and 300 m in soft bottom areas of the northern Barents Sea[67], the polychaetes Maldanesarsi, Spiochaetopterus typicus, and Chone aucibranchiata are among the dominant species.

Some crustaceans occur or have occurred in the arctic regions at densities sufficient for commercial interest. These include the deepwater prawns Pandalus borealis[68] and Pandalopsis dispar, and several crab species: red king crab (Paralithodes camtschatica[69], Lithodes aequispina,Tanner and snow crab (Chionoecetes spp.), and Dungeness crab (Cancer magister[70]). Commercially harvested arctic mollusks include clams (Mya truncata, M. arenaria), blue mussel (Mytilus edulis), and Iceland scallop (Chlamys islandica). Commercial fisheries and aquaculture are addressed in detail in Chapter 13.

Fish (9.3.1.5)

 

caption Fig. 9.18. Distribution of capelin (green) (based on Vilhjálmsson[2]).

 

Arctic or Arctic-influenced waters are inhabited by more than 150 species of fish[71]. Few are endemic to the Arctic, unlike the situation in the Southern Ocean where endemic species predominate. Most fish species found in the Arctic also live in boreal (northern) and even temperate regions. Arctic fish communities are dominated by a small number of species. The most abundant being Greenland halibut (Reinhardtius hippoglossoides), polar cod, Atlantic and Pacific cod (Gadus morhua and G. macrocephalus), Greenland cod (G. ogac), walleye pollock (Theragra chalcogramma), capelin (Mallotus villosus), long rough dab, also known as American plaice (Hippoglossoides platesoides), yellowfin sole (Pleuronectes asper), Atlantic and Pacific herring (Clupea harengus and C. pallasi), and redfish (Sebastes spp. e.g., S. mentella, S. marinus).

Greenland halibut, polar cod, and capelin have a circumpolar distribution. Greenland cod is a predominantly arctic species that is restricted to Greenland waters. The other species principally occur in waters to the south of the Arctic Ocean, except for parts of the Barents and Chukchi Seas.

Capelin

Capelin is a small circumpolar pelagic fish (Fig. 9.18). It is planktivorous (i.e., eats plankton), feeding mainly on copepods, followed by krill and amphipods. It is particularly abundant in the North Atlantic and the Barents Sea[72], and around Iceland[73]. In the eastern Bering Sea, capelin tend to occur in cooler or more northerly areas. Capelin populations are subject to extreme fluctuations[74] in their distribution and abundance. Capelin is heavily exploited in the Atlantic but not the Pacific sector of the Arctic.

Capelin is important in the diet of other fishes, marine mammals, and seabirds[75] and is thus regarded as a key prey species. Capelin can provide more than 20% of the food required by seabirds, higher predators, and the capelin fishery collectively in an average year (Table 9.6).

Fluctuations in the abundance of capelin have a big impact on their predators, particularly cod, seals, and seabirds. The growth rate of cod and their somatic and liver condition, for example, are correlated with capelin population abundance[76].

Table 9.6. Annual productivity and food requirement of higher trophic levels: average for the whole Barents Sea over several years[77].

 

Annual production (mg C/m2)

Food requirement (mg C/m2)

Capelin

280

 

Cod

90

550

Whales

3.6

360

Seals

0.8

95

Seabirds

0.5

78

Capelin fishery

 

200

Total

375

128

Herring

Atlantic herring is generally restricted to waters south of the Polar Front, for example in the Nordic Seas and the Barents Sea[78]. Like capelin, Atlantic herring is planktivorous, feeding in highly productive frontal areas of the open sea. Larval herring are important prey for seabirds. Adult herring is an important food item for larger fish and marine mammals.

The principal population of Atlantic herring in the Arctic is the Norwegian spring-spawning stock; one of the largest fish stocks in the world and with a spawning biomass that exceeded ten million tonnes for much of the 20th century. This population, together with the Icelandic spring- and summer-spawning herring make up the Atlanto-Scandian herring group. The migration route from nursery areas to feeding areas to overwintering areas to spawning areas takes the Norwegian spring-spawning herring around the Norwegian Sea (Box 9.8), over a distance of several thousand kilometers, and even into the Icelandic Sea during certain climatic warm periods (see Fig. 9.19 and 9.3.3.3).

