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Under-ice observations of water column temperature, salinity and spring phytoplankton dynamics: Eastern Bering Sea shelf

P. J. Stabeno,1 J. D. Schumacher,1 R. F. Davis2 and J. M. Napp3

1NOAA, Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Seattle, Washington  98115
2Department of Oceanography, Dalhousie University, Nova Scotia, Canada
3NOAA, Alaska Fisheries Science Center, 7600 Sand Point Way NE, Seattle  98115

Journal of Marine Research, 56, 239–255, 1998.
This paper is not subject to U.S. copyright. Published in 1998 by the Journal of Marine Research

1. Introduction

In March 1995 three surface moorings with accompanying acoustic Doppler current profilers (ADCPs) were deployed on the southeastern Bering Sea shelf (Fig. 1) as part of an experiment conducted by Fisheries Oceanography Coordinated Investigations for NOAA's Coastal Ocean Program. The purpose of this experiment was to obtain time series of physical and biological variables to characterize the changing nature of the shelf waters in spring and summer. Surface floats supported instruments that measured a variety of atmospheric variables. Subsurface instruments measured temperature and conductivity (salinity), phytoplankton fluorescence, and chlorophyll-a absorbance. The nearby ADCPs provided time series of currents through most of the water column.

Figure 1. A map of the eastern Bering Sea continental shelf indicating the locations of the three moorings. Maximum ice extent during 1995 is shown by a dashed line. Depth contours are in meters.

While it has long been desirable to measure a suite of oceanographic parameters under an advancing ice field, this experiment was not designed to accomplish this. On March 17, a period of steady winds out of the north pushed the ice ~100 km southward in ~2 days (Fig. 2). The ice reached mooring 2 three days after deployment and dragged the mooring ~9 km southward. We continued to obtain position fixes via Service Argos for ~9 days. Mooring 3 met a similar fate on April 8, when it was dragged 2 km southward by the advancing ice. The ice continued southward, and at its maximum extent, mooring 2 was >100 km from the ice edge. In contrast, mooring 3 remained near the ice edge (within 10 km) until the ice retreated on ~April 20. The ice did not reach the southernmost mooring.

Figure 2. Wind velocity derived from atmospheric pressure and interpolated to a point midway between moorings 1 and 2. North is along the y-axis.

On April 29 the NOAA ship Miller Freeman found and recovered mooring 2, ~14 km south of its deployment site. The surface buoy was badly damaged (approximately two-thirds of the foam had been rubbed away by the ice) and stripped of all meteorological sensors. Six days later mooring 3 was found and recovered ~5 km south of its deployment site. This surface buoy was in slightly better condition, but all surface instruments had been damaged. On both moorings, however, the subsurface instruments escaped damage by ice and provided unique time series of temperature, salinity, and chlorophyll-a estimates underneath an advancing and retreating ice field.

This paper focuses on the observations from the two sites that were within the ice field. In the next section we give a brief background of the region and then describe the mooring design, instruments, and data processing. The time series of temperature and salinity, currents, shear, and chlorophyll-a are presented in the following sections. We close with a discussion of the conclusions derived from this data set.


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