9/97
Principal Investigators
J. D. Schumacher and P. J. Stabeno, PMEL
R. D. Brodeur and J. M. Napp, AFSC
G. L. Hunt, UCI.
Objectives
The objectives of our proposed project are to:
To attain these objectives, biophysical monitoring of the shelf and slope region was conducted during 3 seasons in the following manner: (1) hydrography from stations in the vicinity of Unimak Pass and from stations which span the Aleutian North Slope and Bering Slope Current; (2) hydrography across the shelf and along the 80-m isobath, and nutrient chemistry and plankton biomass at clusters of stations along those transects; (3) hydrography, nutrient chemistry and plankton biomass along four transects radiating from the Pribilof Islands.
Summary of Results
Observations of water properties and nutrients from the vicinity of Unimak Pass provided the basis for characterization of flow onto the southeastern shelf from both the Alaska Coastal Current via the pass itself (Reed and Stabeno, submitted) and from the slope water flowing along the north side of the Aleutian Island chain (Schumacher and Stabeno, 1996; Stabeno et al., in press). Given the limited definition in both time and space provided by the monitoring scheme, it appears that the flow of slope water onto the shelf via Bering Canyon provides a more consistent source of nutrients than does the flow through Unimak Pass. The critical issues are to determine the mechanisms that result in flow up the Bering Canyon and how these are connected to variations in the Aleutian North Slope Current, and the contribution of this source of nutrients to the nitrogen cycle on the southeastern shelf.
Observations of temperature and salinity versus depth were collected
six times from the slope/shelf CTD transect. Preliminary analysis of these
data revealed that in spring 1997 transport in both the ANSC and the BSC
was unusually large. Sections from both currents showed transports greater
than 6 x 10 6 m3 s-1 whereas transport
is typically 2-4 x 10 6 m3 s-1 . Moored current records from the ANSC revealed
a strong consistent flow, supporting ythr inference that flow in the ANSC
was steady this year. Whether or not the enhanced strength of these currents
affects shelf/slope exchange is not known; however, it is vital for understanding
the shelf ecosystem.
For the first time, time series of water properties, current and acoustic backscatter was obtained of the fall evolution from two layers to mixed condition over the middle shelf. Preliminary analysis reveals that well mixed conditions did not occur until late in November 1996. Because there are no other observations of this phenomenon, we cannot say how representative the timing is. The transition from two layers to mixed may be a factor in replenishment of nutrients. The climatological baroclinic flow eastward across the shelf from the vicinity of the Pribilof Islands likely does not exist when the water column is mixed. The later into the fall that this feature can transport nutrients, the greater the supply for the following spring.
The global atmospheric phenomenon that resulted in an El Nino at the equator also caused marked changes in the southeastern Bering Sea. Vast numbers of marine birds died this summer and salmon returns were far below expected. As with the El Nino, departures from the typical climatology were not marked during winter, but became greatly different by early summer. Sea ice conditions exhibited average coverage, latitudinal extent and time of melt-back. Satellite remote sensing provides the following sequence of sea surface temperature conditions over much of the southeastern Bering sea shelf: in early May temperatures were slightly below normal (~0.0 to -1.0°C), by mid-June the anomaly was strongly positive (2..0 -2.5°C above normal) and persisted through August, decreasing to < 1.0°C by mid-September. These changes resulted from local heat exchange with the atmosphere rather than advection of water from the Pacific Ocean.
Two factors contributed to an unusually strong and widespread stratification of the middle and inner shelf regions. Cloud cover was less than usual, with the resulting strong insolation adding to sea surface warming, and the shelf was characterized by extremely calm wind conditions during the spring and summer. This resulted in an early phytoplankton bloom, depletion of nitrogenous nutrients and subsequent settling of the bloom in early May. During the summer, a large portion of the shelf water had high reflectance as a result of a coccolithophorid bloom (high light extinction, but low levels of chlorophyll or particulate organic carbon). We may also have observed a shift in the zooplankton community relative to other years (cf. Napp et al., submitted, Brodeur et al., submitted,a) with a decrease in copepods and an increase in pteropods. The temporal and spatial extent of the bloom is being examined using in situ and satellite image data. Further examination of our plankton and juvenile fish samples will reveal if (and how) this anomalous bloom affected the food web and the diets and condition of juvenile pollock (Brodeur et al., submitted,b). The opaque waters may also have affected foraging of the resident seabirds and been partly responsible for high mortality of adult birds observed this summer.