Spawning occurs at many sites along the Norwegian coast between 58° and 70° N. The spawning grounds comprise five main areas, but their relative importance, the time of arrival on the spawning grounds, and the spawning time have often changed[79]. These changes are not solely due to varying environmental conditions, but are also affected by population structure, and the optimum life history strategy for individual fish under varying levels of food supply. Flexibility in spawning behavior offers an adaptive advantage to the population during changing climates.

Pacific herring are common in the Bering Sea shelf regions[80]. This species is, however, of relatively minor importance for seabirds and marine mammals in that region[81].

Box 9.8. Effects of climate on Norwegian spring-spawning herring

In the Icelandic area, herring was the fish species most affected by the environmental adversities of the 1960s[82].This is not surprising since herring are plankton
feeders and in Icelandic waters are near their northern limit of distribution. Thus, the traditional feeding migrations of the Norwegian spring-spawning herring stock to the waters off northern Iceland (Fig. 9.19a) stopped completely when the Atlantic plankton community collapsed. In 1965–1966, the oldest herring were instead forced to search for food in the Norwegian Sea near the eastern boundary of the East Icelandic Current, i.e., around 150 to 200 nautical miles farther east than previously (Fig. 9.19b). In 1967–1968, the stock migrated north to feed west of Svalbard during summer (Fig. 9.19c).This was also the case in 1969 when the overwintering grounds also shifted from 50 to 80 nautical miles east of Iceland to the west coast of Norway (Fig. 9.19d).The Norwegian spring-spawning herring stock collapsed in the latter half of the 1960s{{ref|80b} and the feeding migrations to the west into the Norwegian Sea ceased altogether (Fig. 9.19e).

The abundance of the Norwegian spring-spawning herring stock increased dramatically in the 1990s.This process has, however, taken about twenty-five years despite a ban on commercial fishing in the period 1973 to 1983. It was not until the mid-1990s that these herring resumed some semblance of their previous feeding pattern.The Norwegian spring-spawning herring still overwinter in fjords in the Lofoten area on the northwest coast of Norway. When and if they will revert completely to the traditional distribution and migration pattern cannot be predicted.

 

 

caption Fig. 9.19. Changes in the migration routes, and feeding and wintering areas of Norwegian spring spawning herring during the latter half of the twentieth century. The plots show (a) the normal migration pattern during the warm period before 1965, (b and c) the pattern following the Great Salinity Anomaly until the stock collapsed in 1968, (d) during years of low stock abundance, and (e) the present migration pattern[3].

 

Polar cod

 

caption Fig. 9.20. Distribution of polar cod (red)[4].

 

Polar cod is a key species in many arctic food chains and forms a major link in the transfer of energy from zooplankton to top carnivores (Fig. 9.20). Large polar cod (23–27 cm) consume mainly fish and are themselves eaten by a variety of large fish as well as by many seabird species and most arctic marine mammals [83]. Polar cod spend much of their time associated with sea ice and stay in arctic waters throughout their life cycle. This species is broadly distributed, from inshore surface waters to very deep waters[84]. Polar cod occur in large schools[85] mostly north of 60° N in the eastern Bering Sea or in the cold pool of the mid-shelf region[86]. The importance of this species is likely to have been underestimated in the past, in part owing to its patchy distribution. Polar cod displays a variety of physiological and biochemical adaptations to life in cold waters, including bioenergetic adjustment to low temperature[87].

Cod

Cod species found in the Arctic include Atlantic cod, Pacific cod[88], Pacific tom-cod (Microgadus proximus), which occurs as far north as the Bering Sea, Greenland cod, and Arctic cod (Arctogadus glacialis) that resides in the Arctic Ocean, but about which little is known[89]. The majority of these species appear regularly in the diet of marine mammals[90].

Atlantic cod is the most abundant gadoid species in the northern North Atlantic. Like Atlantic herring it occurs mainly to the south of the Polar Front, yet can live in temperatures below 0°C by producing antifreeze proteins. Four large cod populations occurred in the arctic areas of the North Atlantic during the 20th century. The Northeast Arctic cod spawns along the Norwegian coast, with more than 50% of this occurring in the Lofoten area. Cod from Iceland spawn around the coast with more than 50% of this occurring off the southwest corner. The cod off Greenland have inshore and offshore spawning components, and an immigrant contribution from Icelandic waters. The history of the increase and collapse of this cod population during the 20th century is described in Section 9.3.3.3. The cod population off Newfoundland and Labrador also collapsed during the 1990s, owing to high fishing mortality combined with adverse environmental changes[91].

