The annual cycle and spatial distribution of zooplankton on Georges Bank are analyzed in Davis (1984a,b,c; 1987a,b). The zooplankton is dominated in numbers and biomass by the copepods Calanus finmarchicus, Pseudocalanus newmanii, Pseudocalanus moultoni, Centropages typicus, Centropages hamatus, Paracalanus parvus, and Oithona similis. Calanus and Pseudocalanus are winter-spring species, while Centropages and Paracalanus are dominant during fall. 0. similis is abundant throughout the year but is not important in terms of biomass or production. Zooplankton production is highest during late summer - early fall due to rapid growth of small warm-water species, with most of the production going into predation by the chaetognath Sagitta elegans, the ctenophore Pleurobrachia pileus, and the omnivorous copepods Centropages spp.
Each of the dominant species has its own characteristic life cycle (Davis, 1987a) and therefore may be impacted differently by advective loss from the bank. Calanus finmarchicus is a large boreal animal which reaches maximum abundance in June accounting for the major portion of the spring zooplankton biomass peak. It enters diapause as fifth stage copepodids in mid-summer and spends the warm stratified months at depths of 200-300m in the Gulf of Maine and Slope Water. C. finmarchicus spawns on Georges Bank in February and produces two generations during its spring appearance there. During its growing season Calanus abundance is highest in the deeper regions of the bank (60-100m) than in the well mixed area, Gulf of Maine, or Slope Water. Calanus likely undergoes diapause to avoid the warm oligotrophic fall conditions. Calanus undergoes diel and seasonal vertical migration which depend on life stage. The life cycle of Pseudocalanus moultoni is similar to Calanus' in that it begins its population growth during the winter when it is carried onto the northwestern edge of Georges Bank by prevailing currents. Pseudocalanus (including P. newmanii, Frost, 1989) reaches maximum abundance in spring (May/June). Pseudocalanus spp. abundance decreases markedly after June as it gives way to Centropages hamatus, C. typicus, and Paracalanus parvus. The latter two species, during peak abundance, inhabit the warm surface layer on Georges Bank and the Gulf of Maine undergoing little or no diel migration. Their distributions are less restricted to the bank as is the case for their spring counterparts. P. parvus is not likely to be food limited on Georges Bank whereas C. typicus growth and reproduction are inhibited at mean bank food levels (Davis and Alatalo, 1990). C. hamatus lays bottom resting eggs which overwinter in the sediments and hatch out from August-September giving rise to a large fall population. This species, like other resting egg layers, has a well defined distribution restricted to the well mixed region. At present, we have only a limited understanding how physical processes interact with dynamics of dominant zooplankton species on Georges Bank.
In short, the effects of large scale physical forcing on ecological efficiency and consequences for recruitment at higher trophic levels are poorly understood. Significant insights can be gained by modeling interactions between physical transport and simple food chain dynamics as well as dominant or characteristic zooplankton species. Each species has evolved certain characteristics which are affected differently by advective transport out of favorable growth areas.
Changes in global climatic conditions can potentially have dramatic effects on Georges Bank plankton. The most direct effects might be through changes in sea surface temperature (SST). Prevailing winds from the North American continent cause an unusually large seasonal range in SST in the Georges Bank region. Since this area represents a faunal transition zone between colder boreal plankton to the north and warmer water species to the south, any general trends in land air mass could alter the SST and cause latitudinal shifts in this transition zone away from the Georges Bank region. Temperature changes larger than 2°C could cause significant latitudinal displacement in the relative abundance of planktonic species. Thus this region, due to its strong seasonality, may be relatively more sensitive to changing climatic conditions than other areas.
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