area Bering - Descriere Marea BeringCuriozitati Geografice > Despre ape

Cea mai nordica mare a Oceanului Pacific. Are o suprafata de 2 270 000 km2 si o adîncime de 4 191 m. Ea scalda tarmurile a doua continente, Asia si America de Nord. Tarmurile sînt în mare parte muntoase si puternic crestate. Prin strîmtoarea Bering se scurge o parte din apele Oceanului Înghetat formîndu-se curentul rece (Oya- sivo) care transporta gheturi pîna în apropierea Aleutine. La începutul secolului al XIX-lea, geografii i-au dat acest nume, în amintirea marelui explorator Vitus Jonassen Bering (1681 - 1741), care a descoperit-o cînd a condus doua expeditii în Kamceatka (1725 - 1730 si 1733 - 1741).

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Marea Bering (sau Imarpik) este situata la nord si separata de Oceanul Pacific de nord de catre Alaska si Insulele Aleutine. Acoperind mai mult de doua milioane de kilometri patrati, este marginita la est si nordest de Alaska, la vest de Siberia si Peninsula Kamceatka, la sud de Peninsula Alaska si de Insulele Aleutine si în nordul îndepartat de Strâmtoarea Bering care separa Marea Bering de Marea Chukchi din Oceanul Arctic. Numele sau este cel al primului explorator european care a navigat pe apele sale, navigatorul danez Vitus Bering.

În timpul celei mai recente ere glaciare, se crede ca nivelul marii era suficient de jos pentru a permite oamenilor si animalelor sa migreze din Asia în America de Nord pe jos, pe unde acum se afla Strâmtoarea Bering, localizata în partea nordica a marii. Acest loc este cunoscut ca "Trecatoarea Bering" si unii oameni de stiinta cred ca a fost locul prin care au ajuns în America primii locuitori.

Marea Bering este unul din locurile cele mai bogate în peste, si pescuitul în apele sale reprezinta jumatate din industria pescuitului din Statele Unite. Datorita schimbarilor climatice, evolutia climei si ecosistemului Marii Bering este nesigur. Aceasta este o problema în doua sensuri: schimbarile climatice influenteaza ecosistemele si ecosistemele servesc ca indicatori pentru schimbarile climatice.

Marea BeringDe la Wikipedia, enciclopedia liberă

Fotografie din satelit a Mării Bering

Marea Bering şi Oceanul Pacific de Nord

Marea Bering cu Peninsula Kamceatka şi Alaska

Marea Bering (sau Imarpik) este situată la nord și separată de Oceanul Pacific de nord de

către Alaska și Insulele Aleutine. Acoperind mai mult de două milioane de kilometri pătrați, este mărginită la est

și nordest de Alaska, la vest de Siberia și Peninsula Kamceatka, la sud de Peninsula Alaska și de Insulele

Aleutine și în nordul îndepărtat de Strâmtoarea Bering care separă Marea Bering de Marea

Chukchi din Oceanul Arctic. Numele său este cel al primului explorator european care a navigat pe apele sale,

navigatorul danez Vitus Bering.

În timpul celei mai recente ere glaciare, se crede că nivelul mării era suficient de jos pentru a permite oamenilor

și animalelor să migreze dinAsia în America de Nord pe jos, pe unde acum se află Strâmtoarea Bering,

localizată în partea nordică a mării. Acest loc este cunoscut ca "Trecătoarea Bering" și unii oameni de știință

cred că a fost locul prin care au ajuns în America primii locuitori.

Marea Bering este unul din locurile cele mai bogate în pește, și pescuitul în apele sale reprezintă jumătate din

industria pescuitului din Statele Unite. Datorită schimbărilor climatice, evoluția climei și ecosistemului Mării

Bering este nesigur. Aceasta este o problemă în doua sensuri: schimbările climatice influențează ecosistemele

și ecosistemele servesc ca indicatori pentru schimbările climatice.

