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The Alaska North Slope Environment The North Slope of Alaska includes about 230,000 km2 (89,000 mi2) north of the crest of the Brooks Range, an area slightly larger than Minnesota. It encompasses the drainage basins that empty into the Arctic Ocean and the Chukchi Sea, including the Kongakut River on the east and small drainages east of Point Lay in the west. The land slopes gradually from the crest of the Brooks Range northward to the Arctic Ocean. During the nine-month winter, tempera- tures can plunge to -50 °C (-58 °F). Annual snowfall aver- ages less than 50 cm (20 in.), but the nearly constant winds produce drifts that are as much as 6 m (20 ft) deep. From November 18 to January 24, the sun never rises above the horizon in Barrow, the northernmost part of the North Slope, although there is a little midday twilight. Conversely, the sun does not set from May 10 until August 2. Annual pre- cipitation ranges from 12 to 20 cm (5 to 8 in.) along the coast and up to 1 m (40 in.) in the highest elevations of the Brooks Range. Low temperatures reduce evaporation, and perma- nently frozen soil prevents vertical drainage of water. As a result, extensive areas of the North Slope are covered by thaw lakes, ice-wedge polygons, frost boils, water tracks, bogs, and other features typical of permafrost regions. The patterns created by these features are often difficult to per- ceive on the ground but are striking from the air. They are particularly well expressed in the Prudhoe Bay region. To set the stage for the committee's analyses of cumula- tive effects, we next describe the diverse terrestrial, freshwa- ter, and marine environments of the North Slope. TERRESTRIAL ENVIRONMENT Geology The North Slope is the largest coherent geological prov- ince in Alaska. Rocks exposed in the sea cliffs of the Chukchi Sea on the west can be identified in outcrops all the way to the Canadian border on the east. Long ridges of sandstone 24 that continue for many miles in the foothills maintain their east-west orientation and define and expose a giant trough of folded sedimentary rocks, called the Colville Basin or Colville Syncline. The trough extends west to the Chukchi Shelf, where the associated folded structures turn to the northwest and are cut off by vertical faults that mark the eastern border of the Chukchi Basin (Grantz et al.1994~. To the south, that trough is bounded by the overthrust front of the Brooks Range. To the north, it is bounded by, and sepa- rated from, the Canada Basin of the Beaufort Sea by a buried ridge of older rocks, a composite structural feature com- monly called the Barrow Arch. At Barrow the top of this ridge is only about 700 m (2,300 ft) deep. The arch plunges east to a depth of about 4,000 m (13,000 ft) in the Prudhoe Bay area and then continues east until it loses its identity as a major structural feature in the Arctic National Wildlife Refuge (Bird and Magoon 1987~. The arch extends west into the Chukchi Shelf, and to the north it slopes gently offshore to underlie the Beaufort Shelf. The south flank of the Barrow Arch forms the primary trap for the Prudhoe Bay oil field (Morre et al. 1994~. Carbon-rich sedimentary rocks primar- ily of Mesozoic age are believed to be the source for the oil accumulations that have been found in nearly all of the sedi- mentary rock units of the Colville Basin. Permafrost Permafrost is earth material that stays frozen year-round. On the North Slope it extends to below surface depths of 200-650 m (650-2,000 ft), with the deepest permafrost oc- curring at Prudhoe Bay. It is insulated from the ground sur- face by an "active layer," which thaws each summer to a depth of 20 cm (8 in.) in some peats to more than 2 m (80 in.) in some well-drained inland gravels. The active layer is sub- ject to continuous natural change, but its disruption by, for example, destruction of the organic insulating mat or im- poundment of surface water can initiate permafrost thawing
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THE ALASKA NORTH SLOPE ENVIRONMENT and conspicuous surface changes. In extreme cases, called "thermokarst," the differential settlement and loss of strength creates thaw pits, ponds, retreating scarps, or mud flows. To maintain permafrost in its natural frozen condition and to avoid destructive surface settlement, roads, and work areas must be built on thick gravel foundations, heated buildings and pipelines must be elevated on piling, and off-site activi- ties must be carefully controlled. Surficial Geomorphic Features The North Slope has three distinct regions: the Arctic Coastal Plain, the Arctic Foothills, and the Brooks Range (Gallant et al. 1995~. All North Slope oil extraction has oc- curred on the Arctic Coastal Plain, but there has been