The goal of monitoring studies is to acquire observations of physical, chemical
and biological aspects of the environment to investigate interannual variability
over an extended period of time. These time-series serve several purposes: 1)
they provide the observational basis to develop indices either directly related
to zooplankton success or to the success of their predators and prey; 2) they
allow comparisons among habitats and years; and 3) they provide an environmental
data base complimentary to modeling and process oriented studies.
The first objectives are to establish those aspects of the environment which
indicate or significantly influence the status of zooplankton populations; and
identify where critical or pulse-points of the system exist. This can be
achieved using existing knowledge of the ecosystem augmented by retrospective
and modeling studies.
In the southeastern Bering Sea, physical processes and topography results in
several domains that provide different conditions/habitats for zooplankton and
thereby are candidates for monitoring:
Instruments on existing moored platforms can provide single or multiple point
time-series of atmospheric and oceanographic parameters, including: downwelling
irradiance, wind, air and water temperature, salinity, nitrate, currents,
calibrated acoustic backscatter, and detritus. Satellite-tracked buoys and
shipboard surveys expand the limited spatial scale of moored current,
temperature and ocean color observations. The addition of efficient underway
sampling during the ongoing annual bottom-trawl and tri-annual hydroacoustic
surveys of the eastern shelf conducted by the AFSC provides an excellent
opportunity to enhance the Bering Sea monitoring effort. Close coordination
with ongoing Japanese research provides similar opportunities.
- The Bering Slope Current (BSC) and its upstream source, the Aleutian
North Slope Flow (Reed and Stabeno, submitted) provide the seaward boundary of
the shelf and bathes the outer shelf with heat, salt, nutrients and plankton.
Further, this is the region where high primary (~365 gC m-2 yr-1: NRC report)
and secondary production (64 gC m-2 yr-1) result in the feature known as the
"Green Belt" (Springer and McRoy, 1996) (Fig. 7). The BSC originates as a flow
along the north side of the Aleutian Island chain (Stabeno and Reed, 1994: Reed
and Stabeno, 1994; Schumacher and Stabeno, in press). This region north of the
Aleutian Islands is biologically important because salmon occupy this region
during their summer feeding migrations. Transport through Amukta Pass augments
this flow, influencing the subsurface thermal environment (Reed 1995) and
generating eddies that can contain a high abundance of larval pollock
(Schumacher and Stabeno 1994).
- The Pribilof Islands provide a natural laboratory to examine many aspects
of the ecosystem. The nearshore structural front (Schumacher et al., 1979) and
to a lesser degree the more seaward middle front are a focus for higher trophic
level organisms (Springer, 1993; Kinder et al., 1983). Coincident variations in
physical and biological characteristics are a marked characteristic around the
islands (Napp et al., 1995; Brodeur et al., 1996).
- The Outer Shelf has features distinct from both slope and middle shelf
habitats: for example, water column structure, amount/duration of ice cover,
plankton community and carbon flux dynamics (Cooney and Coyle, 1982).
Interannual variations in ice cover as great as 40% (Niebauer 1988) occur.
Ice-edge melt initiates a phytoplankton bloom and an estimated 10-65% of the
annual production can occur during melt-back (Niebauer et al., 1995). Winds,
directly related to convective cooling, ice formation and transport, also have a
profound impact on low-frequency horizontal kinetic energy (Schumacher and
Kinder, 1983) and vertical mixing. Nitrate-uptake exhibits a nonlinear
relationship to wind induced mixing; the timing of storms relative to the phase
of the production system (i.e., respiration or nutrient limited period) is
critical (Sambrotto et al., 1986). While ice and wind are dominant features of
the entire shelf, they exhibit differences among the shelf habitats north and
south of the Pribilof Islands.
- The Middle Shelf has a carbon cycle that tends to enrich the benthos
rather than being utilized by the pelagic community as occurs over the outer
shelf; there is a 10-fold larger infaunal biomass found here than on the outer
shelf (Cooney and Coyle, 1982; Walsh and McRoy, 1986). Cooling by convection,
ice cover and the resultant cold pool exert a momentous influence on the
ecosystem, with impacts noted from primary production to distribution of adult
fish (Niebauer et al., 1995; Wyllie-Echeverria, 1995; Ohtani and Azumaya, 1995).
- The Unimak Pass region is dominated by flow of Alaskan Coastal Current
(ACC) water through the pass (Schumacher et al., 1982) which forms the major
shelf-to-shelf connection between the Gulf of Alaska and the Bering Sea and
provides a sizable fraction of the net northward flow through Bering Strait
(Schumacher and Stabeno, in press). This region is a locus of pollock spawning
(Hinckley 1987) and a pathway for fish and marine mammal migrations to and from
the Bering Sea. The flow of slope water onto the shelf here provides a second
source of nutrients and biota (Napp et al., 1996) that influence this habitat
and may be important throughout the outer shelf habitat.