Abstract.— Annual and seasonal 

 variability of Georges Bank zoo- 

 plankton biomass and dominant spe- 

 cies abundance are described and 

 related to variations in mean sur- 

 face temperature and average depth 

 distribution. Data were obtained 

 from plankton samples collected bi- 

 monthly with a 0.333-mm mesh net 

 throughout a ten-year period: 1977- 

 86. Biomass was measured by dis- 

 placement volume and the dominant 

 species analyzed were the copepods 

 Calanus finmarchicus, Pseudo- 

 calanus minutus, Centropages typi- 

 cus, Centropages hamatus, and 

 Metridia lucens. 



Biomass levels were high in 1977 

 through 1979, low in 1982 through 

 1984. Biomass and copepod abun- 

 dance in the spring of 1977 were ex- 

 traordinary. Measurements over the 

 entire bank were two to three times 

 above a ten-year median. Unlike the 

 first five years of monitoring, the av- 

 erage seasonal biomass cycle was not 

 coherent from 1982 through 1986. 

 Departures from the average sea- 

 sonal cycle occurred several times 

 during the second half of the time 

 series. 



Calanus finmarchicus and Pseudo- 

 calanus minutus abundance trends 

 were nearly identical, suggesting 

 that their populations may be af- 

 fected by similar factors. Centro- 

 pages hamatus abundance in the 

 central shoal depth zone (<61m) was 

 related to surface temperature vari- 

 ability and its spring abundance es- 

 timates were indirectly proportional 

 to the abundance of other dominant 

 copepods. Centropages typicus counts 

 in autumn 1985 were nearly double 

 all other years, and Metridia lucens 

 abundance surged in late spring 

 1979 but was low from 1983 through 

 1986. 



Variability of zooplankton biomass 

 and dominant species abundance 

 on Georges Bank, 1977-1986 



Joseph Kane 



National Marine Fisheries Service. NOAA 



28 Tarzwell Drive 



Narragansett. Rhode Island. 02882 



Manuscript accepted 16 April 1993. 

 Fishery Bulletin 91:464-474 1 1993). 



Zooplankton biomass has long been 

 recognized as an important index for 

 estimating the seasonal and annual 

 variability of secondary production in 

 marine ecosystems. Zooplankton play 

 a key role in pelagic food chains, serv- 

 ing as the connecting link between 

 primary producers and secondary 

 consumers. The availability of zoo- 

 plankton as food for larval fish is 

 thought to be one of the key factors 

 determining year class strength of 

 commercial fish species (Cushing, 

 1978). 



The rich fishing grounds of Georges 

 Bank in the northwest Atlantic have 

 been the focus of zooplankton studies 

 since the turn of the century. The Ma- 

 rine Resources Monitoring, Assess- 

 ment, and Prediction (MARMAP) pro- 

 gram (Sherman, 1980) has monitored 

 the U.S. Northeastern continental 

 shelf marine ecosystem from 1977 

 through 1987 with bimonthly surveys, 

 measuring a variety of biological and 

 physical parameters. During the first 

 five years of MARMAP monitoring, 

 zooplankton biomass on Georges Bank 

 formed a coherent seasonal pattern, 

 not changing significantly from year 

 to year (Sherman et al., 1983). Com- 

 parison of this data to that collected 

 by Bigelow (1926) from 1912 to 1920 

 showed that biomass levels, species 

 composition, and abundance estimates 

 of dominant species were essentially 

 the same in both studies. Sherman et 

 al. (1987) described in greater detail 

 the seasonal cycle of Georges Bank 

 zooplankton and how it relates to 

 ichthyoplankton life histories. Addi- 



tional studies (Davis, 1984; Meise- 

 Munns et al., 1990) of Georges Bank 

 zooplankton have used subsets of this 

 large data base to help define and 

 simulate seasonal cycles of dominant 

 species in relation to environmental 

 parameters. 



The purpose of this paper is to fur- 

 ther describe the Georges Bank zoo- 

 plankton community by utilizing data 

 collected during MARMAP surveys 

 from 1977 to 1986. The annual and 

 seasonal variability of zooplankton 

 biomass captured with 0.333-mm 

 mesh nets is reported and related to 

 changes in the abundance of the five 

 dominant zooplankton species (Sher- 

 man et al., 1987). The average depth 

 distribution of biomass and dominant 

 species abundance is described and 

 departures from it are compared to 

 overall population variability. 



The sensitivity of the above param- 

 eters to surface water temperature 

 readings is also examined to consider 

 the potential effects of climatic 

 change on Georges Bank zooplank- 

 ton populations. This study is part of 

 a continuing long-term investigation 

 by the National Marine Fisheries 

 Service (NMFS), which monitors the 

 zooplankton component of the U.S. 

 Northeast shelf ecosystem. 



Methods 



Plankton samples were collected at 

 monthly to bi-monthly intervals at 

 32 station locations during MARMAP 

 surveys on Georges Bank (Fig. 1). 



464 



