In contrast to Antarctica, the physical environment in the Arctic is a consequence of recent glaciation and the relatively short time span for ecosystem and faunal diversification.  Polar ecosystems are characterized by extreme environmental conditions induced by cold temperatures, extensive snow and ice cover with abbreviated periods of solar radiation and primary productivity.  In general the productivity in Arctic freshwater and marine systems is concentrated over short periods of time centered around ice breakup when nutrients become available and light becomes more prevalent and water temperatures warm as the ice cover is reduced.  These environmental constraints concentrate recolonization during a few months of the year resulting in low species diversity due to extremely fast population growth of key zooplankton species responding to the release of nutrients which then contribute food to slower growing and longer-lived species. The objective of this chapter is to present an overview of the physical environment of the Arctic and indicate important characteristics that determine presence of Arctic ecosystem components.

The Arctic Ocean and associated waters comprise one of the most unique marine ecosystems in the world.  Two sources of productivity are instrumental in actively replenishing nutrients over short time periods:  ice algae, and riverine input.  It has been estimated that ice algae contributes 10-70% of annual productivity (AMAP 1998).  River discharges to the Arctic shelf regions augment nutrients, organic materials, and sedimentation on a seasonal basis.  The marginal seas are either influenced by the Atlantic or Pacific Oceans (Nordic, Barents, Northern Labrador Sea and Bering, Chukchi Seas, respectively), or are relatively isolated and border the Asian or North American continents (Kara, Laprev, East Siberian, and Beaufort, respectively).  The Chukchi, Bering and Barents Seas are among the most seasonally productive ecosystems.  The seas bordering the continental landmasses are influenced by freshwater runoff from the river systems and prior to the onset of recent increased warming trends had landfast ice associated with the shorelines for most of the year.  Freshwater discharge from rivers leads to earlier open water in the nearshore zone.  Maximum productivity is limited to open coastal waters during spring/summer months or to polynyas between landfast ice and the polar pack ice (occurring near the continental shelf edges).  Organic material not cycled through organisms or advected to the central Arctic basin is incorporated into sediments, producing localized areas of high organic enrichment near the mouths of major rivers and to locally deposited sections of tundra breaking off from shore locations.  Nearshore river systems are characterized by estuarine conditions, i.e. higher water temperatures, lower salinity, increased turbidity, as well as increased productivity due to the organic enrichment.  In most other areas the benthic standing crop decreases with increasing depth; ridges such as the Lomonosov Ridge have higher benthic standing crops than adjacent basins (Kröncke 1994; Kröncke et al. 1994).  The age of the organisms creating these standing crops on ridge environments may be quite old and reflect development over many decades but this has been a difficult environment to investigate so little is well known.

The two main deep basins, the Eurasian and the Canadian are separated by the transpolar Lomonosov Ridge (Figures 1-2 and 1-3).  The Canadian Basin which is < 3500 m in depth is transected by the Alpha Cordillera ridge into the Makarov and Canada Basins.  The Eurasian Basin is deeper, reaching depths of 4000m, and is divided by the Nansen Cordillera into Amundsen and Nansen basins.  While the continental shelf generally extends 50 to 100 km offshore, the shelf is broad north of Siberia, extending up to 900 km offshore. 

Figure 1-2.   Basins and ridges of the Arctic Basin  (Mike Norton)

Figure 1-3.  Main water bodies of the Arctic (Source: AMAP 1998)

Most oil and gas development activities, shipping routes, as well as major fishing grounds occur along the margins of the Arctic Ocean at the interface of land and sea edges.  Figure 1-4 highlights the main shipping routes transiting Arctic waters and areas of active oil and gas exploration.  Increased periods of ice-free conditions along shipping routes will result in increased vessel and fishing activities.   It should be noted that increased commercial fishery pressure itself may lead to changes in fishery stocks and biodiversity in the Arctic.