Câteva insule din Marea Bering:

Insulele Pribilov

Insulele Komandorski , inclusiv Insula Bering

Insula St. Lawrence

Insulele Diomede

Insula King

Insula St. Matthew

The Bering Sea Ecosystem: The Big Picture

Introduction

One hundred years ago, biological exploration differed substantially from the research carried out today. The Harriman Expedition scientists described and collected animals, birds and plants, and made very good observations; however, the concept of an ecosystem was to come far in the future, and there was absolutely no understanding

that the complex biological structure of a region constantly changes in response to what we call environmental forcing functions -- which means climate, weather and ocean conditions. Even less that global processes have regional effects. This presentation differs, therefore, from the kind of descriptive presentation which we might have enjoyed on board the George W. Elder.

Map of the Bering Sea, showing the extensive shallow shelf and deep basin.Click image for a larger view.

The eastern Bering Sea shelf supports a diverse, abundant and highly productive marine biota. It is home to a rich variety of biological resources, including the world's most extensive eelgrass beds; at least 450 species of fish, crustaceans and mollusks; 50 species of seabirds; and 25 species of marine mammals. From the earliest days of its exploration, the potential for an immense harvest was recognized and exploited. Today, the U. S. Bering Sea fishery contributes over half of the nation's fishery production, mostly walleye pollock, but the Bristol Bay sockeye salmon fishery is a major component also, as has been the snow crab fishery; Dutch Harbor is the top fishing port in the United States.

In common with most of the rich fishing grounds of the world, the weather in the Bering Sea is often inclement. Calm, peaceful, warm seas are not very productive from the harvesting point of view. Situated between the North Pacific Ocean and the Arctic Ocean, the Bering Sea shares characteristics of each and is influenced by both. However, it is neither truly north temperate or arctic. The relationship between ocean conditions and the high marine biodiversity and productivity is my topic today. Why is this sea which lies between the Arctic Ocean and the North Pacific Ocean so rich in numbers and species at multiple trophic levels? Given current events, a second question, perhaps more important is "what are the effects of climatic and oceanic changes and variability on the biological regimes?"

Climatic and oceanographic conditions over the southeast Bering Sea shelf vary significantly from year to year. A dramatic "regime shift" occurred in the late 1970s, producing a major increase in temperature and a reduction in seasonal ice cover. The term "regime shift" means a clear jump from one state to another which is then

maintained. Along with these dramatic oceanic changes, there have been large changes at the top predator levels.

Today, we recognize that what we see now incorporates a long historic legacy of exploitation. For example, the current Steller sea lion problem can be understood when you realize that the young used to be harvested for their fur, and that, as the fishing intensified, the adults were shot by fishermen for interfering with their activity &endash; in other words, stealing fish. Recently, trawling has been banned in Steller sea lion critical habitat, with the closed area extending 20 miles offshore of about 40 sea lion rookeries and 82 haulouts in the eastern Bering Sea and Gulf of Alaska, plus three foraging areas, effective August 8, 2000. Subsequently, the political consequences have included attacks on a controversial biological opinion document prepared by the National Marine Fisheries Service. This is just one policy issue. Others include the effects of drastically reduced, although variable, runs of salmon in Bristol Bay and in the Yukon/Kuskokwim Delta area. These runs not only support commercial fisheries, but also subsistence use. One result has been a shortage of sled dog food in the area. One more example: Native subsistence hunters are very worried, saying that the gray whales, which use to congregate in certain areas, are now more dispersed. We believe that there has been a reduction in the ocean bottom productivity of benthic invertebrates in the shallow northern shelf region. The point is that we are seeing changes, but most of these changes are probably not due to short term human effects, but rather to natural cycles or long term effects via the global climate, as well as the historic ruthless exploitation.

The Bering Sea Environment

The Bering Sea is influenced by atmospheric and oceanic processes in the Arctic Ocean to the north and the North Pacific Ocean to the south. It shares properties of both. It is neither truly polar nor typically north temperate in character. The Bering Sea is the world's third-largest semi-enclosed sea, the wide eastern shelf makes up about half of its total area. Most of the shelf is extremely shallow, in many places less than 60 m in depth, whereas the basin is deep, exceeding 3,000 m. There are several huge undersea canyons which cut into the shelf at the edge.

