Overview of Georges Bank Plankton

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 (<60 m) and April in deeper areas (60-100 m); diatoms remain dominant in the well-mixed region all year while dinoflagellates dominate the stratified deeper area during summer and fall, with maximum abundance near the seasonal pycnocline (Figure 10; O'Reilly et al., 1987; O'Reilly and Busch, 1984). High biomass and productivity in the well-mixed area are maintained through tide-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 well-mixed area (Harrison et al., 1990).

On an annual basis, there is a low ratio of secondary to primary production compared to the North Sea, but a high ratio of fish production to secondary production (Cohen and Grosslein, 1987). It has been suggested that differences in apparent transfer efficiencies between trophic levels in the different ecosystems may be due to advective loss of zooplankton (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). Klein also noted that his physical loss rates were long term averages of short term events including storms and rings. Klein further discussed the need for more realistic zooplankton models which include population dynamics and spatial distributions outside the well-mixed area. Lacking any direct measurements of secondary production disallows anything more than speculation about the relative importance of in situ growth vs. advective exchanges (gains and losses) as factors controlling copepod population size.

The annual cycle, spatial distribution and production 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. O. similis is abundant throughout the year. Very little is known about their growth rates, but since they are small in size their growth rates are probably high. Because of this, their contribution to total production may be higher than one might guess from knowledge of their biomass alone. Zooplankton production is highest during late summer and early fall due to rapid growth of warm-water species that are small in size. Most of the production is thought to go 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 gain of individuals from surrounding regions to the Bank and loss from the Bank (Figure 11). Questions about the isolation of Bank populations from contiguous areas (e.g., Gulf of Maine, Slope Water) and the importance of local recruitment vs. immigration of new recruits from any "source" regions are unresolved. Stephenson and Kornfield (1990) concluded that recruitment of herring on the Bank resulted from reproduction of the indigenous population following a ten-year period of very low population density, because the pre- and post-crash populations were genetically indistinguishable. Population genetic and life history analysis of the target species will be required to address questions of the degree of reproductive isolation and the source(s) of immigrants to Bank populations .

Calanus finmarchicus is a large boreal copepod which reaches maximum abundance in June, accounting for the major portion of the spring zooplankton biomass peak. It enters diapause as fifth-stage copepodid in mid-summer and spends the warm stratified months at depths of 50-300 m in the Gulf of Maine (Bigelow, 1926; Clarke, 1933, 1934; Mullin, 1963) and 200-500 m in Slope Water (Miller et al., 1991). C. finmarchicus spawns on Georges Bank in February and produces two generations during its spring appearance there. During its growing season Calanus abundance is higher in the deeper regions of the Bank (60-100 m) than in the well-mixed area, Gulf of Maine, or Slope Water. 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, in press). 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 species which produce resting eggs, has a well defined distribution restricted to the well-mixed region.

Other larger crustacean zooplankton, including the mysid, Neomysis americana, the amphipods, Monoculodes edwardsi, Gammarus annulatus, and Themisto gaudichaudi, and the euphausiid, Meganyctiphanes norvegica, can serve as prey of late stage cod and haddock larvae (Lough et al., 1989), as well as predators on early stages. The role of these larger zooplankton as predators on the copepod populations may be important based on their abundance and large size. These species are all benthoplanktonic and are known to undergo pronounced diel vertical migration (Whitely, 1948). M. norvegica and the two amphipod species are more abundant in the deeper regions (60-100 m; Davis, 1987b). Very little is known about the life cycles of these species on Georges Bank (see review in Davis, 1987b).

Gelatinous species of zooplankton including medusae and ctenophores can be important predators on copepods and fish larvae, and larvaceans (Fritillaria borealis and Oikopleura dioica) may be important grazers of phytoplankton on the bank. Gelatinous predators thought to be important on Georges Bank include the scyphomedusa Cyanea capillata (250-1200 mm), the large hydromedusa Staurophora mertensii (100-200 mm), the trachymedusa Aglantha digitale (20-40 mm), and the ctenophore Pleurobrachia pileus (20-40 mm) (Davis, 1987b). Commensal associations of pelagic juvenile haddock with the hydromedusae Cyanea capellata have been reported (Lough and Potter, in press). Data on the abundance of these species in time and space on Georges Bank is limited due to a general lack of quantitative sampling methods for these delicate forms.

Species of zooplankton targeted for study include those which are dominant on Georges Bank and also have an important impact on the life cycle of larval cod and haddock. These species include the copepods Calanus finmarchicus and Pseudocalanus spp. As mentioned above, other copepods, larger crustaceans, chaetognaths, mysids, and gelatinous predators should be studied at the same time.