Materials and Methods 



Study Area and Slalion Locations 



The second leg of ihe 1988 US-USSR cruise aboard the 

 research vessel Akademik Korolev focused pri manly on the 

 shallow Chirikov basin (<50 meters), the region between 

 St. Lawrence Island and the Bering Strait (see Frontispiece). In 

 the Chirikov basin, three major water types occur and are 

 bathymetrically steered northward across this northern Bering 

 Shelf. 



Anadyr water (AW) is located in Soviet waters along the 

 western boundary of the system. The Bering Shelf water 

 ( BSHW) is restricted to the central basin; and Alaskan Coastal 

 water ( ACW) is located near the Alaskan coast and forms the 

 eastern boundary of the northern shelf ecosystem. These 

 waters are identified by temperature/salinity profiles and bottom 

 water properties. In the summer, AW is characterized with 

 salinities >32.5"/(ki and temperatures <I.5°C, BSHW with 

 salinities of 31.8 to 32.5"/iki and temperatures of 0-1.5°C. and 

 ACW with salinities of <31.8"A«i and temperatures of >4°C 

 (Walsh f/«/., 1989). 



Two additional areas were also made in deep water of the 

 south Bering Sea basin near the Aleutians (see Frontispiece). 

 Bacterioplankton dynamics were measured at Station 1 10 in 

 the South Polygon (53°9'N. 175°9'W) and at Station 113, the 

 old GEOSECS station in the eastern Bering Sea basin (53°2'N, 

 177°3'W). 



Physical Measurements 



Salinity, temperature, and depth data were collected using 

 a Sea-Bird SBE9 CTD/General Oceanic Rosette System. This 

 information was used to select water depths for bacterioplankton 

 samples. 



TCA and were dissolved in Aquasol. Radioactivity, corrected 

 for counting efficiency using an internal 'H standard, was 

 detennined by liquid scintillation. 



Bacteria in 10-ml water samples were preserved with 

 0.2 |im filtered formaldehyde (2% final concentration) and 

 stored at 5°C. One to 3 weeks following the cruise, bacteria 

 were stained with Acridine Orange and filter on 0.2 |im 

 Nuclepore filters for counting total bacteria in 10 microscopic 

 fields filter ' (Hobbie etal.. 1977), along with dividing cells in 

 20 microscopic fields filter ' (Hagstrom el al., 1979). The 

 frequency of dividing cells were calculated relative to the total 

 number of single plus dividing cells. Bacterial numbers and 

 dividing cells were determined by epifluorescence microscopy . 



Estimates of cell production and growth rates in natural 

 populations of bacteria were calculated using two different 

 techniques: thymidine incorporation into DNA material using 

 the theoretical conversion factor of 2 x 10'* cells mole ' 

 thymidine incorporated (Fuhrman & Azam, 1982) and the 

 frequency of dividing cells using the empirical relationship 

 between FDC and specific growth rate (u) of In u = 0.81 

 (FDC) -3,73 for southern ocean bacterioplankton (Hanson 

 etal., 1983). 



Growth rates were calculated from estimates of cell 

 production divided by standing stocks of bacteria. To convert 

 cell production and standing stocks to carbon, an estimate of 

 cell carbon was assumed to be 10 fentogram C cell ' based on 

 estimates from Antarctica and British Columbia (Fuhrman & 

 Azam, 1980; Fuhrman, 1981). 



Statistics Analysis 



Data transformation and statistics analysis (i.e., 

 correlations, slope analysis, t-test, ANOVA) were computed 

 with SAS, Inc., software. 



Bacterioplankton Measurements 



On the shallow Bering Sea Shelf, water samples for 

 bacterioplankton measurements were collected from ? depths 

 at 12 stations using 1.7-1 Niskins bottles on the rosette. Sample 

 depths were chosen to represent surface mixed waters, 

 hydrographic conditions within the region (i.e.. themiohalocline 

 or midwater column), and near-bottom waters. In the deep 

 Bering Sea at Stations 110 and 113, water samples were 

 collected from 1 2 depths: 6 depths in the upper 250 m of the 

 water column and 6 depths over the remaining water column 

 down to the bottom. 



Bacterioplankton measurements included 



l"H-methyl|thymidine incorporation, bacterial numbers and 

 frequency of dividing cells, and empirical growth experiments. 

 For thymidine incorporation (Fuhrman & Azam. 1982). 

 unaltered ?0-ml water samples were transferred to an 8-oz 

 sterile Whiri Pak bag. | 'H-methyl|thymidine (84.8 Ci mmol ' ) 

 was added to obtain a final concentration of 20 nmoles 1 '. 

 Water samples were incubated in the dark at in situ surface 

 temperatures. After 3 h. samples were chilled in an ice bath and 

 ice-cold trichloroacetic acid (TCA) was added to a final 

 concentration of S'/f TCA. After30minonice, 25-ml replicate 

 samples were filtered on 25-min Millipore cellulose acetate 

 filters of 0.45 |i pore size. The filters were rinsed with cold 57c 



Results and Discussion 



Bacterioplankton. Thymidine Incorporatum. and Frequency 

 of Dividing Cells 



Anadyr water ( AW) : Rates of thymidine incorporation by 

 bacterioplankton along the western boundary of the Chirikov 

 basin remained constant with depth, except at Station 86 where 

 rates refiected the themiohalocline in the Bering Strait 

 (Frontispiece, Figs. Ia,b). Ratesaveraged 1.37 pmolesl' h ' in 

 AW (Table I ). Salinity profile at Station 86 characterized low- 

 salinity ACW in the upper 15 m and high-salinity AW below 

 30 m. Rates of thymidine incorporation correlated strongly 

 with temperature but not with the index of the population 

 growth rate (i.e., specific activity of thymidine incorporation) 

 (Fig. 2, Table 2). 



Bacterial populations were generally more abundant in 

 surface waters in AW and averaged 3.7 x 10" cells 1 '. Highest 

 numbers occurred in the surface waters of the Bering Strait 

 (Station 86, Fig. la). Bacteria in these waters correlated with 

 the narrow range in water temperatures (Fig. 3. Table 2). This 

 relationship to the index of population growth rate was similar 

 to that found for ACW (Fig. 2, Table 2). 



The frequency of dividing cells, an index of cellular 

 growth rate, ranged from 4 to 8% dividing cells in surface 



61 