Continental runoff is a major source of freshwater, terrigenous materials and nutrient loads to the Arctic Ocean.  Information gathered from the Regional Arctic Hydrographic Network data set (R-ArcticNET) indicates that the overall annual discharge is ~3,300 km³ y^-1.  This buoyant freshwater contributes to a low saline layer (upper 200 m) of the Arctic Ocean which is isolated from the warmer, saltier Atlantic layer by a strong halocline (Fichot et al. 2013).  Rivers account for 2% of the influx of water to the Arctic region resulting in highly productive areas, particularly during the peak flow occurring from April to July (~60%).  The rivers in Siberia (Ob, Yenisey, Lena, and Kolyma) and in North America (Yukon, and Mackenzie) provide the majority of continental fresh water to the Arctic Ocean.  The Arctic Great Rivers Observatory (Arctic-GRO) monitors the discharge of these six river systems.

River systems generally increase productivity due to the availability of increased organic nutrients in the areas of riverine discharges.  Figure 1-5 shows the locations of the major rivers in the Arctic.   The regions along the continental shelf may be the most likely to encounter oil spill incidents, either due to oil and gas production activities or vessel mishaps. In determining environmental impacts of released oil, areas of special importance are the interfaces where oil components may re-concentrate (e.g. current convergence zones, pycnoclines, upwelling or downwelling of water masses, shoreline stranding or concentration at air-water, ice-water, or sediment-water interfaces).

Figure 1-4. Circumpolar regions of activities (AMAP)

Figure 1-5. Major river systems in the Arctic

Ocean temperatures vary widely depending on latitude and proximity to warm Atlantic or Pacific Ocean waters.  For the Arctic Ocean, temperature variation between winter and summer is small (remains close to freezing year round) and salinities vary between 30 and 33.  In the coastal shelf areas, surface water temperatures range from -1 °C to 4-5 °C, winter to summer, respectively while salinities may be <30, especially in areas receiving freshwater from rivers and ice melt.  In areas where oceanic mixing occurs, the temperature remains higher than 0 °C throughout the year.  In the Kara Sea and Siberian shelf, salinity is <20 throughout the year, and may drop to 10 during the summer (Figure 1-5; AMAP 1998).  Arctic seas are primarily ‘beta oceans’, i.e. a salinity profile is the most important permanent stratification feature (Carmack and Wassmann 2006).

Figure 1-6. Temperature, salinity, and density profiles (AMAP 1998)

Warm ocean currents flow northward from the Atlantic and Pacific Oceans and cold Arctic countercurrents flow southward (Figure 1-6).  Arctic Ocean water mainly flows from the Atlantic Ocean (79%), while inflow from the Bering Strait accounts for only 19%.  Concern with thinning of Arctic ice cover over recent decades has generated numerous investigations of the ocean-ice heat flux and changes in the Arctic Ocean surface layer (ASL).  The ASL is generally insulated from the warmer and higher salinity bottom water (Atlantic water layer; AWL) by a well-developed halocline (cold halocline layer, CHL).  The halocline establishes as a strong barrier to upward mixing by turbulence, and consequently most of the ocean heat flux in the central basins of the Arctic is generated by solar heating through open leads and thin ice during the summer months.

Figure 1-7. Arctic currents (AMAP 1998)

Researchers have found that although changes in surface velocity and surface stress in the open ocean reflects large scale atmospheric pressure fields (the Arctic oscillation, AO), the properties of sea-ice concentrate energy into relatively narrow zones of intense shear which can raise the pycnocline.  Such events may greatly enhance ocean-to-ice heat transfer (McPhee et al. 2005).  Examination of hydrographic records indicated that the CHL dissipated during a low pressure system from 1988 to 1997 in the Nansen and Amundsen Basins, resulting in an increased rate of melting sea-ice (15-25 cm/y).  The lateral Ekman currents moved in a counter-clockwise direction, resulting in upwelling of warmer, saltier water from the AWL and raising a weakened halocline layer about 50 m.  Average seasonal changes in sea-ice conditions (concentration and movement), sea level pressure, Ekman transport vectors, and upwelling patterns are presented in Figure 1-7 (Yang 2008).  

Figure 1-8. Seasonal sea-ice and ocean movement averaged over a 28 year period (Yang 2008)

Closer to shore, the seasonal cycle of Ekman transport is highly influenced by high sea level pressure in fall and winter.  For example, strong offshore transport along the Beaufort, Chukchi and East Siberian coasts intensifies coastal upwelling and downwelling in the interior Canadian Basin.  The offshore transport also pushes low-salinity warm shelf water toward the deeper basins, reducing salinity and increasing temperature.  

