42 



of the Shelf Water Mass comprises the Subsurface Water Unit. 

 It is considered to be characteristic of the Shelf Water Mass 

 proper, and its upper boundary lies at the thermocline. At 

 this level more or less well-defined temperature discontinu- 

 ities are present throughout the year. Although they vary 

 greatly in the magnitude of the temperature change, there is 

 usually no difficulty in defining. the boundary between the 

 Surface and Subsurface Water Units. 



Within the Surface Water Unit, a Sewage Field Sub-unit 

 has been defined, characterized by high temperatures and low 

 salinities. 



Annual Ranges in Temperature 



The limits of the temperature range within the bay for 

 increments of ten feet of depth are shown in Figure 13. This 

 represents the yearly range at each of these depths over the 

 entire bay. It should be noted that in particular areas of 

 the bay the ranges are much smaller? for example, the tempera- 

 tures at particular localities, or at depths of 100 feet or 

 more, probably change less than 3°F. 



The Surface Water Unit is the one of greatest tempera- 

 ture change because of seasonal variations in solar heating, 

 evaporation, and other factors (Fig. 14). During the summer 

 and early fall the inshore surface water may reach tempera- 

 tures of 72°F. In the winter the same area may have a surface 

 temperature of 58°F. In the central shelf area, summer tem- 

 peratures may approach 67°F and winter temperatures, 56°F. 

 For the bay as a whole, surface temperatures may range over 



