926 BXTLLETIX OF THE BUREAU OF FISHEEIES 



that the consumption of slope water is most rapid. From March to April, however, 

 vertical currents in the coastal belt between Cape Cod and Cape Ann are suddenly 

 opposed by the increase in stability effected by the combined effect of the freshen- 

 ing of the surface by the river freshets and of the rising surface temperature. 

 Together these processes produce an average difference of about 2 to 3.5 units of 

 density between the surface and the 40-meter level in this zone by May (fig. 185). 

 The most stable state yet recorded in the gulf was off the mouth of the Saco River 

 on April 10, 1915 (Bigelow, 1914a, p. 417), when very low surface salinity (26.74 per 

 mille) was responsible for a vertical range of 4.53 in density within this stratum, 

 showing that vertical mixings had virtually ceased, for the time being. May also 

 sees the rather sudden establishment of a high degree of stabiUty in the Bay of 

 Fundy consequent on the sudden lowering of the salinity of the surface by the fresh- 

 ets from the St. John River (p. 808), Mavor (1923) having recorded a difference of 

 about 3.7 in density in the upper 40 meters on May 4, 1917, at Prince station 3, 

 where the water had been virtually homogeneous on April 9. 



The Penobscot freshet apparently has much less effect on the stability of the 

 water off its mouth; and without sufficient inrush of fresh water along the coast 

 between Penobscot Bay and Grand Manan to offset the active tidal mixing, we find 

 that in May the upper stratum of the gulf is most stable in its two opposite sides, 

 viz, Massachusetts Bay to Cape EUzabeth in the west and in the train of the St. 

 John River in the Bay of Fundy in the east. Consequently, the active vertical 

 circulation that characterizes the Bay of Fundy during most of the year is tempora- 

 rily interrupted there at this time. 



This period of temporary quiescence for the Bay of Fundy is of brief duration, 

 Mavor (1923, p. 375) showing the 40-meter stability decreasing again by June to only 

 about one-fourth of the May value as the river water is incorporated into the water 

 of the bay. 



I can not state the stability along western Nova Scotia for May; but it is not 

 likely that the small amount of fresh water emptying in along this sector of the coast 

 line can offset the active mixing which the strong tidal currents tend to effect there. 



In the offshore parts of the gulf, to which the freshening effect of the increased 

 discharge from the rivers has not yet extended, the superficial stratum is but little 

 more stable in May than in April, the average difference in density between surface 

 and 40 meters rising only to about 0.3 over the basin generally. The Nova Scotian 

 current, as it flows into the gulf from the east, is so nearly homogeneous, both in 

 temperature and in salinity, that it, too, is but slightly stable, though considerably 

 lighter than the warmer but much more saline water in the eastern side of the trough 

 over which it floats (cf. the density at station 10270, p. 988). 



In the southwestern part of the gulf generally, where tidal currents are weaker 

 than in the northeast, their mixing action is not suflacient to prevent a progressive 

 development of stability in the upper 40 meters through April, May, and June as the 

 surface warms; and as soon as the surface temperature has risen appreciably above 

 that of the underlying water, upwellings are readily recognized by their chilling effect. 



As remarked in another chapter (p. 550), water often wells up from below along 

 the western side of the gulf in spring, when offshore gales drive the surface water out 

 to sea. Bathers on New England beaches also are famiUar with this same event in 



