Holdfasts of adult kelp Ecklonia maxima provide refuges from grazing for recruitment of juvenile kelps

The distribution of young sporophytes (up to 0.25 m stipe length) of the kelp Ecklonia maxima, on various substrata, at depths of 2.5 to 5.0 m, was studied at 8 sites on the southwest coast of South Africa. The most common substratum available was rock (bare or covered with encrusting coralline algae), followed by kelp holdfasts, and the ascidian Pyura stolonifera at some sites. A disproportionately high ratio (relative to the available substrata) of young sporophytes grew on the holdfasts of mature kelps at most sites, but particularly where high densities of benthic invertebrate grazers were present (mainly the urchin Parechinus angulosus, also abalone Haliotis midae, limpets Patella spp, and gastropods Turbo spp, and Oxystele spp.). Jacobs' index of electivity was used an indicator of 'preference for' (interpreted as indicating survival on) the substratum type. This showed a statistically significant 'negative selection' of rock as a substratum at sites where grazers were numerous. The ratios of young sporophytes on holdfasts/young sporophytes on rock were directly proportional to grazer densities when sites were compared (r = -0.90, p = 0.002); supporting the hypothesis that mature holdfasts are an important refuge for recruitment of E. maxima sporophytes. There was an inverse relationship between percentage cover of understorey algae and grazer densities (r = 0.92, p = 0.001). In general, sites east of Cape Point (west coast/south coast transition zone) have far more grazers and reduced understorey algal biomasses compared to west coast sites. There thus appear to be fundamental differences in some of the major ecological processes in kelp beds in these 2 areas, with important implications for commercial kelp harvesting.