Pacific cod is a mixed feeder that consumes a wide variety of fish (primarily walleye pollock), shellfish, and invertebrates in the eastern Bering Sea[92].

Walleye pollock

Walleye pollock is the single most abundant fish species in the Bering Sea, comprising the bulk of the commercial catch in this area[93]. It is mainly semi-pelagic, dominating the outer shelf regions. Walleye pollock is primarily planktivorous, feeding on copepods and euphausiids but adults become cannibalistic, feeding on juveniles seasonally[94]. Juvenile pollock is an important prey item for other fish species, marine mammals, and seabirds[95].

Redfish

Several redfish species are broadly distributed and common in arctic deep waters (100 to >500 m). They are slow-growing and long-lived species. The three common species which are exploited in the northern North Atlantic are Sebastes marinus, S. mentella, and S. viviparus, but the latter, which is the smallest of the three, is not caught in significant amounts[96]. There are two distinct populations of S. mentella. These vary in their habitat and fishery and are commonly known as deep-sea redfish and oceanic redfish, respectively. The relationship between the two forms and the extent to which the populations are separated spatially is not clear[97].

Oceanic redfish are caught in the Irminger Sea during the summer at depths of 100 to 200 m and water temperatures of 5 to 6°C. Mature fish feed on krill and small fish such as capelin and herring and undertake extensive feeding migrations. They mate in early winter and the female carries the sperm and eggs, and later larvae, which are born in April/May[98]. The juveniles stay near the bottom, along the edge of the continental shelf.

Greenland halibut

Greenland halibut is commercially important in the North Atlantic and the Pacific, and is an important food item for deep-feeding marine mammals (e.g., narwhal and hooded seals) and sharks feeding on benthos such as the Greenland shark (Somniosus macrocephalus). During their first four to five years as immature fish in the eastern Bering Sea, the Greenland halibut inhabit depths to 200 m. On the Atlantic side, immature fish occur mainly between 200 and 400 m depth. Adults mainly occupy slope waters between 200 and 1,000 m or more[99]. Walleye pollock and squid are the main prey items for Greenland halibut in the eastern Bering Sea[100].

Other flatfish

Other arctic flatfish include the long rough dab, which is an abundant bottom-dweller in some parts of the Arctic seas, including the Barents Sea[101]. On the Pacific side in the eastern Bering Sea, yellowfin sole, flathead sole (Hippoglossoides elassodon), rock sole (Pleuronectes bilineatus), Alaska plaice (Pleuronectes quadrituberculatus), and arrowtooth flounder (Atheresthes stomias) are important members of the groundfish community[102]. Yellowfin sole, Alaska plaice, and rock sole consume mostly infaunal prey such as polychaetes, clams, and echiuran worms. These fish are distributed at depths generally less than 50 m. The highly piscivorous (i.e., fish-eating) arrowtooth flounder is found mostly on the outer shelf area, as is flathead sole, which mainly consumes brittle stars.

Marine mammals and seabirds (9.3.1.6)

Arctic marine mammals to a large extent escaped the mass extinctions that affected their terrestrial counterparts at the end of the Pleistocene[103]. Like fish, mammals and birds have the advantage of having great mobility and hence are good colonizers. Thus, it is not surprising that these groups dominate the arctic marine megafauna, represented both by resident and migratory species. Their high abundance was a major attractant for people to this region historically, becoming the mainstay of the diet of coastal communities throughout the Arctic (Chapter 12) and later the subject of extreme levels of commercial exploitation. The massive harvests of marine mammals and seabirds that began in the 1600s and lasted for several hundred years decimated many arctic populations. Bowhead whales (Balaena mysticetus) and sea otters (Enhydra lutris) were almost driven to extinction throughout the Arctic[104], while the great auk (Pinguinus impennis) and Steller sea cow (Hydrodamalis gigas) did become extinct. Walruses (Odobenus rosmarus) were all but extirpated in some arctic regions[105]. Polar bears, all the great whales, white whales (Dephinapterus leucas), and many species of colonially nesting seabird were dramatically reduced. Harvesting of marine mammals and seabirds is now undertaken in accordance with management schemes based on sustainability in most Arctic countries, although overexploitation of some species is still occurring[106].

