72 



Fishery Bulletin 92(1). 1994 



volumes of high density (high salinity) bottom wa- 

 ter in 1985, 1986, and 1989 compared with other 

 years. The upper mixed layer generally was deep- 

 est on the northern end of the transect, near the 

 Alaska Peninsula, with a steeply sloping density 

 gradient near the middle. The exception, in 1988, is 

 discussed later. Averaged across the Strait, the up- 

 per mixed layer was deepest in 1985 and shallow- 

 est in 1986 and 1987. 



Observations of phytoplankton clogging sampling 

 nets during the cruises showed that the spring 

 bloom of large diatoms occurred latest in 1985. By 

 this approximation, what probably was the major 

 spring bloom in the Strait began after the first week 

 of May in 1985, whereas it already was well under- 

 way when we began sampling in early May 1986 

 and 1989 and late April 1988. A grid of sampling 

 stations that extended to the northern end of the 

 Strait in 1985 showed that the bloom in that year 

 formed first in a band along the middle of the Strait 

 for virtually its full length of 300 km. Our grid in- 

 terval was not sufficiently fine to resolve the width 

 of the bloom feature, but our findings are consistent 

 with a diameter <25 km. 



Our samples were dominated numerically by cope- 

 pod nauplii, which composed from 46 to 827c of all 

 organisms sampled along the transect over the five- 

 year period (Table 1), followed in most years by cope- 

 pods eggs, from 3.5 to 35 f /r. Of the remaining taxo- 

 nomic categories, only a few ever contributed more 

 than 57c of the total organism count: small copep- 

 ods (including copepodid stages), tintinnids, rotifers, 



Nauplii 

 Late April - Early May 



1.8 x 10 6 



o 1.2 x 10 6 



<d 6 x 1 o b 



E 



1 x 10 s 



Kt 



57 58 59 



1985-1989 



60 61 



7 Km 



Figure 3 



Across-strait patterns of integrated abundance (No. m' 2 ) and 

 average concentration mo. L ' ) of copepod nauplii from surface 

 to 60-m depth during spring at the primary time-series sta- 

 tions, marked with asterisks. Upper and lower lines describe 

 the maximum and minimum values observed, 1985-89. 



and polvchaete larvae. None of these ever exceeded 

 15'7r of the total count. 



The integrated (0-60 m depth) abundance of 

 microzooplankton at the primary sampling stations 

 increased across the Strait from south to north (see 

 Fig. 3 for copepod nauplii). Average abundances of 

 nauplii, eggs, and all other organisms were highest 

 in 1986 and 1987. For copepod nauplii, abundance 

 was lowest in 1985 and intermediate in 1988 and 

 1989 (Table 1). In 1985, near-surface concentrations 

 averaged 869}- of those at 10-m depth. Therefore, the 

 assumption of uniform concentration of organisms 

 in the upper 10 m may have introduced a small 

 upward bias in the integrations from 1986 onward. 

 The 7-km resolution of microzooplankton obtained 

 across the Strait in 1988 (Fig. 4) shows a more com- 

 plex pattern of distribution than suggested by other 

 transects. Specifically, comparatively large numbers 

 of nauplii and other microzooplankton were found 

 at stations 56 and 57, nearly equivalent to popula- 

 tions at the two northern stations. Both groups of 

 stations were marked by waters of lower surface 

 nitrate concentration (Fig. 2) associated with flow 

 around a dynamic high in the middle of the Strait 

 (Fig. 5). The two groups of stations differed from 

 each other in the composition of planktonic eggs 

 (greater concentrations at stations 60, 61) and other 

 microzooplankton (greater at 56 and 57) and in tem- 

 perature and salinity. The southern "limb" of the 

 anticylconic feature was about 0.1 C warmer and 

 0.05 g kg~ ! more saline than the northern limb. 

 Chlorophyll data show high chlorophyll-a concentra- 

 tions (up to 6 ug Lr x ) and high integrated 

 chlorophyll-a (140-180 mg m" 2 , 0-100 m) 

 in the two limbs of the ACC surrounding 

 the anticyclonic feature; the lowest chlo- 

 rophyll-a (10 mg m -2 ) was found in the 

 middle. 



Copepod nauplii were found mostly in 

 the upper 30 m, though they extended 

 deeper at some stations in 1985 and 1988 

 (Fig. 6). Naupliar concentrations were 

 greater in the northern half of the 

 transect in 1985, 1986, and 1987; they 

 were distinctly bipolar in 1988; and in 

 1989 maximum concentrations of both 

 nauplii (Fig. 6) and chlorophyll-a (Fig. 7) 

 occurred in the center of the Strait. Maxi- 

 mum naupliar concentrations encoun- 

 tered at any depth across the Strait per 

 transect ranged from 18 L "' in 1985 to 144 

 L ' in 1987, both at 20 m depth at station 

 60. Planktonic copepod eggs also occurred 

 mostly in the upper 30 m but exhibited a 

 variety of across-shelf patterns that were 



30 



20 E 