some exploration in the foothills. Arctic Coastal Plain The coastal plain is generally flat with large oriented thaw lakes and extensive wetlands. The plain is about 150 km (93 mi) wide south of Barrow, and it narrows toward the east. The Prudhoe Bay oil field is within an exceptionally flat portion of the coastal plain (flat thaw lake plains) be- tween the Sagavanirktok and Kuparuk Rivers (Walker and Acevedo 1987~. Drainage systems in this portion of the coastal plain are often poorly defined, and much of the run- off occurs in sheet flows, which can shift direction depend- ing on the volume of discharged water. The Kuparuk oil field is in a somewhat hillier portion of the coastal plain (gently rolling thaw lake plains) (Walker and Acevedo 1987~. The hilly aspect of this region is caused in part by large broad-based low hills, or "pingos," created by permafrost and generally 5-20 m (16-65 ft) tall (Walker et al. 1985~. Gently rolling plains occur east of the Saga- vanirktok River. Those regions have better-defined drainage networks, with more runoff channeled into streams instead of sheet flows. The dominant geomorphic characteristics of the flat coastal plains are thaw lakes, drained lake basins, polygonal patterned ground, and pingos. Frost boils or consorted circles (Washburn 1980) cover large areas of the coastal plain and foothills. Those features typically measure 1-2 m (3.2-6.5 ft) in diameter and are the result of frost heave (Peterson and Krantz 1998~. They are highly sensitive to off-road vehicle disturbance, such as that caused by seismic operations. Thaw lakes are formed by thawing of the frozen ground (Britton 1967, Hopkins 1949) and have a distinct directional orientation attributed to the action of wind (Carson and Hussey 1959, Rex 1961~. The lakes grow until they breach other lake basins or stream channels, at which point they empty, leaving drained lake basins (Britton 1967, Peterson and Billings 1980~. Ice-wedge polygons dominate the ter- rain between lake basins. The micro-elevation differences associated with ice-wedge polygons are only a few centime- 25 ters (1-2 in.), but soil-moisture differences associated with those small changes in elevation influence the distribution of plants on the landscape. Pingos are common in drained lake basins, particu- larly where the water had been deep enough to cause deep thaw zones in the permafrost (Mackay 1979~. When lakes drain, those thawed areas are exposed to the weather, and permafrost re-forms. Water is expelled from the freezing soil and an ice core develops, which expands and deforms the soil, eventually forming a hill. Pingos are very stable because of their gravelly parent material and the cold climate. Rivers west of the Colville meander sluggishly in val- leys incised between 15-100 m (50-330 It); rivers to the east of the Colville are fast flowing, braided, and have extensive delta systems. River systems support a diversity of plant and animal life and can serve as corridors for migrating mam- mals and birds. The Beaufort Sea coastline is irregular and contains many small bays, lagoons, spits, beaches, and barrier islands. Extensive mud flats occur in the deltas of the rivers. Most of the coastline is low lying, with only small bluffs less than 3 m (10 ft) high. At Camden Bay, the land rises more steeply from the sea, and the bluffs are up to 8 m (26 ft) high. Arctic Foothills The Arctic Foothills is a band, roughly 50-100 km (30- 60 mi) wide, of generally smoothly rounded hills between the Arctic Coastal Plain and the Brooks Range. Major drain- age systems form broad valleys between the masses of hills. Numerous east-to-west linear bedrock outcrops occur within the foothills, reflecting the orientation of the underlying sedi- mentary deposits. Most of the hills have gentle slopes with parallel, closely spaced, shallow channels that are unique to permafrost regions (Cantlon 1961~. The northern sector of the foothills is smoothly eroded. The hills are covered with late Tertiary to mid-Pleistocene-age glacial till, capped with more recent windblown glacial silt deposits. The southern sector was glaciated more recently (late Pleistocene), and it has many irregular glacial features. The basins between hills have peat deposits and a variety of wetlands (Walker and Walker 1996~. Brooks Range The Brooks Range extends almost across the width of Alaska, centered at about 68° north latitude. It is a complexly folded sedimentary mass made up of shale, slate, sandstone, schist, conglomerates, limestone, marble, and granite (BLM 1998~. It is incised by north-south river valleys on its north slopes. Maximum elevations reach only about 3,000 m (9,800 ft), but because of the mountains' northern location, they form a barrier to many plants especially trees that occur on the south slopes.
