Biotic and abiotic influences on rocky intertidal biomass and richness in the southern Benguela region

Abstract

Species richness and biomass of 12 rocky shores subject to various conditions of substratum type, sea temperature regime, and degree of wave exposure around the Cape of Good Hope were analysed. Zonation and characteristic dominant species are described. Richness values were high (total of 310 species) and were influenced by substratum stability and temperature regime (being highest where the south and west coast biota overlapped) but not rock type or exposure. Richness was greatest where the biomass of either filter-feeders or macroalgae was high otherwise declining upshore. High limpet biomass in the cochlear zone resulted in low algal biomass owing to grazing and an interruption of this vertical gradient. Biomass was influenced by substratum stability and exposure, but not temperature regime. Forty-five species (of which 37 were macroalgae, cirripeds or molluscs) accounted for an average of over 92% of total biomass on each shore. Twenty-one of these favoured exposed conditions resulting in significantly higher total biomass on exposed shores. Biomass also showed an upshore decrease interrupted in the cochlear zone. Herbivore biomass was related to beach slope and was greatest on gently sloping shores. Biotic interactions, particularly grazing and competition for space, are discussed within this framework of abiotic factors.