Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 28
The Bering Sea Ecosystem: Geology, Physics, Chemistry, and Biology of Lower Trophic Levels MARINE GEOLOGY The Bering Sea is framed by the Seward and Chukchi peninsulas on the north, by the Kamchatka Peninsula on the southeast, and a 1,900 km long ridge and island chain that comprise the Aleutians to the south and southwest (Figure 3.1). Geographically, the Bering Sea lies between 52° and 66° north latitude, and 162° east and 157° west longitude. The narrow, 85 km long passage of the Bering Strait connects the Bering Sea on the north to the Chukchi Sea and the Arctic Ocean. This subarctic Bering Sea lies in the northern part of the Pacific Basin. It exchanges water with the Arctic Ocean (through Bering Strait) and with the Pacific Ocean, into the Bering Sea from the Gulf of Alaska through the Aleutian Islands and into the northwest Pacific Ocean through Kamchatka Strait. The Bering Sea covers almost 3 million km2 and is unusual in having an extremely wide continental shelf, ranging from 500 km wide in the southeast region to over 800 km wide in the north. The Bering seafloor is partitioned into a series of sedimentary basins, the principal of which are (1) the Aleutian Basin, just north of the Shirshov Ridge, Bowers Bank, and Aleutian Islands; (2) Komandorsky (Commander) Basin, adjacent to the Komandorsky Islands, Kamchatka Peninsula, and Shirshov Ridge; (3) Bowers Basin, enclosed by the arm of Bowers Bank; (4) Anadyr Basin, encompassing the Gulf of Anadyr; (5) Chirikov Basin, adjacent to the Chukotka Peninsula and Bering Strait; (6) Norton Basin, between Alaska's Seward Peninsula and Yukon-Kuskokwim delta; (7) Bristol Basin, between the Alaska Peninsula and mainland, and (8) the Beringian Shelf, which encompasses the remainder of the continental shelf from Chukotka Peninsula, St. Lawrence Island, and Alaska mainland southwestward to the extensive submarine canyon system described below (Hood and Kelley, 1974; Sharma, 19771. The Bering Sea region shelf is unusual from the global perspective in being extremely smooth and generally featureless (Figure 3.2), with the exception of three large and some small islands. Its gradient is among the gentlest in the world (0.24 m/km), with bathymetry less than 200 m deep and a very steep continental margin (Sharma, 19771. This continental shelf is incised by seven of the largest submarine canyons in the world. From north to south, they are Navarinsly, Pervenets, St. Matthew, Middle, Zhemchug, Pribilof, and Bering canyons. The abyssal Aleutian Basin, which lies at depths of between 2,800 and 3,600 m, constitutes the southwest portion of the Bering Sea and is adjacent to the Aleutian Islands, Komandorsky Islands, and Kamchatka Peninsula. Two submerged mountain chains, the Bowers and Shirshov 28
OCR for page 29
Geology, Physics, Chemistry, and Biology of Lower Trophic Levels ~- 9'~, / Aorta Boy |,g~ Pa t~-McNeil ~ a ~ R I N G S ~ ~ t"~. Allison Basin · ~I ~ ~-~ ~ __ ~ _ a 29 ~0 ~I ~ ~e~ e ~ ~e ~ ~e ~ ~- ~ ~- ~ ~ ~- ~- '' P£ ~: > A R C l I C 0~~ ~ ,, 1 [~W ~ -1 I,__ ~ ,: ~ ~yaw ~_-'/ ~ \ ~S e a ~-~ / ~ I / ~ _ - ,pluTors~, if_ ~y,-~ronofskyBd\t \~ / ,,' Isles ~ \1 V, 1 en. 1 ~ Clonic ,- Ws`1 ~ Ad* O. 200 400 600 Ken ALEUT. AN ~. a Figure 3. 1 Physiographic features of the Bering Sea seafloor (adapted from Hood and Kelley, 1974).
OCR for page 30
30 :. ::: ; .i The Bering Sea Ecosystem .... _' ~ At.. _ ~.,~,,,,,,,~,,.~.~ ......... .. :. ~ Figure 3.2 A side-scan sonar image of the Bering Sea seafloor (EEZ-SCAN Scientific Staff, Atlas of the U.S. Economic Zone, Bering Sea, U.S. Geological Survey miscellaneous investigations series 1-2053 1991~.
