Intertidal habitats on rubble 

 structures may serve as partial refuges 

 for palatable algal species that are 

 seasonally eaten to extinction in subtidal 

 habitats by herbivorous fishes (e.g., Hay 

 1981c, 1984a, 1985; Hatcher and Larkum 

 1983). Since seaweed productivity in 

 subtidal communities is often much less 

 than in intertidal communities, the 

 effects of herbivory on seaweeds can be 

 much greater in the subtidal zone (Hay 

 1981c; Hatcher and Larkum 1983). This 

 occurs because productivity often 

 decreases rapidly with depth due to 

 decreasing light and turbulence, which 

 provides nutrients by breaking down 

 diffusion gradients around the algal 

 thallus. Thus, the proportion of net 

 production lost to grazers can be greater 

 in deeper than in shallower waters even if 

 absolute grazing rates are equal. As an 

 example, Vine (1974) and Hay (1981b) found 

 that seaweed production was 27 to over 400 

 times greater at 2 to 3 m deep than at 13 

 to 20 m deep on tropical reefs. Thus, if 

 herbivores removed equal amounts of plant 

 material from deep and shallow sites, the 

 effects on the deep habitat plants would 

 be greater since losses would be a larger 

 portion of net growth and take longer to 

 replace. This pattern is compounded by 

 the fact that seaweeds in deeper areas are 

 always available to herbivorous fishes 

 while those in shallower areas 

 periodically escape fishes during low 

 tides and periods of turbulent seas. 

 The effects of herbivorous fishes are 

 discussed at greater length in the 

 following section on the organization of 

 sunlit, subtidal communities. 



The effects of other herbivores on 

 intertidal communities have not been 

 studied. The most common sea urchin, 

 Arbaci a punctul ata , is very prone to 

 desiccation and appears to have little 

 impact in intertidal communities. It is 

 possible that herbivorous amphipods, 

 isopods, or polychaetes could graze 

 intertidal algae and avoid desiccation by 

 sheltering in the bases of algal turfs 

 during low tide. Some of these small 

 mesograzers consume macroalgae (Glynn 

 1965; Martin 1966; Greze 1968; Nicotri 

 1977, 1980; Zimmerman et al . 1979; Lewis 

 and Kensley 1982; Norton and Benson 1983; 

 D'Antonio 1985; Gunnill 1985; Hay et al . 

 1987, 1988), but their effects in the 



intertidal zone along this coast have not 

 been studied. Herbivorous gastropods have 

 been shown to have a substantial impact on 

 intertidal algal communities in New 

 England and elsewhere (see the review by 

 Hawkins and Hartnoll 1983), but their 

 effects on intertidal hard substrates in 

 the bight have not been studied. 



Competition has been demonstrated to 

 play a substantial role in the 

 organization of intertidal algal 

 communities in New England and on the west 

 coast of the United States (Dayton 1971, 

 1975; Lubchenco 1978, 1980; and others). 

 Descriptive studies of algal seasonality 

 and zonation on jetties in the bight have 

 suggested that competition among seaweeds, 

 and between seaweeds and invertebrates is 

 important in determining community 

 organization in the intertidal zone 

 (Williams 1949; Kapraun and Zechman 1982). 

 However, no experimental evidence is 

 available to either support or refute 

 these contentions. 



4.3 ORGANIZATION OF SUNLIT, SUBTIDAL 

 COMMUNITIES 



In the shallow subtidal zone, 

 seaweeds tend to be the dominant members 

 of the sessile community. In the one 

 location where succession has been 

 studied, the algal community appeared to 

 be the result of a 3-year-successional 

 process (Van Dolah et al . 1984). Van 

 Dolah and coworkers found that the mussel 

 Brachidontes exustus dominated the 

 subtidal zone in the first year after 

 construction of the jetties at Murrells 

 Inlet. Predation by the starfish Asterias 

 forbesi i , and sheepshead appeared to 

 result in the replacement of mussels by 

 hydroids, bryozoans, and tunicates after 

 the first year. These groups in turn were 

 replaced by red and green algae by the 

 third year. This successional process was 

 not entirely predictable; it was observed 

 only on the north jetty. IL_ exustus 

 dominated the subtidal zone on the south 

 jetty for the two years that it was 

 studied (Van Dolah et al . 1984). However, 

 B_;. exustus is largely absent from the 

 subtidal zone of jetties in North Carolina 

 (J. Sutherland and M. Hay; pers. obser.). 

 It is likely that mussels are limited to 

 the intertidal zone by predation. The 



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