the eastern area and Eurytemora herdmani in 

 the central area in summer 1968 (table 3). 



HYDROGRAPHY AND ZOOPLANKTON 



Observations were made of the seasonal 

 changes in the temperature and salinity of the 

 coastal waters and the effects of temperature 

 changes on zooplankton abundance. 



Surface Temperature and Salinity 



Surface temperatures and salinities in each 

 of the areas varied seasonally. In the western 

 and central areas temperatures increased from 

 a winter low to a summer high and decreased 

 in autumn; in the eastern area the annual max- 

 imum was reached in autumn. The salinity 

 maximum was in autumn in the eastern area and 

 during winter in the central and western sec- 

 tions. Surface conditions were similar in all 

 three areas in winter. In other seasons tem- 

 peratures decreased and salinities increased 

 from west to east along the coast (fig. 4). 



The areal differences in tennperature and 

 salinity are a result of local environmental 

 conditions, rather than of large-scale advection 

 of waters. The low temperatures and high 

 salinities of the eastern area are produced 

 through vertical mixing by tidal stirring; the 

 higher temperatures and lower salinities of the 

 western region result from increased stability 

 of the water column, reduced tidal mixing, and 



1968 



>^„s-^\*^^ ..# 















Flgure 4. — Mean seasonal surface temperature and salin- 

 ity for the western, central, and eastern areas of the 

 coastal Gulf of Maine in 1967 and 1968. 



large-scale runoff from rivers (Bigelow, 1927; 

 Sherman, 1966). Bathythermograph traces 

 made along the coast support earlier reports 

 of vertical mixing in the east and stratification 

 in the west (fig. 5). Large-scale seasonal 

 changes in the temperature of coastal waters 

 of the Gulf of Maine are controlled by the sea- 

 sonal variations in solar radiation, prevailing 

 wind patterns, and river runoff (Bigelow, 1927). 

 The long-term annual changes in temperature 

 appear to depend on shifts in the relative posi- 

 tion of coastal and oceanic water masses along 

 the edge of the Continental Shelf (Colton, 1968a, 

 b, and c). 



Variations in Temperature and Zooplankton 

 Abundance 



Thermal stratification of coastal waters 

 begins in spring and is most pronounced in 

 summer. With the onset of vernal warming, 

 the overwintering populations of adult zoo- 

 plankters produce swarms of nauplii, cope- 

 podites, and other young fornns. The timing of 

 the spring swarming, however, varies with 

 temperature. In spring 1967, when sampling 

 was earlier (March 28 to April 13) than in 1968 

 (May 15-22), the coastal waters were cool 

 (< 3° C.) and vertically mixed from Cape Ann 

 to Machias Bay, In spring 1968, the waters 

 were warmer and a thermocline was developed 

 in the western and central areas (fig. 6); 

 eastern waters were also warmer, although 

 vertically mixed because of tidal stirring. 

 Zooplankton volumes in spring were not 

 significantly different between the two years 

 (U = 56, P > 0.05), but the size composition of 

 the dominant zooplankter- - Calanus finmar- 

 chicus - - showed the effects of the earlier 

 sampling in 1967; the adult overwintering form 

 was dominant in 1967, whereas copepodite 

 stages three and four predominanted in 1968. 



10 15 



TEMPERATURE C. 

 O 5 10 15 



10 15 



25 



50 



75 



K 100 



1968 



EAST 



Figure 5. — Bathythermograph traces tor selected stations 

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

 GuU of Maine in spring 1967 and 1968. 



