Survival of sand-burial by seaweeds with crustose bases or life-history stages structures the biotic community on an intertidal rocky shore

Abstract

Responses of a rocky intertidal community to seasonal sand-inundation were investigated on the cool-temperate west coast of South Africa by experimentally testing the hypothesis that the crustose components in certain macroalgae survive burial, enabling them to dominate the community when the shore is sand-free. Twelve 0.25×0.25 m plots served as controls in the mid-eulittoral and 12 in the upper eulittoral zone. Treatment 1 (T1) comprised a further 12 plots in each zone that were cleared of all biota (scraping, wire-brushing and burning) soon after the sand moved off in year 2. Treatment 2 (T2) comprised clearing a further 12 plots in each zone at the end of Year 2, just before sand returned, in case T1 had inadvertently removed early-settling microscopic stages. Mid-eulittoral zone controls developed similar communities each year dominated by Mazzaella capensis, Gymnogongrus complicatus, barnacles, mussels and small limpets, with smaller covers of several other macroalgae. Both treatments significantly reduced M. capensis, G. complicatus and G. glomeratus, but did not affect covers of other species. These three species persisted as crustose holdfasts, and the latter two also as crustose tetrasporophytes; the crusts probably comprise coalescent sporelings. All other species recruited annually as the sand moved off. In the upper eulittoral zone, T2 had no effect on the biota that developed, indicating that none of the species (including the dominants Porphyra capensis and Chthamalus dentatus) relied on persistent elements to survive burial. These results confirm that sand-disturbed communities comprise a mixture of opportunistic and disturbance-tolerant species, but experimentally show the importance of crustose elements for survival of these red algae.