Kane. Zooplankton biomass and species abundance on Georges Bank 



473 



studies (Marshall and Orr, 1955; Corkett and McLaren, 

 1978) have shown that both C. finmarchicus and 

 P. minutus grow and reproduce within the upper range 

 of spring temperatures. There was no evidence in 

 our data that either species had a strong response to 

 spring surface temperature variability. One possibility 

 is that the omnivorous C. hamatus may have depressed 

 production in the other species by preying on their egg 

 and naupliar stages, as Davis (1984) suggests they do 

 in autumn months. Physical or behavioral responses 

 to other changing spring conditions, such as daylight or 

 thermocline formation, cannot be eliminated as poten- 

 tial factors that triggered or limited their production. 



There is growing evidence that the earth's climate is 

 changing (Mitchell, 1989). The impact of global warm- 

 ing on Georges Bank zooplankton could be substantial 

 because the area is a faunal transition zone between 

 northern boreal species and southern warm water 

 plankton (GLOBEC, 1991). The sensitivity of the 

 Georges Bank C. hamatus population to temperature 

 indicates that the effects of a long-term warming trend 

 might first affect the life cycle of this species. Cen- 

 tropages hamatus virtually disappears from the water 

 column when surface temperature falls below 5°C. 

 Marcus (1989) has shown it produces diapause eggs 

 and reports that nauplii appeared after incubation of 

 Georges Bank sediment. Centropages hamatus appar- 

 ently overwinters as bottom resting eggs that hatch 

 when water column temperatures rise to some thresh- 

 old. If this phase of its life cycle is shortened or elimi- 

 nated by global warming, growth and reproduction in 

 the central shoals could continue year round and po- 

 tentially limit the production of other copepod species. 

 The early to late autumn rise of C. hamatus abun- 

 dance from 1982 to 1986 suggests dormancy was post- 

 poned and an additional generation produced in these 

 years. Since monthly anomalies of sea surface tem- 

 perature for 1981-86 in the area indicate a warming 

 trend (Wood and Tang, 1988), this may be the first 

 signal that climatic change is affecting the marine eco- 

 system. 



Interannual variability in the amount of food avail- 

 able to larval fishes is believed to be an important 

 determinant of their survival and subsequent recruit- 

 ment to adult populations. Field evidence for the link- 

 age between larval survival and zooplankton prey con- 

 centrations is poor (Laurence and Lough, 1984; Leak 

 and Houde, 1987). Though this report was not designed 

 to examine the relationship between zooplankton vari- 

 ability and its effect on fishery resources on Georges 

 Bank, it should be noted that zooplankton biomass 

 patterns closely resembled those of northern sand lance 

 [Ammodytes dubius) population estimates. Their popu- 

 lation surged in the late seventies, responding to a 

 reduction in predation and competition pressure caused 

 by the fishery-induced collapse of herring and mack- 



erel populations (Sherman et al., 1981). Relative abun- 

 dance of the zooplanktiverous sand lance in survey 

 trawls increased dramatically between 1977 and 1981, 

 decreasing thereafter through 1986 (Nelson, 1990). The 

 sand lance explosion may have been fueled by the high 

 concentrations of zooplankton food stocks available in 

 the late 1970's. 



Interannual variations in time of sampling can bias 

 estimates of biomass and abundance for predefined 

 seasons. Early spring on Georges Bank is especially 

 sensitive to this bias because the zooplankton popula- 

 tion is beginning to harvest the late winter phytoplank- 

 ton bloom and is rapidly transferring it to higher lev- 

 els of the food chain. Obviously, the calendar definition 

 of early spring used here to subset data may not be 

 real in nature. The question arises whether the bio- 

 mass estimates recorded in early spring 1984 were 

 truly low or was sampling conducted too early? Me- 

 dian surface temperature in 1984 was only 0.3°C be- 

 low the ten-year median of 5.5°C. There were other 

 years with lower temperatures that had higher esti- 

 mates of biomass. A biological sign of spring's arrival 

 on Georges Bank is the presence of early copepodite 

 stages of C. finmarchicus in the water column. The 

 spring phytoplankton bloom triggers their spawning 

 and, if spawning had not yet occurred, only overwin- 

 tering stage-5 copepodites would be present (Davis, 

 1987). However, in 1984, 52.7% of the population was 

 stage-2 and stage-3 copepodites, similar to the high 

 biomass year of 1977 where 56.2% of the population 

 were these early developmental stages. Thus, the low 

 zooplankton biomass measured in early spring 1984 

 was probably real. 



There is presently only a perceptual understanding 

 of how physical processes affect the abundance and 

 distribution of Georges Bank zooplankton. The success 

 of a population depends not only on food availability 

 and predator abundance but also upon the dynamics 

 of its physical environment, which influence feeding 

 efficiency, susceptibility to predation, transport, and 

 recruitment success. Sea-surface temperature was the 

 only physical parameter discussed in this report and 

 its narrow range of variability could not be correlated 

 to the comparatively large fluctuations of the zooplank- 

 ton population. Monthly mean-derived wind stress com- 

 ponents in the area (see Ingham and Wood, 1987) and 

 anomalies in the volume of Georges Bank shelf water 

 (Mountain, 1991) were also examined from 1977 to 

 1986 and no persistent correlation to biomass variabil- 

 ity was evident. Ongoing studies are presently analyz- 

 ing historical time series of physical and biological pa- 

 rameters to help direct future research efforts 

 attempting to couple the physics and biology of the 

 marine environment. Future monitoring surveys of the 

 U.S. Northeast shelf ecosystem will continue to mea- 

 sure the variability of zooplankton and gather infor- 



