PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 725 



of the gulf. The minimum for the coastal sector between Cape Sable and St. Marys 

 Bay can not be stated for want of observations close in to the land at the critical 

 season, but may be set (tentatively) at about 31 per mille, contrasting with 28 to 

 29 per mille in the opposite side of the gulf (p. 702). 



In 1925 the surface salinity of the Isles of Shoals-Cape Ann sector had de- 

 creased to 28.7 to 29.1 per mille by April 7 to 8, a change of more than 1 per mille 

 since March 12 {Fish Hawk cruises 9 and 11). Up to that date, however, freshen- 

 ing from the land had hardly affected the surface at the mouth of Massachusetts 

 Bay, which was still 31.9 to 32 per mille, with 31.2 per mille in its inner waters 

 near Plymouth {Fish Hawk stations 10 and 31 to 34, cruise 11). So little change 

 took place in the surface state of the bay during the next two weeks that the Fish 

 Hawk again had 31.1 per mille to 32 per mille there on AprU 21 to 23. 



The reason the surface of Massachusetts Bay does not experience a drop in 

 salinity as early or as sudden as the coast sector north of Cape Ann, only a few 

 miles away, is simple: No large streams empty into the bay, so that the only source 

 from which it can receive large volumes of land water are the rivers tributary to 

 more northerly parts of the gulf. Naturally the freshening effect of these is not as 

 pronounced at a distance from their mouths as it is near by, nor is it felt as soon. 

 This explanation is corroborated also by the fact that the lowest salinities recorded 

 for the Massachusetts Bay region for AprU 21 to 23, 1925, took the form of a tongue 

 extending southward past Cape Ann, obviously with its source to the north — i. e., 

 from the Merrimac (fig. 102). 



The general surface chart for April, 1920 (fig. 101), is made one of the most 

 interesting for the year by its demonstration that the freshening effect of the river 

 freshets continues strictly confined to the coastal zone until late in the month and 

 does not spread out over the surface of the gulf generally, as might, perhaps, have 

 been expected. By contrast, the basin of the gulf outside the 100-meter contour alters 

 so little in salinity from March to April that- the greatest change there from the one 

 month to the next in 1920 was only about 0.5 per mille for any pair of stations. 

 The surface also remained unaltered over the eastern end of Georges Bank (we have 

 no April data for the western end), where the extreme variation in salinity from 

 March to AprU of that year was only about 0.1 per mUle. Mr. Douthart found a 

 similar gradation (though with actual values 0.5 to 1 per mUle higher) on April 27, 

 1913, from 31.5 in Massachusetts Bay to 33.1 to 33.3 per mUle on the southwestern 

 part of the basin and along the northern half of Georges Bank. The contrast in the 

 salinity of the surface water between inshore and offshore stations is greater in April, 

 in fact, than in any other month. On the other hand, the pool of high surface salin- 

 ity (32.8 per miUe) that occupied the southeastern part of the basin of the gulf and 

 the inner end of the Eastern Channel in March, 1920 (p. 704, fig. 91), had been 

 entirely dissipated by the middle of the following month, leaving this whole area 

 uniformly about 32.5 to 32.6 per mille at the surface; but in its stead the surface 

 salinity at one station in the eastern side of the basin, off Lurcher Shoal, had been 

 increased to an equaUy high value (32.89 per mUle) by some local disturbance of 

 water. 



The discovery of these pools of high salinity in different localities in different 

 months — one of them, at least, short lived — is more interesting than the slight actual 