OCR for page 31
Geology, Physics, Chemistry, and Biology of Lower Trophic Levels 31 ridges, divide this basin into the separate Aleutian, Bowers, and Komandorsky basins (Cooper et al, 1987a). The Bering Sea became separated from the rest of the Pacific Basin through evolution of the Aleutian Island arc during the Eocene. This arc of islands with active volcanoes was established at the current site of continental drift and collision, where the northward-moving Pacific is subducted beneath the North American Plate. Preceding the formation of this island arc, the Beringian margin was the site of plate subduction, with fault block basins forming to the southwest in response to tectonic forces. The rocks found in the Alaska-Bering Sea-Siberia region appear to have moved several thousand kilometers northward during the Mesozoic era and were accreted in their approximate current locations by the Paleocene epoch. Bedrock geology of the offshore areas of the Bering Sea is not well known; only a few rock samples have been collected or drilled, leaving seismic reflection geophysical data to provide basic information. The Aleutian Basin is believed to be underlain by Mesozoic oceanic crust, and has been a center for sediment deposition of between 2 and 9 km since formation of the Aleutian Island arc. Deposition in smaller basins, such as the Bowers and Komandorsky, consists of a thinner sedimentary section of between 1 and 3 km. Bedrock underlying the Beringian Shelf is comprised primarily of Paleozoic, Mesozoic, and Cenozoic sedimentary rocks, and Tertiary and older volcanic rocks. Formation of the submarine canyon system of the Beringian margin occurred in Cenozoic glacial periods during low stands of sea level (Carlson and Karl, 1988). For more detailed discussions of the geologic evolution and history of the Bering Sea region, see Bogdanov and Neprochnov (1984), Carlson and Karl (1988), Churkin (1972), Cooper et al. (1991, 1987a, 1987b), Dundo and Lopatin (1989), Grantz et al. (1970a, 1970b), Hood and Kelley (1972), Lisitsyn (1966), Marlow et al. (1976), Scholl et al. 1966, 1968, 1978), and Sha~ma (1977). Sediments The configuration of the Bering Sea continental shelf, margin, and abyssal basins, in association with weather patterns of the region, influences the basal physical oceanographic conditions present in the Bering Sea. Circulation models indicate that tides and the Bering Slope Current flow to the northwest paralleling the continental slope, although they are complicated by countercurrents and eddies developed near the canyons (Kinder et al., 1975). The breadth and size of the continental shelf contribute to the huge storm waves of the Bering, which are capable of moving sediment at outer shelf and slope depths. Principal unconsolidated seafloor sediments are gravels, sands, silts and clays derived from previous glacial action, river erosion and deposition, and ice rafting. Again, the shelf is very flat, except for a few basins and banks, with transitory swales and ridges, channels, and depressions. Sediments of the partially enclosed bays, such as Bristol Bay, Norton Sound, and Chirikov Basin, and areas adjacent to islands of the shelf have more sands and gravels than clays, but clays and silts predominate on the deeper shelf. Accumulations of organic materials in shallow sediments of Norton Sound, Chirikov Basin, and elsewhere in the northern Bering Sea have undergone chemical transformation into natural eases. Gases ranging from methane to carbon dioxide have been found to seep into the
OCR for page 32
32 The Bering Sea Ecosystem water column at a variety of seafloor locations either derived from circular blowout pits or along faults. It has been postulated that the gas has a proclivity for release during storms when shallow sediments are disturbed by wave-action. These sites can be extensive: 7,000 km2 in Norton Sound alone are underlain by areas of potential shallow gas pockets (Larsen et al., 19801. The effects of gas seepage on marine life in the Bering Sea, positive or negative, are unknown at this time. Sediments deposited annually in the Bering Sea are derived primarily from the Yukon River (~S to 100 million metric t), with lesser amounts from the Kuskokwim (4 million t) (Sharma, 1977) and the Russian coast. Plumes of sediment from the rivers draining the Russian ~ these plumes concentrate and Alaskan coasts are easily seen in satellite photography, and suspended sediment load near the coastal regions. Sharma (1977) details the mechanisms of ice rafting of sediment: fine sediments are apparently incorporated during ice formation from suspended sediment in water and resuspension of bottom sediment by storm waves in shallow water; ice-rafted clay, silt, and biogenic materials drop into the water column after the winter months along the receding ice edge margin-amounts have not been quantified. Bottom sediments derived from the Alaskan mainland rivers and coastal erosion are deposited on the seafloor and then swept by northward currents toward the Bering Strait. Sedunents from the Russian coast deposited on the Bering Shelf are swept northward and eastward toward the Bering Strait as well. Materials derived from the Aleutian Islands and Kamchatka Peninsula are mostly deposited into the abyssal basins. Figure 3.3 shows sediment distribution on the northern Bering Sea shelf. Coastal Geography Geography of the coastal areas bordering the Bering Sea is strongly influenced by the geologic forces that shaped and are shaping the region: the relentless erosional forces of the Bering itself and the subarctic climatic conditions. Alaska Regional Profiles (undated) contains detailed maps and descriptions of the coastal regions of Alaska, while Landscape Atlas of the USSR (Plummer et al., 1971) profiles aspects of the Russian coast. The local topography and resultant water, surface materials, and vegetation affect the presence of local and regional biota. The southern border of the study region is bounded by the Aleutian TslancIs a chain of volcanic islands. many of which are still active. driven bv tectonic forces. The islands extend more than 1,100 miles (1,770 km) and consist of more than 50 islands, in five groups, separating the Bering Sea from the northern Pacific Ocean. The Aleutian and Shumagin islands are low mountains with steep to moderate slopes and rolling topography. Plateaus and uplands occur in some places in the chain. Elevations of the islands range from sea level to nearly 5,000 feet (1,524 m). A good number have wave-derived terraces up to 600 feet (~83 m) above sea level, and are bordered by lower sea cliffs from previous sea level stands. Generally broad and flat intertidal platforms derived from glacial period sea level changes surround some islands. Those islands with peaks higher than 3,000 feet (914 m) were heavily glaciated and include fjords extending up to 2,000 feet (610 m) into the sea. Frequent lakes occur in ice-derived basins on islands showing signs of glaciation, while streams move water in high-gradient descent
