PHYSICAL OCEANOGBAPHY OF THE GULF OP MAINE 



747 



season is afforded by the station data for 1920 at the mouth of Massachusetts Bay 

 (station 20120) for May 4, when the upper 15 meters was near its minimum salinity 

 for the year and homogeneous (29.1 to 29.2 per mille), but with the salinity 

 increasing by 2 per mille in the next 15 meters of depth to 31.13 per mille at 30 meters. 

 A vertical distribution of this type, coupled with the fact that the deeper water there 

 was less saline on that date than it had been two weeks previous (station 20092), 

 is evidence that when the tongue of water of low salinity described above (p. 741) 

 first spread southward past Cape Ann, vertical mixing was active enough for it to 

 dUute the whole column of water at the mouth of the bay. The latter, however, 

 was followed in turn by an increase in the salinity of the whole column during the 

 next 12 days, resulting primarily from a movement of more saline water inward 

 over the bottom (fig. 122; stations 20120 and 20124). 



Events seern to have followed a similar course in the Isles of Shoals region in 

 1913, when Mr. Welsh recorded a progressive increase in the mean salinity of the 

 whole column of water, in depths ranging from 36 to 48 meters, from about 31.1 



Fig. 122.— Vertical distribution of salinity at the mouth of Massachuietts Bay. A, April 20, 1920 (sta- 

 tion 20119); B, May 4, 1920 (station 20120); C, May 16, 1920 (station 20124) 



per mille on May 10 to 13, 31.5 per mille on the 13th, and 32.7 per mille on the 16th, 

 resulting in the i-ecovery of the bottom salinity (32.2 to 32.6 per mille) almost to 

 the April value (32.5 to 32.8 per mille). Evidently the absorption of freshet water 

 from the rivers into the general circulation was accompanied by some indraft of 

 water of high salinity from offshore in this region; otherwise the mean salinity of 

 the column of water would not have increased as it did. 



On the other hand, the salinity of the bottom water of Massachusetts Bay 

 changed very little from April to May in 1925 '" at depths greater than 40 meters, 

 except for a slight decrease near Cape Ann, reflecting the surface drift from the north 

 (p. 741). It is certain, therefore, that bottom water does not enter the bay every 

 May in as great volume as it did in 1913 and 1920. 



In the coastal sector between Cape Cod and Penobscot Bay the vertical range 

 of salinity is wider in May than at any other time of year — widest of all off the 

 river mouths and along the track followed by the discharges from the latter. Off 

 the mouth of the Kennebec, for example, the surface had freshened to 29.6 per mille 

 by May 13, 1915, a value about 3 per mUle below that of the 50-meter level (about 



' Fisli, Hawk cruises 12 and 13. 



