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Fishery Bulletin 9 1(3). 1993 



CO 



E 



o 



o 



100 



80 



Early Spring 



□ Surr 





1977-86 



1983 



1985 



Year(s) 



Figure 3 



Departures from the ten-year (1977-86) median biomass cycle occurred between 

 early spring and summer in 1983, 1985, and 1986. 



out the winter time series, C. hamatus 

 was virtually absent at stations where 

 surface temperature was below 5°C 

 (Fig. 6). Median surface temperatures 

 for the years 1983-86 were all above 

 5°C (Table 2) and significantly higher 

 (P<0.05) than the earlier years, which 

 were all sampled later in the season 

 when the annual minimum tempera- 

 ture on Georges Bank is normally 

 reached. Biomass and the abundance 

 of the other four copepod species were 

 not affected by winter temperature 

 regimes. 



dance decreased from early to late autumn. During 

 the next five years, increases were recorded between 

 seasons (Table 1, E-F). After an unusual pulse in early 

 spring 1986 (Table IB), C. hamatus declined in late 

 spring and then rebounded to their average summer 

 abundance. In one year, 1977, its annual high was 

 delayed until early autumn (Table 1, D-E). 



Surface temperature variability was examined by 

 fitting seasonal medians (Table 2) to a harmonic re- 

 gression (Fig. 5). The model depicted a strong annual 

 cycle; r = 0.91. Certain seasons in specific years had 

 above or below average temperatures, but there were 

 no prolonged warm or cool periods. 



Interannual variability by season 



Winter Zooplankton standing stock reaches its an- 

 nual low and exhibits little interannual variation dur- 

 ing the cold winter months (Table 1A). The only re- 

 markable year was 1978, when median biomass was 

 nearly double all other years. However, this may have 

 been an artifact of survey logistics, rather than en- 

 hanced winter productivity. None of the usually low 

 biomass stations along the northern and southern pe- 

 rimeter of Georges Bank were sampled that year. 



Centropages hamatus was the only dominant species 

 that demonstrated substantial interannual variation 

 during the winter season. Population estimates in the 

 central shoal depth zone from 1983 to 1986 were well 

 above the ten-year median (Table 1A). These depar- 

 tures can be directly related to surface-temperature 

 variability caused by dates of survey coverage. Through- 



Early spring Zooplankton biomass in 

 early spring was high at the begin- 

 ning and end of the time series; 1977 

 and 1986 (Table IB). Standing stock 

 in 1984 was very low; significantly less 

 (P<0.10) than all other years except 

 1982. 

 C. finmarchicus and P. minutus 

 abundance (Table IB) was similar to biomass trends 

 in some years. Like biomass, abundance estimates for 

 both species were highest in 1977 and low in 1984. In 

 1986, an unusual early spring pulse of C. hamatus 

 (Table IB) in the central shoals depth area elevated 

 standing stock levels. The above average biomass lev- 

 els recorded in 1985 could not be related to the low 

 abundance estimates of the dominant species in that 

 year (Table IB). Notations made by shipboard person- 

 nel indicated that nets were frequently clogged with 

 dense concentrations of phytoplankton. High biomass 

 in 1985 was likely elevated by phytoplankton and en- 

 trapped organisms not usually captured with 0.333- 

 mm mesh nets. 



As in winter, the C. hamatus population was de- 

 pressed by cold temperatures in early spring. Their 

 highest early spring abundance occurred in 1986 when 

 surface waters in the central shoals depth area were 

 warmest (median=6.5°C) in this season. Calanus 

 finmarchicus were also more abundant at stations 

 where surface temperatures were 6°C or more. An- 

 nual changes in biomass and the abundance of other 

 dominant species could not be related to surface tem- 

 perature variability in early spring (Table 2). 



Late spring Zooplankton biomass surged in the late 

 spring of 1977 (Table 1C). The median volume was 

 nearly three times higher than the seasonal 10-year 

 median, and significantly different (P<0.05) from all 

 years except 1978 and 1979. Though biomass declined 

 from this peak in 1978 and 1979, median estimates in 

 these two years were nearly double those recorded in 