OCR for page 33
Geology, Physics, Chemistry, and Biology of Lower Trophic Levels 66 ° ' Cat S E WA R D -9 650 64° - 630 172° 170° 168° 166° ~/,<3 C O A R SE ~3 GRAVEL S A N D Y G R A V E L GRAVE LY SAND 33 S I L T Y SAN D _ _; CLAYE r _ SILT Figure 3.3 Sediment distributions of the Bering seafloor based on data from Sharma (1977) and Hood and Kelley (1974~. to the ocean. Where volcanoes have erupted in the recent past, the presence of ash, cinders, or lava has either killed local vegetation or provided a poor substrate that is resistant to revegetation for a number of years. The Pribilof Islands are five small islands in the Bering Sea that lie 200 miles (322 km) north of the Aleutian island Unalaska. St. George, one of two populated islands in the group, has hills and ridges with steep cliffs rising up to 900 feet (274 m), whereas St. Paul has a rolling plateau with some extinct volcanic peaks. The islands of St. Matthew, Pinnacle, and Hall are located in the Bering Sea north of the Pribilofs and approximately 220 miles (324 km) west of mainland Alaska. These islands have volcanic ridges separated by valleys; highest elevations of approximately 1,500 feet (458 m) are associated with volcanic cones. Shorelines of these islands are mostly quite steep, and St. Matthew has some lagoons and freshwater lakes. Islands of the Bristol Bay area of Alaska have extensive glacial outwash and morrainal deposits at the base of volcanic islands at their cores. These sediments are bisected by streams and small lakes, and include beaches with sand dunes. Other parts of the southwestern Alaska coast have unconsolidated gravel, sand, silt, and clay of glacial origin. Lowland deltas and
OCR for page 34
34 The Bering Sea Ecosystem coastal plains have formed in the material, with extensive beaches, spits, and offshore bars dunes, and islands. Permafrost permanently frozen ground, remaining frozen at least one winter to the next-is found throughout much of Alaska and the Russian Far East, with depths of less than 1 m up to 610 m (2,000 feet) or more bordering the Arctic Ocean. In general, permafrost is mostly absent in the Aleutians and Alaska Peninsula, discontinuous in the area south of the Kuskokwim River (60° north latitude), and continuous to the north along the coast. Similar depths have been reported in Russia, with the southern permafrost limit found in the south- central Kamchatka Peninsula, then stretching north to the Bering Strait; the highest content of ice is found in the deposits of the northern coastal plains and islands of Novosibirsk, where ice may constitute 10 to 15 percent of the soil at depths up to 10 m (30 feet). Permafrost limits rooting depth of plants. Where an active layer that thaws in summer and refreezes in winter may be present in the permafrost, the melting results in small pools of standing water conducive to marsh and tundra development and accumulation of peat deposits. Certain kinds of features are formed in permafrost environments that border the Bering Sea coast including polygonal ground, stone nets, thaw lakes, and beaded drainage (Ferrians et al., 1969). Small areas of high brush, spruce-hardwood forest, and spruce-poplar forest occur only along drainages of the major and some medium sized rivers that drain toward the sea (the Kuskokwim and Yukon, for example). High brush may be found along the largest of the Aleutians in southerly-facing mountain slopes primarily bordering the Pacific Ocean. The southwestern part of Alaska is one of the most seismically active areas of North America and the world. Over the last 100 years, nine Alaskan earthquakes have been recorded with magnitude of 8 or greater on the Richter scale, with a large number of additional earth- quakes registering 7 or more. The results of these earthquakes include faulting, cliff collapse, landslides and slumping, changes in water drainage patterns, tsunamis with their resultant wave- based erosion and destruction, underwater slumps, and increased water turbidity (Eckel, 1970). The most recent destructive earthquake in the region (magnitude 7.5) struck the Kamchatka Peninsula in June 1995. Volcanoes and volcanic ranges border nearly the entire Bering Sea coast. Nearly 30 active volcanoes are found in the southwestern part of Alaska, almost all of which have been active since 1760 (Alaska Geographic, 1976). Kamchatka has 127 volcanic cones, of which approximately 15 are active (National Geographic, 1994). Volcanic eruptions result in earthquakes, magma and ash flows into coastal and marine areas, land and water temperature induced changes, changes in local and regional substrates, and increased ocean turbidity in the vicinity and downwind of volcanic ash or magma eruptions. The impacts on local and regional flora and fauna include die-offs from smothering or heat effects, and migration due to changed ecological conditions. In addition, the mountain ranges formed by the volcanoes, especially those along the Russian coast, intercept the landward flow of air from the Pacific, causing localized high precipitation zones. The Yukon delta consists of tidal flats and lowlands with lakes, ponds, and tundra, and also some highlands, such as the Cape Romanzoff Bluffs. The river deltas of the Yukon and Kuskokwim are significant because they provide a major nesting area for waterfowl, including geese, ducks, and swans. Raptors are not common in tundra habitats, but they are found in cliff The cliffs, such as those at Nunivak, also provide suitable areas near rivers and the ~ermg Lea.
