708 BULLETIN OF THE BUEEAU OF FISHERIES 



outflow from the Merrimac produced a slightly greater vertical range of salinity 

 (average difference of 1.5 per mille between surface and 40 meters) in the region 

 between Cape Ann and the Isles of Shoals by March 12 {Fish HawJc cruise 9, stations 

 20 to 28), though its full effect was not felt until a month later (p. 725). 



Unfortunately, the water samples for these Fish Hawk stations and for the Alba- 

 tross station off Boothbay for March 4, 1920 (station 20058), were not taken at vert- 

 ical intervals close enough to show whether the river water was then pouring into 

 the gulf in volume great enough to maintain a sharply defined stratum of low salin- 

 ity at the surface. It is more likely that vertical stirring by tides and waves still 

 continued active enough to produce a more even gradation from the surface down- 

 ward. However, its effect was certainly greatest close to the surface and perhaps 

 not appreciably deeper than 20 to 40 meters until later on in the season. 



40 METERS 



Thanks to the homogeneous state that characterizes the superficial stratum of 

 the whole gulf (with the exceptions just noted) diu-ing the late winter and early 

 spring, the regional distribution of salinity for February and March is much the same 

 down to a depth of 40 to 50 meters as it is at the sm'face (fig. 91). The agree- 

 ment is especially close for the isohaline for 32.5 per mille, which shows the same con- 

 trast at 40 meters (fig. 93) between fresher water near land and salter offshore all 

 around the guff as at the surface, and with the same expansions of low salinity out 

 over the western half of Georges Bank, southward into the central part of the basin 

 off the Penobscot Bay region, and out from Nova Scotia across the Northern 

 Channel to Browns Bank. 



The isohalines for the 40-meter level (fig. 93) likewise parallel those for the sur- 

 face in locating the axis of the freshest band on the Shelburne profile (< 32 per 

 mille) as lying over the outer part of the shelf, not close in to that coast as we have 

 found it later in the season (fig. 132) . However the rather abrupt east-west transition 

 in salinity from this tongue to higher values over Browns Bank and in the Eastern 

 Channel (32.86 per mille, station 20071) is sufficient evidence that the Nova Scotian 

 current had not appreciably affected the sahnity so deep as this farther west than 

 longitude 65° up to this date, though some slight movement of water may already 

 have taken place in this direction at the surface (p. 703). 



The distribution of water salter than 32.5 per mille is also very nearly the same 

 at 40 meters as at the surface in March, with the same gradation lengthwise of 

 Georges Bank from lower values (about 32.4 per mille) at the western end to higher 

 values (about 32.6 to 32.7 per mille) at the eastern, and to slightly more saline water 

 (32.8 to 33 per mille) in the Eastern Channel and in the southeastern part of the 

 basin. 



It is interesting to find a circumscribed pool of very high salinity ( > 33 per mille) 

 in the eastern side of the basin at this level, which could have resulted only from 

 some local upwelling. 



In winter and early spring, when the water has little vertical stability to resist 

 vertical currents, events of this sort are to be expected locally over small areas as 

 the result of tidal churnings, or caused by the wind. The distribution of salinity at 

 different seasons shows that the basin is most subject to them in its eastern side, and 



