PHYSICAL, OCEANOGRAPHY OF THE GULF OF MAINE 



765 



The maximum alteration that took place in the surface salinity at any one of 

 these locaUties during the interval of from three to nine weeks was thus only 0.6 

 per mille; in most cases it was less than 0.2 per mille; several times it was too smaU 

 to be measured, a statement covering both sides of the basin of the gulf as weU as 

 the coastal belt, and applying to one locahty or another in three different 

 years. Among the islands or off headlands where the tide runs strong the surface 

 would not show this uniformity, because the salinity in such situations varies widely 

 with the stage of the tide. Even if the observations were taken at the same stage 

 of tide, variation would be expected with the varying interaction between current 

 and wind. UpweUings, for instance, such as follow offshore winds (p. 588) , will bring 

 up water appreciably Salter, as well as colder, from below, along the western shores 

 of the Gulf of Maine, even if the updraft comes from a depth of only a few meters. 



It is probable that the high salinity of the surface stratum recorded near 

 Gloucester on July 9, 1912 (station 10001, 32 per mille) is to be explained on this 

 basis. The saUnity of the whole upper 40 meters, or so, of water may, in fact, be 

 expected to vary considerably along the northern shore of the bay within brief periods, 

 depending on the direction of the wind as this drives the surface water onshore or 

 offshore. Unfortunately, however, our observations do not throw much light on the 

 fluctuations in salinity of this sort, except on one occasion at a locality 3 to 5 miles 

 off Gloucester, where the surface salinity, as calculated from hydrometer readings,'' 

 increased by about 0.7 per mille between July 9 and 11 in 1912, with a correspond- 

 ing decrease of 4.5° in surface temperature, the latter usually a sure evidence of 

 upwelling thereabouts. In the eastern parts of the gulf, however, where the water 

 is more nearly homogeneous vertically, winds and tides affect the surface salinity 

 chiefly by the on and off shore interchange of Salter and less saline waters. Cope- 

 land (1912), for example, found the salinity of Passamaquoddy Bay varying with 

 the tide (as well as locally in the bay) according to the relative outflow from the St. 

 Croix River. Swirling tidal currents are also partly responsible for the regional 

 variations recorded by Vachon (1918) and by Mavor (1923) in the surface salinity 

 of Passamaquoddy Bay and of the Bay of Fimdy, where, however, they also record 

 a general increase in surface salinity during July and August, as follows: 



In every August of record— 1912 (Bigelow, 1914, pi. 2), 1913 (fig. 135), 1914 

 (fig. 136), or 1915 (fig. 137) — the surface salinity has been highest over the north- 



»• Both taken with the same instrument. 

 37755—27 17 