OCR for page 35
Geology, Physics, Chemistry, and Biology of Lower Trophic Levels 35 habitat for seabird rookeries (for kittiwakes, murres, puffins, and gulls). Passerines are less abundant on the deltas than shorebirds, and leave by mid-September. The beaches, sea cliffs, tundra, and willow-lined water courses bring in many smaller birds, including Old World species. The extensive beach environments, ranging from rocky to muddy, attract intertidal fauna that are food sources for large populations of birds and marine mammals. Sandy beaches are particularly important for harbor seals, whereas Steller sea lions are found along the rocky coast. Fur seals prefer the coasts of the Pribilofs. Sea otters inhabit the shallow waters along the islands, where offshore reefs and kelp beds provide safety and food. Erosion of beaches and coastal marshes generally occurs at a slow rate across southwestern Alaska. However, storms can severely affect local or regional areas through storm surges and waves, as undercutting of cliffs and bluffs by waves results in collapse and cliff retreat. The fallen materials become available for redistribution in the sea or down current along the coasts. PHYSICAL OCEANOGRAPHIC STRUCTURE Physical Characteristics and Bathymetry of the Northern North Pacific and Bering Sea The circulation of the northern North Pacific and Bering Sea, seen in a global context, serves to transport heat and freshwater poleward. It also replenishes the nutrients in the surface layers to support biological productivity. Changes in this transport system affect heat, salt, and food supply for the Bering Sea ecosystem. The Basins The northeast Pacific Ocean contains the Gulf of Alaska, a feature bounded on the northern and eastern sides with a moderately wide (100 km) shelf. It has relatively free communication to the south and west. Water depths are in excess of 4,000 m; the deepest feature is the Aleutian Trench, in close proximity to the Aleutian Island arc (Figure 3.4). The Aleutian-Komandorsky Island arc forms a semipermeable boundary between the Gulf of Alaska and the Bering Sea. The ability for water exchange between the North Pacific and Bering Sea depends on the water depth in the passes. Since there are no deep passes east of 180° W. deep water exchange is restricted to the western side of this boundary. Gulf of Alaska Circulation Deep ocean circulation for the Gulf of Alaska consists of a large (1,000 km) cyclonic (counterclockwise) gyre that advects warm water northward along the British Columbia coast and southeast Alaska. In its northward flow off the British Columbia shelf break, the current
OCR for page 36
36 The Bering Sea Ecosystem 6~ 6Z 6C Sb sly t~J6 t 160-E 164'£ 16~t 1~£ 176~£ IRO I?6-W 172~W 168-W 4i4_ 160~ IS6~ IS2~W ~_ ^ ~if ~IAVARtN Hi ~'!~ ~'~ ~ -~ ,'..~-.f ST LAWRENCE ISLAND A L ~ 5 K A 4 ~I I ~1 1 1 1 · ' ME '7rw AL~rI~'V I?~5/W L ~rKA Has BLAND ElJ lSLANO ~Q~ , . . . , . , , . , . , 176'E ~176~W 1 Figure 3.4 Bathymetric map of the Bering Sea (Sayles et al., 1979). ISLAND OAK , .,,, ~ PRIM .'t ,': LAVA ISLANDS <~/~ 5 ~ ~ ~ I : l ~I \ 172-w ~W 164-W Go S. is wide and unorganized (Tabata, 19821. As it is diverted westward at the apex of the gulf, it forms a western boundary current and becomes a much more coherent flow. The currents have speeds of 25 to 75 cm s~i and transports on the order of 10 to 30 Sverdrups (Sv) (! Sv = ~ million m3 so) (Dodunead et al., 19631. The current continues southwestward along the shelf break in the western Gulf of Alaska, closely following topography, until it reaches a longitude of about IS0° W. Here, it tends to continue on a zonal, or sometunes a southwestward, trajectory (Thomson, 19721. The northern North Pacific is the terminus for the world ocean's deep circulation. Deep water formed in other high-latitude regions of the North Atlantic and Southern Ocean reaches the Gulf of Alaska after traveling for centuries. Gulf of Alaska deep waters are therefore some of the oldest waters in the world's ocean and have very high nutrient concentrations. Unlike most other high-latitude regions, the North Pacific is not a site for the formation of deep water. The reason for this absence of deep water is the relatively low density surface water that is created by high rates of precipitation and runoff in the region. The northeast Pacific has been thought of as analogous to an estuary (fully and Barber, 1960), with the halocline and accompanying pycnocline serving as a cap on the ocean in the North Pacific and Bering Sea. The deep ocean circulation is accompanied by a very active coastal circulation. High rates of precipitation over the Gulf of Alaska and coastal freshwater discharge around the
OCR for page 37
