720 BULLETIN OF THE BUEEAU OF FISHERIES 



west as Browns Bank to a value (32.5 per mille) appreciably lower than had probably 

 prevailed there a week or two earlier in the month. This locates the first extension 

 of this comparatively fresh current as directed toward the southeast and not around 

 Cape Sable into the inner part of the gulf, though there is evidence that some of 

 this Nova Scotian water drifts right across the Eastern Channel later in the season 

 and far westward along the outer side of Georges Bank (p. 848). 



LIMITS OF WATER MORE SALINE THAN 34 PER MILLE 



Salinities higher than 34 per mille, whenever encountered in the deep trough of 

 the gulf, are unmistakable evidence that indraft is either taking place from the 

 region off the mouth of the Eastern Channel at the time, or has taken place so 

 recently that the saline water from this source has not yet been appreciably diluted 

 during the sojourn in the basin of the gulf by mixture with the less saline water 

 beneath which it spreads. A chart of the depth to which it would have been neces- 

 sary to descend to find water as salt at 34 per mille in the gulf in March, 1920, as 

 well as its horizontal limits, irrespective of depth (fig. 100), is therefore instructive 

 as graphic evidence of the recent activity of this movement. The gradient there 

 shown, with upper boundary of 34 per mille water Ij^ing 100 meters deeper at the 

 two heads of the two branches of the Y-shaped trough than in the Eastern Channel, is 

 proved the normal state by close correspondence with April (fig. 118) and midsummer 

 (fig. 152). It represents the consumption of this water in the inner parts of the gulf 

 as vertical mixing destroys its identity, and has an important bearing on the circu- 

 lation of the gulf from this standpoint (p. 849). 



Comparison with the corresponding isothermobath (fig. 20) shows that salinity 

 corresponds more closely to the contour of the bottom than to temperature at this 

 season, there being no reason to suppose that water as saline as 34 per mille 

 encroaches at all on Georges Bank in spring. The north-south ridge, which culmi- 

 nates in Cashes Ledge, also influences the salinity of the bottom water more than 

 its temperature. 



BOTTOM 



The salinity on bottom is interesting chiefly for the biologist who is concerned 

 with the physical conditions to which the bottom fauna is subject. In any small 

 subdivision of the Gulf of Maine this is governed directly by the depth, with the 

 water saltest where deepest; but when the survey is expanded to cover the area as a 

 whole, account must also be taken of the regional differences just described, especially 

 of higher salinities in the eastern side than in the western, and of freshenings of the 

 coastal zone, whether by river freshets or,by the Nova Scotian current. Early in 

 the spring, before these last influences have altered the water appreciably from its 

 winter state, the differences in salinity between the two sides of the gulf are widest 

 in the mid depths. Consequently we find the regional variation in bottom salinity 

 is then widest somewhat more than rnidway down the slopes of the basin, near the 

 100-meter contour. 



In March, 1920, the bottom water of this belt varied in salinity from about 

 32.3 per mille to 32.5 per mille, along the western- and northern margins of the gulf, 

 to about 33.5 per mille on its eastern slope, with a corresponding west-east grada- 



