COHERENCE IN ZOOPLANKTON OF 

 A LARGE NORTHWEST ATLANTIC ECOSYSTEM 1 



K Sherman, 2 J. R. Green, 2 J. R. Goulet, 2 and L. Ejsymont 3 



ABSTRACT 



Mesoscale measurements of zooplankton of the continental shelf off the northeast United States reveal pre- 

 viously unreported large-scale temporal and spatial coherence in the Gulf of Maine, on Georges Bank, off 

 Southern New England, and in the Mid- Atlantic Bight. Unlike the apparent decline in zooplankton over the 

 30 years reported for the North Atlantic and North Sea, the zooplankton of the northeast shelf have not 

 undergone any large-scale change in abundance or species composition since initial measurements made 70 

 years ago. Recent declines in fish populations of the shelf appear related more directly to excessive fishing 

 mortality than to any changes in the abundance of zooplanktoa 



Zooplankton in marine ecosystems function as links 

 between primary producers (phytoplankton) and 

 predatory populations of fish, marine birds, and 

 mammals. Mesoscale changes in zooplankton abun- 

 dance have been associated with disruption of 

 predator- prey relationships resulting in econo- 

 mically disastrous declines in fish stocks (Glover 

 1957; Glover et aL 1 96 1 ; Williamson 1961; Jacobsen 

 1980). Although it has been demonstrated that large- 

 scale (100-1,000 km) seasonal and annual variability 

 in abundance of zooplankton has been associated 

 with advective processes in the northeast Pacific and 

 northeast Atlantic (Wickett 1967; Colebrook 1977, 

 1978a, b), we have not observed any large-scale 

 changes in abundance of zooplankton off the 

 northeast coast of the United States. The region has 

 been under investigation since the turn of the cen- 

 tury, but previous studies of zooplankton have been 

 limited to restricted areas of the northeast shelf and 

 covered relatively short periods of time (Fish 1925, 

 1936a, b; Bigelow 1926; Clarke and Zinn 1937; 

 Bigelow and Sears 1939; Clarke 1940; Clarke et aL 

 1943; Deevey 1952, 1956, 1960; Grice and Hart 

 1962; Sherman 1968, 1970, 1976; Malone 1977; 

 Judkins et al. 1980). 



METHODS 



Our findings are based on 32 surveys of zooplankton 

 conducted by the United States, Poland, Soviet 

 Union, and German Democratic Republic between 



'MARMAP Contribution No. MED/NEFC 82-68. 



2 Northeast Fisheries Center Narragansett Laboratory, National 

 Marine Fisheries Service, NOAA, South Ferry Road, Narragansett, 

 RI 02882, U.S.A. 



'Morski Instytut Rybacki, Zaklad Sortowania i Oznaczania 

 Planktonu, 71-550 Szczecin, Poland. 



Manuscript accepted August 1983. 

 FISHERY BULLETIN: VOL. 81, NO. 4, 1983. 



1 97 7 and 1 98 1 , as part of a joint MARMAP study of the 

 ecosystem of the northeastern shelf (Sherman 1 980). 

 Between 6 and 8 surveys were done per year. Sampling 

 was done in four subareas: Gulf of Maine, Georges 

 Bank, Southern New England, and Mid-Atlantic 

 Bight, each characterized by distinct bathymetry and 

 circulation (Emery and Uchupi 1972; Butman et aL 

 1982) (Fig. 1). Zooplankton were collected at an 

 average of 129 locations per survey situated 25-35 

 km apart, resulting in a total of 3,568 samples. The 

 time-series analyzed for each subarea is shown in 

 Figure 2. At each sampling location, tows for 

 zooplankton, using a paired bongo- type sampler 

 (Posgay and Marak 1980) with 60 cm openings and 

 nets of 0.333 and 0.505 mm mesh, covered the water 

 column obliquely from 5 m above bottom to the sur- 

 face. These nets were towed at ship speeds from 1.5 

 to 3.5 kn, and were lowered at a wire speed of 50 m/ 

 min and retrieved at 20 m/min. Water filtered 

 through the net was measured with a flowmeter and a 

 time- depth recorder was used to measure the towing 

 profile of the sampler. 



Zooplankton samples were sorted, identified, and 

 counted at the Plankton Sorting Center, Szczecin, 

 Poland. The biomass of zooplankton is expressed as 

 cc/100 m 3 of water strained; numerical abundance is 

 expressed as numbers of zooplankters/100 m 3 of 

 water strained. Patterns of abundance of the domi- 

 nant zooplankters are based on the analysis of the 

 size-fraction retained in the 0.333 mm net, which 

 primarily captured late juvenile and adult copepods. 



RESULTS 

 Coherent Patterns of Biomass 



Displacement volumes expressed as cc/100 m 3 of 



855- 