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26 CUMULATIVE EFFECTS OF ALASKA NORTH SLOPE OIL AND GAS The Canning River looking south to the Brooks Range. July 2001. Photograph by David Policansky. FRESHWATER ENVIRONMENTS Rivers and Streams Several types of streams are found north of the Brooks Range (Craig and McCart 1975~. Mountain streams, such as the Colville, Sagavanirktok, Ivishak, and Canning rivers, which originate in the Brooks Range, are the largest river systems that cross the Arctic Coastal Plain. Smaller moun- tain streams include the Shaviovik and Kavik rivers and most of the streams between the Canning River and the Mackenzie delta in Canada. Spring streams are spring-fed tributaries, generally less than 1.5 km (1 mi) long and a few meters wide, that feed the upper reaches of mountain streams. Short, meandering tundra streams drain the tundra-covered slopes of the Brooks Range foothills and the coastal plain. They are either tributary to mountain streams or flow directly into the Beaufort Sea. Larger tundra streams include the Ikpikpuk, Meade, Inaru, and Kuparuk rivers. During winter, river flow ceases except in perennial springs (Walker 1983), and ice forms to a thickness of about 1.8 m (6 It). Smaller streams typically freeze completely; larger streams have water in discontinuous, deep pools. Stream habitat is reduced by 98% during winter (Craig 1989~. More than half of the annual stream flow is discharged from Arctic Coastal Plain streams during the 2- to 3-week ice break-up each spring (Sloan 1987~. Lakes and Poncis Lakes and ponds are among of the most striking land- forms of the coastal plain, particularly when viewed from the air. Most lakes in the oil-field region between the Sagavanirtok and Colville rivers are shallow, typically less than 1.8 m (6 ft) deep (Moulton and George 2000~. In the Colville delta, site of the Alpine oil-field development, the mean maximum lake depth is 4.5 m (15 It). Lakes are deeper to the west and south, with a mean maximum depth of more North Slope tundra stream. July 2001. Photograph by David Policansky. than 9 m (30 ft) in lakes south of Teshekpuk Lake. Many of the lakes are oriented in a north-south direction, a striking feature of the landscape. Lakes on the coastal plain are typically covered with ice from early October until early July. Maximum ice thickness typically reaches 1.8 m (6 ft) by April, but can exceed 2.4 m (8 ft) in some years (Sloan 1987~. Shallow ponds become ice-free by mid to late June, with deeper lakes retaining ice into early July. Teshekpuk Lake, the largest lake on the coastal plain (816 km2 [315 mi21) retains its ice cover into late July or early August. Because of the dry climate of the North Slope, a sub- stantial amount of surface water evaporates during the short summer (Miller et al.1980~. Much of the snowmelt runoff in the coastal plain during break-up goes to replenish pond and lake water lost to evaporation in summer. In the Barrow area, only about half of the snowmelt becomes runoff; the rest goes into ponds (Miller et al. 1980~. In contrast, 85% of the precipitation becomes runoff in the steep drainage basins of the Brooks Range. MARINE ENVIRONMENTS The Chukchi Sea extends from the 200 m (660 ft) isobath of the Arctic Ocean to the Bering Strait (Weingartner 1997~. The Alaska Beaufort Sea extends from Point Barrow to the Canadian border (Norton and Weller 1984~. The sea- floor slopes gently for 50-100 km (30-60 mi) to form the Beaufort Sea shelf, which is among the narrowest of the con- tinental shelves in the circumpolar Arctic. A series of linear shoals landward of the 20 m (66 ft) contour (Reimnitz and Kempema 1984) determines where ice ridges and hummocks form. The larger rivers that discharge into the Beaufort Sea form depositional delta shelves that can extend several kilo- meters from the shore. Some areas of the coast are directly exposed to the wind, wave, and current action of the open ocean. Other stretches of shore are protected by chains of
