Incze and Ainaire: Distribution and abundance of copepod naupln 



69 



pling begins along this transect near the time of 

 larval hatching each spring and proceeds down-cur- 

 rent (westward) over time. In this paper we report 

 on across-shelf patterns of abundance and vertical 

 distribution of copepod nauplii and other small zoop- 

 lankton from 1985 through 1989 and relate these 

 patterns to hydrographic conditions, chlorophyll 

 concentrations, and distributions of selected taxa of 

 adult female copepods. 



Materials and methods 



For convenience, we use the term microzooplankton 

 to refer to small zooplankton captured and pre- 

 served by methods described below. Hydrography, 

 nutrients, and microzooplankton were sampled with 

 a CTD and rosette sampler along a transect of sta- 

 tions across Shelikof Strait, Alaska, during spring 

 from 1985 through 1989 (Fig. 1) (sampling dates are 

 listed in Table 2). Hydrographic (CTD) data were 

 obtained near bottom at 7 stations at 7-km inter- 

 vals and were processed to give 1-m averaged data 

 of salinity, temperature and density. Nutrients were 

 sampled at five or more stations on the transect by 

 removing water samples from 10-L Niskin bottles 

 tripped at standard depths of 10, 20, 30, 50, 75, and 

 100 m; below this depth we sampled with lower reso- 

 lution, generally at 50-m intervals, plus a sample 

 near bottom. Nutrient concentrations were deter- 

 mined after the cruise by using standard 

 autoanalyzer techniques on frozen samples 

 (Whitledge et al., 1981 3 ). Chlorophyll data were 

 obtained from nutrient sampling depths in the up- 

 per 100 m in 1988 and 1989. Analyses were con- 

 ducted on board the vessel following methods of 

 Yentsch and Menzel (1963) as modified by Phinney 

 and Yentsch (1985) with 0.45-|im Millipore HA ac- 

 etate filters. Microzooplankton was sampled from 

 Niskin bottles were tripped at 10-m intervals from 

 to 60 m in 1985 and from 10 to 60 m in other years. 

 We used the same bottles as for nutrient and chloro- 

 phyll samples for those depths which were common to 

 all. The number of stations sampled varied over the 

 years, beginning in 1985 with stations 55, 58, and 61. 

 In 1986 and 1987 we included station 60. In 1988 we 

 sampled all seven stations along the transect, and in 

 1989 we sampled all except station 57. 



Niskin bottles were sampled for nutrients and 

 chlorophyll when called for; the remaining contents 

 of the bottles were filtered through small (6 x 18 cm) 



3 Whitledge, T. E., S. C. Molloy, C. J. Patton, and C. D. Wirick. 

 1981. Automated nutrient analyses in seawater. Tech Rep. No. 

 BNL-51398, Brookhaven Natl. Lab., Upton, NY. 



conical nets made of 41-um mesh nylon netting. 

 Material retained on the netting was flushed into 

 4— ounce (120 mL) glass jars by using 0.45-um fil- 

 tered seawater and was preserved in a final solu- 

 tion of 5% formalin:seawater. Larger zooplankton 

 was sampled at all seven stations by using 60-cm 

 diameter bongo samplers equipped with 333-um 

 mesh nets and towed in double-oblique fashion from 

 the surface to about 10 m off bottom. From 1986 

 onward, a 20-cm bongo sampler with 150-um mesh 

 nets was attached to the towing wire 1 m above the 

 larger sampler to try to improve on the sampling of 

 smaller copepods. Properties of each tow were moni- 

 tored by time, wire angle from the towing block, 

 mechanical flowmeters mounted across the mouth 

 of each net, and a bathykymograph attached to the 

 bridle of the large bongo. 



In the laboratory, each microzooplankton sample 

 was filtered onto a 41-um mesh sieve, stained over- 

 night in Rose Bengal, transferred to a 10-mL scin- 

 tillation vial and examined in approximately 2-mL 

 aliquots. Microzooplankton was analyzed by using 

 a stereo dissecting microscope equipped with an 

 image analysis system consisting of a high-resolu- 

 tion video camera and computer software to make 

 measurements and record data (Incze et al., 1990). 

 The microscopist made identifications, placing each 

 organism into one of thirteen categories (Table 1), 

 and directed the orientation of measurements. Cope- 

 pod nauplii were measured for total length (TL) and 

 maximum width. Total length was the carapace 

 length ("prosome"), plus the abdomen ("urosome") 

 when present. The latter section often was curled 

 beneath the carapace, necessitating measurement 

 along a curved line. We measured the diameter of 

 eggs and only the total body length of all other or- 

 ganisms. In most cases the entire sample was ana- 

 lyzed, but 25% of the original sample sometimes pro- 

 vided adequate counts, which we established as at 

 least 50 nauplii per sample. Subsampling was done 

 by increasing the stored sample volume to 200 mL, 

 dividing as necessary, then recondensing the mate- 

 rial for examination. Subsampling was checked for 

 accuracy by completely analyzing both half-portions 

 from 30 samples. Final counts of microzooplankton 

 were corrected for the subsampling fraction and for 

 differences in the original volume of water filtered 

 and are presented as number of organisms per li- 

 ter. Integrated abundances (No. m~ 2 ) were estimated 

 for the upper 60 m of the water column by using a 

 trapezoidal algorithm. 



Vertical and horizontal patterns of micro- 

 zooplankton distribution were plotted by using an 

 inverse distance gridding technique ("Surfer", 

 Golden Software, Inc., Golden, CO) with a grid size 



