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Prolonged Biological Production, Ocean Color and Eddies in the Green Belt of the Southeastern Bering Sea
Prepared by Drs. J. D. Schumacher and P.J. Stabeno, both at PMEL
The shelf edge in the eastern Bering Sea is a region of prolonged productivity as a result of the continued introduction of nutrients into the euphotic zone (Springer et al., 1996). This band of enhanced primary and secondary productivity is generally referred to as the Green Belt (Figure 1). Eddies are a common feature of this region (Schumacher and Stabeno, 1997: Schumacher and Reed, 1992). They can contain high concentrations of pollock larvae (Schumacher and Stabeno, 1994) and are hypothesized to interact with topography to cause upwelling of nutrients which fuel Green Belt productivity. A knowledge of eddies and their biophysical dynamics is vital toward understanding natural variability in this region of prolonged production (Springer et al., 1996).
To locate an eddy in a timely fashion so that field operations can examine these features is a challenge we have surmounted. During NOAA's Coastal Ocean Program (COP) Bering Sea FOCI, coordination between researchers at PMEL (Dr. Phyllis Stabeno) and Colorado Center for Astrodynamics Research (CCAR: Dr. Robert Leben) occurred. In order to direct field operations for her study of ocean color (funded in FY97 for one year by COP's Southeastern Bering Sea Carrying Capacity program), Dr. Stabeno needed information on the presence of eddies in the Bering Slope Current . She contacted Dr. Leben who now provides satellite measurements of anomalies of sea surface height (ssh) through the world wide web in near-real time.
A positive anomaly (clockwise eddy) located south of St. George Island (approximately at 55 N 170 W) has consistently appeared for several months. (Figure 2) This feature was chosen for the SEBSCC ocean color study of production within and outside of an eddy over the slope as observed from satellite tracked drifters equipped with ocean color sensors (Figure 3). A cross-section of density (sigma-t) and the inferred geostrophic velocity (Figure 4)reveal an asymmetric distribution of mass and speed. This may indicate that the eddy is "rubbing" against bathymetry of the inner slope/outer shelf and experiencing the affects of friction. During this process, nutrients may become displaced vertically and thus become available to phytoplankton in the euphotic zone. The distribution of chlorophyll across the eddy (not shown) reveals that higher concentrations of chlorophyll existed within the eddy and over the shelf edge than over the basin. The use of near-real time satellite altimetry together with satellite tracked drifters equipped with ocean color sensors provides an important tool for examination of biophysical processes in the Green Belt. The drifters will continue to monitor chlorophyll concentration and circulation for the next six months, thereby providing a time history of the eddy and associated prolonged production.
References:
Schumacher, J.D., and P.J. Stabeno (1997). The continental shelf of the Bering Sea. In: The Sea, Vol. 11 - The Global Coastal Ocean: Regional Studies and Synthesis, John Wiley & Sons, Inc., New York, in press.
Schumacher, J.D., and P.J. Stabeno (1994). Ubiquitous eddies of the eastern Bering Sea and their coincidence with concentrations in larval pollock. Fisheries Oceanography, 3(3), 182-190.
Schumacher, J.D., and R.K. Reed (1992). Characteristics of currents over the continental slope of the eastern Bering Sea. Journal of Geophysical Research, 97(C6), 9423- 9433.
Springer, McRoy and Flint (1996): The Bering Sea Green Belt: shelf-edge processes and ecosystem production. Fish. Oceanogr., 5, 205-223.
Figures:
A composite pattern of primary productivity in the Bering Sea. The high productivity of the Green Belt (indicated by the dark green) is centered on the shelf edge. (From Springer et al., 1996).
Contours (5 cm interval) of sea surface height anomaly from satellite altimetry on 12 June 1997. The eddy selected for investigation is located south of the Pribilof Islands at approximately 55.5 N, 170.5 W. The center of this eddy has surface relief of about 20 cm.
Shown are the trajectories of 2 satellite tracked drifters deployed four weeks prior to 12 June and a set of 4 drifters deployed between June 11 and 13 in and around the eddy. These latter drifters carried ocean color sensors to measure an index of production within the eddy and in the adjacent Bering Slope Current water. The drifter trajectories reveal clockwise rotation of the eddy of ~40 cm-1.
The distribution of density has been used to provide an estimate of geostrophic velocity (cm s-1) in the eddy. Note the reduced speed in the eastern half of the eddy. This is associated with shoaler water and then spin down of the eddy as it interacts with topography. The highest concentrations of chlorophyll were found at the eastern most station and within the eddy, with very low concentrations found at the western edge of the transect.