Geology, Physics, Chemistry, and Biology of Lower Trophic Levels 31' ,7 perimeter create a low-salinity surface layer that contributes to a cyclonic flow over the shelf, especially adjacent to the coast (Reed and Schumacher, 1986). On average, more than 23,000 m3 s~t of terrestrial freshwater discharge enters the coastal region, creating a buoyancy-influenced alongshore flow in the Gulf of Alaska that is constrained along the coast by the cyclonic (downwelling) wind system. Current speeds in excess of 175 cm s-i have been reported for this flow called the Alaska Coastal Current (Royer, 1982). The width of this current is generally less than 20 km. It flows westward in the northern Gulf of Alaska and along the Aleutian Island arc to Unimak Pass, where a portion of it enters the Bering Sea (Figure 3.5). This coastal flow continues around the Bering Sea in a cyclonic sense, until it flows through Bering Strait into the Arctic Ocean. The exchange of water between the North Pacific Ocean and Bering Sea through the passes in the island arc is quite uncertain. Best estimates are that there is an outflow through Kamchatka Pass (21.0 Sv) (Arsen'ev, 1967) and inflow through Near Strait (14.4 Sv), with smaller inflows through Kiska, Buldir, and Semichi passes (West Aleutian group) of 0.7 Sv and inflows through the Central Aleutian group (Tanaga and Amchitka passes) of about 4.4 Sv. There is also a net inflow below 3,000 m through Kamchatka Strait. New observational results on inflow into the Bering Sea show that flow through Amukta Pass is larger than previously expected (-1 Sv). This inflow is comparable to the flow measured through the much deeper Amchitka Pass. The flow through Amukta Pass is particularly important as a source of warm (>4° compared to -3.5° C), subsurface (-200 m) water in the southeastern Bering Sea basin. This flow is intermittent, however. The balance of all of these flows includes a net outflow through Bering Strait of about.0.8 Sv, which is quite certain (Coachman and Aagaard, 1988). Thus, there is something less than 5 percent net throughflow in the Bering Sea; it is extremely important, however to global freshwater flux. Bering Sea Circulation and Hydrographic Structure Flow descriptions for Bering Sea circulation have usually been based on inferences from water properties (Figures 3.5 and 3.6), though Stabeno and Reed (1994) use drifter climatology. There is a general cyclonic flow within the basin, with an intensified western boundary current, the Kamchatka Current, and a northwestward-flowing eastern boundary current associated with the eastern continental slope. The eastern boundary current flow is characterized as weak and variable, typical of eastern boundary currents elsewhere (Schumacher and Reed, 1992). There are two circulation features along this slope, one being the large, persistent salinity front, and the other being the Bering Slope Current, which transports about 5 Sv of wafer northwest along the shelf edge at speeds in the range of 10 to 20 cm~' s~i (Kinder et al., 1975, 1986), although the speed in winter may be slower (Royer and Emery, 1984). The current intensifies as a western boundary current as it bifurcates and travels north around the Gulf of Anadyr and through Bering Strait, and south along the Kamchatka Peninsula (Kinder et al., 1986). The physical basis of the Bering Slope Current is poorly known. Kinder et al. (1975) have suggested that it is driven by planetary waves. There is theoretical and observational evidence that it exists-at times-as a series of eddies, or that it includes eddies, some possibly as permanent features that are generated by topographic interactions, baroclinic instabilities, or
OCR for page 38
38 The Bering Sea Eco~ystem ~ ~ ~ : ~ ~ . ^~;~: :': ', ; :W, ~ ~c~ Water SY~em AG Anedyr Gult w.~, AS Alask~n Streem BSW Bering Ses water CA Convective ares CW Coa~tel watsr IFA Ics forming eres NS Norton Sound water _ . ~; Figure 3.5 Bering Sea circulation schemes with water masses (Takenouti and Ohtani, 1974~. /~.'' ~'': ~R~ RUSSIA: .. . . : ,,/~; ~ ~ ~1 1 ,~ 1 . :.: .. :. ':,-'-'-< ' ~.~2~''.'',U.S.'' ' ~ ~. - ~L~ o w>. .,: \ ~I . ~*- ~ C, ~I J -.,> ~_-~ ~! ~\- - B~stol ~y -~.~ /k . , _,N,: ' ~_ ~ _ ~ I J ~ ~ ~ 1 60°E 165° 170° 175° 180° 1 75°W 170° 165° OC-I~ 60° 58° 56° 540 s2O _ ~ 5oo Figure 3.6 Schematic of flow through the passes of the Aleutian-Komandorski chains (Stabeno and Reed, 1994~.
OCR for page 61
Geology, Physics, Chemistry, and Biology of Lower Trophic Levels 28F t tuE 24 E 22 I O 18 ° 16 C, 14 tar 12 Z 10 8 29 10 40 4 26 T 33: - 21 29 ~ ~ r7 is I I ~ :~] 61 2t it' ~ · May - Oct 0 Jan - Mar °119641 '65 1 '66 1 '67 1 '68 l.6-9-r-~70 1 '71 1 '72 1'73 1 .74 1 '7s 1 '76 1'77 1 '78 1 '79 ~ '80 1 '81 1 '82 1 '83 1'84 1 '85 YEAR Figure 3.16 Index of phytoplankton production in the central North Pacific (observations of integrated chlorophyll a in the CAP). Bars indicate the 95 percent confidence intervals of the mean x = r | (:n; the number of observations is shown above each bar. Winter values (open squares) and values before 1968 are excluded from our analyses. Definitions: t, 97.5 percentile of t statistic with n - ~ tiff;;, the variance of the observations (Venrick et al., 19871. 260 S 200 60 001 ~O1 . o 0 60 100 160 200 A o o ~ - o . . .- ., . . - a . - Ekman TV Report Figure 3.17 Relationship between wind stress (Ekman transport) at 60° N. 149° W in the northern Gulf of Alaska and yearly zooplankton biomass values for the periods 1956-62 (circles) and 1980-89 (squares) (Brodeur and Ware, 1992~.
