reported for the 2-unit operational period 

 (NUSCO 1987), indicating that the local commu- 

 nities, and the factors responsible for structuring 

 them, have remained relatively stable since Unit 

 3 began operation. Second, the community at 

 Fox Island- Exposed continued to appear dissim- 

 ilar to those at other rocky intertidal stations. 

 These hypotheses can be tested by analyzing lo- 

 cally prevalent species whose patterns of abun- 

 dance are important in structuring local intertidal 

 communities (e.g., Chondrus, Fucus, Balanus, 

 Mytilus), and those which, although not dominant 

 in terms of percentage of cover, interact with and 

 exert influence on other components of the com- 

 munity (keystone species sensu Paine (1966)), e.g., 

 predatory and grazing snails. 



Barnacles and predatory snails 



Barnacles have been shown to influence the 

 structure of intertidal communities by providing 

 surface heterogeneity and spatial escapes from 

 consumers for settling plants and animals that 

 would otherwise be quickly eliminated from 

 smooth substrata (Lubchenco 1983). Under 

 3-unit operational conditions, barnacles (mostly 

 Balanus halanoides) exhibited a characteristic pat- 

 tern of abundance. Balanus settlement on local 

 shores occurred as early as December- January, 

 but the period of maximum settlement was 

 February- March, earlier than reported for north- 

 ern New England (Grant 1977). Growth of newly 

 settled barnacles continued through summer, 

 when barnacles occupied almost all available pri- 

 mary space; barnacle abundance ranged from 55% 

 to 95% in Zone 2 in May (Fig. 5). A decline in 

 abundance began in late summer and barnacle 

 coverage was lowest in winter (e.g., 6-45% cover 

 in November, Zone 2). In most cases, the rock 

 surfaces exposed by the mortality of barnacles 

 remained free of macroscopic cover until the fol- 

 lowing spring. This pattern of barnacle abun- 

 dance was similar to that observed by Katz ( 1 985) 

 in local populations. 



Barnacle abundance varied among stations and 

 zones (Fig. 5). The lowest abundance of barnacles 

 was evident in the high intertidal zone, because 



of reduced settlement and slower growth due to 

 physical factors such as decreased immersion time 

 and probable desiccation (Menge 1976; Wethey 

 1985). Sheltered stations such as FS and GN 

 had a lower abundance of barnacles in Zone 1 

 ( < 10%) than at exposed stations such as FE and 

 BP (ca. 30%) where the spray zone increased 

 available moisture for settlement and survival, es- 

 pecially in cracks or crevices that retained mois- 

 ture. Competition for space was not the limiting 

 factor for barnacle abundance in the high intertidal 

 zone. - 



Conversely, at times of maximum cover (early 

 summer), barnacles covered virtually all available 

 surfaces in the mid and low intertidal zones, al- 

 though competition for space with the perennial 

 alga Chondrus crispus was responsible for the 

 lower absolute abundance of barnacles in Zone 3 

 as compared to Zone 2. Particularly in Zone 2, 

 intraspecific competition for space was a source 

 of barnacle mortality. When barnacles settle 

 densely and grow rapidly, they crowd and cannot 

 expand laterally. The consequent upward expan- 

 sion of the individuals results in relatively small 

 basal areas for attachment, and the hummocks 

 (cf. Connell 1961; Grant 1977) were lost during 

 storms. 



Predation by carnivorous snails (most com- 

 monly Urosalpinx cinerea and Thais lapillus) also 

 influences barnacle abundance, especially in the 

 low intertidal zone (Connell 1961; Menge 1976). 

 In the 3-unit operational period to date, 

 Urosalpinx (primarily) and Thais (to a lesser ex- 

 tent) were present throughout the year, but were 

 most abundant and active in late summer, and in 

 Zone 3. Maximum coverage was less than or 

 equal to 3%, but this population was sufficient 

 to reduce average barnacle cover to ca. 3% by 

 November. Similar predator abundances, and 

 similar control of barnacle populations, have been 

 reported by others (e.g., Menge 1976; Katz 1985). 



In general, abundances of barnacles and preda- 

 tory snails and their seasonal cycles during 3-unit 

 operation were similar to those of the pre-op pe- 

 riod. The single exception to these basic trends 



24 



