448 VADAS, KESER, AND LARSON 



species might be an ideal candidate for monitoring stress in marine 

 ecosystems. The use of Ascophyllum has several advantages, in- 

 cluding its v^ide distribution, the similarity in phenology throughout 

 its range (MacFarlane, 1932; Printz, 1956), and the simplicity with 

 which plants can be tagged and monitored. Additionally, we have 

 shown that small- to-moderate temperature changes (natural or 

 artificial) are detectable in the field. Perhaps short-term field 

 measurements (monthly or weekly) or short-term laboratory growth 

 (Stromgren, 1977), combined with respirometry (Kanwisher, 1966), 

 might provide a more rapid assay for thermal stress. The potential of 

 Ascophyllum as an indicator organism for other stresses (pollutants) 

 remains to be tested. Rueness (1973), studying a.d\x\i Ascophyllum 

 transplanted to the polluted inner Oslofjord, observed no differences 

 in growth in transplanted vs. control plants. His sample size may have 

 been too small to detect differences, however (Rueness, 1973, 

 p. 452). 



Studying the sequence of stresses placed on the populations of 

 Ascophyllum and Fucus in Montsweag Bay provides insight into the 

 response of individual organisms to stress but little into the impact of 

 stress on community organization. Although the dominant species, 

 Ascophyllum, was disturbed significantly during surface discharge, 

 the population showed considerable resilience. This was aided by 

 power-plant shutdowns during the summers of stressed years when 

 plants were most vulnerable. Irrespective of the initial thermal 

 impact, enough individuals survived to prevent Ascophyllum from 

 being overgrown or outcompeted. Moreover, the most likely com- 

 petitor, Fucus, was also severely stressed. A few species colonized the 

 recently exposed surfaces (Vadas, Keser, and Rusanowski, 1976), but 

 none were able to displace the dominant algae. In a sense the basal 

 system of Ascophyllum is analogous to the underground perennial 

 systems of higher plants that undergo regular physical or biological 

 disturbance. Regeneration also occurs normally in wave-exposed 

 populations of Ascophyllum (Baardseth, 1970). This resilience may, 

 in fact, contribute to its continued and long- recognized dominance in 

 sheltered and moderately exposed coasts of the North Atlantic. 



The effects of thermal stress on Ascophyllum and the possible 

 ecological consequences of this stress are given in Fig. 5. In one sense 

 the graphic model is a simplification of Shelf ord's Law of tolerance, 

 where organisms respond differentially over a gradient of stress. In 

 this case, however, the stress declined exponentially with distance 

 from the source. Theoretically, the resultant zones of stress should 

 be concentric, but, because of the restriction of macroalgae to rocky 

 substrates, the actual zones are linear along the nearest shore. 



