EFFECTS OF REDUCED TEMPERATURES 447 



regenerated from stunted and moribund thalli. Growth during 

 posts tress periods in this population was equivalent to prestress 

 growth and to growth at control sites. With few exceptions, apical tip 

 mortality neither increased nor decreased as a result of amelioration 

 of the thermal stress. Plant survival increased markedly at previously 

 stressed sites, however. Increased plant survival and increased apical 

 growth at Foxbird Island brought about a significant increase in 

 plant biomass by 1977. 



Several aspects of our data suggest that thermal enhancement 

 (Gibbons, 1976) occurred in Ascophyllum. These effects were best 

 seen in the growth studies at experimental sites (Fig. 3 and Vadas, 

 Keser, and Rusanowski, 1976, Fig. 3). First, as we reported 

 previously, growth was initiated earlier in the spring at sites 

 influenced by warmer waters. Second, higher temperatures during 

 the summer accelerated growth processes, as predicted by Kanwisher 

 (1966). Thermal enhancement in Montsweag Bay also occurred 

 naturally and was best seen in 1976, when growth at all sites except 

 Hodgson Island was significantly higher than normal. Summer 

 temperatures in the Gulf of Maine in 1976 were the highest recorded 

 for the last 20 years (Welch, 1977). Averaging Welch's temperature 

 data (taken daily) over the maximum growth periods of 

 Ascophyllum (April through July) yields means of 11.4, 11.9, and 

 11.4°C for 1955, 1957, and 1976, respectively. A wide shoulder on 

 all Montsweag Bay and regional control temperature graphs for 1976 

 (Fig. 2) and the data of Vadas, Keser, and Larson (1977, Fig. 89) 

 support this trend for our study sites even though these temperatures 

 were measured monthly. Growth at Bailey Point illustrates both 

 types of thermal enhancement, artificial in 1973 and 1974 and 

 natural in 1976. The enhanced growth rates during poststress periods 

 at Foxbird Island and Young Point are mainly the result of natural 

 warming processes. 



These data provide strong support for the potential use of 

 Ascophyllum as an indicator of environmental stress in the North 

 Atlantic (Vadas, Keser, and Rusanowski, 1976). Borowitzka (1972) 

 suggested that benthic organisms, because of their attached habit, 

 would be expected to integrate and reveal the cumulative effects of 

 long-term exposure to stress. In areas of high industrial and sewage 

 pollution, the diversity of benthic algae declined (Grenager, 1957; 

 Borowitzka, 1972; Edwards, 1975), whereas stimulation of particular 

 species has also been observed (Cotton, 1911; Hartog, 1959; 

 Klavestad, 1967; Edwards, 1972). 



The reversal in Ascophyllum growth after elimination of thermal 

 stress and the appearance in our relatively long-term measurements 

 of changes in growth caused by thermal alterations suggest that this 