In 1997, the numbers of foraging seabirds recorded south of St. George Island appeared reduced compared with records from the 1980s. This may have reflected a shift in foraging grounds. Preliminary inspection of data from the 1980s suggested that the numbers and foraging activity of picivorous seabirds changed between years in which reproductive success of birds on St. George differed. In 1987, a year in which seabirds had low reproductive success, considerable numbers of birds foraged to the southwest of St. George Island. In contrast, in 1988, a year in which reproductive success was high, more birds were observed foraging, and more birds foraged to the southeast of St. George Island, over the head of the Pribilof Canyon. The reduction in the numbers of birds south of the Pribilof Islands in 1997 may indicate a shift in the local availability of forage fish.
Results from the ongoing National Science Foundation study of the Inner Front near Nunivak Island revealed that the front there was not as well developed as previously reported. The water in the coastal domain was more stratified than typical because unusually weak winds were not able to mix the water column. This may have effectively shut-down inner front dynamics which normally supply nutrients to the upper layer. The extent of nutrient depletion north of St. Paul, south of St. George Island and across the outer shelf between the Pribilof Islands and Unimak Pass was noticeable. Likewise, preliminary estimates of biomass of chlorophyll were lower than expected.
Equipment from moored platforms and one platform itself was lost during 1996/97. This is the first large loss since we began our Bering Sea research programs funded by the Coastal Ocean Program (1992). In 1997, due to equipment failures, we were unable to accomplish our planned deployment of acoustic Doppler current profilers at sites 2 and 3 (for a continuous time-series of currents and zooplankton backscatter, a proxy for zooplankton biomass). Next year we will be without any instruments at site 3 and without subsurface mooring at site 4, unless lost or broken equipment is replaced.
Recommendations
1. The program needs to replace lost equipment if monitoring at the present level is to be maintained.
2. the capability to monitor dissolved nitrate and nitrite concentration from the moorings is necessary to examine biophysical coupling of nutrient dynamics.
3. The importance of the ANSC/BSC to the supply of nutrients on the outer shelf requires more extensive monitoring. We recommend deployment of satellite tracked buoys near mooring site 6.
4. An additional two to four days of surveys south of St. George Is. would be desirable to examine how changes in flow up the Pribilof Canyon affect the distribution of juvenile pollock and the birds that forage on them. Additional nutrient and water property samples would also permit examination of the potential transport of nutrients onto the shelf in this region. The later issue is an objective of an ongoing Arctic Research Initiative (ARI) study of the Green Belt. Our work could be coordinated with the ARI field operations and both programs would benefit.
Products
Brodeur, R.D., M.T. Wilson, and L. Ciannelli. Spatial and temporal variability in feeding and condition of age-0 walleye pollock in frontal regions of the Bering Sea. Ices J. Mar. Sci., (1997b) Submitted.
Brodeur, R.D., M.T. Wilson, J.M. Napp, P.J. Stabeno, and S. Salo. Distribution of juvenile pollock relative to frontal structure near the Pribilof Islands. Alaska Sea Grant AK-SG-97-01, 1997b.
Hunt, G. L., Jr. and G. V. Byrd, Jr. Climate change, carrying capacity and marine bird populations of the eastern Bering Sea. In: The Bering Sea: Physical, Chemical, and Biological Dynamics. Loughlin, T. R., and K. Ohtani (eds.), Alaska Sea Grant Press, submitted.
Napp, J.M., R.D. Brodeur, D. Demer, R. Hewitt, P.J. Stabeno, G.L. Hunt Jr., and J.D. Schumacher. Observations of nekton, zooplankton and seabird distributions at tidally-generated shelf fronts in the eastern bering sea. Mar. Ecol. Prog. Ser., submitted (1997).
Reed, R. K., and P. J. Stabeno. Magnitude and time scales of inflow through Unimak Pass, Alaska. Geophys. Res. Lett. submitted.
Schumacher, J.D., and P.J. Stabeno. The continental shelf of the Bering Sea. In: The Sea, Vol. XI. The Global Coastal Ocean: Regional Studies and Synthesis. John Wiley, Inc. New York. in press.
Stabeno, P.J., J.D. Schumacher, R.F. Davis, and J.M. Napp. Under-ice observations of water column temperature, salinity and spring phytoplankton dynamics: Eastern Bering Sea shelf, 1995. J. Mar. Res. in press.
Stabeno, P.J., J. D. Schumacher, K. Ohtani, and S. Gladychev. Overview of the Bering Sea. In: The Bering Sea: Physical, Chemical, and Biological Dynamics. Loughlin, T. R., and K. Ohtani (eds.), Alaska Sea Grant Press, in press.