algae, barnacles, and mussels are probably 

 controlled by physical factors (e.g. Van 

 Dolah et al . 1984). Chthamalus fraqilis 

 is probably restricted to the upper levels 

 of the barnacle zone through competition 

 with larger, faster growing Balanus spp. 

 (Wethey 1983, 1984). Similarly, the lower 

 distributional limit of barnacles is 

 probably set by competition with the 

 mussel Brachidontes exustus since 

 predation on barnacles in the intertidal 

 is minimal (Ortega 1981; Van Dolah et al . 

 1984). The lower limit of the mussel beds 

 at mean low water (Figure 5) is probably a 

 result of predation by the starfish 

 Asterias forbesii , the sheepshead, (Van 

 Dolah et al . 1984), and the Atlantic 

 oyster drill (Wood 1968). Experimental 

 studies have shown that oyster abundance 

 on exposed jetties is low because exposure 

 to heavy wave action restricts growth and 

 survival and because oysters are 

 outcompeted by the mussel Brachidontes 

 exustus (Ortega 1981). 



In protected waters it is also likely 

 that the upper limits of each zone are 

 determined by physical factors while the 

 lower limits are determined by biological 

 factors. Again, Chthamalus fragil is is 

 probably excluded from lower levels 

 through competition with the larger 

 Balanus spp. (e.g. Wethey 1984). 

 Brachidontes exustus is absent from 

 protected waters and Ortega (1981) has 

 shown that oysters competitively exclude 

 barnacles from the oyster zone. In waters 

 of relatively high salinity there is an 

 abrupt end to the oyster zone at mean low 

 water (Wells 1961). This is probably a 

 result of several biological processes, 

 including predation by oyster drills, 

 Urosal pinx cinerea (Chestnut and Fahy 

 1953) and Thais floridana (Wells and Gray 

 1960), and shell erosion by the boring 

 sponge CI iona celata (Lunz 1943). Oysters 

 are found subtidally only where low 

 salinity excludes these other species 

 (Wells 1961). 



Physical and biological disturbances 

 can cause considerable changes in the 

 abundance of organisms in each zone. For 

 example, near Beaufort, NC, Brachidontes 

 exustus was absent from pilings on the 

 open beach from May through August 1977 

 (Ortega 1981). Abundance increased to 

 nearly 100% cover in September 1977, 



remained high until February 1978, and 

 decreased again to near 0% after March. 

 Mortality seemed to be a direct result of 

 wave actioiT on mussels which had increased 

 in size during a winter of growth. It is 

 possible that the wooden substrate 

 provided a less secure attachment site 

 than the granitic rock of which jetties 

 are usually made. However, similar 

 changes in the abundance of mussels were 

 observed on jetties at Murrells Inlet, SC, 

 by Van Dolah et al . (1984), where 

 mortality was a result of winter feeding 

 by ruddy turnstones and gulls. 



The intertidal community of 

 invertebrates is resilient (Hoi 1 ing 1973); 

 the general pattern of zonation is 

 restored by recruitment within a year or 

 two after experimental clearing (Ortega 

 1981) or predation by birds on mussels 

 (Van Dolah et al . 1984). 



Although experimental data are 

 lacking, the most likely factors affecting 

 the biomass and species composition of 

 intertidal algal communities on jetties in 

 the South Atlantic Bight are desiccation 

 during low tides and grazing by fishes 

 during high tides. Green algae in the 

 genera Ul va , Enteromorpha , CI adophora , 

 Ulothrix , Chaetomorpha , and Bryopsis and 

 red algae in the genera Gel idium , 

 Ervthrotrichia , and Audouinella are 

 commonly among the more abundant seaweeds 

 in the intertidal zone (Williams 1949; 

 Kapraun and Zechman 1982). Feeding 

 preference and gut content studies show 

 these algae to be preferred or heavily 

 used by omnivorous fishes common to the 

 bight. Ogburn (1984) noted that 

 sheepshead tended to feed in the 

 intertidal zone during periods of high 

 tide and that more than 70% of the algae 

 and invertebrates in their stomachs 

 occurred primarily in the intertidal zone 

 at her study site. Hay (1986) documented 

 some of the effects of jetty fishes on 

 patterns of seaweed distribution. 

 Palatable seaweeds like Ulva and 

 Enteromorpha were almost completely 

 excluded from subtidal habitats during 

 warm periods of the year when fishes were 

 common. During cold seasons when fishes 

 were rare, Ulva and Enteromorpha were 

 common in subtidal areas. 



37 