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THE ALASKA NORTH SLOPE ENVIRONMENT barrier islands composed of sand and gravel that enclose shallow lagoons. Ocean Processes Surface circulation in the Beaufort Sea is dominated by the southern edge of the perpetual clockwise gyre of the Canadian Basin (Selkregg et al. 1975~. Most of the year the gyre moves surface water and ice shoreward. The subsurface Beaufort Undercurrent flows in the opposite direction, to the east, over the outer continental shelf (Aagaard 1984~. Cur- rents in the shallower waters of the inner Beaufort Sea shelf are primarily wind driven and, thus, can flow either east or west. Because the principal wind direction during the sum- mer ice-free season is from the east, nearshore flow is gener- ally from east to west (Wilson 2001a). East winds generate west-flowing surface currents that are deflected offshore in response to the Coriolis effect (Niedoroda and Colonell 1990~. This offshore deflection of surface waters causes a depression in sea level (negative storm surge), which is partially compensated for by an on- shore movement of underlying marine water. Under persis- tent east winds, bottom marine water can move onshore, where it is forced to the surface. This upwelling of marine water can cause some otherwise brackish and warm areas along the coast to become colder and more saline (Man- garella et al. 1982; Savoie and Wilson 1983, 1986~. Under strong and persistent east winds, the negative storm surge causes nearshore water levels to drop as much as 2 m (6.5 ft). When westerly winds prevail, the Coriolis effect deflects surface waters onshore, causing nearshore water levels to rise. That onshore transport of surface waters is balanced by offshore transport at depth, resulting in regional down- welling along the coast. Those wind-driven marine surges are the principal forces that determine sea level along the coast. Lunar tides along the North Slope are very small, av- eraging 20-30 cm (8-12 in.) (Norton and Weller 1984, Selkregg et al. 1975, USACE 1998~. The Chukchi Sea receives water flowing northward through the Bering Strait, driven by the half-meter drop in sea level between the Aleutian Basin of the Bering Sea and the Arctic Ocean (Overland and Roach 1987~. Pacific waters are an important source of plankton and carbon in the Chukchi and Beaufort seas (Walsh et al. 1989), influencing the distribution and abundance of marine biota and seasonal migrations (Weingartener 1997~. The deeper waters (100 m [330 ft]) offshore in the northern Chukchi Sea are a poten- tially important source of nutrient-rich waters. Waters up- welled from greater depths (250 m [800 ft]) contain nutrients and change the temperature-salinity structure of the northern Chukchi (Weingartner 1997~. Sea Ice The Beaufort Sea is covered with ice for about 9 months each year. The Chukchi Sea is covered for 8 months of the 27 year. The ice that first forms is weak and easily displaced by wind and waves, often forming pileups and ridges. By late winter, however, land-fast ice about 2 m (6.5 ft) thick ex- tends from the shore to the zone of grounded ice ridges or to a depth of about 15 m (50 ft) (MMS 1987a, Selkregg et al. 1975~. Nearshore waters shallower than 2 m (6.5 ft) freeze to the bottom. Seaward of the 2 m isobath, land-fast ice floats and can be displaced during winter into ridges. The shear zone is a bank of deformed and dynamic ice that extends over waters that are 15-45 m (50-150 ft) deep (Barnes et al. 1984~. Here, land-fast ice is sheared by the constantly moving mobile pack ice, resulting in an extensive pressure ridge system of massive ice buildups. Ridge build- ups and the accumulation of old ice can be so extensive that large pieces of ice frequently gouge and plow the bottom. The pack ice zone is seaward of the shear zone. It con- sists of first-year ice, multi-year ice floes, and ice islands. The neck ice moves from east to west in response to the Beaufort Sea gyre at rates that range from 2.2 km to 7.4 km (1.4 to 4.6 mi) per day (MMS 1987b). Retreat of sea ice becins in June and usually attains its farthest north position (approximately 72° N) by mid-September (NOCD 1986~. High rates of biological primary productivity are normally associated with the ice edge (Niebauer 1991) and with areas of upwelling. By mid-July, the Beaufort Sea is usually ice-free from the shore to the edge of the pack ice, which by late summer retreats from 10 km to 100 km (6 to 60 mi) off shore. River runoff, coupled with the melting of coastal ice, creates brack- ish (low to moderate salinity) conditions in nearshore areas, particularly near the mouths of rivers. The relatively warm water discharged by rivers and insolation elevate nearshore water temperatures. As summer progresses, this nearshore coastal band of warm, brackish water begins to cool as it mixes with the large sink of cold, arctic marine water. By late summer it is gone, and nearshore waters remain cold and saline until they freeze again in September or October (Wil- son 2001a). Sea ice off Barrow in late summer. September 1992. Photograph by David Policansky.
