clarity of water and absence of herbivores at Ammen Rock Pinnacle. 

 Both factors are important but light levels undoubtedly are more 

 important at this site. Although extinction depths vary greatly 

 depending on water clarity, traditionally the percent of surface 

 illumination correlating with algal depth limits is thought to 

 remain rather constant (Hiscock, 1986). Our data conforms with 

 other published studies for algal crusts (.02% of surface 

 illumination, Sears, 1987 but see Luning and Dring, 1979; Hiscock, 

 1986), and foliose algae (.1% of surface illumination, Luning and 

 Dring, 1979; Hiscock, 1986; Sears 1987). However, we found kelp 

 growing at significantly lower light levels (0.44%) than 

 previously reported (0.7% Luning and Dring, 1979 and Sears, 1987; 

 1% Hiscock, 1986). Littler et al . (1985), however, observed lower 

 light intensities for tropical algae (.05% foliose algae, .0005% 

 crustose corallines). Thus percent surface illumination may 

 provide a good rule of thumb for certain regions and forms but the 

 values are not absolute. 



Although the depth maxima of algae are set by light, the 

 virtual absence of herbivores from the pinnacle is significant. 

 Sea urchins are dominant structuring forces in much of the 

 Northwest Atlantic (Breen and Mann, 1976) and Gulf of Maine 

 (Sebens, 1985; Witman, 1985; Vadas et al . , 1986). The causes for 

 their absence from the pinnacle is unknown, although preliminary 

 experiments suggest predation from fishes is involved (Steneck and 

 Vadas, unpublished data). Nonetheless it is the absence of 

 herbivores that allows the presumed physiological depth limits of 

 these fleshy algae to be expressed. 



Another unusual feature is the local dominance of anemone 

 patches (M. senile ) on vertical rock walls and prominences near 

 the summit of the pinnacle. In part this may be due to the 

 formation of dense mats (possibly clones) on most shallow 

 outcrops. Clonal development in marine organisms is an effective 

 strategy for occupying primary space in the subtidal zone (Jackson 

 1979) and thus competition may be preventing locally the extension 

 of kelps and other algal groups into these patches. 



There is a clear pattern in the zonation of algal species and 

 functional groups on the pinnacle. Both species and functional 

 group diversity is greatest at the shallowest depths. The 

 successive loss with depth of leathery macrophytes, corticated 

 macrophytes, noncalcified crusts and finally calcified crusts seen 

 at Ammen Rock Pinnacle (Fig. 2, Table 2) is also a global 

 phenomenon (Table 1, Fig. 4). Although it has long been 

 recognized that shallow zones dominated by large leathery 

 macrophytes also have a diverse understory of species and other 

 functional groups such as, corticated macrophytes, foliose, 

 filamentous and crustose forms (Dawson et al . , 1960, Neushul, 

 1967), the universality of this subtidal zonation pattern at a 

 functional group level has gone unrecognized. Lamb and Zimmerman 

 (1964) and Neushul (1967) recognized a tripartite pattern of 

 zonation but these zones were identified by the dominant species 

 not morphologies. For example, they identified 



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