Table <+. — Mean monthly discharge (in cubic 

 feet per second) of the major rivers 

 emptying into the Gulf of Maine, April and 

 May 1963 and 196^^ 



River 



St. Croix 



Machias 



Penobscot. . . . 

 Sheepscot. . . . 

 Androscoggin. 

 Sacc 



Total 



Year 



1964 



6,697 



3,025 



33,470 



788 



19,107 



10, 307 



73,394 



■'■ Data from the U.S. Geological Survey, Dis- 

 trict Engineer, Surface Water Branch, Vickery- 

 Hill Building, Augusta, Maine. 



Bigelow attributed the higher salinities and 

 cooler waters of the eastern Gulf to vertical 

 mixing. Lower salinities and warmer waters 

 of the western sector were related to river 

 discharge and thermal stratification. During 

 winter, one would expect wind-induced vertical 

 mixing and cool air moving from the land to 

 produce relatively homogeneous hydrographic 

 conditions. This situation existed during the 

 winters of 1963 and 1964. Profiles of tempera- 

 ture and salinity corroborate Bigelow's con- 

 clusions regarding vertical mixing in the 

 eastern Gulf and stratification of water in the 

 western area during the warmer months (fig. 

 10). 



Distribution of copepods and circulation 



The general decrease in zooplankton vol- 

 umes from west to east, differs from the 

 areal distribution which would be expected 

 from the cyclonic drift theory applied to the 

 calanoid community of the offshore Gulf of 

 Maine by Redfield (1941). He showed a progres- 

 sive west-to-east seasonal increase in the 

 Gulf calanoid population from a low in winter 

 to a high in late summer and early fall. This 

 change occurred during an annual circuit of 

 the Gulf in the cyclonic nontidal drift. If the 

 circulation theory presented by Redfield for 



offshore Gulf waters were applicable in the 

 coastal region, one would expect an increase 

 in the density of the calanoid population along 

 the coast from west to east; maximum volumes 

 should occur off Mt. Desert Island in early 

 fall. The generally decreasing volumes from 

 west to east in 1963 and 1964, and similar areal 

 decline in numbers of the eight dominant cala- 

 noid copepods suggest that local hydrography 

 exerts a greater influence on the abundance 

 and distribution of the coastal zooplankton 

 than does the cyclonic-eddy system of the Gulf 

 proper. Periodic offshore-inshore incursions 

 of calanoid species and other zooplankters 

 from outer Gulf waters have been reported 

 however (Bigelow, 1926; Fish, 1936a, 1936b 

 Fish and Johnson, 1937; Redfield, 1939, 1941 

 Redfield and Beale, 1940). During 1964 two 

 calanoid species known to breed in both in- 

 shore and offshore waters were responsible 

 for apparently anomalous west-to-east dis- 

 tributions of copepods. During the winter 

 Metridia lucens accounted for the eastern- 

 area peak in copepod numbers, and during the 

 fall Calanus finmarchicus was responsible for 

 the central-area peak. These anomalous occur- 

 rences may have resulted from incursions of 

 outer Gulf waters. 



Areal differences in zooplankton volumes 

 (low in the eastern area, nnoderate in the 

 central area, and high in the western area) 

 may be caused by dissimilar hydrography. 

 In the eastern area the unstable water column, 

 minimal runoff (J. J. Graham, unpublished 

 data), and lack of appreciable influx of zoo- 

 plankton from the north and east (Bigelow, 

 1926; Redfield, 1941) lead to minimal condi- 

 tions for population growth. In contrast, opti- 

 mal conditions of maximum runoff and asso- 

 ciated nutrient effluent, increased stability of 

 the water colunnn, and higher spring and 

 summer water temperatures in the western 

 area provide a favorable environment for 

 growth and developnnent of coastal zooplankton; 

 conditions in the central area are transitional 

 between the two extremes. Preliminary infor- 

 mation indicates that coastal eddies form along 

 the periphery of the Gulf coast under the 

 interaction of river discharge and nontidal 

 drift (J, J. Graham, unpublished data). Little 

 is known about the seasonal permanence of 

 these eddies. They may be of prime import- 

 ance, however, in the maintenance of popula- 

 tions of coastal zooplankton. 