OCR for page 62
62 The Bering Sea Ecosystem high, in spite of only moderate primary production, because of inefficient grazing by the herbivorous zooplankton community. In the northern Bering and southern Chukchi, Benthic carbon supply results from prodigious primary production that far surpasses the ability of the zooplankton to control. The latitudinal gradient in Benthic biomass mirrors the high production on the northern shelf (Ficure 3.181. . Benthic infaunal biomass in the southeastern Bering Sea averages 1 to 10 g C m~2 in the inner domain, 1 to 20 g C me in the middle domain, and 1 to 11 g C m~2 in the outer rlomain (Table 3.3; Figure 3.19; Feder et al., 1980; Haflinger, 1981; Stoker, 1981). ~. . . _ It is lowest along the Alaska coastline in the northern Bering and Chukchi seas, where it is limited by ice gouging, river runoff, high current scouring, and low overlying water production (Figure 3.20; Grebmeier et al., 1988, 1989; Stoker 1981). Benthic biomass increases to the west to 20 to 40 g C me southwest of St. Lawrence Island and in regions of the Gulf of Anadyr (Table 3.3; Figure 3.21; Sirenko and Koltun, 1992; Grebmeier, 1993; Grebmeier and Cooper, 1995). The infaunal community structure is generally dominated by bivalves and polychaetes (Grebmeier, 1993; Stoker, 1981), although it becomes dominated by amphipods in the Chirikov Basin, north of St. Lawrence Island (Table 3.3), with biomass ranging from 10 to 30 g C m~2 (Figure 3.20; Feder et al., 1985; Grebmeier et al., 1988, 1989; Highsmith and Coyle, 1992; Stoker, 1981). Infaunal biomass peaks at 50 to 60 g C m~2 in the southern Chukchi Sea, where a downstream deposition of organic materials occurs (Grebmeier and McRoy, 1989; Grebmeier, 1993). Epifaunal benthos on the eastern Bering Sea shelf (Figure 3.19) is numerically dominated by bivalves, then arthropods and echinoderms; however, nearly 70 percent of the total epibenthic biomass represents sea stars (Jewett and Feder, 1981). The most important epifauna on the southeastern shelf include four species of commercially harvested crabs: king crabs Paralithodes camtschatica (red) and P. platypus (blue), and the Tanner (snow) crabs Chionoecetes opilio and C. bairdi (Figure 3.22). In comparison, sea stars (Asterias amurensis, Evasterias echinosoma, [eptasterias Polaris acervata, and L. nanimensis) dominate in the northeast Bering Sea. Limited studies of fauna of the shelf break and deep basin in the eastern portions of the Bering Sea indicate the Benthic fauna is dominated by polychaetes of low biomass, about 78 g wet wt me, or less than 5 g C me (Grebmeier, 1993; Sirenko and Koltun, 1992). Benthic populations in the basin are limited by depth and the reduced carbon supply to the bottom. Few studies are available in western literature related to the western Bering Sea, but it is reasonable that the high primary production reported for this region, similar to the upper estimate for production in the northern branch of the green belt, would directly influence Benthic faunal populations and sediment processes on underlying shelf systems that it traverses. Petersen and Curtis (1980) hypothesized that polar regions exhibit a stronger benthic-pelagic coupling than temperate and tropical areas, and a number of field studies have confirmed this hypothesis (e.g., Grebmeier, 1993; Grebmeier and McRoy, 1989; Stoker, 1981). Regions of high overlying water column production in the Bering and Chukchi seas have a direct influence on underlying Benthic biomass (Grebmeier, 1993; Grebmeier et al., 1988; Rowe and Phoel, 1991). Additional studies support these patterns of direct coupling of water column production and underlying benthos in the southeast Bering Sea (Rowe and Phoel, 1991), and on the continental shelf of the Barents Sea, where sediment bacterial growth rates were higher where carbon deposition was greater than in higher-latitude regions (Pfannkuche and Thiel,
OCR for page 63
Geology, Physics, Chemistry, and Biology of Lower Trophic Levels 35 30 Z 25 CD L) I' 20 0 y cat to In ~10 z - z o 57 5 8 59 60 6 1 62 1 1 1 1 1 ran r ~ \~ / rid m I 1 ~ 63 64 65 66 67 68 69 70 71 72 LATITUDE (°N) 63 Figure 3 . IS Variation of benthic biomass with latitude on the Bering and Chukchi sea shelves (vertical lines indicate standard deviation and brackets indicate the coefficient of variation around the mean) (Stoker, 1981.)
OCR for page 64
64 The Bering Sea Ecosystem Table 3.3 Comparison of phytoplankton and zooplanDton production, benthic biomass, and sediment oxygen uptake rates for different regions on the continental shelves of Bering and Chukchi seas Phytoplankton Zooplankton Primary Secondary Benthic Sediment Oxygen Production Production Biomass Uptake Rates Area (g C m~2Y-~) (g C m-2 ye) (g C me) (mmol O. me d-') Southeastern Bering Sea Inner domain 50-80 (1) - <10 (2) 3-8 (3) Middle domain 166 (1) 8-30 (4, 5) 10-20 (2, 6) 3-8 (3) Outer domain 162 (1) 30-50 (4, 5) < 10 (2, 6) 5-10 (3) Oceanic domain 50 (7) 20 (4) < 5 (2) Northern Bering Sea Alaska Coastal Water 80 (8) 5 (9) <10 (10, 11) <10 (11, 12) Bering Shelf Water/ 80-480 (8, 13) 9 (9) 10-30 (10, 11) 10-30 (11, 12) Anadyr Water Southern Chukchi Sea Alaska Coastal Water 80 (8) - < 10 (11, 12) < 10 (11, 12) Bering Shelf Water/ 470-720(8, 14) - 10-60(10, 11) 1040(11,12) Anadyr Water Northern Chukchi Sea "Southern group" 50-100(15, 16) - 1-11 (17) 5-10 (11, 18) "Northern group" 50-100(15, 16) - 2-20 (17) 5-10 (18) Source: Modified from Grebmeier et al. (1995).
