U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications

Factors influencing physical structure and lower trophic levels of the eastern Bering Sea shelf in 2005: Sea ice, tides and winds.

Phyllis Stabeno1 Jeffrey Napp2, Calvin Mordy3, and Terry Whitledge4


1National Oceanographic and Atmospheric Administration, Pacific Marine Environmental Laboratory, 7600 Sand Point Way NE, Seattle, Washington, 98115-0070, USA

2National Oceanographic and Atmospheric Administration, Alaska Fisheries Science Center, 7600 Sand Point Way NE, Seattle, Washington, 98115-6349, USA

3University of Washington, JISAO, 7600 Sand Point Way NE, Seattle, Washington, 98115-0070, USA

4University of Alaska, Fairbanks, 245 O'Neil Building, P.O. Box 757200, Fairbanks, Alaska 99775-7200, USA

Progress in Oceanography, 85(4), 180–196, doi:10.1016/j.pocean.2010.02.010.
© 2010 Elsevier Ltd.

2. Methods and Data

2.1 Hydrography

This study uses hydrographic data collected during two cruises in 2005—one on the R/V Thomas G. Thompson (cruise TN179 leg 3: 10 May–25 May) and the second on the NOAA ship Miller Freeman (MF05-13: 21 September–4 October). Conductivity-temperature-depth (CTD) measurements were collected with a Seabird SBE911 plus system (reference to trade names does not imply endorsement by NOAA) with dual temperature and salinity, oxygen (one SeaBird SBE 43 on TN179 and two on MF05-13), solar radiation (Biospherical PAR QSP-200L, 400–700 nm), and chlorophyll fluorescence sensors (WETStar WS3S). Fluorescence from both the moored fluorometers and the fluorometer on the CTD was converted to chlorophyll concentration (μg 1−1) using the relationships provided by the manufacturer for each instrument during annual service and calibration. Data were recorded during the downcast, with a descent rate of 15 m min−1 to a depth of 35 m, and 30 m min−1 below that. Salinity calibration samples were collected on most casts and analyzed on a calibrated laboratory salinometer. The distance between stations along the 70-m isobath and on the cross-shelf transects was ~20 km. Each cruise occupied the same CTD stations along the 70-m isobath.

During both cruises, water samples for dissolved inorganic nutrients were collected at each station using Niskin bottles. Nutrient samples were analyzed onboard for dissolved phosphate, silicic acid, nitrate, nitrite, and ammonia (only during TN179) using protocols of Gordon et al. (1993) and the ammonia protocol available at http://chemoc.coas.oregonstate.edu/~lgordon/cfamanual/whpmanual.pdf.

In situ oxygen sensors were calibrated by the manufacturer prior to each cruise, but a titrator was not available for ground truth measurements during the cruises. On TN179 there was one sensor, while on MF05–13 there were two sensors. The mean difference between the two sensors on each cast ranged from 3.9 to 6.7, with a mean of 5.6, and for each cast the correlation between the two sensors was greater than 0.99. We recognize the problem of not having titrated water samples for in situ calibration, but feel that the oxygen values provide patterns of variability that are informative and important in our descriptions of conditions on the shelf.

2.2 Modeled winds

The modeled winds were estimated using daily data from the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) Reanalysis (Kalnay et al., 1996). We follow the procedure used in Bond and Adams (2002) to specify particular elements of the atmospheric forcing on a daily basis for selected periods and interpolated wind velocity to the locations of two moorings, M2 and M8 (Fig. 1). The daily winds from the reanalysis are reliable in this region (Ladd and Bond, 2002). Meteorological variables, including wind velocity, were also measured on the surface mooring at M2 as described below.

2.3 Moorings

Four biophysical mooring sites (M2, M4, M5, and M8) are the cardinal locations of our observing network (Fig. 1). The moorings are recovered and redeployed twice a year, once in the spring (April/May) and again in the late summer or early fall (September/ October). Moorings at M2 have been maintained almost continuously since 1995. In addition, a series of moorings have been deployed at M4 since 1996 (continuous since 2000). M5 and M8 have been maintained since 2005 (current measurements since 2004) and 2004, respectively.

