NWA - Chapter 8.4

## Georges Bank Holozooplankton

### by Cabell Davis

The plankton ecology of Georges Bank has been reviewed extensively in Backus (1987) and can be summarized as follows. A spring diatom bloom occurs in March in the well mixed region (<60m) and April in deeper areas (60-100m); diatoms remain dominant in the well mixed region year-round while dinoflagellates dominate the stratified deeper area during summer/fall, with maximum abundance near the seasonal pycnocline (O'Reilly et al., 1987; O'Reilly and Busch, 1984). High biomass and productivity in the mixed area are maintained through tidally induced vertical mixing together with physical input of new nitrate (Walsh et al, 1987). About half the nitrogen demand of the primary production is supplied as nitrate input along the edges of the bank but the physical exchange mechanisms are poorly understood; recycling accounts for about 1/3 of the phytoplankton demand during winter and spring and about 2/3 during summer and fall (Walsh et al., 1987; Loder et al., 1982). Recent studies by Canadian researchers on the northeast flank suggest that tidal mixing processes may be the dominant physical factor controlling cross-frontal exchange in the mixed area (Harrison et al, 1990). On an annual basis, there is a high ratio of primary to secondary production compared to the North Sea, perhaps due to advective loss of plankton (Cohen and Grosslein, 1987). Estimates for seasonally averaged turnover rates of the bank water mass support this view (Mountain and Schlitz, 1987), but the time dependence on sub-seasonal scales is not known. In particular, the impact of storms and Gulf Stream rings on Georges Bank trophodynamics has not been examined (Mountain and Schlitz, 1987; Klein, 1987). Klein (1987) used a simple nutrient-phytoplankton-zooplankton model coupled to a kinematic model of circulation in the well mixed region of the bank. His model showed large loss rates of plankton from the region which were comparable to those found by Walsh et al. (1987), but he also noted that his physical loss rates were long term averages of short term events including storms and rings. He further discussed the need for more realistic zooplankton models which include population dynamics and spatial distributions outside the well mixed area.

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.

#### 8.4.1 References

Backus, R. H. 1987. (ed) Georges Rank. MIT Press, Cambridge, Massachusetts, 593 pp.

Cohen, E. B. and M. D. Grosslein. 1987. Production on Georges Bank compared with other shelf ecosystems. In: R.H. Backus (ed), Georges Bank, MIT Press, Cambridge, Massachusetts, p.383-391.

Davis, C. S. 1984a. Interaction of a copepod population with the mean circulation on Georges Bank. J. Mar. Res. 42, 573-590.

Davis, C. S. 1984b. Predatory control of copepod seasonal cycles on Georges Bank. Mar. Biol., 82, 31-40.

Davis, C. S. 1984c. Food concentrations on Georges Bank: non-limiting effect on development and survival of laboratory reared Pseudocalanus sp. and Paracalanus parvus (Copepoda: Calanoida). Mar. Biol. 82, 41-46.

Davis, C. S. 1987a. Components of the zooplankton production cycle in the temperate ocean. J. Mar. Res. 45, 947-983.

Davis, C. S. 1987b. Zooplankton life cycles. In: R. H. Backus (ed.), Georges Rank. MIT Press, Cambridge, Massachusetts, p. 256-267.

Davis, C. S. and P. Alatalo. 1990. Effects of food concentration on growth and reproduction in Centropages typicus (Copepoda: Calanoida) reared in continuous laboratory culture. Submitted to Limnol. Oceanogr.

Frost, B. W. 1989. A taxonomy of the marine calanoid copepod genus Pseudocalanus. Can. J. Zool. 67, 525-551.

Harrison, W. G., E. P. Home, B. Irwin, and T. Platt. 1990. Biological production on Georges Bank: are tidal fronts primary sources of new production in summer? EOS 71, 96.

Klein, P.1987. A simulation of some physical and biological interactions. In: R.H. Backus (ed), Georges Rank, MIT Press, Cambridge, Massachusetts, p. 395-405.

Loder, J. W., D. G. Wright, C. Garrett, and B. A. Juszko. 1982. Horizontal exchange on central Georges Bank. Can. J. Fish. Aquat. Sci. 39, 1130-1137.

Mountain, D. 0. and R. J. Schlitz. 1987. Some biologic implications of the circulation. In: R.H. Backus (ed), Georges Rank, MIT Press, Cambridge, Massachusetts, 392-394.

O'Reilly, J. E., C. E. Evans-Zetlin and D. A. Busch. 1987. Primary production. In: R. H. Backus (ed), Georges Rank, MIT Press, Cambridge, Massachusetts, 220-233.

O'Reilly J. E. and D. A. Busch. 1984. Phytoplankton primary production on the northwestern Atlantic shelf. Symposium on the biological productivity of north atlantic shelf areas, Kiel, West Germany, Mar. 2-5, 1982. Rapp. P-V Reun. Cons. int. Explor. Mer. 183(0), 255-268.

Walsh, J. J., T. E. Whitledge, J. E. O'Reilly, W. H. Phoel, and A. F. Draxler. 1987. In: R. H. Backus (ed), Georges Rank, MIT Press, Cambridge, Massachusetts, p. 234-246.