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OCR for page 24
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.
OCR for page 26
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
OCR for page 27
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.
OCR for page 28
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.
OCR for page 29
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
OCR for page 30
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
OCR for page 31
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:
coastal plain
