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

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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. Die spesierykheid en biomassa van 12 rotsstrande, blootgestel aan verskeie toestande van substratumtipe, seetemperatuur-regime en golfaksie om die Kaap die Goeie Hoop, is geanaliseer. Sonering en tipiese dominante spesies word beskryf. Spesieryk-heidswaardes was hoog (totaal van 310 spesies) en is beïnvloed deur bodemstabiliteit en temperatuurregime (die hoogste waardes is aangetref waar die suid-en weskus-biota oorvleuel het), maar nie deur rotstipe of blootstelling nie. Rykheid was eendersyds die hoogste waar die biomassa van òf die filtreerders óf makroalge hoog was; andersyds het dit teen die strand op verminder. Hoe klipmosselbiomassa in die cochleasone het 'n lae biomassa van die alge veroorsaak, as gevolg van weiding en die onderbreking van hierdie vertikale gradiënt. Biomassa is beTnvloed deur bodemstabiliteit en blootstelling, maar nie deur die temperatuurregime nie. Vyf-en-veertig spesies (waarvan 37 makroalge, Cirripe-dia, of Mollusca was) het gemiddeld meer as 92% van die totale biomassa op elke kus verteenwoordig. Hiervan het 21 blootgestelde toestande verkies, wat 'n beduidend hoêr totale biomassa op blootgestelde strande tot gevolg gehad het. Biomassa het ook verder teen die strand op verminder, wat in die cochleasone onderbreek is. Herbivoorbiomassa hou verband met die strandhelling, en was die hoogste op strande met lae hellings. Biotiese wisselwerking, veral weiding en kompetisie vir ruimte, word teen die agtergrond van hierdie abiotiese faktore bespreek.

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