PLANKTON OP THE GULF OF MAINE 
415 
taking place a month or so earlier in the season. With similar emendation in date in 
one direction or the other, it would apply to Massachusetts Bay equally in an “ early” 
year, such as 1913, or in a late year. 
The mutual fluctuations of G. tripos and C. longipes may be summarized as 
follows for the Gulf of Maine as a whole: 
Early in spring when the vernal augmentation of diatoms is at its height, Cera- 
tium (and indeed all peridinians) practically vanishes from the gulf, an event taking 
place first along the northwest coast, where the diatoms flower earliest, and soo 
afterwards in other parts of the gulf. After the flowerings of diatoms dwindle, C 
longipes (fig. 112) multiplies until July, when all the gulf, except for a narrow zone 
along its northeast and east coasts, supports an abundant Ceratium plankton. 
During July C. tripos (figs. 113 and 114) multiplies in the central deeps. As the 
summer advances the area of abundance of C. tripos expands coastwise and the stock 
of C. longipes dwindles until tripos becomes predominant along the southwestern, the 
eastern, and finally along the northwestern and northern coasts of the gulf, with C. 
longipes persisting latest as an important factor in the plankton in the region between 
Cape Elizabeth and the Grand Manan Channel. C. tripos predominates throughout 
the winter, but even then, when C. longipes is at its lowest ebb, the latter has occurred 
in small numbers at most of our stations; nor does either species vanish wholly 
from the gulf at any season, though either may be so scarce when the other is at 
its peak of abundance, as well as during the flowering period of the diatoms, that 
careful search of considerable amounts of plankton may be required to reveal its 
presence. 
The seasonal changes in the relative abundance of these two peridinians must 
not, of course, be understood to take place in as orderly a manner as they are repre- 
sented here, for they are undoubtedly accompanied by temporary interruptions and 
even reversals, which would alter the smooth curves to a succession of zig-zags, were 
daily or weekly records available. In fact, such a reversal is known to have taken 
place in 1915 off Machias, Me., where longipes was predominant on July 15 (in the 
proportion of 16 longipes to 3 tripos at station 10301), was outnumbered by tripos 
on September 11 (station 10316), was again predominant on October 9 (station 
10327), and would doubtless have been found outnumbered by tripos a month after 
that, had we visited that region again later in the season. Sporadic alternations of 
this sort do not weaken the general thesis that the succession, as here outlined, is a 
regular and characteristic feature of the planktonic cycle of the gulf, however, though 
its time table varies from year to year, as do all other seasonal changes in the sea. 
In the foregoing account I have purposely refrained from alluding to the status 
of the two leading species of Ceratium on Georges Bank in late summer or autumn 
( longipes predominates there in spring and early summer (p. 408) as it does elsewhere 
in the gulf), because no collection of phytoplankton has yet been made on the bank 
during the half year, August to February 1 . 
A fourth species of Ceratium — C. fusus — has been taken so often in our tow 
nets that it deserves brief mention, though it is never predominant in the Gulf of 
Maine. C. fusus has been found at most of the stations where the genus as a whole 
