810 BXTLLETIN OF THE BUKEATJ OF FISHERIES 



from its maximum to its minimum during the year illustrated, the 100-meter level 

 by about as much, though the surface freshened by upwards of 4 per mille. This 

 secular change also culminates later in the season with depth, just as vernal warming 

 does (p. 664), with the mid-stratum least saline about the first of September or four 

 months after the salinity of the surface has passed its minimum for the year. 

 The progressive freshening of the 75 to 100 meter stratum was also interrupted in 

 the July in question by some temporary welling up of more saline water from below. 



The graph (fig. 165) is also instructive for its demonstration that the incorpora- 

 tion of the vernal outpouring of river water into the supei-ficial strata of the bay has 

 little, if any, effect on the salinity at depths greater than about 140 to 150 meters. 

 Consquently the periodic variations that take place in its deepest waters reflect cor- 

 responding variations in the volume and precise salinity of the inflow over its rim 

 from the open basin of the gulf outside. SHght undulations in the curve for the 175- 

 meter level (fig. 165) show a sort of irregular pulse in this respect, in which the 

 annual variations seem (from available data) wider than the seasonal variations. 



This graph is a striking illustration of the general rule that the vertical range of 

 salinity is widest in coastwise boreal waters, generally, at the time of the vernal fresh- 

 ening of the surface; narrowest in autumn and winter, when little land water enters 

 and when winds, waves, and tidal currents stir the water most actively. 



MASSACHUSETTS BAY REGION 



The regional distribution of salinity in and abreast of Massachusetts Bay is such 

 that a difference of 3 to 5 miles in the location, nearer to or farther from shore, is 

 associated with wide differences in salinity, especially at the surface, so closely does 

 the freshest water hug the land during most of the year. 



The accompanying composite graph (fig. 166), based on monthly averages for 

 various years 8 to 12 miles of Gloucester, is offered as an approximation of the sea- 

 sonal progression to be expected in years neither unusually salt nor unusually 

 fresh, unusually late in seasonal schedule nor unusually early;'* and it pre- 

 tends to nothing more. It does not represent any one year; in fact, some of the 

 individual readings have differed considerably from the smoothed curve laid down 

 here, differences reflecting the annual variations described in the preceding pages. 



The curve for the surface corroborates an earlier graph, based on less extensive 

 data (Bigelow, 1917, p. 207, fig. 42), to the effect that the superficial stratum of 

 water may show vernal freshening as early as the end of February or a month earlier 

 than in the Bay of Fundy (p. 808) ; but additional records for the spring months have 

 proven that the minimum salinity for the year is to be expected considerably earlier 

 in the season in Massachusetts Bay than I formerly supposed, and that the salinity 

 falls to a much lower value there at its annual minimum. It is a fortunate chance 

 that our survey has included one spring (1920) that may be described as "fresh" in 

 this region, and one (1925) as "salt." These two years differed little during the first 

 half of April (p. 728; 32 to 32.4 per mille) , and the surface seems to have freshened to 

 its minimum about the last of April or first of May in both years. '^ However, while 



i< The station occupied at this general locality in July, 1916, is omitted, that being an unusually fresh year. 

 IS Observations were not taken at intervals close enough to establish the date more closely than this. 



