Such studies generally support the hypothesis that the lower limits of 
intertidal species are mainly biologically controlled while upper limits 
are more likely set by abiotic factors (Connell, 1972). Lewis (1977) 
cautioned that neither biological nor physical factors should be under- 
estimated in the distribution of rocky shore communities. 
At Murrells Inlet, physical stress most likely controlled the upper 
limits of Chthamalus fragilis and blue-green algae; no evidence of 
significant barnacle predation was observed at the higher levels. 
Although not tested, we believe the lower distribution of C. fragilis 
may be limited by competition for space with B. exustus, which formed 
dense mats of biota at 0.5 m above MLW. The relative distribution of 
both these species parallels that noted by Menge (1976) for Balanus 
balanotdes and Mytilus edults in a New England rocky intertidal system. 
Even when mussel density declined with time, other dominant forms replaced 
mussels on the bare rock space and it is likely that C. fragilis was 
still competitively excluded. 
The upper distribution of the mussels B. exustus and algae Ulva sp., 
Hypnea musctformis and Gracilaria foltifera also appeared to be regulated 
by physical factors since the mat of mussels stopped abruptly just above 
the 0.5-m level and the algae were rarely found above MLW even though 
there was space available for all these species to colonize the rocks. 
Within the intertidal and subtidal zones covered by B. exustus, mussel 
density may have been influenced by biotic factors. Intertidally, large 
numbers of birds, particularly overwintering ruddy turnstones and various 
species of gulls, were observed on the jetties during February. These 
birds were seen feeding around jetty rocks, and shell fragments of B. 
exustus were abundant in bird excrement on the jetties. Subtidally, the 
mussel predator Astertas forbesti was often observed on the rocks, 
apparently feeding on B. exustus. Therefore, it is likely that the decline 
in mussel density after the first year was due to predation since the 
natural life span of most mussels is longer than one year (Woods Hole 
Oceanographic Institution, 1952) and mussels are generally competitive 
dominants in rocky intertidal systems (Paine, 1974; Menge, 1976). 
In terms of the overall sessile community composition on the jetty 
rocks, species composition and vertical distribution patterns in the 
more physically stressed intertidal zone appear to have approached 
relative stability quickly. The well-defined bands of blue-green algae, 
barnacles, oysters and mussels were established within the first 12 
months after rock emplacement, and alterations in invertebrate community 
structure were relatively minor thereafter. Subtidally, epibenthic 
communities appeared to be less stable over the four-year study. Mussels, 
which initially dominated the subtidal community, were replaced by 
bryozoans, ascidians, cnidarians and algae with major changes occurring 
in the yearly dominance of taxa. Additionally, differences in community 
composition were observed between jetties in subtidal areas sampled 
during the same season, and even between sides on the same jetty. These 
differences were probably due to differences in time of rock submersion 
and wave exposure (Woods Hole Oceanographic Institution, 1952; Calder and 
Brehmer, 1967; Connell, 1972; Osman, 1977). However, the observations 
during this study support Sutherland's (1974) and Sutherland and Karlson's 
44 
