Physical modeling studies are needed in the Bering Sea U.S. GLOBEC program to
diagnose and project the effects of climate change on physical features (e.g.,
currents, vertical structure, ice), to generate velocity and scalar (e.g.,
temperature, salinity) fields as a complement to field investigations, and to
drive spatially-explicit biological models of the Bering Sea ecosystem. To
serve such purposes, a model must have sufficiently small horizontal spacing to
resolve inflows and outflows through passes, flows along submarine canyons that
cut across the shelf, and mesoscale eddies which range in size from 20-200 km
(Schumacher and Reed, 1992). Vertical resolution must be sufficient to allow
decoupling of flow from topography under stratified conditions, to permit the
development of appropriate shears when flow is baroclinically unstable, and to
at least partially resolve boundary layers at the top and bottom of the water
column. Ice must ultimately be included, or at a minimum parameterized as a
surface buoyancy forcing, to generate the cold pool in appropriate years. Tides
or at least tidal energy must be included in modeling efforts on the Bering Sea
The domain for the physical model is the eastern Bering Sea shelf, extending out
into the Bering Sea basin, and including at a minimum, the Bering Strait, Unimak
Pass, and Amukta Pass. The inflow from Amchitka Pass must also be considered.
Grid spacing should be approximately 5 to 50 km, varying by location. Finest
resolution should be employed near the Pribilof Islands, because of their
special biological significance, and near Unimak Pass, to better capture the
detailed structure of that inflow. Fine vertical spacing would be needed near
the sea surface.
Knowledge of the temperature, salinity and velocity fields is crucial for
understanding multiple trophic levels. Primary production depends on the
interplay of light, predation, advection and vertical mixing, and varies widely
in space. Both vertical and horizontal advection and diffusion supply
nutrients, whereas excessive vertical mixing can deprive phytoplankton of
Secondary production is in turn strongly dependent on the magnitude of this
primary production. Species at higher trophic levels, such as pollock, can be
strongly affected by the circulation field as spawned individuals are advected
to food-rich or food-poor environments. Hydrographic features such as the cold
pool may act to segregate predators from their prey (e.g., adult pollock from
The physical models described above can be coupled with a suite of biological,
biophysical and ecosystems models. Development of biological models should
occur concurrently with development of the physical model. Four types of
biological or biophysical models are recommended (See below). Linking outputs
from each of these models will allow the examination of ecosystem level
questions regarding top down or bottom up controls in determining pelagic
production in the Bering Sea.
In addition to the above, specific biological models, such as one concerning the
bioenergetics of juvenile stages of key fish species, and fine-scale biophysical
models, such as one focusing on predator-prey interactions at fronts around the
Pribilof Islands would be useful to examine areas of critical importance to the
- A spatially-explicit, individual-based model (IBM) of the early life
stages of key fish species (e.g., walleye pollock and/or sockeye salmon smolts).
This model should be coupled with a physical model, and eventually, with both
lower and upper trophic level models (see below). Such a model could be used to
examine hypotheses relating biotic (e.g., zooplankton prey production,
predation, cannibalism) and abiotic (e.g., temperature, salinity, advective
patterns, the role of the cold pool and fronts, climate change) factors to the
population dynamics of key species.
- A lower trophic model, a spatially-explicit
Nutrient-Phytoplankton-Zooplankton (NPZ) model, should be developed to
investigate questions of primary and secondary production as they relate to
physical forcing and control by upper trophic level processes. As mentioned
above, this model should also be coupled with the IBM.
- A spatially resolved multispecies model of upper trophic levels, which
focuses on adult pollock and sockeye salmon, their predators and competitors.
This model would be useful in investigating hypotheses regarding bottom-up and
top-down control of ecosystems processes, including the central issue of
cannibalism as a major controlling factor in pollock population dynamics. This
model may use physical forcing by the hydrodynamic model, lower trophic level
input from the NPZ model, and may be coupled with the IBM in order to track the
early life stages of key fish species.
- An aggregated ecosystems model, which includes basic trophic components and
a simple spatial structure. This simplified model would be intended primarily
as an exploratory tool. Questions about the factors that might lead to shifts
in the broad ecosystem structure of the Bering Sea, about net production
("carrying capacity") of the systems, and about the effects of climate change on
ecosystem structure could be addressed with this type of model.