The continental shelf zones comprise about 50% of the Arctic Ocean surface and there are marked regional differences.  Narrow shelves are characteristic of the North American continent, and wide shelves with very steep slopes are typical of Eurasian continents (Table 1-1). Carmack and Wassmann (2006) group shelf areas according to function:  1) Inflow shelves (Bering Strait/Chukchi Sea, Barents Sea); 2) 2) Interior shelves (Beaufort and Kara/Laptev/East Siberian Seas); and 3) Outflow shelves (Canadian Archipelago and East-Greenland shelf).   The pan-Arctic shelves are estuarine in nature in that waters originating from the Atlantic and/or the Pacific Oceans mix with inflowing river waters.  Although there are regional differences in source water characteristics, all shelf areas are generally associated with the nutrient enrichment, productivity, and biodiversity.  Figure 1-8 is an excellent illustration of the dynamic physical, chemical, and biological interactions occurring in the Arctic shelf regions in the vicinity of receiving waters of major river systems.  In general, the nearshore ecosystems are characterized by relatively few trophic links and low biodiversity, and would be more sensitive to increased climatic warming than temperate regions (Carmack and Wassmann 2006).

Table 1-3. Characteristics of Arctic shelves (after Carmack and Wassmann 2006)

Figure 1-9. Dynamic shelf processes augmented by nutrient loading from riverine discharge (Source: AMAP 1998)

Sea ice forms when the temperature of the ocean falls below the freezing point, effectively converting the seawater to ice (-1.8 °C at 33 ‰ salinity).  Ice is characterized by where it is located:  landfast ice lines the shoreline and first year ice is intermediate between the multi-year ice and open water.  The annual changes in pack ice are determined by temperature, winds, and ocean currents (such as the Alaska, Labrador, East Greenland currents and the warm West Spitsbergen and North Cape currents).  Multi-year ice averages 2.5 to 4 m in thickness and may be intersected by leads such as shown in the photo above (~1% open in winter to 10-20% open in summer; Gow and Tucker 1990).  The two main ice circulation systems in the Arctic are the clockwise Beaufort Gyre in the Amerasian Arctic and the Transpolar Drift in the Eurasian Arctic (migrating east to west, exiting via the Fram Strait).  The North Cap current keeps the southern Barents Sea ice-free during the winter.

Photo 1-2. Ice lead (Lionel Camus)

Photo 1-3.  Polynya

Ice leads and open-water lenses (polynyas) form in nearshore or ocean areas and are characterized by cold, highly saline water (Photos 1-2 and 1-3).  The processes of ice production, salt flux, and heat transfer from leads and polynyas are important contributors to the larger scale climate events in the Arctic.  Polynyas occur for the most part during winter and harsh weather conditions and may be formed by two processes: 1) Mechanically forced (wind-driven; strong winds force ice cover away from the coast; 2) Convectively forced (ice subsidence; subsurface heat is transferred to the surface water by the upwelling of warmer, deeper water); refer to Figure 1-10.   Improved satellite technologies (e.g. synthetic aperture radar, SAR) and mathematical algorithms (e.g. polynya signature simulation method, PSSM) have made it possible to define the size and shape of polynyas as well as differentiate open water, new ice, and young ice (Dokken et al. 2002).  The heat exchange from the ocean to the atmosphere can be orders of magnitude larger in polynyas compared to the surrounding ice pack, and polynyas can contribute ~50% of the seasonal mean of annual ice production in some areas (Winsor and Bork 2000).  Winsor and Bjork (2000) estimated the mean ice production and corresponding salt flux from Arctic polynyas and found that the salt flux represents about 30% of that necessary to maintain the CHL.    The marginal seas contributing the most to the CHL are the Barents, Kara, Chukchi, and Bering; the Chukchi Sea is the only sea contributing actively to deep water formation.  The North Water Polynya is located between Greenland and Canada in northern Baffin Bay.  It is 85,000 sq. km in area and creates a warm microclimate that is one of the most biologically productive marine areas in the Arctic Ocean.  Polynyas are of critical importance to significant populations of marine birds, mammals, and fish.

