444 VADAS, KESER, AND LARSON 



normal later in the year (Vadas, Keser, and Rusanowski, 1976). 

 During poststress years growth at both these sites was significantly 

 increased over stressed years. This was caused by: (1) increased 

 temperatures from insolation to newly exposed mud flats and the 

 subsequent conduction of heat to the water column, (2) the regional 

 warming effect in 1976, and (3) possibly the diffuser discharge. 



The effectiveness of the diffuser was evident at Bailey Point, the 

 site nearest to it. With the exception of 1976—1977, growth there 

 was significantly lower than in stressed years. To fully understand 

 the growth patterns at Bailey Point, we must understand the effects 

 of the surface discharge. Previously, a flow of warm water moved 

 northward from the southern end of Foxbird Island, past Bailey 

 Point, on the ebb tide. The periodically elevated temperatures 

 increased the growth of Ascophy Hum from spring to fall, as shown in 

 Fig. 3. Thus reduced growth in 1975—1976 and the early part of 

 1977—1978 indicates that discharge through the diffuser has less 

 effect at this site than the surface discharge had. These data suggest 

 that seasonal or local variability alone (e.g., in 1976—1977), may be 

 more important to the growth of Ascophyllum in Montsweag Bay 

 than thermal stress from the diffuser discharge. 



Mortality 



Except in 1976, plant mortality was consistently lowest (10 to 

 20%) at Hodgson Island (Fig. 4). This site provided an excellent 

 control on tag losses, which rarely exceeded 1%. The reasons for the 

 unusually low survivorship in 1976 are not known. It is possible that 

 seaweed harvesters cut plants at this site. The sustained mortality 

 throughout the rest of the year may have been induced by weakening 

 of holdfasts from the initial raking and pulling process, which ac- 

 celerated mortality in all periods. Yearly plant survivorship at Bailey 

 Point was similar for stressed and poststress years, ranging from 40 to 

 60%. Survivorship at Young Point and Foxbird Island, with the 

 possible exception of 1977—1978 at Young Point, increased sub- 

 stantially with the relocation of the thermal discharge. It is possible 

 that plants at Young Point were harvested in 1977. 



Survival of apical tips was considerably lower and less variable 

 than plant survival, even at Hodgson Island. This supports the view 

 that plant holdfasts were weakened at Hodgson Island from 

 harvesting or other causes. Tip survival at the three Montsweag Bay 

 sites was quite similar during stressed and poststress phases, with the 

 exception of Foxbird Island in 1974—1975 and possibly Young 

 Point in 1977-1978. 