Circulation of the Bering Sea, showing the major currents.Click image for a larger view.

The shelf is divided into a number of domains, separated by oceanic frontal systems, and each domain has distinct characteristics. The overall basin circulation comprises a huge cyclonic gyre, with a western boundary current. The Kamchatka Current, along the west side of the basin. Water from the Alaska Stream enters the Bering Sea through a number of passes in the Aleutian Chain, primarily Near Strait, but also Amchitka, Buldir, and Amukta Passes. This is largely balanced by outflow through Kamchatka Strait (Stabeno and Reed 1994).

High primary productivity (plant growth) over the shelf results from the northward movement of nutrient-rich water entering the Bering Sea through the passes from the Gulf of Alaska onto the outer eastern continental shelf (Stabeno and Schumacher, 1999). Once on the shelf, this water moves westward and then north, ultimately passing through the Bering Strait into the Chukchi Sea; its nitrogenous nutrient content is sufficiently high to preclude exhaustion as the water passes through the Bering Sea. Moving up onto the shelf, the water passes westward across the outer shelf as a slope current, and in the western Bering Sea splits into northward and southward components. Most of the northward flow passes to the west of St. Lawrence Island. The productivity of the southeast Bering Sea middle shelf, between 50 and 100m in depth, depends on nutrients transported into the area or regenerated in situ, and over this part of the shelf the northward flow is insufficient to compensate for utilization by the primary producers in spring and summer. Mesoscale processes such as upwelling and transport by eddies are important in nutrient supply . We suspect that the undersea canyons which run up into the shelf from the deep basin also are important. One result of this hydrographic regime is a belt of high productivity in summer along the outer edge of the shelf, the so-called "Green Belt", which lies seaward of the 160m isobath

The position and strength of the Aleutian low pressure system has a large influence on the wind direction and oceanic regime of the southeast Being Sea. One approach to looking at variability is the Pacific Decadal Oscillation (PDO), developed by Mantua et al. (1997) as a function of North Pacific sea surface temperatures. We must also consider the El Nino/Southern Oscillation (ENSO) events which occur periodically. Although equatorial controlled, these affect the subarctic seas through telecommunication, and as will be shown below, the 1997-1998 El Nino had a major impact on the southeast Bering Sea. The Arctic Ocean affects the Bering Sea, especially ice formation and transport.

Sea Ice and the Biological Regime

The duration and extent of sea ice depends primarily on atmospheric processes to the north. Sea ice affects spring phytoplankton production, especially the spring bloom, which at these latitudes signals the start of the biological season. The timing, composition and fate of this bloom is important. The ice extent in late winter can vary from 700 km south of the Bering Strait in a light ice year to 1,100 km in heavy ice years. Following the regime shift, there was a clear decrease in sea ice extent. The spring bloom is a major event inn high latitude systems--it signals the onset of spring, and its timing and productivity are important to the ecosystem.

The southerly extent of sea ice over the Bering Sea shelf in January, showing contrast between before and after the 1977 regime shift.Click image for a larger view.

In ice covered seas, as the ice begins to retreat, a layer of meltwater, lighter then the more saline water below, forms at the surface. In the absence of ice, it does not happen till the sun warms the surface, producing warmer less dense water. The spring phytoplankton bloom occurs in this surface water, where sunlight can penetrate and provide the energy bloom rapidly. Ice-elated blooms occur several weeks earlier that the thermal blooms, and spring is advanced. This is not trivial at these high latitudes!

Other Factors

Recently, especially in the last five years, we have had a period of unprecedented and rapid change in the Bering Sea ecosystem, with major die-offs of seabirds and declines in populations of some marine mammal species, as well as a severely reduced returns of salmon to the Alaskan Bering Sea coast. The causes of these changes are not as yet clear, but we do know that since the regime shift of the late 1970s, with its "step" increase in temperature, there has been a warming trend.

