Biological and Physical Factors Controlling the Spatial Distribution of the Intertidal AlgaGelidium Pristoidesin the Eastern Cape, South Africa

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Abstract
Gelidium pristoidesdominates the lower and mid-eulittoral zones of rocky shores in the eastern Cape, South Africa. A disproportionately high percentage of the plants is attached to barnacle and limpet shells, or restricted to rock crevices. Our experiments at Port Alfred indicate that this distribution is caused by grazing by the limpetPatella oculusand/or strength of attachment of the plants to the different substrata. Exclusion of limpets caused an approximately four-fold increase in the cover of G.pristoideson rock, and an increase from almost 0 to 80% cover on sterilised (dead) limpet shells. Measurements showedG. pristoidesto be more than 50% more strongly attached to barnacle and limpet shells than to rock. The distribution ofG. pristoideson the various substrata is largely determined by limpet grazing and possibly the different strengths of attachment to the different substrata. The upper distribution limit ofG. pristoidesis set by physical effects of emersion and was largely unaffected by limpet exclusion. Competition with other algae is important in setting the lower limit: in the sublittoral fringe, although limpet exclusion enhanced recruitment, juveniles were later displaced by articulated corallines, and adult transplants senesced because of encrusting coralline epiphytes.

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