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28 BIOTA Plants and Vegetation Arctic vegetation patterns and dynamics are strongly influenced by topography, climate, and soils (Walker et al. 2001 a,b; 2002~. For the purposes of this report, we divide the vegetation of the North Slope into six general categories (Table 3-1, Figure 3-1~. Vegetation patterns in the Brooks Range are complex, but the dominant vegetation on well- dra~ned, wind-blown slopes is generally dry tundra dom~- nated by arctic evens (Dryas) (Unit 1~. The Arctic Foothills are dominated by moist tussock tundra (Unit 4) with tussock TABLE 3-1 Area, Percentage Cover of Land-Cover Classes CUMULATIVE EFFECTS OF ALASKA NORTH SLOPE OIL AND GAS cottongrass, abundant shrubs, and mosses. The Arctic Coastal Plain's wetlands are an intricate mosaic of wet, moist, and aquatic vegetation types. Wet tundra (Unit 6) is dominated by sedges, and mosses. Moderately drained (moist) areas on the coastal plain have either moist nonacidic tundra (Unit 2) with sedges, mosses, and low-growing and creeping (prostrate) shrubs or, on sandy substrates, a dwarf form of moist acidic tussock tundra (Unit 3~. Climate vanes greatly with distance from the coast. A narrow band along the Beaufort Sea coast is influenced by the ice pack and by cold ocean waters; mean July tempera- tures are about 4-7 °C (39-45 OF). Shrubs near the coast are Arctic Coastal Plain Arctic Foothills Brooks Range Arctic Slope Unit Vegetation km2 % km2 % km2 % km2 % 1 Dry prostrate dwarf-shrub tundra and barrens 1,778 3.58 1,493 1.56 13,566 24.31 16,887 8.37 2 Moist sedge, dwarf-shrub tundra (nonacidic) 12,088 24.32 20,340 21.29 11,248 20.16 43,676 21.72 3 Moist tussock-sedge, dwarf-shrub tundra (sandy, acidic) 4,958 9.97 2,540 2.66 0 0.00 7,499 3.73 4 Moist tussock-sedge, shrub tundra (nonsandy, acidic) 5,693 11.45 38,728 40.53 11,101 19.90 55,522 27.61 5 Shrub tundra and other shrublands 1,969 3.96 26,117 27.33 9,252 16.58 37,338 18.57 6 Wet sedge tundra 13,303 26.76 3,702 3.87 1,020 1.83 18,025 8.97 7 Water 9,874 19.86 2,369 2.48 535 0.96 12,778 6.36 8 Ice 17 0.03 97 0.10 1,283 2.30 1,397 0.69 9 Shadows 0 0.00 164 0.17 6,881 12.33 7,045 3.50 10 No data 31 0.06 6 0.01 909 1.63 946 0.47 Total 49,711 100.00 95,556 100.00 55,795 100.00 201,062 100.00 SOURCE: Modified from Muller et al. 1999. Dry Prostrate DwarF~shrub Tundra arid Barrer~s Moist Sedge, Dwarf-shrob Tundra Acidic ~ Water Moist Tussock-~cige, DwarI-shrob TundraL (sandy, acidic) I 1 Ice n Moat Tussock-sedge, ~ ~ Shrub Tundra (nor~sancly, nonacidlc) Shrub Tundra arid other Shrublands ~ Wet Seclge Tundra · Sh~owsIbl ~ Data 50 100 150 ~QQkm FIGURE 3-1 Major North Slope ecological regions and vegetation types. SOURCE: Data from Alaska Geobotany Center, University of Alaska Fairbanks, 2002.
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THE ALASKA NORTH SLOPE ENVIRONMENT low growing or prostrate. Local flora near the coast consists of fewer than 150 vascular plant species. Most of the coastal plain is somewhat warmer in summer, with mean July tem- peratures of 7-9 °C (45-48 °F); the flora includes 150-250 plant species. Shrub heights in open tundra reach about 40 cm (16 in.) near the southern edge of the coastal plain. In the foothills, mean July temperatures are about 9-12 °C (48- 54 °F). Tussock tundra covers vast areas, and the local flora exceeds 400 species. In the warmer areas of the foothills, shrub tundra occurs with shrubs that are taller than 40 cm (16 ink. Willows taller than 2 m (6.5 ft) and alders grow along the rivers in the foothills. Cottonwoods grow in the warmest oases and at some springs along the rivers. Soil pH varies considerably across northern Alaska, and it is an important factor in controlling patterns of vegetation and many other ecosystem processes. It also affects the dis- tribution of wildlife. Much of the Arctic Foothills and a large sandy area west of the Colville River on the coastal plain have acidic, nutrient-poor soils that support tussock-tundra vegetation types dominated by tussock cottongrass, dwarf shrubs, and mosses (Units 3 and 4~. Those vegetation types generally have few plant species that have low nutrient con- centrations and high concentrations of anti-herbivore pro- tective chemical compounds. In contrast, moist nonacidic tundra (Unit 2) occurs in areas with mineral-rich soils, such as loess (windblown glacial silt) deposits, alluvial flood- plains, and late-Pleistocene-age glacial surfaces. These ar- eas have relatively high soil pH; shallow organic layers; rela- tively warm, deeply thawed soils; more plant species; and plants with fewer anti-herbivore chemical compounds than those found in areas of acidic tundra (Walker et al. 1998~. The importance of moist nonacidic tundra to wildlife has not been studied specifically, but the combination of the factors described above and the fact that all of northern Alaska's caribou herds calve in areas dominated by nonacidic tundra suggests that it is important wildlife habitat (Walker et al. 2001b). Wildflowers on Arctic Slope near Dalton Highway. July 2001. Pho- tograph by David Policansky. 