TEMPERATURE 'C. 



0= I 



HI 



I- 

 bi 



Z 



25 



50 



75 



100 



"T 1 — 7~<~ 



Stotion 12 



Station 21 



WEST 



CENTRAL 



EAST 



Figure 6. — Bathythermograph traces for selected stations 

 in the western, central, and eastern areas of the coastal 

 Gulf of Maine in summer 1967 and 1968. 



The displacement volume of individuals is con- 

 siderably less for copepodites than for adults 

 but since the copepodites were more numerous 

 in 1968 than the adults in 1967, the standing 

 crop estimates in spring were not significantly 

 different in the 2 years. 



The between-year difference in the timing 

 of the spring sampling was also reflected in 

 the abundance of other zooplankton commonly 

 eaten by juvenile herring. Decapod larvae, 

 cladocerans, and cirriped larvae were more 

 numerous in the spring samples in 1968 than 

 in 1967 (table 2). 



The seasonal peak in zooplankton abundance 

 in 1963-66 was in summer (Sherman, 1965, 

 1966, 1968). Whether the low summer standing 

 crop in 1967 and 1968 is a result of a timing 

 gap in sampling, which missed a period of 

 swarming, or an actual low in biomass is not 

 known. Several of the copepod species abundant 

 in coastal waters of the Gulf produce a new 

 generation within 30 days during spring and 

 summer (Fish, 1936a, b, and c), and it would 

 be possible to miss an entire generation in 

 sampling limited to a single 10-day cruise in 

 each season. A semimonthly series of zoo- 

 plankton samples are now obtained from waters 

 near Boothbay Harbor to supplement coastal 

 monitoring and provide information on seasonal 

 succession in the plankton. 



Previous investigators have observed the 

 general decline in zooplankton abundance from 

 west to east and reported that this decline 

 is a result of dissimilar hydrography along 

 the coast (Bigelow, 1926; Fish and Johnson, 

 1937). In the east, the unstable water column 

 and the lack of appreciable influx of zooplank- 

 ton from the north and east lead to relatively 

 poor conditions for population growth. The 

 higher spring and summer temperatures in the 

 western and central areas, where the water 



column is relatively stable and stratified, 

 provide an increasingly favorable environment 

 for growth and development of zooplankton 

 from Mt. Desert to Cape Ann (Bigelow, 1926; 

 Fish and Johnson, 1937; Sherman, 1968). 



Circulation and Zooplankton Abundance 



The intensity and duration of river discharge 

 influence the annual development of nontidal 

 drift in the Gulf of Maine (Bigelow, 1927; 

 Bumpus, 1960; Bumpus and Lauzier, 1965). 

 Runoff in the eastern half of Maine was less in 

 1968 than in 1967 (Anonymous, 1969). This 

 difference could have contributed to the ap- 

 parent anomalous concentrations of three of 

 the more numerous copepod species in summer 

 1968. Concentrations of Calanus finmarchicus , 

 Pseudocalanus minutus , and Termora longi- 

 cornis were similar in the eastern and western 

 areas of the coast in summer 1968, when the 

 reduction in runoff would have advected less 

 zooplankton out of the eastern area in the domi- 

 nant southwesterly drift along the coast. These 

 species followed the more usual pattern ofde- 

 clining abundance from west to east in the 

 summer of 1967 following the spring freshets. 



The annual differences in the abundance of 

 zooplankton from west to east along the coast 

 (fig. 3) corroborate similar differences found 

 both in the early decades of the century, and 

 more recently in each year in 1963-68. Al- 

 though the annual trend in zooplankton abun- 

 dance is well established, the seasonal differ- 

 ences among the three coastal areas are more 

 variable (table 1). Significant variations in 

 abundance can occur between 2 years at the 

 same season of the year, depending on the 

 timing of sampling- -as evidenced by the differ- 

 ences in the spring abundance estimates of 1967 

 and 1968. Local differences in circulation can 

 affect the areal abundance of zooplankton, as 

 observed in summer of 1968. The holoplankters 

 found in coastal waters are widely distributed- - 

 particularly the dominant copepods. Concen- 

 trations of the predominant species, Calanus 

 finmarchicus and Centropages typicus , occur 

 in offshore waters (Bigelow, 1926; Fish, 1936a), 

 and offshore waters are advected periodically 

 into the coastal region to compensate for estu- 

 arine discharge (Graham, in press). The 

 influence of the offshore populations on the 

 abundance of zooplankters along the coast is 

 being investigated. 



LITERATURE CITED 



ANONYMOUS. 



1969. Water resources data for Maine, 

 1968. U.S. Geolog. Surv., 98pp. 

 BIGELOW, HENRY B. 



1926. Plankton of the offshore waters of the 

 Gulf of Maine. U.S. Bur. Fish., Bull. 

 40: 1-509. 