Walruses in the northern Bering Sea.Click image for a larger view.

Other significant ecological changes have been noted The shift in phytoplankton, proportionally although certainly not absolutely, from bloom-forming diatoms to small slow-growing flagellates would provide less energy and slower supply to the highest trophic level, especially mammals and birds. Nutrient supply and water column stability are the keys.

The population of a jellyfish, Chrysaora melamaster, has jumped at least 10-fold over the past decade. These are large jellyfish, which compete for food with young pollock, and also feed on them. They consume 5% of the annual crop of zooplankton, and 3% of new born pollock according to Rick Brodeur, who has been studying them in the Bering Sea. One area of especially high concentration has been termed "Slime Bank". Dr. Tim Parsons in a recent lecture presented upon his award of the Japan Prize, suggested that the surge of jellyfish could be due to the excessive removal of fish; he believes that the fact that fish-eating birds have decreased while plankton-eating birds have increased support this idea. However, given that jellyfish have very low energy requirements compared with fish (20 times less) and mammals (200 times less than whales) on a per unit weight basis, it could also be that they are prevailing due to the reduced nutrient/lower-energy plankton regime. Don Schell has shown a long-term change in the Bering Sea based on stable isotope analysis of historical archived baleen samples as well as modern; he concluded that there has been a change in productivity, although this could be a change either in productivity or in productivity regimes, from a larger proportion of fast-growing cells to slower growing.

The policy implications of all this are enormous. Forty million has been added to the current National Marine Fisheries Service budget to address the Steller sea lion issue. Why are they declining in their western range? What about the salmon returns?

One phenomenon that has amazed us is the bloom of Coccolithophrids which appeared over the shelf in 1997, but which has persisted each years since, probably as a result of the low nutrient warm conditions. This has never been described prior to 1997.

On the plus side, Pacific right whales have been seen in the Bristol Bay area over the past few years, after a long absence. While we have no idea how many there remain in the Pacific Ocean, we hope that the population might recover.

Bering Sea and Strait, Russian Beringovo More and Proliv Beringa,   northernmost part

of the Pacific Ocean, separating the continents of Asia and North America. To the north the Bering Sea connects

with the Arctic Ocean through the Bering Strait, at the narrowest point of which the two continents are about 53

miles (85 kilometres) apart. The boundary between the United States and Russiapasses through the sea and the

strait.

The Bering Sea roughly resembles a triangle with its apex to the north and its base formed by the 1,100-mile-long arc

of the Alaska Peninsula in the east; theAleutian Islands, which constitute part of the U.S. state of Alaska, in the

south; and the Komandor (Commander) Islands in the west. Its area is about 890,000 square miles (2,304,000

square kilometres), including its islands. The maximum width from east to west is about 1,490 miles and from north to

south about 990 miles.

The Bering Strait is a relatively shallow passage averaging 100 to 165 feet (30 to 50 metres) in depth. During the Ice

Age the sea level fell by several hundred feet, making the strait into a land bridge between the continents of Asia

and North America, over which a considerable migration of plants and animals occurred.

In addition to the Aleutian and Komandor groups, there are several other large islands in both the sea and strait.

These include Nunivak, St. Lawrence, and Nelson islands in Alaskan waters and Karagin Island in Russian waters.

Physical features

Physiography

The Bering Sea may be divided into two nearly equal parts: a relatively shallow area

along the continental and insular shelves in the north and east and a much deeper area

in the southwest. In the shelf area, which is an enormous underwater plain, the depths

are, in most cases, less than 500 feet. The deep part in the southwestern portion of the

sea is also a plain, lying at depths of 12,000 to 13,000 feet and divided by separate

ridges into three basins: the larger Aleutian Basin to the north and east, the Bowers Basin

to the south, and the Komandor Basin to the west. The sea’s deepest point, 13,442 feet

(4,097 metres), is in the Bowers Basin.