29 Tundra Ecosystems Tundra ecosystem productivity is limited by the short Arctic growing season, by low temperatures, and because plant growth cannot begin in spring until thawing of the ac- tive layer releases nutrients and water. Much of the initial growth of tundra plants in spring and early summer is sup- ported by stored nutrients, not by current uptake (Chapin and Shaver 1985, 1988; Chapin et al. 1980, 1986~. An ad- equate supply of nutrients, especially nitrogen and phospho- rus, is required at the start of the growing season when growth is most rapid. Nutrients stored in the plants' tissues are available when most needed and are replenished later in the growing season when the soil is thawed more deeply and when aboveground growth has greatly diminished. Nitrogen- fixing species, such as legumes, alder species, and several species of moss and lichen (Chapin and Bledsoe 1992), rarely dominate tundra vegetation, but they control the input of ni- trogen during vegetation succession. Nutrient storage re- duces annual variations in community productivity because growth rates are strongly influenced by average conditions over several years rather than by those of the current year. Plant species affect the quality of the soil substrate for microbes, the primary decomposers of litter. In general, de- ciduous leaf litter decomposes faster than does evergreen leaf litter. Mosses, lichens, roots, and woody stems decom- pose even more slowly (Clymo and Hayward 1982, Nadel- hoffer et al. 1992~. Plants also influence microbial activity by altering soil temperature, moisture, pH, and redox poten- tial. Mosses promote low soil temperatures and permafrost development by conducting heat under cool, moist condi- tions and by insulating soils under warm, dry conditions (Oechel and Van Cleve 1986~. Plant species influence biogeochemistry by affecting rates of herbivory. In general, browsers and grazers prefer deciduous and graminoid (grasses and sedges) species to evergreens. If plants have a low tolerance for herbivory be- cause of their low nutrient availability or low regrowth po- tential, sustained herbivory can shift the community com- position toward less palatable species, thereby reducing nutrient recycling rates (Pastor et al. 1988~. The most important consumers of living and dead plant tissues in terrestrial Arctic tundra are mammals (hoofed mammals, rodents), birds (geese, ptarmigan), arthropods (in- sects, mites, tardigrades), and nematodes. Vertebrate herbi- vores of the Arctic tundra all have varied diets, but the mix- ture of graminoids and woody species normally eaten varies among species. Few species feed heavily on lichens, other than caribou, which depend on lichens for winter feeding. Arthropods are abundant in tundra ecosystems, but the diets of most species are not well known. About half of the insect fauna in the Arctic consists of flies (ranks 1990~. The larvae of some of these species eat living plant tissues, but most of them live in the soil or mud in tundra ponds and feed on dead plant material. Other consumers of plant material
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30 are springtails (Collembola), moths (Lepidoptera), and beetles (Coleoptera), but their relative importance in tundra ecosystems is unknown. Water bears (tardigrades) and nema- todes are abundant in tundra soils, but most species are undescribed and their feeding habits are unknown. Earth- worms are unaccountably absent from North American tun- dra although they are found in Eurasian tundra (Chernov 1995). Lapland longspurs and snow buntings and several species of plovers and sandpipers are important avian preda- tors of tundra arthropods. Herbivores are the food resource for an array of carni- vores that spend part or all of the year on the North Slope. The mammalian carnivores of the Arctic Coastal Plain- wolf, arctic fox, and ermine are active year-round. Om- nivorous mammals red fox, wolverine, and brown bear eat plant and animal matter and are important scavengers. Except for the bears, which hibernate during winter, all are active throughout the year. Raptors in the Arctic Coastal Plain (snowy and short- eared owls and northern harriers) are ground-nesting species because of the small number of cliff nest sites in this region (Ritchie and Wildman 2000). Abundances of these species are low and highly variable (Batzli et al.1980), and they fluctuate in synchrony with the lemming cycle (Batzli et al.1980). Per- egrine falcons, gyrfalcons, golden eagles, and rough-legged hawks are concentrated in the foothills of the Brooks Range, where they nest on cliffs, shale, and soil cut-banks adjacent to rivers and some lakes (Ritchie