OCR for page 65
Geology, Physics, Chemistry, and Biology of Lower Trophic Levels 180v ·"c ,70 165~ 160~ . . _ t.~4, c, ~ ~ ', ~ - . ~, \_ ~ . - 1 _~' m) ~ 6, _ C,p''~ _ ,. 53~ ~ _ TOTAL INFAUNAL BIOMASS S.E. 13er~ng Sea l (g/m: 1 1 · < 50 50 ~ ~ < 1 1 00 c C) < 1 50 150 C C) < 200 200 C 2~ ~; C: t80 ~175' 170° 165 160° ~. _ 1 \ ''\, ~ ~\ _ ~/ ' - . i~ ~W . `` . ~_ :~^ . ~, ~ __ ,, C'' 56 53 ~o _ _ ~ _ . _ TOTAL EPIFAUtVAL 810MASS S.~. Bertng Sea Biomass tgim: · <3 3c. ~ c6 6C (: <9 gc 2 . ~ lo ~ oN ~ ' 1 ~ ° °l ° o ;31 7; i. ~~ _,=~ _~_ _~ 165' Sg-' 53 53 160° Figure 3.19 Total infaunal and epifaunal biomass in the southeastern Bering Sea (Haflinger, 19811. 65
OCR for page 66
66 The Bering Sea Ecosystem KEY · 1984-1986 o 1988 Benthic biomass go m-2 · 1-10 · 10-20 ~20-30 · 30-40 40-50 50-60 Q CHUKOT PENINSULA )~ ~ O ~ o a a o a , .. .. 180 176 _ . CHUKCHI SEA . · · o o o O O -69 O ·., ) ~ it'. ~ Cat · ·. ~-68 O. ·2 O --. o -67 ~ ~ O ,,.,.'' /~ ·. ~ /~ At- ~ Cam ALASKA 0 ~ ·; ~ · ~.. · . ·. · . · 0 Q. - J ~~ . frontal zone 0 · ~ ...' O .'. ~ . BERING SEA ~ , ~/ 170 165 -66 -65 -64 -63 -62 -61 Figure 3.20 Distribution of macrofaunal benthic biomass on the shelves of the northern Bering and Chukchi seas (Grebmeier, 1993~.
OCR for page 67
Geology, Physics, Chemistry, and Biology of Lower Trophic Levels 65o N ~4 63 62 61 'I ~ RUSSIA ~> GUM OF ANADYR Matthew I. . - ~·. ^v Benthic biomass USA -2 (gC m ) 1 -5 5-10 10-15 1 5-20 30-40 40-50 . 60 ~unn~ak I BERING SEA 59° , . . . . . . . N ~ 3: ~` ~0 0 <' 67 Figure 3.21 Distribution of macrofaunal biomass on the northern shelf of the Bering Sea, June 1990 (Grebmeier and Cooper, 1995).
OCR for page 68
68 The Bering Sea Ecosystem 65 60 55' 65' In 180° 170° 160° .:> A, ... ;i' W.'. MS~: ~ 'a I!ql~ndc t~ 160° 180 170 160 ~^' Is. ~ A,.... Norton i: . Sound ~ ~+ / ~')J Matthew ~J is. ~ TANNER CRAB (c. bairdi} Nunivak Is. l l 170° 160° 65 65° 60' 60° s5c 55o ~,]~ Ad'' Hi.; . 170 ~160° fiRo 60° All o TANNER CRAe rc. ODi/io) 160° Figure 3.22 Distribution of king and Tanner crabs in the eastern Bering Sea (vertical lines indicate areas of high abundance) (Otto, 1981)
OCR for page 69
Geology, Physics, Chemistry, and Biology of Lower Trophic Levels 69 1987). However, it is not known whether temporal patterns of vertical flux (continuous versus pulsed) give rise to different benthic communities in polar systems. In the northern Bering and southern Chukchi seas, benthic standing stock is correlated with wafer column production (Grebmeier, 1993; Grebmeier and McRoy, 1989). Highest values occur under the Anadyr Water and Bering Shelf Water in regions influenced by high water column production, low zooplankton grazing, and presumably high carbon flux of phytoplankton to the benthos. This conclusion is also invoked for the relatively high benthic biomass occurring in the middle domain of the southeastern Bering Sea (Haflinger, 1981; Jewett and Feder, 1981). Sediment respiration experiments on shallow continental shelves provide an indication of organic carbon deposition to the benthos, which in the "hot spots" of the northern Bering and southern Chukchi seas ranged from 20 to 30 mmol O2 m~2 d-i, or an average carbon requirement of 0.5 to 1.0 g C m~2 do. These values are similar to that obtained in a sediment trap experiment in the Chirikov Basin, where carbon flux averaged 0.5 g C m~2 d-i in an area where sediment respiration rates indicated an average benthic requirement of 0.48 g C m~2 d-i (Fukuchi et al., 1993; Grebmeier and McRoy, 1989). Sediment oxygen uptake under the waters near the Alaska coastline in both the Bering and Chukchi seas indicates reduced carbon flux to the benthos, with values averaging less than 10 mmol O2 m~2 d-i (Grebmeier, 1993; Grebmeier and Cooper, 1995). Sources and Magnitude of Variability Benthic populations provide a long-term, integrated value for processes occurring in the overlying water column. Both physical and biological parameters directly influence benthic populations. For example, changes in current transport and water mass conditions influence water temperature, nutrient influx, and oxygen concentrations to various regions of the Bering Sea ecosystem, thus influencing subsequent water column production and potential food supply to the sediments and underlying benthic fauna. Changes in physical transport mechanisms also influence sediment transport and deposition; sediment composition determines benthic community structure, whereas food supply drives benthic biomass (Grebmeier et al., 198S, 1989). Sediment processes can provide indications of variability in the water column. Sediment oxygen uptake rates reflect short-term, seasonal carbon supply to the benthos. Cycling of organic carbon back into inorganic carbon dioxide and nutrients and/or organic compounds (e. g., dissolved organic carbon) can influence both overlying and downstream production in the system. Therefore, any climatic or human-induced changes that effect the ecosystem will have long-term impacts on benthic populations and carbon turnover on these shallow shelves; the actual magnitude is as yet unknown. The green belt regime, described in the previous section (see Figure 3.15), extends along the shelf break of the entire Bering Sea continental shelf and across the northeastern shelf through Bering Strait and into the Chukchi Sea. Sediment carbon is higher on the slope than in the shelf system (Lisitsyn, 1966), and it is proposed that high production in the green belt may account for this accumulation. Benthic production reaches its highest levels in both the Bering and Chukchi seas under the northern branch of the green belt in regions influenced by nutrient-rich Anadyr Water (see Table 3.3). If changes in the Bering Sea ecosystem were to occur that directly influenced green
OCR for page 70
