572 EIGHTH PACIFIC SCIENCE CONGRESS 



and strong thermal gradients. In the winter there is much greater pre- 

 cipitation but the fresh water is mixed to great depths by the violent 

 winter winds, which also reduce the thermal gradient. 



The seasonal variation occurs only in the surface zone, and never 

 extends below the salicline. Thus the properties of the water at this 

 boundary are associated with the extreme of winter conditions. Due 

 to the strong salinity gradient, it is the zone of permanent stability 

 maximum below the depth of seasonal influence. There is always a 

 temperature discontinuity of some degree coincident with the salicline. 

 Usually there is a small negative (decreasing) temperature gradient; 

 however, there were positive (increasing) gradients in the northern 

 part of the area in 1950, and in the southern part in 1951. In some 

 cases these amounted to more than a degree Centigrade and were stable 

 in the presence of large salinity gradients. 



Beneath the salicline and extending to the maximum depth sam- 

 pled, is the lower zone, where both the temperature and salinity gra- 

 dients show marked regularity. In fact they fitted the logarithmic struc 

 ture suggested by Tully (1953) within the limits of observations. 



As shown in Figure 3 the surface salinity is lowest at the coast 

 and increases seaward throughout the area, and in all seasons. The 

 lowest salinities appear to be associated with Juan de Fuca Strait, and 

 Queen Charlotte Sound, and suggest outflows of coastal waters. The 

 runoff from the land is a substantial factor in reducing the salinity of 

 the surface zone. Even in March when the offshore region was essen- 

 tially homogeneous to 100 metres depth, appreciable gradients were 

 observed near the coast. Along Vancouver Island the local runoff is 

 a maximum in the winter (Pickard and McLeod, 1953). However, the 

 greatest runoff from the coast as a whole occurs in June, when the 

 large mainland rivers flood due to melting snow in the mountains. 

 Following these floods the fresh water influence is a maximum and 

 extends far offshore. More detailed investigation immediately adja- 

 cent to the land would undoubtedly reveal a complex series of systems, 

 varying with tide, as well as the local runoff and discharges from the 

 major drainage systems. 



In the offshore region the salinity was relatively constant during 

 each survey, but changed significantly from summer to winter. In the 

 summer, the fresher waters tend to be conserved near the surface, hence 

 the salinity is less than in the winter when the waters are mixed to grear 

 depth by the violent storms. In this region the vertical salinity gradient 

 is small at all times, but tends to be greater in summer than in winter. 

 The summer gradients are presumably due to the great precipitation 

 in this area, and are preserved under conditions of strong thermal 

 stability. 