Figure 1-10. Polynyas and Sensible-Heat Exchange [Based on image modified from Ocean Circulation, 2nd Edition by Open University, Butterworth-Heinemann Publishers, page 219; Source: National Sea Ice Data Center (NSIDC)]

The term seasonal ice zone (SIZ) describes the presence of transitional ice (i.e. freezes and melts annually) whereas marginal ice zone (MIZ) is used to describe the areas where open water meets ice cover during any season.  About half of the Arctic sea-ice freezes and melts annually.  During summer months, narrow MIZ and wide SIZ bands circumscribe the polar region.  It is along this band of expanding and shrinking open water linkages that most of the Arctic Ocean productivity takes place and where climatic changes would be most evident (Carmack and Wassmann 2006).  In recent years, the ice-free season in the Arctic increased at a steady rate of 1.1 days/year.  A record low in sea ice was recorded in 2007, with 168 ice-free days (Rodrigues 2009).  Increasing seasonal open water periods is coincident with anticipated increases in shipping traffic via the polar routes (Northwest Passage and Northern Sea Route).

Biological productivity hinges upon solar irradiance and nutrient availability.  These two resources are regulated by salinity stratification of the Arctic waters.  In general stratification is expressed as buoyancy frequency relating vertical temperature and salinity gradients with consideration of gravity, thermal expansion, and haline contraction coefficients (Carmack and Wassmann 2006).

The annual productivity/carbon cycle in the Arctic is illustrated in Figure 1-10.  The sun appears over the horizon in April, supplying light to support the growth of ice algae and phytoplankton. Production peaks in June when the sun is at its highest; zooplankton thrive on this superabundance of food. The production gradually declines during the season as the phytoplankton use up the nutrients in the water and as the sun sinks below the horizon, the plankton hibernate until the next growing season (http://www.arcticsystem.no). For areas in the Canadian high Arctic, Welch et al. (1992) estimated the relative importance of contributors to primary productivity as: phytoplankton (90%) > ice algae (10%) > benthic algae (1%).  This relationship likely varies in different locations.

Figure 1-11. Productivity/carbon cycle in the Arctic [Original illustration: Alexander Keck & Paul Wassmann (1993), modified by Frøydis Strand, NFH, University of Tromso; with adaptation from Forest et al. 2013)]

Physical processes inherent in ice field characteristics, water column stratification, vertical mixing, light and nutrients in combination with phytoplankton determine the balance of productivity and biomass build-up in the euphotic zone and its transfer to the aphotic zone.  The fate of suspended and sinking biogenic matter is based on the magnitude of export production and biological activities such as grazing in the upper water column.  The transfer of carbon from the surface layer to the deep ocean is accomplished via the passive sinking (exported) or the active transport of organic material (harvested, fecal production).   The greatest congregation of zooplankton often occurs just below the euphotic zone (Olli et al. 2006); excess detrital matter ultimately sinks into the aphotic zone.  In some regions such as the Barents Sea, the pelagic food web is positively correlated with benthic standing stocks (Carmack and Wassmann 2008).   There is considerable variability in sediment community structures and biomass, depending on carbon development in the upper layers.   Ice cover and other physical factors have been observed to limit primary production and benthic biomass (Grebmeier et al. 1995). 

The highly seasonal attributes of the Arctic influence species abundance and distribution patterns.  The lack of sunlight during the Arctic winter in combination with ice-covered waters limits primary and secondary production when solar irradiance is unavailable.  The periods of low production are punctuated by extremely high production during breakup and seasonal open water.  The combination of open water, long days, and high nutrient concentrations during break up and early summer creates intense periods of primary production, grazing by primary consumers, and predation by the higher trophic levels.  Whereas the aquatic communities are limited during the winter, the diversity and abundance of species increases dramatically in the summer. Key Arctic species have adapted their life cycles to take advantage of this period of high production, and larval or juvenile life stages are adapted to capitalize on the short period of enriched food resources.

The initiation and duration of summer production follows a latitudinal gradient from the southern Barents Sea in March – April to August – September in the Fram Strait and Arctic Ocean (Figure 2-5; Falk-Peterson et al. 2007) with substantially shorter periods of high production in the higher latitudes.  This suggests not only a strong temporal component, but a strong regional component. The largest seasonal effects are seen in the surface and nearshore waters, however there are seasonal patterns in the deeper waters as well, with Arctic cod (adults, YOY and larval cod) and copepods moving to deeper waters during the winter months. 

Seasonal migrations and changes in abundance, particularly of the younger life stages are an important consideration for environmental consequence analysis (ECA) and the relative vulnerability of different VECs.  Based on information provided in this review, there are some general pan-Arctic patterns of seasonal sensitivity.  However, for the purposes of assessing the spill response options, a regional approach is recommended.  Surface waters along ice-edges and in leads, particularly during the breakup and early ice-free period are important concentration points for copepods and juvenile cod, as well as migratory birds and mammals.  During the ice-free period, the ice edge continues to be an important feeding ground for large marine mammals as well as ice-seal pups.  Nearshore waters, are also a concentration point for water column resources, particularly juvenile forms and migratory species (e.g. toothed whales, sea birds).  As in more temperate waters, estuarine lagoons and rocky intertidal habitat during the ice-free season are important nursery grounds and act as a convergence point for aquatic, avian, and terrestrial species (Dunton et al. 2006).  During the open water season, nearshore waters and shallow shelf waters are also important feeding and breeding grounds for walrus.  Portions of the nearshore and shallow shelf are staging and feeding grounds for nesting sea birds and their chicks (Robertson 2013).  The midwater pelagic region is a concentration point for shoals of arctic cod and feeding migratory whales.  In the winter months, Arctic cod and Calanoid copepods move to deeper waters.  Leads and polynyas are concentration points for VEC species during the winter, in particular toothed whales.

For further detailed information on Arctic species, communities, and trophic linkages, please refer to Section 2.

The review of the physical environment described by the authors in this section led to suggestions of further research which can reduce remaining uncertainties; these recommendations are summarized below while recommendations that are particularly important for improving Arctic NEBA are listed separately. 

1. Seasonal and interannual fluctuations. The prevailing environmental characteristics such as extreme cold temperatures, variable solar irradiation and ice coverage affect the marine ecosystem and its populations. Seasonal productivity ties in with the degree of solar irradiation and ice coverage and directly affects species population recruitment and migration patterns.
   1. A better understanding of ice, and the corresponding variance of faunal distributions and life-history patterns on a spatial (regionally and throughout the pan-artic region) and temporal (annual and interannual) scale would be especially useful to protecting areas of densely populated VECs. 
2. Interface habitats. The physical structure of the Arctic provides additional interface as well as open water environments that are occupied by VECs.
   1. Interface environments that are stable (e.g. shorelines, sediment/water interface, edges of multi-year ice, convergence of water masses near subsurface topographic features) have generally been mapped throughout most of the Arctic. Where they have not been mapped they need to be. Seasonal use patterns by VECs need to be summarized by these interface ECs (see Section 2). 
   2. Interface environments that are variable (annual ice edges including polynyas and under-ice brine channels, fresh water releases and their transport, barrier island beaches/shorelines should be characterized by indicating most likely locations in the Arctic for these less permanent features. Understanding of patterns of use by VECs will be strengthened under work proposed in Section 2. 
3. Alterations in chemical processes. We need to better understand how cold temperatures and extreme weather interacts with broken and more permanent ice to affect the chemical processes associated with the fate of oil compounds and natural biodegradation rates (Sections 3 and 5). 

The recommendations presented below indicate where increased knowledge of physical environment would result in reducing existing uncertainties in NEBA assessments. No prioritization has been made to the list; for some of the recommendations, surrogate data may be already available. 

1. Arctic ecosystem consequence analyses need to include impact assessments of open pelagic waters as well as shorelines, river discharges, sediment/water interfaces, ice/water interfaces, convergence zones and surface layers near the air/water interface. These ECs may receive concentrated oil exposure depending on the type of OSR actions employed, and should receive further study.
   1. The location of primary ECs used by VECs could be added to regional sensitivity maps already available. 
   2. The locations where less permanent ECs can be prevalent are also important to VEC distribution and need to be located generally so that site specific assessments of their presence can be added to environmental sensitivity maps for reference during response actions. The environmental features that control the locations of these less permanent features and means to determine their presence during a response action are an area of additional study.

Authors Jack Q. Word and Lucinda S. Word (ENVIRON)