Incze and Ainaire: Distribution and abundance of copepod nauplii 



73 



not always the same as those found for nau- 

 plii. Maximum egg concentrations ranged 

 from 2.2 L" 1 in 1988 (at 30 m depth) to 

 45 L" 1 in 1986 (10 m depth), both at station 

 59. Most eggs and nauplii were in the up- 

 per mixed layer. Since sampling in 1987 oc- 

 curred in late May, the relatively high abun- 

 dance of nauplii may be attributed to time 

 of year. Consequently, a statistical compari- 

 son between transects focussed on the other 

 four years, which were sampled the last 

 week of April and first week of May. This 

 time period is close to the time of peak lar- 

 val hatching. Abundance was statistically 

 different among transects (Quade test 0.025 

 < P < 0.05). The lowest (1985) and highest 

 (1986) concentrations were significantly dif- 

 ferent at a = 0.05; the intermediate concen- 

 trations of 1988 and 1989 differed from 

 those in 1985 (but not 1986) at a = 0.10 

 (Multiple comparisons of ranks). 



The lengths of sampled nauplii showed 

 positively skewed frequency distributions 

 with peak abundance between 100 and 150 

 urn TL in all years and nearly identical cu- 

 mulative distribution functions (Fig. 8). 

 Median size differed by less than 15 urn 

 among years and averaged 140 urn during 

 the five-year period. The average length: 

 width ratio of nauplii measured in this study 

 was 2.2, with a standard deviation of 0.1 

 (rc = 1500). Consequently, our mesh, 41 urn on 

 a side and 58 urn on the diagonal, should 

 have retained some nauplii >90 urn long and 

 all nauplii >128 urn. Our data showed a 

 steep decline in frequency of nauplii with 

 length <110 um, between the above esti- 

 mates, and width <50 um, corresponding to 

 the relationship 110/2.2 = 50. Most of the 

 nauplii did not have urosomal segments, so 

 total length and maximum width are equiva- 

 lent to prosome length and width for most 

 of our data. 



The abundance and size distribution of 

 eggs differed substantially between years 

 (Fig. 8). The greatest number (and smallest median 

 size [ca. 75-um diameter]) of planktonic eggs was 

 present in 1986; the fewest eggs occurred in 1988, 

 when median size was the largest (ca. 165 urn). 



Abundances of potentially significant contributors 

 to the standing stocks of copepod eggs and nauplii 

 are given in Table 2. Among the taxa of interest, 

 Calanus pacificus had low adult female numbers 

 because most individuals were in copepodid stage 5 

 (C5) during spring. Other adult female copepods 



Chl-a (mg m 2 ) 



Distance (km) 



20 30 



Figure 4 



Mean number of nauplii and total microzooplankton per li- 

 ter in the upper 60 m across the study transect in April 1988 

 (top panel), viewed looking westward. Numbers at the top 

 of the panel show integrated (0-100 m) chlorophyll-a con- 

 centrations (mg m -2 ). Temperature (°C) and salinity (g kg -1 ) 

 are shown in the middle and bottom panels, respectively. 

 Data can be compared with nutrient distributions (Fig. 2), 

 dynamic topography (Fig. 5), and depth distributions of 

 nauplii (Fig. 6). 



were broadly distributed across the Strait, but the 

 maximum concentration of each taxon occurred in 

 the northern half (among stations 58-61) in all but 

 one instance. The across-Strait patterns of low and 

 high abundances within species were similar from 

 year to year and statistically significant (Spearman 

 rank correlation test, P<0.05). The shift in mesh 

 sizes for Pseudocalanus spp. collections limits the 

 between-transect comparisons that can be made. 

 (Note that there are interspecific differences within 



