Advection and Food Webs in the
Western Arctic: Retrospective Sample and Data Analysis and Modeling
Sharon L. Smith, Nasseer Idrisi
and Peter Lane
Part I:
Retrospective Studies
The
present study focuses on the influences and effects of large bodied copepods
on the food chain, carbon cycle and biological energy budgets in the western
Arctic. The region of study includes
the northern Bering Sea, the Chukchi Sea, the western Beaufort Sea and the
Canada Basin adjacent to the Chukchi and Beaufort Sea continental shelves.
In
the Chukchi Sea, the wide shelf is influenced by water from the central Bering
Sea, which flows northward and mixes with Alaskan Coastal Water (Coachman and
Shigaev, 1992), and water from the Arctic Ocean. The circulation of the Chukchi Sea is not fully understood
(Coachman and Shigaev, 1992), but there are many fronts and areas of
interleaving of warm and cold water (Paquette and Bourke, 1979), troughs in the
seafloor are implicated in steering warm water flow (Paquette and Bourke, 1974,
1981), and upwelling or flow up canyons onto the shelf may occur (English and
Horner, 1977). There is no doubt
that some flow from the Arctic Ocean onto the shelf of the Chukchi Sea occurs
because species of Arctic Ocean plankton are found on the shelf of the eastern
Chukchi Sea from Bering Strait to Pt. Barrow (Johnson, 1958; English and
Horner, 1977; Kulikov, 1992; Zaitzev, Polischuk and Alexandrov, 1992; S. Smith,
unpublished data). Only one investigation
found essentially no taxa of Arctic Ocean origin on the Chukchi shelf (Springer
et al., 1989). This study revealed the important role
of the Anadyr Water, coming from the slope of the Bering Sea, in transporting
large-bodied copepods whose life cycle includes a period of diapause (Neocalanus plumchrus, Neocalanus cristatus, Eucalanus
bungii) onto the western side of the
Chukchi shelf. The transport of
zooplankton through Anadyr Strait has considerable variability, declining 75%
during the summer of 1985 for example (Springer et al., 1989).
Since transport and biomass of large bodied copepods are directly
related, delivery of the large copepods, which are crucial for a pelagic food
web, to the Chukchi shelf is highly variable. Large variability in the processing of carbon in the Chukchi
ecosystem would result. The
transport through Shpanberg Strait had little variability, but since it carried
smaller bodied copepods, input from Shpanberg Strait would have less impact on
the carbon cycle of the ecosystem.
We
have utilized zooplankton samples and data collected on several research expeditions
conducted over the past several decades including the Beaufort Sea expeditions
in 1950 and 1951; Processes and Resources of the Bering Sea (PROBES) in 1980
and 1981; Inner Shelf Transfer and Recycling (ISHTAR) in 1985 and 1986; and
a Science of Opportunity (SOO) cruise in 1998 (Figure
1). Samples and data collected during the
Beaufort Sea Expeditions proved to be the most useful for the present study
because sea surface temperatures were several degrees warmer in 1951 than
in 1950, allowing us to contrast biological observations made from the same
region under different environmental conditions.
Our
analyses of historical samples and data have focused on the large calanoid
copepod species associated with the Bering Sea (Neocalanus spp.) the
Chukchi Sea (Neocalanus spp. and Calanus marshallae) and the
Arctic Basin (Calanus hyperboreus and C. glacialis) (Figure
2). Although these species
are not always numerically dominant, their relatively large body sizes compared
with the relatively small, though often more abundant, small copepods (e.g.,
Oithona and Pseudocalanus) (Figure
2), causes them to dominate the zooplankton community in terms of biomass.
Historical samples and data indicate that the large calanoid copepods
Neocalanus cristatus, N. plumchrus and/or N. flemingeri
are advected north onto the Chukchi shelf and east along the continental shelf
from the Chukchi Sea to the Beaufort Sea (Figure
3& Figure
4). For example, during the Beaufort Sea expedition in 1951, Neocalanus
sp. stage 5 copepodites were collected over the shelf north of Point Barrow
(Figure
3). Late stage copepodites
of the arctic species C. hyperboreus were found co-occurring with Neocalanus
sp. in this region, indicating a mixture of water masses
with sources in the sub-arctic Bering Sea and the Arctic Ocean (Figure
3).