The main mooring at each site is constructed of heavy chain to protect it from loss due to sea ice and the heavy fishing pressure in the region. In 2005, data collected by instruments on the moorings included temperature (miniature temperature recorders, SBE-37 and SBE-39), salinity (SBE-37), nitrate (In Situ Ultraviolet Spectraphoto-meter, discussed below), and chlorophyll fluorescence (WET Labs DLSB ECO Fluorometer). Currents were measured using an upward-looking, bottom-mounted, 300 kHz Teledyne RD Instruments acoustic Doppler current profiler (ADCP) deployed next to the main mooring. All instruments were prepared, calibrated prior to deployment, and the data were processed according to manufacturer's specifications.

In 2005, we used an In Situ Ultraviolet Spectrophoto-meter (ISUS; Satlantic, Inc.) to estimate dissolved inorganic nitrate at M2. This instrument can sample hourly for as long as 12 months. ISUS uses optical technology (UV spectra) to provide chemical-free measurements of in situ nitrate and has been field tested on drifting buoys, towed vehicles, moorings, and CTD profilers. The instrument is solid state with no moving parts and has a sensitivity of 0.25 lM and 1% accuracy with post-processed CTD corrections. A discrete sample collected at the mooring site in May agreed to within 0.4 lM of the ISUS measurement. Data were collected hourly.

The depths of the shallowest instruments on the main moorings vary from 1 to 20 m dependent upon the mooring location and the time of year (http://www.pmel.noaa.gov/foci/foci_moorings/mooring_info/mooring_location_info.html). When the mixed layer shoals above 11 m, the upper mixed layer measurements in the summer are sometimes under-represented in our data set (Stabeno et al., 2007). Sampling intervals varied for the different instruments and range from every 10 min to once an hour. During late spring and summer (the ice-free period), the mooring at M2 included a surface toroid buoy with an aluminum tower where a full suite of meteorological variables was collected (Gil WindSonic for winds, Vaisal HMP-50 for air temperature and humidity, Setra 270 for atmospheric pressure and Eppley BSP for PAR). Winds were measured at a height of ~3 m. This surface mooring also permitted measurement of ocean temperature and salinity at ~1 m.

2.4 Zooplankton

Samples for mesozooplankton were collected using double-oblique tows of paired bongo frames (60-cm frame with 0.333 mm mesh and 20-cm frame with 0.150 mm mesh). Tows extended from the surface to within 5 m of the bottom. A SeaBird SeaCat SBE19-plus was attached above the top bongo frame and telemetered net depth in real time to the operator. Each net mouth contained a calibrated General Oceanics mechanical flow meter. The samples were preserved in a sodium borate buffered 5% formalin: seawater solution and then sent to the Polish Plankton Sorting and Identification Center (Szczecin, Poland) for processing. Organisms were identified to the lowest possible taxonomic level and then enumerated. All enumerated organisms were returned to AFSC in Seattle, Washington, for quality control.

2.5 Sea ice

We used weekly data on ice extent and concentration from the National Ice Center (NIC). The NIC published a CD of ice extent and concentration from 1972 to 1994. The CD specifies ice concentration and extent in 0.25° latitude and longitude bins. After a break of several years, the NIC resumed posting ice concentration information in GIS format on their websites (http://www.natice.noaa.gov/pub/Archive/arctic/ and http://www.natice.noaa.gov/products/archi/index.htm). Data from 1995 to 2006 were converted to 0.25° bins to make them comparable to the earlier (1972–1994) data. The NIC assimilates data from satellites (Radarsat, Defense Meteorological Satellite Program, and Envisat) as well as aerial reconnaissance, local information, climatology, meteorological information, and models to produce their estimates of sea-ice extent and concentration.


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