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Map
of the Bering Sea, showing the extensive shallow
shelf and deep basin. |
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.
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Circulation
of the Bering Sea, showing the major currents. |
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.
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The
southerly extent of sea ice over the Bering Sea
shelf in January, showing contrast between before
and after the 1977 regime shift. |
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.
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Walruses
in the northern Bering Sea. |
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.
Selected Bibliography
Alexander, V. and H. J. Niebauer. 1981. Oceanography of the eastern Bering Sea ice-edge zone in spring. Limnol. Oceanogr. 26:1111-1125.
Baduini, C. L., K. D. Hyrenbach, K. O. Coyle, A. Pinchuk, V. Mendenhall, and G. L. Hunt. 2001. Mass mortality of short-trailed shearwaters in the southeast Bering Sea during summer 1887. Fish. Oceanogr. 10(1):117-130.
Brodeur, R. D., C. E. Mills, J. E. Overland, G. E. Walters, and J. D. Schumacher. 1999. Evidence for a substantial increase in gelatinous zooplankton in the Bering Sea, with possible links to climate change, Fisheries Oceanography 8(4):296-306.
Iverson, R. I., L. K. Coachman, R. T. Cooney, T. S. English. J. J. Goering, G. L. Hunt, Jr., M. C. Macauley, C. P. McRoy, W. S. Reeburgh, and T. E. Whitledge. 1979. Biological significance of fronts on the southeastern Bering Sea. In Ecological Processes in Coastal and Marine Systems (Robert J. Livingston, Ed.). Plenum Publishing Corp. pp. 437 &endash; 465.
Kruse, G. H.1998. Salmon Run Failures in 1997-1998: A Link to Anomalous Ocean Conditions? Alaska Fishery Research Bulletin 5(1):55-63.
Niebauer, H. J. and V. Alexander. 1985. Oceanographic frontal structure and biological production at an ice edge. Cont. Shelf Res. 4:367-388.
Niebauer, H. J., N. A. Bond, L. P. Yakunin and V. V. Plotnikov. 1999. An Update on the Climatology and Sea Ice of the Bering Sea. In: Dynamics of the Bering Sea. Thomas R. Loughlin and Kiyotaka Ohtani, eds. Alaska Sea Grant College Program, Fairbanks. Pp. 29-59.
Stabeno, P. J., J. D. Schumacher, R. F. Davis and J. M. Napp. 1998. Under-ice observations of water column temperature, salinity and spring phytoplankton dynamics: Eastern Bering Sea shelf. Jour. Mar. Res. 56:239-255.
Stockwell, D. A., T. E. Whitledge, S. I. Zeeman, K. O. Coyle, J. M. Napp, R. D. Brodeur, A. I. Pinchuck and G. L. Hunt. 2001. Anomalous conditions in the south-eastern Bering Sea, 1997: nutrients, phytoplankton and zooplankton. Fisheries Oceanography 10(1):99 - 116.
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For information on the Harriman Retraced Expedition e-mail: harriman2001@science.smith.edu |