Marine mammals are the top predators in the Arctic other than humans. Virtually all large-scale taxonomic groupings of marine mammals have arctic representatives[107].

Polar bear

The polar bear, the pinnacle predator, has a circumpolar distribution and is dependent on sea ice to provide for most of its needs[108]. Polar bears feed almost exclusively on ice-associated seals[109]. Adult bears can swim quite long distances if required, but mothers with cubs depend on ice corridors to move young cubs from terrestrial denning areas to prime hunting areas on the sea ice[110]. Pregnant females dig snow dens in the early winter and give birth several months later. This requires a significant depth of snow, thus females return year after year to land sites that accumulate sufficient snow early in the season. A mother that emerges from the den with her young has not eaten for five to seven months[111]. Therefore, successful spring hunting is essential for the family’s survival and largely dictates condition, reproductive success, and survival for all polar bears[112]. Factors that influence the distribution, movement, duration, and structure of sea ice profoundly affect the population ecology of polar bears, not least due to their influence on the principal prey species, ringed seal (Phoca hispida)[113]. The global polar bear population is estimated at 22,000 to 27,000[114].

Walrus

caption Fig. 9.21. Walrus routinely use sea ice as a haul-out platform in shallow areas where they feed on benthic fauna. (Photo supplied by Kit Kovacs & Christian Lydersen, Norwegian Polar Institute)

Walruses, like polar bears, are circumpolar, but with a more disjointed distribution. Two sub-species are recognized: the Pacific walrus (Odobenus rosmarus divergens) and the Atlantic walrus (O. r. rosmarus)[115]. The global walrus population is estimated at about 250,000, of which 200,000 belong to the Pacific sub-species. The Atlantic walrus is distributed from the central and eastern Canadian Arctic eastward to the Kara Sea[116], including several more or less well-defined sub-populations[117]. Walruses haul-out on pack ice most months of the year (Fig. 9.21), using land-based sites only during summer when sufficient sea ice is unavailable. Walruses have a narrow ecological niche, depending on the availability of shallow water (<80 m) with bottom substrates that support a high production of bivalves[118].

Seals

Ringed seals represent the "classical" arctic ice seal, being uniquely able to maintain breathing holes in thick sea ice. Thus, they can occupy areas far from sea-ice edges, unreachable by other seal species. They are distributed throughout the Arctic, even at the North Pole[119]. They number in the millions and this is by far the most abundant seal species in the Arctic. This species exclusively uses the sea ice for breeding, molting, and resting (haul-out), and rarely, if ever, moves onto land. Although quite small, ringed seals survive the thermal challenges posed by the arctic winter by building lairs in the snow on top of sea ice, where they rest in inclement weather and where they house their new-born pups[120]. Ice amphipods and fish constitute much of their diet[121].

The bearded seal (Erignathus barbatus) has a patchy circumpolar Arctic distribution[122]. This species breeds on drifting sea ice[123] but occasionally hauls out on land during the summer. These animals are mostly benthic feeders, eating a wide variety of fish, mollusks, and other invertebrates in shallow areas. Some bearded seal populations are thought to be resident throughout the year, while others follow the retreating pack ice in summer, and then move southward again in the late autumn and winter[124]. The global population has not been assessed but is thought to number in the hundreds of thousands in the Arctic[125].

Harbour seals (Phoca vitulina) have one of the broadest distributions of the pinnipeds, from temperate areas as far south as southern California to arctic waters of the North Atlantic and into the Bering Sea in the Pacific[126]. They are coastal, non-migratory, and aggregate in small numbers on rocky outcrops, beaches, or inter-tidal areas[127]. They are opportunistic feeders that eat a wide variety of fish species and some cephalopods and crustaceans[128]. Harbour seals are not numerous in the Arctic and several of the populations that live north of the Arctic Circle are very small[129].

