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

3. Methods

Immediately following deployment and before recovery of each mooring, a conductivity-temperature-depth (CTD) cast with Niskin bottles was done at each site. Discrete water samples for determination of chlorophyll-a , pheophytin-a, and nutrients were obtained in the vicinity of the moorings. Aliquots of seawater were filtered through Whatman GF/F filters and stored at -70°C until 24-hr cold extraction in 90% acetone and fluorometric analysis could be performed (Parsons et al., 1984). Nutrient samples were frozen for later colorometric analysis (Whitledge et al., 1981).

b. Surface moorings

The surface moorings consisted of chains supported by a surface toroid. The length of the tether was ~50% longer than the water depths, permitting horizontal motion. The mooring float was a 2.3-m-diameter foam-filled fiberglass toroid with a rigid 2-m-long bridle. The meteorological instruments were mounted on a 3.5-m aluminum tower. Oceanographic instruments were mounted on the buoys' bridle and mooring lines. SBE 16-03 SEACATs recorded temperature, conductivity (salinity), and usually pressure, to accuracies of 0.01°C, 0.001 practical salinity units (psu), and 0.14 decibar (db), respectively. Temperature was also recorded by miniature temperature recorders (MTR). Chlorophyll absorption meters were on each mooring and, in addition, fluorometers were used on mooring 2 in conjunction with the SEACATs. The positions of all equipment on each mooring are provided in Table 1. Two of the SEACATs (11 and 22 m) flooded due to failure of the O-rings, likely caused by going from the cold temperature on deck to the relatively warm water (~0.5°C). In addition, the chlorophyll absorption meter at 14 m on mooring 2 ceased functioning on March 25, and the data are not presented.

If your browser cannot view the following table correctly, click this link for a GIF image of Table 1

Table 1. Mooring location, water depth, deployment period, and depths (m) of subsurface instruments. ADCPs (with SEACAT) were deployed within 1 km of surface mooring. Instruments that failed and provided no data are indicated by asterisks (*).
	MTR				SEACAT				Fluorometer				A-3
Mooring 1		1
55°04 : 164°31		15				10								5	*
65 m		20				26								21
March 14-June 15		32				44
600 KHz ADCP		38				60
		50
Mooring 2		1
56°36 : 164°37		15				11	*			11	*			7
75 m		20				22	*			22	*			14	*
March 14-April 29		32				44				44
150 KHz ADCP		38				68
		50
		56
Mooring 3		1				10								7
56°02 : 166°23		15				26
123 m		20				44
March 13-May 8		32				80
150 KHz ADCP		38				118
		50
		56

Most instruments were calibrated before deployment and after recovery. The time base was adjusted, if necessary, following checks for missing records and clock drift. Spikes were removed from the time series. These were most common in the salinity time series due to the different response times of the temperature and conductivity sensor.

The chlorophyll-a absorption meters were A-3s from Western Environmental Technology Laboratories and measured in situ absorption at 650, 676, and 710 nm. Long-term deployment of these instruments is described in some detail in Davis et al. (1997). Absorption values were converted into estimates of chlorophyll-a concentration using standard spectrophotometric techniques (Parsons et al., 1984) and a constant chlorophyll absorption coefficient of 0.017 m (mg chl) (Davis et al., 1996). Simulated chlorophyll fluorescence was measured using WET Lab's Wet Star fluorometers. In situ fluorescence was converted to chlorophyll-a concentration using instrument-specific empirical relationships determined in the laboratory at two temperatures using cultured phytoplankton.

c. Acoustic Doppler Current Profiler

A 150-KHz ADCP was deployed at mooring sites 2 and 3, and a 600-KHz ADCP at site 1. The extensive fishing in this region necessitated the use of trawl-resistant cages (TRAP). Each TRAP is a five-sided structure approximately 3.5 m in diameter and 0.8 m tall. Each contained an upward-looking ADCP with a 90° head, an external alkaline battery pack, two recovery floats with 400 m of line attached to each, and two releases. In addition, one SEACAT with a 5-m-long tube was positioned in the cage to measure temperature and salinity at 5 m above the bottom.

The 150-KHz (600-KHz) ADCP measured averaged velocities at half-hour intervals in 4-m (2-m) bins. The time series of currents presented in this paper have been low pass filtered with a 35-hour, cosine-squared, tapered Lanczos filter to remove high-frequency variability (especially the diurnal and semi-diurnal tidal signals), and then resampled at 6-hour intervals.

d. Winds

Since no winds were obtained at the mooring sites 2 and 3, we use geotriptic winds, computed from 12-hourly atmospheric surface pressure. These surface winds (a balance of Coriolis, pressure gradient, centrifugal, and friction forces) were rotated 15° counterclockwise and reduced in speed by 30% from the geostrophic wind. They were interpolated to a point halfway between moorings 2 and 3 (Fig. 1). Such winds are in good agreement with measured winds at these latitudes (Macklin et al., 1993). The winds at site 1 were strongly influenced by the proximity of the mountainous Alaska Peninsula.

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