atures 3-4 °C above ambient for 4-5 hours per 

 tidal cycle. 



Ascophyllum growth patterns at the control sites 

 have remained consistent throughout the pre-op 

 and 3-unit operational periods (Figs. 12b and c). 

 Total lengths (84-89 mm) and periods of peak 

 growth (late July) were similar between stations 

 and between operational periods. 



In summary, local Ascophyllum populations 

 have not been affected by operation of Unit 3 to 

 date. Growth rates were similar to those reported 

 for Ascophyllum throughout its geographical range 

 (cf. Vadas et al. 1976, 1978; Stromgren 1977; 

 Wilce et al. 1978). 



Mortality 



Ascophyllum mortality, determined as thaUus 

 breakage, is a result of mechanical and environ- 

 mental stress. Thallus breakage could occur be- 

 low the base tag, between the base tag and the 

 colored tie wrap used as a tip tag, or between the 

 tip tag and the growing apex. Our measure of 

 tip mortality represents either loss of the tip tag, 

 or loss of all viable apices on a tagged tip. Loss 

 of tip tags implies mechanical removal and im- 

 mediate loss of plant material. Loss of viable 

 apices and/or damage to the apical cell implies a 

 potential loss of biomass due to lack of growth. 



Locally, mortality is variable from year to year. 

 Factors that contribute to mortality include degree 

 of exposure, grazing, wave-force and movement, 

 temperature, competition for space, increased drag 

 due to epiphytization, ineffective reproduction, 

 and thallus breakage (Vadas et al. 1976, 1978; 

 Bokn and Lein 1978; Seip et al. 1979). 



Mortality at each Ascophyllum site is illustrated 

 by plots of the number of remaining tips (Fig. 

 13) and number of remaining plants (Fig. 14). 

 Means of monthly values from 1979-86 (pre-op) 

 are plotted with their ranges, together with 1986-87 

 (3-unit operation) data, for the control stations 

 WP and GN. Fox Island Ascophyllum mortality 

 data are divided into three periods, corresponding 



to the temperature regimes specified in the 

 Ascophyllum growth section: 1979-84 (FL, pre- 

 op) with means of monthly values plotted with 

 their ranges; 1985-86 (FN, pre-op); and 1986-87 

 (FN, 3-unit operational). The 1984-85 pre-op 

 data are excluded because of the elimination of 

 Ascophyllum at FL in September 1984. 



Under 3-unit operational conditions, the control 

 stations had very similar mortality rates, and the 

 experimental station showed a different, higher 

 rate of mortality than the control stations for 

 both tips and plants. Tip mortality was 94% at 

 FN, and 59% and 73% at WP and GN, respec- 

 tively; plant mortality was 86% at FN, and 40% 

 and 46% at WP and GN (Figs. 13 and 14). 

 Mortality rates at FN were more precipitous than 

 at the control stations, but at all stations, greatest 

 loss in tips and plants occurred in early autumn. 



Similar patterns of Ascophyllum mortality were 

 evident in pre-op mortality data. The greatest 

 loss of tips and plants was seen at Fox Island, 

 while WP and GN had mortality rates similar to 

 each other under pre-op conditions. Approximate 

 tip and plant losses, respectively, at Fox Island 

 were 80% and 60%, at both WP and GN 75% 

 and 50% (Figs. 13 and 14). Mortality rates at 

 Fox Island were more sudden than at the control 

 stations. Greater mortaUty is typical of exposed 

 stations, such as FL and FN (cf. Jones and 

 Demetropoulos 1968; Baardseth 1970b; Seip 

 1980). 



Average Ascophyllum plant and tip losses in the 

 3-unit operational period are similar to losses re- 

 corded in the pre-op period, excluding 1984-85 

 data from FL (during and after elimination of 

 Ascophyllum at FL). I^cal plant loss data are 

 similar to those of Chock and Mathieson (1983) 

 who reported 50% of the autumn's Ascophyllum 

 standing crop in New Hampshire was lost to 

 storms and ice-rafting. Other researchers have 

 noted extensive losses of Ascophyllum axes (e.g., 

 Vadas et al. 1978; Wilce et al. 1978; Topinka et 

 al. 1981; Mathieson et al. 1982) suggesting that 

 decomposition of the fragments provides an im- 

 portant source of nitrogen to the detrital pool. 



Rocky Intertidal Studies 



49 



