Effects of benthic grazers on microalgal communities of morphologically different encrusting corallines: implications for abalone recruits

Abstract

Recruits of the South African abalone Haliotis midae are more abundant on irregularly textured than on smooth encrusting corallines, and experimental removal of the urchin Parechinus angulosus leads to the virtual disappearance of H. midae recruits. Explanations include the following; (1) In the absence of larger grazers, prostrate diatoms that are preferentially consumed by abalone recruits become displaced by less suitable overstorey diatoms, filamentous algae or blue-green algae; (2) textured corallines protect pockets of suitable diatoms, whereas large grazers eliminate these from smooth corallines. We tested these possibilities in aquarium experiments in which microalgal communities were grown on artificial coralline mimics with different textures and then exposed to different grazers at densities simulating those in the field. Microalgal standing stocks (measured as chlorophyll a) increased in the absence of grazers, but this was reflected only in the amounts of fila- mentous and blue-green algae. Diatoms remained unchanged in quantity and community composi- tion between different grazing treatments and substratum textures. Changes in diatom communities following urchin removal are thus unlikely to explain the ensuing disappearance of abalone recruits, although changes in filamentous and blue-green algae may do so. Textured surfaces did, however, consistently support higher stocks of microalgae than smooth surfaces (in either the presence or the absence of grazers), which may account for the preference of abalone recruits for irregular encrust- ing corallines in the field.