The continental crust is more than 12 miles thick along the shallow shelves and in the

Aleutian Islands. The thickness decreases in the slope areas, and in the deep part of

the sea the crust is 6 to 9 miles thick.

Enormous quantities of sedimentary material enter the sea annually from the land as a

result of erosion of the shore. Plant and animal life at the surface also produce

sedimentary material, but very little reaches the bottom, and consequently most of the

sediment on the floor of the sea is from the land. Along with a great deal of silica, the

bottom ooze holds a large quantity of boulders, pebbles, and gravel torn from the

shores by ice and carried out to sea. In the southern part, the sediments are rich in

material of volcanic origin.

Climate

Although the Bering Sea is situated in the same latitude as Great Britain, its climate is

much more severe. The southern and western parts are characterized by cool, rainy

summers with frequent fogs and comparatively warm, snowy winters. Winters are

extreme in the northern and eastern portions, with temperatures of -31° to -49° F (-35°

to -45° C) and high winds. The summers in the north and east are cool, with

comparatively low precipitation. Snow persists on the Koryak coast for as long as 8

months and on the Chukchi Peninsula for nearly 10 months, with a snow cover one to

two feet thick. The annual precipitation in the southern part of the sea is more than 40

inches (1,000 millimetres), mainly in the form of rainfall, while in the northern part the

precipitation is less than half as much and is mainly snow.

Mean annual air temperatures range from -14° F (-10° C) in the northern areas to about

39° F (4° C) in the southern parts. Water temperatures on the surface average from 34°

F (1° C) in the north to 41° F (5° C) in the south. The period without frosts lasts for

about 80 days in the northern part of the sea, where snow is common even in the

summer and maximum temperatures are only 68° F (20° C). In the southern area there

are nearly 150 days without frost, and the temperature seldom falls much below

freezing. January and February are the coldest months, July and August the warmest.

Severe storms caused by strong centres of low atmospheric pressure occasionally

penetrate the southern part of the sea.

Hydrology

Practically all of the Bering Sea water comes from the Pacific Ocean. The salinity of the

surface water is relatively low, 31 to 33 parts per thousand; in the deeper parts of the

sea the salinity increases to 35 parts per thousand near the bottom. In winter the

northern portion of the sea is covered with ice, and even in summer the water below the

surface retains a subfreezing temperature. The structure of the Bering Sea waters in

general is subarctic, characterized by the presence in summer of a cold intermediate

layer with warmer waters above and below. During the summer the surface water is

heated, but a considerable layer of water that was cooled during the winter remains cold

and is known as the cold intermediate layer. The maximum thickness of this

intermediate layer is about 475 feet in the northern part of the sea and as much as 280

feetin the south. Underneath this layer is one that is slightly warmer, below which lie the

colder bottom waters. In the northern and eastern shallow regions of the sea, only two

upper layers develop: surface water and a cooler intermediate layer.

Warm oceanic waters from the south enter the Bering Sea through the numerous straits

of the Fox Islands, through the Amchitka and Tanaga passes, and to a great extent

through the Blizhny Strait between Attu and Medny islands. The Attu, Tanaga, and

Transverse currents carry the warm water to the northwest. The Transverse Current,

proceeding along the Asian continental slope in the direction of Cape Navarin, branches in

two: one branch forms the Lawrence Current moving northward, and the other joins the

Anadyr Current, which in turn gives birth to a powerful Kamchatka Current that governs

the southward movement of the Bering Sea waters along the Asian coasts. Near the

Alaska coast the general direction of the water is to the north, a factor responsible for

the less severe ice conditions in that part of the sea as compared with the western part.

Some of the Bering Sea water passes through the Bering Strait into the Arctic Ocean,

but the bulk of it returns to the Pacific. The deep Bering Sea waters rise gradually to the

surface and return to the Pacific as surface waters. Thus, the Bering Sea is an

important factor in the general circulation of the northern part of the Pacific Ocean

waters. The rise to the surface of oceanic waters rich in nutrient salts gives the sea a

high biological productivity.

