PHYSICAL OCEANOGKAPHY OF THE GULF OF MAINE 



811 



this reduced the surface salinity by at least 3.2 per mille between April 9 and May 4 

 (29.1 per mille) in 1920, the lowest value recorded at the mouth of the bay in 1925 

 was 31.3 per mille on April 23 and again on May 22, though it is possible, of course, 

 that the "peak" fell between these two dates, as already remarked (p. 741). 



A considerably higher surface value at this locality on May 4, 1915 (station 

 10266, 32.3 per mille), is reconcilable on the assumption (discussed above) that the 

 effects of vernal freshening were more closely confined to the immediate vicinity of 

 the land in that spring. However, this record is averaged on the graph (fig. 166). 



-Q 





Fig. 166.— Seasonal progression of salinity at the surface at the mouth of Massachusetts Bay, 12 miles off Gloucester, 

 based on monthly averages of the records in the various years. The data for July, 1916, are omitted for the reason 

 given on p. 810 



Taking one year with another, the lowest surface salinity of the year is to be 

 expected at this general locality between the last week of April and last week of May. 

 Surface values lower than 31 per mille (sometimes as low as 29 per mille) are to be 

 expected there at some time during this period — a decrease of more than 2 per mUle 

 from the maximum salinity at the end of winter. 



The vernal freshening at this particular I'egion results chiefly from the discharges 

 from the large rivers to the north (nearest of these is the Merrimac), for no large 

 streams empty in the immediate vicinity. Consequently, any fluctuations in the 

 volume and direction of the drift past Cape Ann will be mirrored by corresponding 