70 The Bering Sea Ecosystem belt production, whether via natural or anthropogenic effects at either primary or secondary production level in the water column, one would expect a direct impact on the carbon being deposited to the benthos and subsequent impact on underlying benthic populations. Changes in benthic populations, particularly fauna that are a food source to commercially harvested demersal fish and crabs, could have a Tong-term impact on these fisheries that are currently so productive in the Bering Sea. Marine mammal populations (e.g., gray whales, walruses, and bearded seals) that feed on the productive benthic fauna could also be affected. Studies of benthic infauna populations have been done since the early 1930s in the Bering Sea (Grebmeier and Barry, 1991; Sirenko and Koltun, 1992, and references therein), but few have been extended long enough to investigate the population dynamics that are needed to differentiate~natural environmental impacts from anthropogenic ones. However, a few studies over multiple years do indicate possible ecosystem-level changes and are especially informative because they occur in the northern Bering Sea, where commercial fishing pressure is minimal. Variability in benthic macroinvertebrate epifauna (crabs) is discussed in the following chapter. A recent three-year study of the dominant ampeliscid amphipods in the northern Bering Sea, one of the most productive benthic communities in the world (Grebmeier et al., 1988; Highsmith and CoyTe, 19901, found a decrease in production from 1986 to 1988 (Hi~hsmith and _ . , Coyle, 19921. The authors indicate, however, that further long-term studies are needed to determine whether this decrease in amphipod production is due to natural predation cycles by benthic-feeding gray whales or to a long-term climate trend (Coyle and Highsmith, 19941. Their modelling results predict that perturbations in predation on the amphipod populations will have long-term effects (tens to hundreds of years) because these invertebrates are sIow-maturing species, a common characteristic of high-latitude organisms. The large species of this amphipod community, Ampelisca macrocephala, requires a high carbon flux to the detrital poo! it utilizes for food, and with low predation rates, which limits its occurrence to the northern cold, productive waters (Highsmith and Coyle, 19911. The modelling study by Coyle and Highsmith (1994) indicates that any decrease in carbon flux would be detrimental to such large species, shifting the population to smaller species that would outcompete the large amphipods, thus changing the benthic community structure. In addition, the low bottom temperatures of the northern Bering Sea exclude most bottom-feeding fish (Bakkala, 1981; lewett and Feder, 1980~; thus, any shift to higher temperatures would extend their range and could increase predation pressure by fish, gray whales, and crabs on these amphipod populations. Another issue is the apparent spatial shift and population decline of a bivalve community, dominated by nuculanid, nuculid, and tellinid bivalves, south and southwest of St. Lawrence Island in the northern Bering Sea. The single most common bivalve, NucuZana radiata, is a surface deposit feeder associated with finer silt sediments, which is consistent with the higher total organic carbon (and lower C/N values) observed in these surface sediments (Grebmeier and Cooper, 1995~. Although benthic sampling in this region has not been extensive, apparent shifts in species dominance over the past few decades suggest that other processes are influencing the biomass and community structure of these Bering Sea benthic communities. Abundant benthic populations and high biomass have been documented near the St. Lawrence Island polynya and westward into the Gulf of Anadyr, including both infauna (Feder et al., 1985; Grebmeier, 1993; Grebmeier et al., 198S, 1989; Ne~man, 1963; Sirenko and
OCR for page 71
Geology, Physics, Chemistry, and Biology of Lower Trophic Levels 71 Koltun, 1992; Stoker, 1981) and epifauna (Jewett and Feder, 1981). The dominant benthic fauna in the Gulf of Anadyr region until the late 1980s were reported to be the common arctic tellinid bivalve Macoma calcarea and the nuculid bivalve Nucula belloti. Since then there has been a change in the dominance and distribution patterns of the major bivalves, however with the surface deposit feeding N. radiata (a more cold-water tolerant species than Macoma) moving west into the Gulf of Anadyr (Grebmeier, 1993; Grebmeier and Cooper, 1995; Sirenko and Koltun, 1992). Both N. belloti and M. calcarea declined in abundance and were observed at more westerly locations (Sirer~ko and Koltun, 1992). In contrast, the Nuculana community was the same in 1960 as in the 1930s, and then started to expand in the late 1970s and early 1980s, reaching its current population structure and position in 1988 (Sirenko and Koltun, 1992). Sirenko and Koltun (1992) suggest that the change in bivalve community dominance in the Gulf of Anadyr may be due either to a northward shift of cold water since the 1930s or to preference by the dominant bivalve fauna for a specific sediment type or associated chemical composition (Nuculana prefers a muddier sediment regime than Macoma). Grebmeier and Cooper (1995) hypothesize that variable transport conditions, such as a decrease in the size of the Gulf of Anadyr gyre or the intensity of circulation around the pyre, could have influenced carbon supply rates to the benthos, leading to increased settling of finer-grained sediments over a larger area, or at least further to the west. A continuing decline in both abundance and biomass values of the dominant bivalve species and associated benthic fauna in the St. Lawrence island Polynya and Gulf of Anadyr region may indicate that a larger, ecosystem-scale change is responsible (Grebmeier and Cooper, 1995, unpublished data). Studies over both interannual and decadal time scales are required to elucidate factors influencing observed changes in benthic populations in the northern Bering Sea.
Representative terms from entire chapter: