Vol. 19: 145-151, 1984 MARINE ECOLOGY - PROGRESS SERIES Mar. Ecol, h o g . Ser. Published August 30 Influence of sea temperature, substratum and wave exposure on rocky intertidal communities: an analysis of fauna1 and floral biomass C. D. McQuaid and G. M. Branch Zoology Department. University of Cape Town. Rondebosch 7700. South Africa ABSTRACT: Effects of sea temperature regime, substratum type and degree of wave exposure on the composition of rocky intertidal communities in the Cape of Good Hope were examined using cluster analyses of data for biomass of 310 species on 12 rocky shores. The Cape of Good Hope forms an overlap area between the south and west coast intertidal biota of South Africa (Stephenson. 1939) and species composition was found to be largely influenced by temperature regime, species richness being greatest within the area of overlap. However, biomass is dominated by very few species, the 10 species with highest biomass on each beach accounting for an average of 92 % of total biornass. If the 10 species exhibiting greatest biornass on each of the 12 beaches considered are pooled, a total of only 45 species is involved; 25 of these reveal biomass trends in response to gradients of wave exposure. Thus, while temperature regime establishes, by exclusion, a framework of species composition on each beach, the actual biomass of the species present is largely influenced by degree of exposure. Cluster analyses based on biomass data reveal groupings based primarily on the degree of exposure and secondarily on temperature. While specific influences of substratum (especially of substratum instabil- ity) were apparent, any overall effects on the biota were masked by those of exposure and temperature. INTRODUCTION Intertidal communities are affected by a wide range of physical factors including the effects of wave action, sea temperature and substratum. Considerable effort has been directed towards an understanding of the influence of these factors on intertidal populations. Temperature has long been recognised as a major factor governing species distribution (e.g. Isaac, 1938; Stephenson, 1944; Lewis, 1964) and mechanisms for this have been examined by Southward (1955, 1958), Crisp (1957), and Lewis (1964) amongst many others. Stability of substratum is obviously important and vari- ous properties of the substratum may lead to signifi- cant differences in the populations present (e.g. Barnes and Powell, 1950; Stephenson, 1961; Williams, 1964; Harlin and Lindbergh, 1977; Sousa, 1979). Likewise the degree of wave action can have important effects on the community (e.g.Kingsbury, 1962; Dayton, 1971; Hallfors et al., 1975) and relative abundance of com- mon species on various shores may be used as a biolog- ical scale for wave exposure (e.g. Ballantine, 1961; Taylor, 1971). The Cape of Good Hope forms an area of overlap between the south and west coast intertidal biota of South Africa (Stephenson, 1939), the east coast of the Cape Peninsula experiencing much warmer sea tem- peratures than the west coast which is cooled by sum- mer upwelling. Rocky shores in the peninsula may be formed by Cape granite, Malmesbury shale or Table Mountain sandstone, which have different degrees of hardness and which weather in characteristic forms, while outcrops of extremely soft, semi-consolidated beach rock occur along the northern shore of False Bay (Fig. 1). In the present study the interplay of the influ- ences of temperature, substratum and degree of expo- sure on rocky intertidal communities was examined on a quantitative basis. Twelve rocky shores were se- lected which covered a range of combinations of temp- erature, substratum and exposure. The effects and rela- tive importance of these physical parameters to the intertidal communities selected were analysed in terms of their influence on species composition and biomass as well as the richness and diversity of the communities. O Inter-ResearchPrinted in F. R. Germany