In the Atlantic sector of the Arctic, there are three additional phocid (i.e., true) seal species: harp seals, hooded seals (Cystophora cristata), and grey seals (Halichoerus grypus). Harp seals are highly gregarious and migratory, moving southward to three traditional breeding sites (off the east coast of Canada, in the White Sea, and between Jan Mayen and Svalbard) for the birthing period on pack ice in March[130]. Following the breeding season, harp seals from each population move northward into molting sites before dispersing into the Arctic for the rest of the year[131]. Adult harp seals feed mainly on small marine fish such as capelin, herring, sculpins (Cottidae), sand lance (Ammodytes americanus), and polar cod[132], and then on krill and amphipods. The global population is thought to exceed seven million animals[133].

The hooded seal is a large, pack-ice breeding northern phocid that ranges through a large sector of the North Atlantic. In spring the adults gather to breed in two main groups; one off the east coast of Canada and the other either in Davis Strait or off East Greenland depending on conditions[134]. Some weeks after breeding, the animals move northward into traditional molting areas before dispersing for the summer and autumn, preferring the outer edges of pack ice[135]. They feed on a variety of deep-water fishes including Greenland halibut and a range of redfish species, as well as squid[136]. The global population is very difficult to estimate because hooded seals are difficult to survey, but is certainly in excess of half a million animals[137].

Grey seals were historically abundant in Icelandic waters and along the coastal regions of northern Norway and northeastern Russia[138]. They have been depleted through hunting and government culling programs[139] and in some areas have been extirpated[140]. A crude estimate of the population of grey seals inhabiting northern Norway and the Murman coast of Russia is 4,500[141].

Two additional ice-breeding seals that occur in the Bering Sea are the spotted seal (Phoca largha) and the ribbon seal (Phoca fasciata). The spotted seal breeds in eight largely discrete birthing areas[142]. They have a coastal distribution during the summer and early autumn, but migrate offshore to the edge of the ice pack for the rest of the year[143]. Spotted seals eat a wide variety of prey, including fish, crustaceans, and cephalopods[144]. There are no recent, reliable population estimates for this species[145]. Ribbon seals are poorly known, pack-ice breeders that congregate loosely in suitable areas of thick pack ice in the North Pacific during the breeding season[146]. They do not haul out on land and are assumed to be either pelagic or northern pack-ice dwellers in summer[147]. They are reported to eat crustaceans, fish, and cephalopods[148]. Current data on population size are not available, but counts in the 1970s revealed 100,000 to 200,000 animals[149].

Northern fur seals (Callorhinus ursinus), Steller sea lions (Eumetopias jubatus), and sea otters all breed terrestrially on the Pribilof, Aleutian, Commander, and Kurile Islands in the North Pacific. The latter two species breed as far south as the Californian coast.

Whales

White whales, narwhal (Monodon monoceros), and bowhead whales live only in the high Arctic[150] and are commonly found in ice-covered waters where they use edges, leads, and polynyas to surface for breathing. Narwhal mainly occur within the Atlantic region, while the others have patchy circumpolar ranges[151]. All three migrate seasonally, largely in relation to the northward retraction and southward expansion of the seasonal sea ice. They prey on small fishes, especially polar cod, although narwhal also eat large quantities of cephalopods, and bowhead whales consume a greater proportion of planktonic crustaceans than either of the other two species.

Other cetaceans also frequent arctic waters in summer, but these remain in relatively ice-free waters and spend most of the year elsewhere. These include white-beaked dolphin (Lagenorhynchus albirostris) in the North Atlantic/ Barents/Greenland Sea and Dahl’s porpoise (Phocoenoides dalli), right whales (Eubalaena glacialis), and grey whales (Eschrichtius robustus) in the North Pacific/Bering Sea. Harbour porpoise (Phocoena phocoena) and killer whales (Orcinus orca) are among the toothed whales, and blue whales (Balaenoptera musculus), fin whales (B. physalus), minke whales (B. acutorostrata), humpback whales (Megaptera novaeangliae), and sei whales (B. borealis) are some of the baleen whales that are regular summer residents in arctic waters. Many of the great whales inhabit the Bering Sea in summer.