Economic aspects

Biological and mineral resources

The existence in the Bering Sea of the cold intermediate layer separating the deep

waters, which are rich in nutrient salts, from the upper photic layer (i.e., the layer

exposed to sunlight) results in two growths of floating plant life during the year. The first

growth occurs in the spring after the mixing of waters in winter, and the second during

the autumnal mixing, when the cold surface waters descend and the deeper waters

come to the surface while there is still sufficient sunlight for plant growth.

This floating plant life consists of some 160 species, of which the most common

are diatom algae. The largest concentration of diatoms have been found in the shallow

part of the sea. Diatoms are the principal producers of organic matter, and they are

consumed by small copepods (microscopic crustaceans), which in turn become the

food of fish and mammals. On the continental shelf there are vast quantities of mollusks,

echinoderms (particularly sea urchins and starfish), and barnacles. Also abundant on the

shelves are sponges, marine worms, and crustaceans. In the southern regions, down to

depths of 100 or 130 feet, populations of giant brown algae grow like forests on the rocky

bottom. There are about 200 species of algae, some reaching lengths of 200 to 300

feet.

The Bering Sea has more than 300 species of fish, including 50 deep-sea species, of

which 25 are caught commercially. The most important among them aresalmon, herring,

cod, flounder, halibut, and pollack. The islands are breeding grounds for the fur seal and

the sea otter. The northern areas are inhabited by thewalrus, seal, and sea lion. Several

whale species, notably gray whales, migrate to Bering waters to feed during the

summer. Intensive fishing in the last half of the 20th century has drastically reduced

some of the most valuable fish species, and this has led to greater exploitation of less

commercially valuable species.

Oil and gas deposits are believed to exist under the Bering Shelf and along the margin

of the Kamchatka Peninsula. The extent of potential reserves, however, is unknown.

Navigation

The Bering Sea is considered to be one of the most difficult bodies of water to navigate.

Winter storms are frequent and severe, often coating the superstructures of ships with

ice. Wave heights may exceed 40 feet. Added to these hazards are powerful tidal

currents in many parts of the sea and fog, rain, and floating ice in the north. In winter the

northern area is covered by ice fields about 4 or 5 feet thick, with hummocks in some

places more than 100 feet high. At its maximum extent in April, the ice reaches as far

south as Bristol Bay and the Kamchatka coasts. Melting begins in May, and by July there

is no ice in the sea except for drift ice in the Bering Strait. Nonetheless, the sea contains

important shipping routes for the Soviet Far East, including the eastern terminus at

Provideniya on the Chukchi Peninsula for the northern sea route to Arkhangelsk in the

west.

Study and exploration

The Bering Strait and the Bering Sea were first explored by Russian ships under

Semyon Dezhnyov, in 1648. They are named for Vitus Bering, a Danish captain who was

taken into Russian service by Peter the Great, in 1724. He sailed into the strait four

years later but did not see the Alaskan coast, although he discovered the islands of St.

Lawrence and Diomede. In 1730 the strait was charted for the first time by Mikhail

Gvozdev and Ivan Fyodorov. Bering sailed again in 1733, leading a large expedition

from St. Petersburg along the northern coast of Siberia, and he reached the Gulf of

Alaska in the summer of 1741. He reconnoitred the southwestern coast of mainland

Alaska, the Alaska Peninsula, and the Aleutians, but misfortune befell him, and he

perished in that year along with many of his men. In 1780, Russian merchants founded

a private company to trade in fur-bearing animals in northwestern America. A geographic

study of the Bering Sea was made at the end of the 18th century and was

supplemented later by hydrographic studies.

Deep-sea studies were begun in 1827 by British explorers. Extensive work was also

done by a U.S. group aboard the American research vessel Albatross in 1893–1906.

Since then the sea has been systematically studied by Soviet, American, and Japanese

investigators. Some of the most detailed study was undertaken by the Soviet

vessel Vityaz in a series of expeditions undertaken in the 1950s and ’60s.