takes place. Coverage can decrease as plants are lost due to senescence 

 or storms, especially in winter and early spring. These phenomena will 

 be discussed in a later section. Aside from the actual loss of plants, 

 apparent (canopy layer) percent coverage decrease can occur as a result 

 of overgrowth of the host plants by ephemeral algae. In Zone III, for 

 example, at most stations Chondrus experienced a decrease in percent 

 coverage in late spring, followed by an increase through the summer, 

 then a decrease in early autumn, and another increase during late autumn 

 and winter. These fluctuations were inversely related to percent coverage 

 of the ephemeral class; since the percentages given represent canopy 

 coverage, the variations represent overgrowth of Chondrus by other 

 species. In the spring, Monostroma pulchrum was the dominant ephiphyte; 

 it disappeared by summer, and Polysiphonia harveyi became the most 

 common ephemeral until autumn. This epiphytism did not appear to 

 produce enough shading to harm the Chondrus , which maintained a high 

 understory percentage, contrary to the findings of Menge (1975) and 

 Lubchenco and Menge (1978) . 



Another illustration of the spatial and temporal variability associated 

 with intertidal communities is shown when percent standardized Bray- 

 Curtis similarity coefficients are represented as clustering dendrograms. 

 Numerical classification was applied to the data, using a hierarchical 

 clustering algorithm (Boesch 1977). Figure 4 represents data from 3 

 years of monthly percent cover determinations (March 1979 - February 

 1982) , analyzed by station and zone. Three groups are apparent at the 

 50 percent level and correspond to the three designated shore zones. 

 The inclusion of FE1 and FS3 in a cluster otherwise made up of Zone II 

 stations is caused by the high proportion of Fucus in the high intertidal 



16 