and Wildman 2000, Wildman and Ritchie 2000). Their abundances have been stable or in- creasing (Wildman and Ritchie 2000). AQUATIC ECOSYSTEMS Freshwater Ecosystems Much of the Arctic Coastal Plain is covered by shallow lakes and ponds. Those deeper than 1.8 m (6 ft) do not freeze to the bottom during winter and typically harbor fish. Shal- lower lakes that freeze completely during winter do not have fish but they have high densities of benthic and planktonic invertebrates. Live and decaying vegetation in those lakes is consumed primarily by larvae of arthropods, principally craneflies and midges. Those insects constitute the primary food supply for the thousands of shorebirds that breed on the wet tundra during the brief summer. Ponds that contain emergent sedges are essential brood- rearing habitats for most ducks (Bergman et al. 1977), and islands in those ponds are preferred nesting sites for some waterfowl. Consequently, bird densities tend to be high in the mosaic of wet meadow, ponds, and drained lake basins near the coast (Cotter and Andres 2000, Derksen et al.1981), and in riparian areas (FWS 1986). The Colville River delta also supports high densities of waterfowl and shorebirds. Lakes northeast of Teshekpuk Lake are important molting areas for brant and white-fronted geese (King 1970). CUMULATIVE EFFECTS OF ALASKA NORTH SLOPE OIL AND GAS Most bird species that breed on Alaska's North Slope nest in tundra habitats, associated wetlands, or adjacent ma- rine lagoons. More than 130 species have been recorded on the coastal plain of the Arctic National Wildlife Refuge (FWS 1986). Dominant groups, both in number of species and in abundance, are waterfowl ducks, geese, and swans and shorebirds. Loons are of interest because their popu- lations are generally declining elsewhere in Alaska (Groves et al. 1996). Yellow-billed loons and eiders are of special concern because their range within the United States is con- centrated in northern Alaska, where they occur in low densi- ties. Some other species are of special concern because they congregate in large numbers to molt in coastal lagoons and wetlands. Marine Ecosystems The Arctic Ocean has very low biological productivity despite supporting a specialized biotic community. In win- ter, when marine nutrient concentrations are at their annual peak, there is little or no sunlight to drive photosynthesis. In summer, when there is ample sunlight, nutrient concentra- tions are low because the lack of mixing results in a stratified water column. The southern Chukchi Sea has high primary production, some of which is exported to the northern Chukchi Sea and the Arctic Ocean (Walsh et al. 1989). The ecology of the northern Chukchi Sea is poorly understood, but the presence of the ice edge and upwelling suggests high biological production (Weingartner 1997). In general, sea ice plays a complex ecological role through spring lead zones, polynyas, and other seasonal changes in structure. Inorganic nutrient concentrations in the surface waters of the Beaufort Sea are typically lowest during summer, when nitrate and phosphate are almost undetectable (Homer 1981) because of phytoplankton uptake and water column stratification. During winter, stratification slows and in- creased vertical mixing replenishes surface-water nutrients. Strong upwelling in some regions of the Beaufort Sea sup- plies deep, nutrient-rich ocean water to nearshore areas. River discharge is another source of nutrients, especially ni- trates and silicates, during the spring thaw when river flows are at their peak (Wilson 2001b). Primary production in the Arctic Ocean is carried out by three groups of organisms: phytoplankton, epontic ice algae (algae that grow on the under surface of ice), and attached benthic macroalgae. Benthic microalgae, which consist pri- marily of diatoms, do not contribute significantly to primary production in the Arctic Ocean (Dunton 1984, Homer and Schrader 1982). More than 100 phytoplankton species, mostly diatoms, dinoflagellates, and flagellates, have been identified from the Beaufort Sea (MMS 1987b). Phytoplankton are gener- ally most abundant in nearshore waters shallower than 5 m (16 ft) (Homer 1984, Schell et al. 1982). Except for isolated areas near Barrow and Barter Island, there are none of the
