1981; Lubchenco 1983). The seasonal cycle is 

 seen most clearly at Seaside Exposed. As Fucus 

 plants mature, they become increasingly suscep- 

 tible to epiphytism (Menge 1975), storm damage 

 and ice-scouring (Mathieson et al. 1982; Chock 

 and Mathieson 1983) in autumn and winter. 

 These processes tend to remove many plants at 

 once, opening new substrata for colonization and 

 perpetuating the cycle of Fucus abundance (cf. 

 Schonbeck and Norton 1980; Keser and Larson 

 1984). If these processes operate on a small scale, 

 removal of old plants from one area will be offset 

 by growth of young plants in a nearby area, and 

 the average Fucus cover at the station will be 

 relatively stable over time (e.g., SS and WP). If, 

 however, the removal process clears a large area, 

 many zygotes will settle, grow, and senesce in 

 synchrony, and produce a long-term cycle in 

 Fucus abundance, based on its 3-5 year lifespan. 

 This phenomenon has been observed locally (e.g., 

 FS and GN) and by other researchers (Niemeck 

 and Mathieson 1976; Keser and Larson 1984). 



Pre-op and 3-unit operational Fucus populations 

 were similar at most stations in the monitoring 

 program. The mid intertidal zone was dominated 

 by a Fucus canopy, and similar spatial and tem- 

 poral patterns were observed for each operational 

 period. Exceptions to these general trends in 

 Fucus abundance were observed at MP and FE 

 in the 3-unit operational period. 



Fucus abundance in Zone 2 at MP steadily de- 

 creased from 45% in 1982 to 1% just before Unit 

 3 began operation in 1986; coverage has ranged 

 from 1% to 4% in the 3-unit operational period 

 (Fig. 7). This decrease in Fucus abundance was 

 initially interpreted as the descending portion of 

 a long-term Fucus cycle, like those seen at FS 

 and at other stations, but more protracted at MP 

 than elsewhere. The delay in increased Fucus 

 abundance may be related to grazing pressure; 

 other researchers have shown that high grazer 

 densities can retard Fucus recolonization for sev- 

 eral years (Lubchenco 1983; Keser and Larson 

 1984). However, the abundance of grazers was 

 stable for most of the study's duration and similar 

 to that at other stations, despite high densities of 



Littorina littorea at MP in November 1984 (over 

 50% cover in some quadrats). Continual low 

 Fucus abundance at MP may be related to water 

 temperature; however, Kanwisher (1966) showed 

 that adult Fucus can tolerate temperatures up to 

 30 "C without thermal injury, and maximum wa- 

 ter temperatures measured at MP were ca. 25 °C. 

 To more fully understand the processes occurring 

 at MP, a schedule of temperature measurement 

 will be developed and a series of exclusion cages 

 is planned in spring 1988. As this site is the 

 second-closest station to the discharges, continued 

 monitoring is needed to determine if MP could 

 be subjected to thermal effects during 3-umt op- 

 eration. 



Fucus abundance at FE has undergone substan- 

 tial changes since the inception of the monitoring 

 program. Fucus coverage was very high from 

 1979 to 1981 (e.g., averaged ca. 60% cover in 

 Zone 2; Fig. 7) and gradually decreased to low 

 abundance (ca. 15% cover in July 1983, ca. 6% 

 cover in July 1984), suggestive of the descending 

 portion of the local 3-5 year Fucus abundance 

 cycle. The thermal impact resulting from 2-unit/ 

 2-cut operation caused elimination of the Fucus 

 population at FE in September 1984, conse- 

 quently interrupting the Fucus population cycle; 

 the expected increase in Fucus cover following 

 settlement of zygotes in spring 1984 was not seen. 

 The abundance of grazers at FE prior to thermal 

 impact was similar to that measured at other 

 rocky intertidal stations, but grazers have been 

 virtually nonexistent at FE since the time of im- 

 pact; therefore, the failure o{ Fucus to recover was 

 not due to grazing pressure. Lethal effects of high 

 water temperatures on Fucus populations have 

 been discussed in past annual reports (NUSCO 

 1985, 1986, 1987) and by other researchers (e.g., 

 Kanwisher 1966; Vadas et al. 1976). Fucus settled 

 in spring 1985, reached about 6% cover, then was 

 eliminated in late summer. Fucus zygotes settled 

 the following spring (1986, beginning of the 3-unit 

 operational period) and germlings grew to achieve 

 a higher abundance (ca. 20% in Zone 2) in sum- 

 mer. These plants survived and eventually reached 

 a peak abundance of over 60% in summer 1987. 

 It is apparent that summer conditions at FE, as 



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