The Distribution, Abundance, Community Structure, and Primary Productivity of Macroorganisms from Two Central California Rocky Intertidal Habitats1

A wave-exposed sea stack and a protected boulder beach at Cayucos Point, California, were compared in terms of their intertidal biota on 17-18 February 1973. The major differences between the two sites appear to be due largely to differences in the shearing forces of waves and habitat structure. The mosaic of crevices, rivulets, and angled substrates in conjunction with a broad gradual slope and reduced wave action at the boulder beach habitat resulted in a predominance of macrophytes and a zonational pattern related to both horizontal location on the shore and vertical tidal level, while sessile macroinvertebrates with zonal patterns closely correlated to tidal height dominated the sea stack. Upward shifts in comparable vertical zones at the sea stack were clearly correlated with increased wetting higher on the shore due to waves and splash, in agreement with similar findings by other workers. The most abundant macrophytes at both sites were blue-green algae and Endocladia muricata,-althoughtheotherabundant.speciesweredifferentat-each site. Eive sessile macroinvertebrates (Mytilus californianus, Chthamalus fissus, C. dalli, Balanus (Balanus) glandula, and Pollicipes polymerus) dominated the sea stack, while only three sessile speci((s (Anthopleura elegantissima, C. fissus, and C. dalll) were prevalent on the boulder beach. Of the mobile macroinvertebrates, Tegulafunebralis was the most numerous species at the boulder beach whereas the limpets Acmaea (Collisella) scabra and A. (Collisella) digitalis occurred most abundantly on the sea stack. Although a greater number of taxa and higher species richness values were recorded at the boulder beach, the evenness index and Shannon's index indicated a higher diversity on the sea stack. At the boulder beach, 12 species assemblages were defined by cluster analysis, while only 6 such groups were identified on the sea stack. The boulder beach macrophytes contributed approximately one-third more to total community primary production than did those of the sea stack (169.7 versus 116.5 net mg C m-z h), due mainly to the greater cover and concomitant production by Cyanophyta and fucalean Phaeophyta. coastal regions subjected to intensive wave exposure. However, considerable portions of the California rocky intertidal coastline consist of boulder beaches that have varying degrees of substrate stability and that receive somewhat reduced wave shock. The structure of biotic communities on rocky shores is profoundly affected by the degree of exposure to wave action. Lewis (1968) described two conof Biosequences of increased wave action: (1) the elevation of species' vertical ranges on a given shore (as a result of increased wave height 293 MUCH OF THE PUBLISHED ECOLOGICAL information on the rocky intertidal has been based, with several exceptions (e.g., Littler and Murray 1974, 1975), primarily on work done at rocky headland habitats along open 1 Manuscript received 20 October 1977. 2 California State University, Department logical Science, Fullerton, California 92634. 3 University of California, Department of Ecology and Evolutionary Biology, Irvine, California 92717.