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THE ALASKA NORTH SLOPE ENVIRONMENT dramatic plankton blooms in the Beaufort Sea that are typi- cal of more temperate waters (Homer 1984~. Rather, there is a gradual, moderate increase in phytoplankton biomass that begins in late spring with ice break-up, peaks in mid- summer when sunlight is most intense, and decreases in late summer when the days shorten. Because of the low primary production, zooplankton communities have few species in low abundance, and slow population growth rates (Cooney 1988~. Herbivorous co- pepods dominate the Beaufort Sea zooplankton (Johnson 1956, Richardson 1986~; amphipods, mysids, euphausiids, ostracods, decapods, and jellyfish (Wilson 2001a) also are present. The abundance and diversity of infauna invertebrates in the substrate tend to be low during summer in nearshore areas shallower than 2 m (6.5 ft) because that zone is covered by land-fast ice in winter. Sedentary infauna are slow to re- colonize the disturbed benthic environment. Biomass and diversity increase with depth, except in the shear zone- 15-25 m (50-80 ft) which is subject to intensive ice goug- ing that presumably destroys substrate-inhabiting organisms. Seaward of 40 m (130 ft), ice no longer disturbs the benthos (Carey 1978~. Infaunal species include foraminifera, polycha- etes, nematodes, amphipods, isopods, bivalves, and priapulids. Organisms that live on the surface (epifauna) are more motile and readily dispersed by currents. Some groups, such as mysids, migrate on- and offshore seasonally (Alexander et al. 1974, Griffiths and Dillinger 1981~. Epifaunal organ- isms are an important food source for several bird and fish species that inhabit coastal waters during summer (Craig et al. 1984~. Epontic communities consist of microorganisms, mostly diatoms, that live on or in the under-surface of sea ice (Homer and Alexander 1972~. Light is the major factor that controls the distribution, development, and abundance of those assemblages (Dunton 1984, Homer and Schrader 1982~. Epontic algae are estimated to contribute 5% of the annual total primary production in nearshore Beaufort Sea coastal waters (Schell and Homer 1981~. Ice algae assem- blages serve as a food source for a variety of invertebrates, including copepods and amphipods, particularly during early spring when other sources of food are in short supply (Wil- son 2001a). Much of the Beaufort Sea floor is covered by silt and sand (Barnes and Reimnitz 1974), but there is an isolated area of rock- and cobble-littered seafloor, called the Boulder Patch, several kilometers offshore from the mouth of the Sagavanirktok River in Stefansson Sound (Dunton and Schonberg 1981, Dunton et al. 1982, Martin and Gallaway 1994~. The Boulder Patch supports a community of several 31 species of large red and brown algae and a diverse assort- ment of invertebrates representing every major phylum (Dunton and Schonberg 2000, Dunton et al. 1982, Martin and Gallaway 1994~. The most conspicuous member of the community is the kelp, Laminaria solidungula. Beneath the overstory is an- other seaweed assembly dominated by several species of red algae. Kelp fix 50-56% of the carbon available to Boulder Patch consumers. Growth of kelp is both energy- and nitro- gen-limited because those two resources are not available in sufficient quantities simultaneously (Dunton 1984~. Sponges and cnidarians are the most abundant and conspicuous in- vertebrates in the Boulder Patch community. Bryozoans, mollusks, and tunicates are common on rocks and attached to other biota. A species of chiton constitutes a large per- centage of molluscan biomass and is one of the few species that graze on kelp. The abundance and diversity of epifauna in nearshore waters that are shallower than 2 m (6.5 ft) in summer is simi- lar to the abundance and diversity in deeper surrounding zones because mobile invertebrates can rapidly recolonize shallows once the ice lifts off the seafloor and the ice cover recedes. Some species find winter habitat in deep holes within the land-fast zone. Mysids and amphipods dominate the nearshore epifaunal community (Griffiths and Dillinger 1981, Moulton et al.1986~. Epifauna from 33 trawls done in the northern Chukchi and western Beaufort Seas in 1977 were described by Frost and Lowry (1983), who identified 238 invertebrate species or species groups and two major community types. The mobility of epifauna, either active or via passive transport, can be critical in maintaining a robust food web. Griffiths and Dillinger (1981) estimated that feeding by birds and fish within Simpson Lagoon would be sufficient to de- plete the basin of mysids rapidly were it not for a substantial and continual immigration of mysids from offshore coastal waters. Seventy-two species of fish have been identified in freshwater and marine habitats on and around the North Slope, although only 29 of them are common. Some 17 spe- cies, of which arctic cisco (Coregonus autumnalis) and broad whitefish (C. nasus) are of highest value, are important for the subsistence harvest. Several bird species use arctic marine environments for food, including gulls, loons, and the sea-ducks. The coastal barrier island and lagoon systems are important molting and staging areas for waterfowl. Most waterfowl species depend more on freshwater than saltwater for their habitat and food requirements. The Beaufort Sea is also important habitat for whales, seals, and polar bears.
Representative terms from entire chapter: