Oceans and seas

Bering Sea

May 13, 2013, 11:21 pm
Content Cover Image

Bering Sea in spring phytoplankton bloom from satellite platform. Source: NASA

The Bering Sea is a marginal sea located on the northern rim of the Pacific Ocean centered at approximately 58 degrees N and 160 degrees W. It is surrounded by Alaska to the east, Siberia to the west and northwest, and the Aleutian Island arc to the south. It has an area of about 3,000,000 km2 and a volume of about 3,700,000 km3. The Bering Sea has some of the largest and gently sloped continental shelves of any sea of the world; furthermore, these seafloors are deeply incised by seven deep submarine canyons.

St. Lawrence Island is the largest island in the Bering Sea. It is part of Alaska (though closer to the Siberian mainland) and home to Yupik Eskimos.

A number of cetaceans are found in the Bering Sea, including Fin whale, Humpback whale, Beluga whale and Gray whale. Some Bering fisheries suffer from overfishing, although the total fish harvest of the Bering Sea is very high by world standards, reflecting the intrinsically high primary productivity. Slight increases in water temperatures have enhanced the productivity of certain Bering fisheries. The terrestrial margin on the Alaska side is chiefly a low-lying tundra, featuring sedges and dwarf shrubs.

caption Regional setting of the Bering Sea.

Circulation and Hydrography

The bathymetry is about equally divided between a vast shelf to the northeast that is at most 200 meters (m) deep and the Aleutian Basin where depths range from 3800 to 3900 m over most of the region. The Shirshov Ridge (along 171 degrees E between 500 and 1000 m depth) and the shallower Bowers Ridge (along 180 degrees E then turning west along 55 degrees N) effectively divide the Basin into three parts. At its northern limit It is connected to the Arctic Mediterranean Sea via the Bering Strait; to the south it merges with the Pacific Ocean via several sills between the various Aleutian Islands, although the main connection is thought to be between 168 degrees E and 172 degrees W where the sill depth is about 1590 m.

The main circulation features include a large part of the westward flowing Alaskan Stream entering the Bering Sea through the passage centered at 170 degrees W, turning east, and driving a cyclonic (counterclockwise) gyre in the Aleutian Basin. This largely barotropic current sees the two main ridges as obstacles which sets up a system of two eddies, one on each side of the Shirshov Ridge. Eddies have been observed separating from the eastern limb (often called the Bering Slope Current) of the Bering Sea gyre, the larger of the two systems. There is a countercurrent further up the Bering Slope whose dynamics are those of an eastern boundary current in a subpolar gyre. A series of currents and related fronts largely driven by Alaska Stream inflow through a shallower passage at 175 degrees W flow north-northwestward on the broad shelf region. 

caption Schematic diagram of circulation and the extent of different water masses in the Bering Sea. Source: AMAP based on Takenouti, et. al.

An important element of circulation in the northern Bering Sea is the Anadyr Current, a chiefly seasonally invariant current flowing northeastward and supplying most of the Bering Strait throughflow. This throughflow, driven by sea level differences across the strait, varies from about 0.1 meter/second (m/s) in the summer to 0.5 m/s in the winter, moves through the Shpanberg Strait seasonally shifting from northward to southward to compensate for the differences. The shelf flows also make a generally unknown contribution to this throughflow. The western limb of the smaller gyre to the west of the Shirshov Ridge contributes to and becomes part of the southwestward flowing Kamchatka Current.

The local water masses are derived from Pacific Ocean water masses transported in to the area and modified by processes on the shelf. This results in a temperature minimum at or below 100 m, low surface salinities rapidly rising to about 300 m, and overall low oxygen concentrations. The water overlying the temperature minimum is surface water imported from the Alaska Stream, and the water below that is Pacific Deep Water.

Water Quality

The Bering Sea has generally low levels of water pollutants and toxic contaminants, but these have been rising over the last 50 years due to increased human activities (mining, fishing, and oil exploration). This increase is linked to the long-range transport of contaminants through the oceans and atmosphere from other regions. Cold region ecosystems such as the Bering Sea are more sensitive to the threat of contaminants because the breakdown of these contaminants is reduced in gelid regions. Also, animals high in the food web with relatively large amounts of fat tend to have high concentrations of organic contaminants such as pesticides, herbicides and PCBs.

Russia today is faced with the environmental problems inherited from the Soviet Union. Russian regional authorities and multi-national oil companies are pushing for further oil exploration and development in these fragile ecosystems. However, the Russian Supreme Court has invalidated a governmental decree issued by former Prime Minister Stepashin that would have allowed marine discharge of toxic wastes from oil drilling off Russia's far east coast.

Bering Strait

See main article: Bering Strait

A northern limit of the Bering Sea, the Bering Strait is a narrow ocean passage separating the North America from Asia. With the Seward Peninsula of Alaska to the east, and Chukotskiy Poluostrovof, Siberia to the west, the Bering Strait separates the USA and the Russia by only 90 kilometers. It is named for Danish explorer Vitus Bering, who passed through the Strait in 1728, and in a later spotted the Alaskan mainland in 1741 while leading an expedition of Russian sailors.

The boundary between the USA and Russia lies between Big and Little Diomede Islands, which are visible in the middle of the Bering Strait.

Marine Ecology

See main articles: East Bering Sea large marine ecosystem; West Bering Sea large marine ecosystem

For ecological analysis the Bering Sea is customarily divided into east and west elements. Both the East and West Bering Sea are classified a Class II, moderately high (150-300 grams of carbon per square meter per year) productivity ecosystem. The East Bering Sea is experiencing a change in species dominance and species abundance in some ecological groups, but it is difficult to establish the causal mechanisms responsible for inducing these changes.The West Bering Sea has the largest fish biomass in the world for Pacific cod and other cod-like fishes. Other west Bering species harvested include salmon, Alaskan pollock, walleye pollock, flatfish, rockfish, halibut, flounder, herring, squid and a variety of crab species and other crustaceans. The Pribilov Islands are the southern limit of the Polar bear (Ursus maritimus) in the North Atlantic.

caption Beluga whale, one of the Bering Sea cetaceans.

The Bering Sea is one of the most significant large marine ecosystem of all the seas of the world. Fisheries of the Bering Sea and Gulf of Alaska comprise half of the marine fish biomass harvest in waters of the USA. Furthermore, the Bering Sea is the locus of numerous large bird and marine mammal populations. The ecosystem of the Bering Sea has changed over the previous fifty years, driven by overfishing and climate forcing. There are two shifts in climate associated with increased warm temperatures and other factors, the first in the late 1970s and again around 2000. There was a major ecosystem reorganization following the late-1970s shift. These changes represent a transition from primarily cold Arctic ecosystems earlier in the 20th century, dominated by sea ice, to sub-Arctic conditions.

Snow crab harvests have diminished in the prior decade, from a combination of warmer temperature and decreased benthic productivity. Fur seal pup births on St. Paul Island declined after the late 1970s and again in the most recent decade. There is some indication of increases in Fin whale and Humpback whale populations at the close of the twentieth century. The pollock biomass harvest rose by a factor of four subsequent to the late-1970s climate shift and has generally stabilized at approximately ten million metric tons, a very large volume by any world fishery standards. Cathes of many other species have declined in the prior three decades; for example, Greenland turbot, which prefers cold benthic waters, declined in catch to one fifth of its peak. Reduced sea ice enhances biological productivity in the upper ocean compared to benthic species. 

Terrestrial Ecology

See main article: Beringia lowland tundra ecoregion

caption Bering Tundra landscape, Bristol Bay lowlands. Photo: National Park Service

The Beringia lowland tundra ecoregion lies along the eastern margin of the Bering Sea. This  ecoregion is formed by three major disjunct areas along the Bering Sea coast of Alaska from the base of the Alaska Peninsula to Kotzebue Sound, as well as one smaller area on the east side of St. Lawrence Island and St. Matthew Island. The ecoregion is characterized by low, flat, or gently rolling terrain, wet soils, and resulting predominance of wet and mesic graminoid herbaceous vegetation. In better drained areas, especially in the somewhat more rolling portions of the section surrounding Bristol Bay, dwarf shrub communities occur interspersed with the wet herbaceous tundra, dominated by sedges, including Eriophorum angustoifolium and Carex spp. Dwarf shrub vegetation is usually dominated by ericaceous species, including crowberry (Empetrum nigrum). In some limited areas of favorable soil drainage and microclimate, stands of black and white spruce (Picea mariana, P. glauca) occur, with understories of alder (Alnus spp.), willow (Salix spp.), and dwarf birch (Betula spp.). Lakes and ponds cover 15-25 percent of the surface area, and wetlands cover between 55 percent (southern portions) to 78 percent (northern portions) of the region.

Much of the landscape is drained by the Nushagak and other large rivers that flow into Bristol Bay; the lowland is dotted with morainal and thaw lakes. Moist and wet tundra communities provide the dominant vegetation. Standing water, mosses, sedges, and low-growing shrubs cover most of the area. Alder, willows, and scattered stands of stunted spruce and birch grow along the major rivers and streams.

Bristol Bay provides staging and migration habitat for large numbers of waterfowl. Ospreys occur more frequently in this province than in any other part of Alaska. Blackpoll warblers are common breeders in conifer stands in the north. Bristol Bay supports the largest run of sockeye salmon in the world. Brown bears are common mammals, partly because of large salmon runs in this area.

Further Reading

  • Peter Saundry, 2011. Seas of the world, Encyclopedia of Earth. Topic editor, C.Michael Hogan, Ed. in chief Cutler J.Cleveland  
  • G. Carleton Ray and Bruce P. Hayden, 1993. Marine biogeographic provinces of the Bering, Chukchi and Beaufort seas. In: Large Marine Ecosystems: stress, mitigation and sustainability, edited by K. Sherman, LM Alexander and B.D. Gold, 1993. ISBN: 087168506X.
  • Arctic Monitoring and Assessment Program (AMPA)
  • L. Zenkevitch. Biology of the Seas of the U.S.S.R. Wiley Interscience, 1963.
  • Matthias Tomczak and J. Stuart Godfrey. 1994. Regional Oceanography: An Introduction. Pergamon.
  • Rhodes W. Fairbridge, editor. 1966.The Encyclopedia of Oceanography. Van Nostrand Reinhold Co.,
  • L. K. Coachman. 1986. Circulation, water masses, and fluxes on the southeastern Bering Sea shelf. Continental Shelf Res., 5:23–108.
  • E. D. Cokelet and P. J. Stabeno. 1997. Mooring observations of the thermal structure, salinity, and currents in the SE Bering Sea basin. JGR, 102:22, 947–22, 964
  • J. D. Schumacher and P. J. Stabeno. 1998. Continental shelf of the Bering Sea. In A. R. Robinson and K. H. Brink, editors, The Sea - Vol. 11: The Global Coastal Ocean, Regional Studies and Syntheses, pages 789–822. Wiley.
  • Phyllis J. Stabeno, James D. Schumacher, and Kiyotaka Ohtani, The physical oceanography of the Bering Sea: A summary of physical, chemical, and biological characteristics, and a synopsis of research on the Bering Sea in Dynamics of the Bering Sea: A Summary of Physical, Chemical, and Biological Characteristics, and a Synopsis of Research on the Bering Sea, T.R. Loughlin and K. Ohtani (eds.), North Pacific Marine Science Organization (PICES), University of Alaska Sea Grant, AK-SG-99-03, 1–28.
  • Takenouti, A.Y. and K. Ohtani, 1974. Currents and water masses in the Bering Sea: A review of Japanese work. In: D.W. Hood and E.J. Kelly (eds.). Oceanography of the Bering Sea with emphasis on renewable resources, pp. 39-57. University of Alaska, Institute of Marine Science, Fairbanks.
  • U.S.National Oceanic and Atmosperic Administration. 2006. Bering Climate
  • U.S.National Research Council. 1996. The Bering Sea ecosystem. Committee on the Bering Sea Ecosystem. National Academies Press. 307 pages

 

Glossary

Citation

Hogan, C. (2013). Bering Sea. Retrieved from http://www.eoearth.org/view/article/150493

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