Preliminary
comparisons of the abundance and distribution of Calanus hyperboreus
and Neocalanus sp. collected during the Beaufort Sea expeditions in 1950 and 1951 indicate that the processes controlling
their distributions were different in each of the two years. Sea surface temperatures from COADS 2
degree enhanced GrADS images averaged over the July to September periods for
each of these two years (NOAA-CIRES/Climate
Diagnostic Center; http://www.cdc.noaa.gov/) imply that 1951 was a warmer
year in this region than 1950 (Figure
5). The likelihood of reduced
ice-cover and increased northerly flow from the Bering and Chukchi Seas associated
with the warmer temperatures in 1951 could explain the apparent decrease in
abundance of C. hyperboreus and the presence of Neocalanus sp. near Pt. Barrow that year (Figure
3 & Figure
6). Note also that the abundance of Calanus sp. (possibly C. marshallae) C-5 copepodites was far greater over the northern
Chukchi shelf in 1951 than in 1950 (21 m-3 vs. 0.1 m-3
respectively; Figure
6). This observation could
be the result of increased northerly transport of the Chukchi Sea population
of C. marshallae in the warmer
year compared with the colder year.
Comparisons
of the abundance of Neocalanus plumchrus collected during the Inner Shelf Transport and Recycling (ISHTAR) project
during summers of 1985 and 1986 in the northern Bering and Chukchi Seas (Turco,
1992) were also made (Figure
7). In general, the abundance
of N. plumchrus was greater in collections made in this region this
region in 1986 than in 1985. This
difference may be the result of increased northward flow of Anadyr water during
1986, or a result of sampling earlier in the summer in 1986 than in 1985,
reflecting a decrease in abundance as the summer season progresses (Springer
et al., 1989).
Sea
surface temperatures from COADS 2 degree enhanced GrADS images averaged over
the July to September periods for 1985 and 1986 (NOAA-CIRES/Climate
Diagnostic Center; http://www.cdc.noaa.gov/) imply that 1986 was a warmer
year in the region than 1985 (Figure
8). In addition to comparing
sea surface temperature, we compared satellite derived monthly ice concentrations
for July, August and September of 1985 and 1986 (NSIDC;
Figure
9). These comparisons showed
less ice cover in 1986 than in 1985, implying a warmer year.
These conditions might also be favorable for increased northerly transport
of large copepods originating in the Bering Sea in 1986.
Summary
1) Comparisons of summer SST data from 1950 and 1951 and SST
and ice cover data from 1985 and 1986 indicate that the Chukchi and western
Beaufort Seas were warmer in 1951 and 1986 than in 1950 and 1985
respectively. These years were
chosen for comparison because of the availability of zooplankton samples and
data from those time periods.
2) Zooplankton data from the Beaufort Sea Expeditions
suggest that there was a greater influence on shelf water biology of the
Chukchi and Beaufort Seas by large calanoid copepods from the Bering Sea during
the warmer year, 1951.
3) Zooplankton data from the southeastern Chukchi Sea during
the summers of 1985 and 1986 show that the maximum observed abundances of Neocalanus
plumchrus were
greater in 1986 than 1985. This
may be an indication of comparatively more copepods being advected north from
the Bering Sea during the warmer year when there was relatively less ice cover.
Implications for field work
1) Our
observations point to the need to address interannual variability with research
cruises, which span a period of several years.
2) From the
biological perspective, plankton data indicate a highly variable environment
across relatively small spatial scales, requiring several multiday sampling periods
over small geographical regions to resolve and quantify small-scale
variability.
3) Regional
studies must include intensive sampling of the shelf break regions to quantify
zooplankton advection on and off the shelves of the Chukchi and western Beaufort
Seas as well as along shelf transport of various species.
4) Biological sampling should include a molecular biology component to resolve questions of closely related sympatric species (Neocalanus plumchrus and N. flemingeri; Calanus marshallae and C. glacialis) which may co-occur and share resources on the shelves of the Chukchi and Beaufort Seas.
References
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The
work presented here is funded by the National
Science Foundation Office of Polar
Programs under grant number OPP9815682 to Sharon Smith.