The copepod C alanus f iamarchicus is the most abundant zooplankton species 

 found in the Gulf of Maine, Bay of Fundy, and surrounding coastal waters, 

 where it often makes up 35% to 70% of the total zooplankton population. It is 

 noted for its importance as the chief diet of herring, shad, and mackerel 

 (Bigelow 1926; Fish 1936; and Sherman 1970). This species is primarily a 

 coastal species that is carried inshore by tidal currents. 



In view of the observations of Bigelow (1926), Fish and Johnson (1937), and 

 Sherman (1965, 1966, 1968, and 1970) on the enormous number of C. f inmarchicus 

 in the Gulf of Maine, it is likely that zooplankton grazing limits 

 phytoplankton populations (Riley 1946 and 1947). Gushing (1968) has shown 

 this to be the case in temperate waters where C. f inmarchicus is the dominant 

 zooplankton organism. It is proposed that during the spring phytoplankton 

 bloom, Calanus lays its eggs; after the eggs hatch, the nauplii grow to adults 

 and consume enough phytoplankton to decrease the standing stock. Sufficient 

 data to test this hypothesis in the coastal areas of the Gulf of Maine are not 

 available at present (Cohen 1975). 



Mean volumes of zooplankton (figure 4-24) are primarily determined by 

 abundances of C. f inmarchicus (figure 4-25; Sherman 1968). An areal decline 

 in zooplankton volumes and C. f inmarchicus abundances from southwestern Maine 

 to northeastern Maine is apparent. Sherman (1968) states that differences in 

 hydrography in the three areas appear to account for this decline. The lower 

 temperatures and unstable water column in the northeastern Gulf in combination 

 with a lack of immigrants from the north and east has been hypothesized as the 

 reason for lower populations in eastern Maine (Sherman 1968). The water 

 column becomes increasingly stable and spring and summer temperatures become 

 higher from northeast to southwest. These factors favor increased abundances 

 of the zooplankton population (Sherman 1968). Based on nutrient data (above), 

 and hydrographic data (above) it would be expected that zooplankton volumes 

 would be higher in the northeastern part of the characterization area (regions 

 5 and 6), because of the potentially high phytoplankton production in this 

 region. It is unknown why this has been found not to be true. 



In Penobscot Bay (region 4) and probably in other bays along the coast, the 

 seasonal cycle of zooplankton is somewhat different from that in the coastal 

 and offshore waters. Copepods comprised more than 907o of the total 

 zooplankton population in Penobscot Bay from August through early March 

 (Bertrand 1977). From late March through July copepods accounted for <50% of 

 the zooplankton. Balanus spp . (barnacle) larvae, cladocerans, and echinoderm 

 larvae (e.g., starfish) made up most of the rest of the zooplankton. Relative 

 percentages of copepod and noncopepod zooplankton in three areas of Penobscot 

 Bay are given in figure 4-26. 



In spring, cirriped (barnacle) larvae are a significant part of the 



zooplankton in coastal waters (Sherman 1968) but never reach abundances 



comparable to those in Penobscot Bay. This spring-summer pulse of principally 



meroplanktonic species is typical of bays and estuaries in coastal Maine. 



The decreased copepod abundance in Penobscot Bay in spring and early summer 

 may be due in part to predation (Bertrand 1977). Important predators present 

 at this time are the ctenophores, Bolinopsis infunidbulum and Pleurobrachie 

 pileus ; the medusae, Cyanea capillata and Aurelia aurita ; the chaetognath, 

 Sagitta elegans ; and the fishes, Atlantic herring and Atlantic menhaden. 



4-68 



