underlying Chondrns (Menge 1975; Lubchenco and Menge 1978); no such detrimental relationship was 

 evident from our data. Chondrus maintained high understory abundance. 



Many other species of ephemeral algae contribute to the local intertidal community. Grouped by 

 division, some patterns are evident (Figs. 9 and 10). Generally, ephemeral algae are more common in 

 Zone 3 than in Zone 2, and more common at exposed than at sheltered stations. Many of these species 

 are opportunistic, and may occur at any time of year (e.g., Uha lactuca, Enteromorpha spp.), but others 

 show seasonal periodicity (cf Qualitative Studies section). For example, an increase in red algal abundance 

 in early spring was attributed to an increase of Bangia and Porphyra spp. Similarly, ephemeral browns 

 were most abundant in early summer (Apr.-July), corresponding to peak abundance of Petalonia and 

 Scytosiphon. Green algae, especially Enteromorpha spp., were most common in early spring and autumn. 

 Some of these abundance patterns were related to numbers and activity of grazers (Figs. 9 and 10). In 

 the MNPS area, littorinid snails (especially Littorina littorea) are the most important grazers, most active 

 in summer. 



The abundances of major components of local rocky shore communities, and therefore the structure 

 and appearance of the communities themselves, vary over time and space. Variations are predictable, and 

 explainable in terms of seasonality, degree of exposure, and life-history of the organisms. Except for the 

 alteration noted at FE following the opening of the second quarry cut, changes to local rocky shore 

 communities have been minor, indicating stable environmental conditions during 2-unit operation. This 

 stability permits a characterization of each rocky shore station in terms of patterns of abundance of its 

 major community components. A measure of this stability is illustrated in Figure 11. Each station/year 

 combination is represented by the average annual percent cover of each species; Bray-Curtis similarity 

 indices were calculated for each pair-wise comparison. When these comparisons are plotted by applying 

 a clustering algorithm to the similarity matrix, the resultant dendrogram shows several levels of grouping. 



The first level of separation distinguishes between unimpacted (group I) and impacted (group II) 

 collections. As previously described, the community that developed at FE in response to elevated water 

 temperatures was more like that found in the effluent quarry than at other sampling sites. Detection of 

 community alterations are evident from analyses of this type. 



32 



