PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 
813 
that this progression may be interrupted by indrafts of water from offshore, or that 
the seasonal schedule may vary from year to year. 
The 100-meter salinities for this locality have averaged about 32.9 to 33 per mille 
for the period February to July (extremes 33.8 and 32.5 per mille), with no definite 
seasonal variation during that period. All but one of the determinations for the 
period August to October have been appreciably lower (32.5 and 32.6 per mille) than 
any for the rest of the year, however. An average seasonal varition of about 0.3 
per mille is thus indicated at 100 meters, reflecting the extreme depth to which vernal 
freshening from above is effective; but here, near its lower limit, this freshening does 
not culminate until a month or two later than at 40 meters, or four months later than 
at the surface. 
The data collected so far fail to show whether any definite seasonal variation of 
this sort can be traced at depths greater than 100 meters at this locality. 
Closer to land, in Massachusetts Bay off Boston Harbor, vernal freshening 
effects about as great a decrease in the salinity of the surface as at the mouth — 
from 32.1 to 32.2 per mille in March (of 1920 and 1921) to about 31 per mille in 
April and to about 30 per mille in May, followed by rather rapid recovery to 31 to 
32 per mille through July and August. The lowest values have been recorded as 
early in the year at 40 meters as at the surface (about 31.6 to 31.7 per mille, April 
and May, 1920). 
OFFING OF THE MERRIMAC RIVER 
The truly remarkable extent to which the vernal discharges from the large rivers 
govern the seasonal cycle of salinity in the coastwise belt of the gulf is illustrated by 
the offing of the Merrimac. To the southward of the Isles of Shoals, in its train, 
vernal freshening is as sudden an event and the decrease in the salinity of the sur- 
face is as great (by about 4 per mille) as in the Bay of Fundy (p. 808); but in 
the trough between the Isles of Shoals and Jeffreys Ledge, only some 20 miles out 
from the mouth of the river, the extreme range of salinity so far recorded at the 
surface for the months of December, March, April, May, July, August, October, and 
November 17 has been only about 1.2 per mille (31.6 to 32.8 per mille) ; nor does ver- 
nal freshening seem to culminate there until August — three months later than along 
shore. Furthermore, its effect is so closely confined to the immediate surface here 
that it has little effect at 40 meters and is not definitely reflected at all in the records 
for 100 meters or deeper where the salinity has proved virtually constant from sea- 
son to season and with but slight variations from year to year. 
NEAR MOUNT DESERT ISLAND 
The vernal freshening of the surface culminates at about the same season near 
Mount Desert Island as in the Bay of Fundy — i. e. late in April or early in 
May. 18 However, this sector of the coast is so much less affected by river water, 
and so much more open to the offshore waters of the gulf, that the seasonal range 
17 A total of 10 stations. 
18 Although only 12 sets of salinities have been taken here, the fact that we have records for 6 consecutive months for 1915, and 
that the other data are consistent with these, makes the graph a reliable picture of the cycle for the half year, May to October, 
which covers the season when the greatest changes in salinity take place. 