Seabirds

Some of the largest seabird populations in the world occur in the Arctic[152]. Over 60 seabird species frequent the Arctic, and over 40 breed there[153]. Many species take advantage of the summer peak in productivity and then overwinter elsewhere. In the extreme, the red phalarope (Phalaropus fulicarius), the northern phalarope (P. lobatus), and the Arctic tern (Sterna paradisaea) spend the summer in the high Arctic and overwinter in the southern hemisphere off Peru or West Africa. In contrast, the spectacled eider (Somateria fisheri), black guillemot (Cepphus grylle), ivory gull, and northern fulmar (Fulmaris glacialis) stay in the Arctic all year round, using the southern edges of the sea ice or open water areas for feeding in winter. Polynyas are extremely important winter habitats for these species[154]. Most of the global population of the threatened spectacled eider overwinters in single-species flocks in a few polynyas in a restricted area of the Bering Sea[155]. In the rare instance that such polynyas freeze for longer than a few days, mass mortalities can occur, altering population growth and affecting the species for decades[156].

Most arctic seabird species nest in large colonies on cliffs, which offers some protection from terrestrial predators such as the Arctic fox (Alopex lagopus). Other species, such as Sabine’s gull (Xema sabini), nest on the ground on isolated islands, while others use burrows either on sloping ground (e.g., little auk) or in rock crevices (e.g., black guillemot). Several of the auk species are among the most abundant nesting arctic seabirds, including the little auk, thick-billed murre, common murre (Uria aalge), and the Atlantic puffin (Fratercula arctica). The blacked-legged kittiwake (Rissa tridactyla) is the most numerous Arctic gull, but glaucous gulls (Larus hyperboreus) are also common. Arctic terns are abundant in some regions, as are common eider (Somateria mollissima). The Pribilof Islands, in the eastern Bering Sea, are breeding sites for large numbers of piscivorous seabirds including black-legged and red-legged kittiwake (R. brevirostris), and common and thick-billed murre.

The foraging ecology and energetics of seabirds have been studied quite extensively in many arctic areas[157] and despite species differences, some basic patterns are evident. Most arctic seabirds forage on small fish and large copepods, primarily in the upper and mid-water column[158]. Foraging is often concentrated in frontal areas or at ice edges, where convergences can concentrate marine zooplankton[159]. Eiders are the exception, foraging in shallow water for benthic animals, particularly echinoderms and mollusks. Polar cod is an extremely important prey item for most arctic seabirds, but other small school-forming species (such as capelin and herring in the Barents Sea) are extremely important regionally. Surface feeders (e.g., kittiwakes and fulmars) forage on the wing, dipping into the water to capture prey, or feed while sitting on the water surface when prey concentrations are high and available within the top few centimeters. The alcids and related species dive to considerable depths[160] in search of prey. They also travel considerable distances and can stay underwater for relatively extended periods, allowing them to take advantage of fish and invertebrates that reside under the sea ice, e.g., euphausiids, amphipods, and polar cod[161]. As foragers, most seabird species are generalists responding to changing spatial and temporal prey availability[162]. However, the little auk and the Bering Sea least auklet (Aethia pusilla), which specialize on calanoid copepods, have a narrow foraging niche[163]. Ivory gulls are one of the most specialized of the arctic seabirds, living in association with pack ice for most of their lives and breeding on exposed mountain peaks in glaciated areas of the high Arctic. One of their favorite foods is the blubber of marine mammals, acquired by scavenging on carcasses. Yet, similar to many arctic seabirds, a large part of their diet comprises polar cod and other small fish and invertebrates. The small fish and invertebrates are usually taken after being washed onto the surface of ice floes and edges[164]. Ross’ gulls (Rhodostethia rosea) also perform this type of foraging behavior.

In addition to seabirds that are strictly marine feeders, skuas, a host of arctic shorebirds, and some ducks (beside the marine feeding eiders), geese, and divers also spend time at sea.

 

Chapter 9: Marine Systems
9.1. Introduction
9.2. Physical oceanography
    9.2.1. General features
    9.2.2. Sea ice
    9.2.3. Ocean processes of climatic importance
    9.2.4. Variability in hydrographic properties and currents
    9.2.5. Anticipated changes in physical conditions
9.3. Biota
    9.3.1. General description of the Arctic biota community
    9.3.2. Physical factors mediating ecological change
    9.3.3. Past variability – interannual to decadal
    9.3.4. Future change – processes and impacts on biota
9.4. Effects of changes in ultraviolet radiation
9.5. The carbon cycle and climate change
9.6. Key findings
9.7. Gaps in knowledge and research needs

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Committee, I. (2012). General description of the Arctic biota. Retrieved from http://www.eoearth.org/view/article/152938

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